Freshwater mussel records collected by the Maryland Department of ...

Freshwater mussel records collected by the
Maryland Department of Natural Resources’
Monitoring and Non-Tidal Assessment Division
(1995-2009): Investigating environmental
Maryland Department of Natural Resoruces
Resource Assessment Service
Monitoring and Non-Tidal Assessment Division
RAS-MANTA-AIM-10-01

12-3112010-443
April 2010

Freshwater mussel records collected by the Maryland Department of Natural
Resources’ Monitoring and Non-tidal Assessment Division (1995-2009):
Investigating environmental conditions and potential host fish of select species
Prepared for
Maryland Department of Natural Resources
Wildlife and Heritage Service
Tawes State Offices Building
580 Taylor Avenue, E-1
Annapolis, MD 21401
Prepared by
Matthew J. Ashton
Maryland Department of Natural Resources
Resource Assessment Service
Monitoring and Non-Tidal Assessment Division
Tawes State Offices Building
580 Taylor Avenue, C-2
Annapolis, MD 21401
December 2009
This study was funded in part by State Wildlife Grants provided to state wildlife agencies
by the United States Congress, and administered through the Maryland Department of
Natural Resources’ Natural Heritage Program

Table of Contents
ABSTRACT ....................................................................................................................................... 1
INTRODUCTION ................................................................................................................................3
METHODS ........................................................................................................................................ 4
RESULTS .......................................................................................................................................... 6
DISCUSSION ................................................................................................................................... 48
REFERENCES .................................................................................................................................. 53
APPENDIX I.................................................................................................................................... 60
ii

List of Figures
Figure Number Page
1. Sites surveyed by MANTA for freshwater mussels during 2009…………………6
2. Locations where Alasmidonta heterodon (dwarf wedgemussel) was collected…..8
3. Box-and-whisker plots of environmental variables collected at sites where
A. heterodon was present (P) or absent (A). Conditions at sites with and
without A. heterodon were compared using the Kolmogorov-Smirnov
(water chemistry, habitat, and land use variables) or Mann-Whitney U test
(habitat metrics and biological indexes).………………………..……………10-14
4. Locations where Alasmidonta undulata (triangle floater) was collected...………15
5. Locations where Alasmidonta varicosa (brook floater) was collected…………..16
6. Locations where Anodonta implicata (alewife floater) was collected…...............17
7. Locations where Elliptio complanata (eastern elliptio) was collected......………18
8. Box-and-whisker plots of environmental variables collected at sites where
E. complanata was present (P) or absent (A). Conditions at sites with and
without E. complanata were compared using the Kolmogorov-Smirnov
(water chemistry, habitat, and land use variables) or Mann-Whitney U test
(habitat metrics and biological indexes)……………………..……………….20-24
9. Locations where Elliptio fisheriana (northern lance) was collected…………….25
10. Box-and-whisker plots of environmental variables collected at sites where
E. fisheriana was present (P) or absent (A). Conditions at sites with and
without E. fisheriana were compared using the Kolmogorov-Smirnov
(water chemistry, habitat, and land use variables) or Mann-Whitney U test
(habitat metrics and biological indexes)………………………..…………….27-31
11. Historic and recent locations where E. lanceolata (yellow lance) was
collected………………………………………………………………………….32
12. Locations where Elliptio producta (Atlantic spike) was collected………………33
13. Locations where Lampsilis sp. was collected……………………………………34
14. Locations where Lampsilis r. radiata (eastern lampmussel) was collected……..35
15. Locations where Lasmigona subviridis (green floater) was collected…………...36
iii

List of Figures
Figure Number Page
16. Locations where Leptodea ochracea (tidewater mucket) was collected………...37
17. Locations where Ligumia nasuta (eastern pondmussel) was collected………….38
18. Locations where Pyganodon cataracta (eastern floater) was collected…………39
19. Box-and-whisker plots of environmental variables collected at sites where
P. cataracta was present (P) or absent (A). Conditions at sites with and
without P. cataracta were compared using the Kolmogorov-Smirnov (water
chemistry, habitat, and land use variables) or Mann-Whitney U test (habitat
metrics and biological indexes)………………………..………..…................41-45
20. Locations where Strophitus undulatus (creeper) was collected………………….46
21. Locations where Corbicula fluminea (Asiatic clam) was collected……………..47
iv

List of Tables
Table Number Page
1. Number of sites sampled by MANTA where live and dead freshwater
mussels were collected in 2009...………………………………………………….7
2. Frequency of occurrence for fishes collected at sites throughout the range
of A. heterodon (AH) in Maryland. Non-native fishes are indicated with an
asterisk (*)…………………………………………………………………………9
3. Frequency of occurrence for fishes collected at sites throughout the range
of E. complanata (EC) in Maryland. Non-native fishes are indicated with an
asterisk (*)……......................................................................................................19
4. Frequency of occurrence for fishes collected at sites throughout the range
of E. fisheriana (EF) in Maryland. Non-native fishes are indicated with an
asterisk (*)……......................................................................................................26
5. Frequency of occurrence for fishes collected at sites throughout the range
of P. cataracta (PC) in Maryland. Non-native fishes are indicated with an
asterisk (*)…..........................................................................................................40
v

ABSTRACT
We summarized freshwater mussel data collected by the Monitoring and Nontidal
Assessment Division (MANTA) of the Maryland Department of Natural Resources
from 1995-2009. Distributional accounts for 14 species of mussels were produced. We
compared water chemistry, physical habitat, biological, and land use data collected at
sites sampled with freshwater mussels to sites where mussels were not encountered.
Alasmidonta heterodon (dwarf wedgemussel), Elliptio complanata (eastern elliptio), E.
fisheriana (northern lance), and Pyganodon cataracta (eastern floater) were present in
streams with lower gradient, greater mean width and discharge, and larger upstream
catchments with minimal urban land cover or impervious surfaces compared to streams
where they were absent. All four species were also found in streams with different
concentrations of total nitrogen and nitrate than streams were they were absent. Physical
habitat metrics provided a narrative of the conditions where mussels were present, but
were poor indicators of an ecological tolerance. Our results indicate that freshwater
mussels may be intolerant to anthropogenic disturbances, such as land use alteration and
subsequent water quality or biological impairment. Fish community data were analyzed
to identify potential fish hosts of freshwater mussels and limitations upon their
recruitment. Some known host fish were rare or absent at sites with mussels, suggesting
host availability may locally limit mussel populations. Several fishes commonly
occurred with mussels, were known glochidial hosts of congeneric mussels or
congenerics of previously confirmed host fish. We recommend further investigation into
potential host of freshwater mussels to better understand and manage this resource in
Maryland.
1

INTRODUCTION
The diversity of freshwater mussels (Family: Unionidae) in North America is
unmatched globally (Bogan 2008). They are also among the most imperiled fauna with
nearly 70% of species listed as endangered, threatened, or of concern (Williams et al.
1993). Furthermore, extinction rates of freshwater mussels currently exceed rates
observed in terrestrial environments (Allen and Flecker 1993, Ricciardi and Rasmussen
1999). The high rate of imperilment and extinction for mussels has been linked to habitat
and flow alteration, invasive species, loss of host fish, increased siltation, and dam
construction (Ricciardi et al. 1998, Brim Box and Mossa 1999, Strayer 1999a, Vaughn
and Taylor 1999, Watters 2000). Poor land use practices, point and non-point source
pollution have further disrupted freshwater ecosystems ultimately leading to the decline
of mussels throughout North America (Bogan 1993). Currently, 16 native species of
mussels are extant in Maryland (Turgeon et al. 1998), of which five are globally
imperiled or rare (Williams et al. 1993). At the state level, 14 species are listed as being
of Greatest Conservation Need (GCN), of which five are also considered state
endangered (MDNR 2007). Alasmidonta heterodon Lea 1829 is a federally endangered
species, while A. varicosa Lamark 1819 and Lasmigona subviridis Conrad 1835 are being
as considered candidates for federal listing.
Freshwater mussels are long-lived, sessile, suspension feeding bivalves that
provide essential ecological services in aquatic environments (Vaughn and Hakenkamp
2001, Strayer et al. 2004). These services include storage and transfer of nutrients from
the water column to substrate, biodeposition of organic material, nutrient mineralization,
habitat modification and engineering, and stimulating multi-level trophic production
(Nichols and Garling 2000, Raikow and Hamilton 2001, Gutierrez et al. 2003, Vaughn et
al. 2004, Spooner and Vaughn 2006, Vaughn et al. 2007, Vaughn et al. 2008). Mussel
biomass historically dominated rivers of eastern North America, and often exceeds that of
other benthic macroinvertebrates in streams, even though densities of macroinvertebrate
families are higher in habitat containing mussels than where mussels are absent
(Parmalee and Bogan 1998, Howard and Cuffey 2006, Vaughn and Spooner 2006).
Living mussels and their spent valves also provide stabile habitat and nutrients for
periphyton, vascular plants, and other benthic organisms (Beckett et al. 1996, Spooner
and Vaughn 2006, Vaughn et al. 2008). The decline of mussel fauna throughout North
America has likely had major implications for the management, conservation, and
restoration of aquatic species, communities, and functioning stream ecosystems.
The Maryland Department of Natural Resources’ Monitoring and Non-tidal
Assessment Division (MANTA) collects chemical, physical, and biological data to assess
the health of Maryland’s 1 st -4 th order wadeable streams through the Maryland Biological
Stream Survey (MBSS) (Stranko et al. 2007). The MBSS includes data from thousands
of sites that span Maryland’s five physiographic provinces, which is ideal for examining
patterns in faunal distribution across the State. Additional surveys conducted by
MANTA collected similar environmental data from areas not typically sampled by the
MBSS. This spatially intensive survey has already discovered previously unknown
mussel populations and helped refine the extent of species distributions. When these data
includes records of freshwater mussel encounters (i.e. presence), environmental
conditions and their effect upon mussel distribution can be examined. With some
3

exceptions, freshwater mussel recruitment is dependent upon the availability of an
appropriate fish host (Lefevre and Curtis 1911, Zale and Neves 1982, Barfield and
Watters 1998). Therefore, stream fish data may be useful to identify potential hosts, or
the lack thereof, for Maryland’s mussels. Describing and ultimately correlating mussel
distribution to chemical, physical, and biological variables will provide the Maryland
Natural Heritage Program (NHP) with critical information necessary to effectively
manage this imperiled faunal group.
The goals of this report are to: 1) incorporate results from sampling during 2009
by MANTA with previously collected freshwater mussel data; 2) describe environmental
conditions found coincident with mussels and throughout their observed range; and 3)
investigate the co-occurrence of potential and known fish hosts for mussels.
METHODS
Since 1995, MANTA has sampled for freshwater mussels at more than 1,600 sites
and Corbicula fluminea Müeller 1774 (Asian clam) at 3,000 sites. At MBSS sites, we
visually searched for live individuals or discarded valves of freshwater mussels and C.
fluminea in suitable habitat, animal middens, and on stream banks for ≥15 minutes. At
the remainder of sites sampled, we recorded incidental observations of mussels if we did
not conduct a visual search. We identified mussels to species in the field and noted their
condition (live or dead). When a live specimen was encountered, it was identified to
species and returned to the location where it was collected. Representative voucher shells
were retained and sent to taxonomical experts for verification of field identifications.
Distributional maps of mussel species within 8-digit watersheds of Maryland (Appendix
I) were created in GIS (ESRI ArcMap 9.3) and include records from all MANTA surveys
where observations of freshwater mussels were made.
While the methods used to collect chemical, physical, and biological data from
sample sites were standardized, the variables collected often changed. As a result, we
limited our analysis of environmental conditions to the MBSS sampling events from
2007-2009 (N = 652). Water chemistry grab samples were collected at sites during
spring base-flow (March - April) and analyzed for acid neutralizing capacity (µeq/L),
chloride (mg/L), sulfate (mg/L), total nitrogen (mg/L), nitrate (mg/L), nitrite (mg/L),
ammonia (mg/L), total phosphorus (mg/L), orthophosphate (mg/L) and dissolved organic
carbon (DOC) (mg/L), using methods described by the U.S. EPA (1987) and APHA
(1998). Slope (%) was calculated from the change in water surface height over the 75-mlong
stream segment using a surveyor’s level and metric stadia. Benthic
macroinvertebrates were collected with a 540 µm D-net from 20, 1 ft² areas of
proportionally available optimal habitat to calculate a benthic index of biotic integrity (B-
IBI) (Stribling et al. 1998). At each site, water temperature was recorded at 20 minute
intervals from June to September with Hobo data-loggers (Onset Corporation). From
these data we calculated an average of the daily mean temperature (N ≈ 92). During
summer base-flow conditions, in-situ water chemistry was measured with a multiparameter
meter for pH, dissolved oxygen (mg/L), and specific conductance (µs/cm).
We collected stream fish within each 75 m segment using a two-pass depletion method
with backpack electrofishing units (one unit per 3 m of stream width) to calculate a fish
IBI (F-IBI) (Roth et al. 1998). From stream fish data we calculated species abundance,
4

species richness, host abundance, and host richness. We visually estimated physical
habitat metrics (instream habitat, epifaunal substrate, velocity depth diversity, pool-glide
quality, and riffle-run quality) from within sample reaches using five metrics scored on a
0-20 scale. Riffle embeddedness was determined by estimating the percentage of gravel
and larger substrates that were surrounded by fine sediment (

RESULTS:
During 2009, 297 sites were surveyed for freshwater mussels throughout
Maryland by MANTA personnel (Figure 1). Live mussels or spent valves were collected
from 54 sites (18%). Typically, only one species was encountered at a site, though two
or more were found at 17 sites. The highest unionid richness (N = 6) was found in the
Potomac River at two locations; downstream of Dam No. 5 and the Marshall Hall boat
ramp. The distribution of live mussels or spent valves observed in 2009 spanned 22
watersheds. Elliptio complanata Lightfoot 1786 was the most frequently encountered
mussel, followed by indistinguishable specimens of Lampsilis cariosa Say 1817 and L.
cardium Rafinesque 1820, herein referred to as Lampsilis sp., and P. cataracta (Table 1).
The remaining species were found at five or fewer sites during 2009 surveys.
Figure 1. Sites surveyed by MANTA for freshwater mussels during 2009.
6

Table 1. Number of sites sampled by MANTA where live or dead freshwater mussels
were collected in 2009.
Number of sites mussel species were collected
Species Total Live Dead
Alasmidonta heterodon 4 1 3
Alasmidonta undulata 3 2 1
Alasmidonta varicosa 2 2
Anodonta implicata 3 1 2
Elliptio complanata 44 30 14
Elliptio fisheriana 3 3
Elliptio producta 5 2 3
Lampsilis r. radiata 1 1
Lampsilis sp. 10 3 7
Leptodea ochracea 1 1
Ligumia nasuta 1 1
Pyganodon cataracta 7 1 6
Strophitus undulatus 3 2 1
7

In 2009, A. heterodon was collected at four sites (Table 1). Since 1995, they have
been encountered 19 times at 11 sites, primarily in the Corsica River watershed (Figure
2). A single confirmed host fish (Etheostoma olmstedi) was commonly collected at sites
where A. heterodon was also present (Table 2). Two additional known hosts (Fundulus
diaphanus and Percina peltata) were rarely collected throughout the range of A.
heterodon in Maryland. Congenerics of other confirmed host fish were also infrequent.
Two fish species (Lepomis gibossus and Semotilus corporalis) previously identified as
hosts of other Alasmidonta spp. were also common at sites with A. heterodon. The
distribution and means of many variables examined were similar where A. heterodon was
present (N = 27) and absent (N = 60) (Figure 3). The pH and specific conductance of
sites with A. heterodon was higher than sites without; however, they were present at sites
with higher chloride levels. They were also primarily collected in streams with total
nitrogen and nitrate concentrations

Table 2. Frequency of occurrence for fishes collected at sites throughout the range of A.
heterodon (AH) in Maryland. Non-native fishes are indicated with an asterisk (*).
Confirmed fish hosts
Frequency of occurrence
AH Present
N=32
AH Absent
N=109
Cottus bairdi Mottled sculpin 0.00 0.00
Etheostoma nigrum Johnny darter 0.00 0.00
Etheostoma olmstedi Tessellated darter 0.90 0.72
Fundulus diaphanus Banded killifish 0.00 0.05
Morone saxatalis Striped bass 0.00 0.00
Percina peltata Shield darter 0.00 0.01
Salmo trutta* Brown trout 0.00 0.00
Congenerics of confirmed fish hosts
Cottus caeruleumentum Blue Ridge sculpin 0.00 0.00
Morone americana White perch 0.00 0.02
Confirmed hosts of congeneric mussels
Catostomus commersoni White sucker 0.32 0.21
Hypentelium nigricans Northern hogsucker 0.00 0.00
Gambusia holbrooki Eastern mosquitofish 0.19 0.08
Lepomis gibbosus Pumpkinseed 0.77 0.49
Micropterus salmoides* Largemouth bass 0.35 0.26
Notemigonus chrysoleucas Golden shiner 0.19 0.34
Noturus insignis Margined madtom 0.32 0.24
Semotilus corporalis Fallfish 0.68 0.27
Perca flavescens Yellow perch 0.03 0.07
9

Figure 3. Box-and-whisker plots of environmental variables collected at sites where A.
heterodon was present (P) or absent (A). Conditions at sites with and without A.
heterodon were compared using the Kolmogorov-Smirnov (water chemistry, habitat, and
land use variables) or Mann-Whitney U test (habitat metrics and biological indexes).
A
P
A
P
A
P
5 6 7 8
pH
p = 0.002
0 100 200 300 400 500
Conductivity (µs/cm)
0 400 800 1200 1600 2000
Acid Neutralizing Capacity (µeq/L)
A
P
A
P
A
P
16 18 20 22 24
Mean Daily Temperature (ºC)
0 2 4 6 8 10
Dissolved Oxygen (mg/L)
0 2 4 6 8 10 12 14 16 18 20 22
DOC (mg/L)
p = 0.29
p = 0.004 p = 0.68
p = 0.07 p = 0.03
10

Figure 3. continued
A
P
A
P
A
P
p = 0.08
0.00 0.05 0.10 0.15 0.20
Total Phosphorus (mg/L)
p = 0.0006
0.00 0.02 0.04 0.06 0.08 0.10
Orthophosphate (mg/L)
p = 0.11
0 2 4 6 8 10 12 14
Total Nitrogen (mg/L)
11
A
P
A
P
A
P
0 2 4 6 8 10 12
Nitrate (mg/L)
0.00 0.01 0.02 0.03 0.04 0.05 0.06
Nitrite (mg/L)
p = 0.11
p = 0.14
p = 0.24
0.0 0.1 0.2 0.3 0.4 0.5 0.6
Ammonia (mg/L)

Figure 3. continued
A
P
A
P
A
P
0 20 40 60 80
Chloride (mg/L)
0 10 20 30 40
Sulfate (mg/L)
0 2 4 6 8 10 12 14
Discharge (cfs)
p = 0.02
p = 0.47
p = 0.08
12
A
P
A
P
A
P
0 1 2 3 4 5 6 7 8
Mean Wetted Width (m)
1 2 3 4 5
F-IBI
1 2 3 4 5
B-IBI
p = 0.002
p = 0.02
p = 0.03

Figure 3. continued
A
P
A
P
A
P
0 5 10 15 20
Instream Habitat
p = 0.07
p = 0.08
0 5 10 15 20
Epifaunal Substrate
p = 0.99
0 20 40 60 80 100
% Embeddedness
13
A
P
A
P
A
P
p = 0.03
0 5 10 15 20
Velocity Depth Diversity
p = 0.18
0 5 10 15 20
Pool Glide Score
p = 0.02
0 5 10 15 20
Riffle Run Score

Figure 3. continued
A
P
A
P
A
P
0 2 4 6 8 10
% Urban
0 20 40 60 80 100
% Agriculture
0 20 40 60 80 100
% Forest
p = 0.12 p = 0.03
p = 0.06
p = 0.06
14
A
P
A
P
A
P
0 1 2 3 4 5
% Impervious Surface
p < 0.001
0 5000 10000 15000 20000
Catchment Acreage
p = 0.02
0.0 0.2 0.4 0.6 0.8 1.0
Gradient (% slope)

Since 2007, A. undulata Say 1817 have been collected at just a few locations
spread across Maryland (Figure 4). In 2009, we collected A. undulata at three sites
(Table 1). Four live individuals were found at a site in the Patapsco River downstream of
Daniels dam and a single individual downstream of Dam No. 5 in the Potomac River. It
was not possible to examine environmental conditions coincident with A. undulata
because of the paucity of collection records.
Figure 4. Locations where A. undulata (triangle floater) was collected.
15

Alasmidonta varicosa had not been previously documented by MANTA surveys
(Figure 5). In 2009, we collected five dead specimens of varied condition from two sites
in the Upper Potomac River (Table 1). It was not possible to investigate environmental
conditions coincident with A. varicosa because of the lack of collection records.
Figure 5. Locations where A. varicosa (brook floater) was collected.
16

Anodonta implicata Say 1829 was encountered at three sites in 2009 (Table 1).
They were found to be common on a gravel shoal in the Susquehanna River upstream of
Port Deposit. Previously, A. implicata was collected 8 times in four 8-digit watersheds
(Figure 6). An earlier record from Flat Run in the Upper Monocacy watershed was
determined to be a misidentification. Anodonta implicata has been collected within or in
the vicinity of tidally influenced water. Due to a lack of collection records with
associated data we were unable to investigate environmental conditions found at sites
with A. implicata.
Figure 6. Locations where A. implicata (alewife floater) was collected.
17

In 2009, E. complanata remained the most frequently encountered mussel;
collected at 44 sites (Table 1). Since 1995, they have been collected 251 times from 209
sites, a distribution spanning 46, 8-digit watersheds in Maryland (Figure 7); most of these
observations were made in the Chester River basin. Five confirmed host fishes were
common to very frequent at sites with E. complanata (Table 3). Anguilla rostrata was
found at nearly every site where E. complanata was also present. Other documented, but
uncommon host fishes were represented primarily by non-native and migratory species.
Congenerics of confirmed host fish and hosts of congeneric mussels were rarely
collected. The distribution and means of nearly every environmental variable examined
was different between sites of E. complanata presence (N = 97) and absence (N = 187)
(Figure 8). Means of 12 water chemistry variables were higher at sites with E.
complanata than sites without. The conditions documented in those variables were also
much lower at sites where E. complanata was absent. Conversely, much of the observed
dissolved oxygen and chloride concentrations at sites with E. complanata fell within a
narrow range compared to the concentrations at sites without. Scores of habitat metrics
at sites they were present usually ranged from marginal to optimal and were higher than
habitat scores at sites where they were absent. Furthermore, biological index scores were
rarely

Table 3. Frequency of occurrence for fishes collected at sites throughout the range of E.
complanata (EC) in Maryland. Non-native fishes are indicated with an asterisk (*).
Confirmed fish hosts
Frequency of occurrence
EC Present
N=194
EC Absent
N=547
Anguilla rostrata American eel 0.92 0.49
Fundulus diaphanus Banded killifish 0.06 0.04
Lepomis auritus Redbreast sunfish 0.63 0.31
Lepomis cyanellus* Green sunfish 0.25 0.23
Lepomis gibbosus Pumpkinssed 0.71 0.34
Lepomis macrochirus* Bluegill 0.69 0.49
Micropterus dolomieu* Smallmouth bass 0.07 0.08
Micropterus salmoides* Largemouth bass 0.46 0.22
Morone americana White perch 0.02 0.00
Perca flavescens Yellow perch 0.17 0.03
Pomoxis annularis* White crappie 0.01 0.00
Congenerics of confirmed fish hosts
Lepomis microlophus* Redear sunfish 0.01 0.00
Morone saxatalis Striped bass 0.01 0.00
Pomoxis nigromaculatus* Black crappie 0.06 0.03
Confirmed fish hosts of congeneric mussels
Etheostoma nigrum Johnny darter 0.00 0.00
Gambusia holbrooki Eastern mosquitofish 0.10 0.06
Dorosoma cepedanium Gizzard shad 0.02 0.01
19

Figure 8. Box-and-whisker plots of environmental variables collected at sites where E.
complanata was present (P) or absent (A). Conditions at sites with and without E.
complanata were compared using the Kolmogorov-Smirnov (water chemistry, habitat,
and land use variables) or Mann-Whitney U test (habitat metrics and biological indexes).
A
P
A
P
A
P
5 6 7 8 9
pH
p = 0.007
0 200 400 600 800
Conductivity (µs/cm)
0 500 1000 1500 2000
Acid Neutralizing Capacity (µeq/L)
A
P
A
P
A
P
p = 0.0007
14 16 18 20 22 24 26 28
Mean Daily Temperature (ºC)
p = 0.005 p = 0.001
p = 0.07
20
0 2 4 6 8 10 12
Dissolved Oxygen (mg/L)
p < 0.001
0 10 20 30 40
Dissolved Organic Carbon (mg/L)

Figure 8. continued
A
P
A
P
A
P
p = 0.0007
0.00 0.05 0.10 0.15 0.20
Total Phosphorus (mg/L)
p = 0.0009
0.00 0.02 0.04 0.06 0.08 0.10
Orthophosphate (mg/L)
p < 0.001
0 2 4 6 8 10 12 14 16
Total Nitrogen (mg/L)
21
A
P
A
P
A
P
0 2 4 6 8 10
Nitrate (mg/L)
0.00 0.01 0.02 0.03 0.04 0.05 0.06
Nitrite (mg/L)
p < 0.001
p < 0.001
p < 0.001
0.0 0.1 0.2 0.3 0.4 0.5
Ammonia (mg/L)

Figure 8. continued
A
P
A
P
A
P
0 20 40 60 80 100 120
Chloride (mg/L)
0 10 20 30 40 50
Sulfate (mg/L)
0 20 60 80
Discharge (cfs)
p = 0.03
p = 0.79
A
P
A
P
A
P
0 5 10 15 20 25 30
Mean Wetted Width (m)
1 2 3 4 5
F-IBI
p < 0.001 p < 0.001
22
1 2 3 4 5
B-IBI
p < 0.001 †
p < 0.001

Figure 8. continued
A
P
A
P
A
P
0 5 10 15 20
Instream Habitat
p = 0.03
p = 0.06
0 5 10 15 20 25
Epifaunal Substrate
p = 0.34
0 20 40 60 80 100
% Embeddedness
23
A
P
A
P
A
P
p = 0.02
0 5 10 15 20
Velocity Depth Diversity
0 5 10 15 20
Pool Glide Score
p = 0.08
0 5 10 15 20
Riffle Run Score
p = 0.0006

Figure 8. continued
A
P
A
P
A
P
0 20 40 60 80 100
% Urban
0 20 40 60 80 100
% Agriculture
0 20 40 60 80 100
% Forest
p < 0.001
p < 0.001
p = 0.0002
24
A
P
A
P
A
P
p = 0.007
0 5 10 20 25 30
% Impervious Surface
p < 0.001
0 20000 40000 60000 80000 100000
Catchment Acreage
p < 0.001
0 1 2 3 4 5
Gradient (% slope)

In 2009, E. fisheriana was collected at three sites (Table 1). Since 1995, they have
been encountered 44 times at 43 sites from 11, 8-digit watersheds of Maryland (Figure 9).
Two fish species confirmed as glochidial hosts were common at sites where E. fisheriana
was present (Table 4). Several congeneric fishes of known hosts were also frequently
collected including E. olmstedi, which co-occurred at nearly every site E. fisheriana was
encountered. Most of the confirmed fish hosts of congeneric mussels were infrequent,
except A. rostrata. Elliptio fisheriana were present (N = 38) at sites with higher pH,
dissolved oxygen, specific conductance, and ANC than sites where they were absent (N =
63) (Figure 10). A majority of sites they occupied had total nitrogen and nitrate
concentrations

Table 4. Frequency of occurrence for fishes collected at sites throughout the range of E.
fisheriana (EF) in Maryland. Non-native fishes are indicated with an asterisk (*).
Confirmed fish hosts
Frequency of occurrence
EF Present
N=54
EF Absent
N=157
Etheostoma nigrum Johnny darter 0.00 0.00
Lepomis macrochirus* Bluegill 0.73 0.63
Micropterus salmoides* Largemouth bass 0.54 0.32
Congenerics of confirmed fish hosts
Etheostoma olmstedi Tessellated darter 0.95 0.69
Lepomis cyanellus* Green sunfish 0.43 0.11
Lepomis gibbosus Pumpkinssed 0.77 0.68
Lepomis auritus Redbreast sunfish 0.63 0.43
Lepomis microlophus* Redear sunfish 0.02 0.00
Luxilus cornutus Common shiner 0.00 0.00
Micropterus dolomieu* Smallmouth bass 0.00 0.00
Confirmed fish hosts of congeneric mussels
Anguilla rostrata American eel 0.98 0.80
Dorosoma cepedanium Gizzard shad 0.02 0.00
Fundulus diaphanus Banded killifish 0.02 0.03
Gambusia holbrooki Eastern mosquitofish 0.18 0.07
Perca flavescens Yellow perch 0.14 0.20
Pomoxis nigromaculatus* Black crappie 0.13 0.06
Pomoxis annularis* White crappie 0.00 0.01
26

Figure 10. Box-and-whisker plots of environmental variables collected at sites where E.
fisheriana was present (P) or absent (A). Conditions at sites with and without E.
fisheriana were compared using the Kolmogorov-Smirnov test (water chemistry, habitat,
and land use variables) or Mann-Whitney U test (habitat metrics and biological indexes).
A
P
A
P
A
P
4 5 6 7 8 9
pH
p < 0.001
0 100 200 300 400 500
Conductivity (µs/cm)
0 400 800 1200 1600 2000
Acid Neutralizing Capacity (µeq/L)
A
P
A
P
A
P
p = 0.33
16 18 20 22 24 26 28
Mean Daily Temperature (ºC)
p = 0.05 p = 0.03
0 2 4 6 8 10
Dissolved Oxygen (mg/L)
p = 0.002 p = 0.42
27
0 5 10 15 20 25 30 35
Dissolved Organic Carbon (mg/L)

Figure 10. continued.
A
P
A
P
A
P
p = 0.02
0.0 0.1 0.2 0.3 0.4
Total Phosphorus (mg/L)
p = 0.09
0.00 0.02 0.04 0.06 0.08 0.10 0.12
Orthophosphate (mg/L)
p = 0.05
0 2 4 6 8 10 12 14
Total Nitrogen (mg/L)
28
A
P
A
P
A
P
0 2 4 6 8 10 12
Nitrate (mg/L)
0.00 0.01 0.02 0.03 0.04 0.05 0.06
Nitrite (mg/L)
p = 0.02
p = 0.09
p = 0.89
0.0 0.1 0.2 0.3 0.4 0.5 0.6
Ammonia (mg/L)

Figure 10. continued.
A
P
A
P
A
P
0 20 40 60 80
Chloride (mg/L)
0 10 20 30 40 50
Sulfate (mg/L)
Discharge (cfs)
p = 0.46
p = 0.76
p < 0.001
0 5 10 15 20 25
29
A
P
A
P
A
P
0 5 10 15 20 25
Mean Wetted Width (m)
1 2 3 4 5
F-IBI
1 2 3 4 5
B-IBI
p = 0.002
p = 0.001
p = 0.001

Figure 10. continued.
A
P
A
P
A
P
0 5 10 15 20
Instream Habitat
0 5 10 15 20
Epifaunal substrate
0 20 40 60 80 100
% Embeddedness
p = 0.002
p = 0.008
p = 0.99
30
A
P
A
P
A
P
p = 0.002
0 5 10 15 20
Velocity Depth Diversity
p = 0.005
0 5 10 15 20
Pool Glide Score
p = 0.01
0 5 10 15 20
Riffle Run Score

Figure 10. continued
A
P
A
P
A
P
0 5 10 15 20
% Urban
0 20 40 60 80 100
% Agriculture
0 20 40 60 80 100
% Forest
p = 0.02
p = 0.07
p = 0.21
31
A
P
A
P
A
P
p = 0.23
0 2 4 6 8 10
% Impervious Surface
p = 0.001
0 15000 30000 45000 60000
Catchment Acreage
p = 0.10
0 2 4 6 8 10 12
Gradient (% slope)

The status of E. lanceolata Lea 1828 in Maryland is currently uncertain because
of its cryptic nature, a lack of verified records, and the poorly understood taxonomy of
lanceolate species (Johnson 1970, Bogan et al. 2009). A single specimen identified as E.
lanceolata was reported from the Southeast Creek watershed in 1995, but after further
scrutiny it was determined to be a misidentification. Though sampling occurred within 8digit
watersheds where E. lanceolata was apparently historically and recently collected,
no individuals were encountered in 2009 (Figure 11).
Figure 11. Historic and recent locations where E. lanceolata (yellow lance) was
collected.
32

Elliptio producta Conrad 1836 has previously been encountered at 11 sites
throughout five watersheds (Figure 12). In 2009, they were collected five times at two
additional sites (Table 1). Investigating environmental conditions coincident with E.
producta was not possible at this time due to the low number of collection records with
associated ecological data.
Figure 12. Locations where E. producta (Atlantic spike) was collected.
33

The status of L. cariosa in Maryland is currently unknown. Lampsilis cardium
Rafinesque 1820, a species native to the greater Mississippi River basin, was introduced
into the Potomac River in the early 1900s (Ortmann 1912). It is believed that L. cardium
can hybridize with L. cariosa (Kelly and Rhymer 2005) and may have displaced L.
cariosa throughout its historic range in Maryland (Strayer 1999a). Furthermore, it has
proven difficult to diagnose these specimens in the field because their periostracums are
often heavily weathered or specimens exhibit characteristics of both L. cariosa and L.
cardium. As a result, specimens exhibiting characteristics of either species are
designated as Lampsilis sp. Since 1995, Lampsilis sp. has been collected 25 times from
19 sites within the Middle and Upper Potomac drainages (Figure 13). In 2009, Lampsilis
sp. was collected at 10 sites; four of these represent new localities (Table 1).
Investigating the environmental conditions found coincident with Lampsilis sp. was not
possible because their collections usually occurred incidentally.
Figure 13. Locations where Lampsilis sp. was collected.
34

Lampsilis radiata radiata Gmelin 1791 has previously been encountered in three
8-digit watersheds, of which a majority came from sites in tidally-influenced locations
which are not well surveyed for freshwater mussels. In 2009, we collected live L. r.
radiata at a location in the Potomac River downstream of Marshall Hall where we also
observed them in 2008 (Figure 14).
Figure 14. Locations where L. r. radiata (eastern lampmussel) was collected.
35

Lasmigona subviridis Conrad 1835 has been previously collected once, though
current and historical records exist near prior sampling sites (Figure 15). They were not
collected in 2009. It was not possible to examine environmental conditions coincident
with L. subviridis because of the paucity of collection records.
Figure 15. Locations where L. subviridis (green floater) was collected.
36

Leptodea ochracea Say 1817 has been previously collected from three locations
in the tidal portion of the Potomac River-Washington Metropolitan basin (Figure 16). In
2009, we observed numerous dead specimens at one of the sites where we previously
observed L. ochracea (Table 1). They remain uncommonly encountered during MANTA
sampling because habitat throughout in Maryland their range is infrequently surveyed.
Figure 16. Locations where L. ochracea (tidewater mucket) was collected.
37

Ligumia nasuta Say 1817 has been previously encountered from four sites in the
upper tidal Potomac River watershed of the Potomac River-Washington Metropolitan
basin (Figure 17). In 2009, we observed live specimens at one of the sites where we
previously collected L. nasuta (Table 1). They remain rarely encountered during
MANTA sampling because habitat throughout their range in Maryland is infrequently
surveyed.
Figure 17. Locations where L. nasuta (eastern pondmussel) was collected.
38

Pyganodon cataracta had previously been encountered 26 times at 24 sites
(Figure 18). In 2009, we collected P. cataracta at 7 sites (Table 1), three of which
represent new watershed records. Two confirmed host fish (L. gibbosus and L.
macrochirus) were frequently collected at sites where P. cataracta was also encountered
(Table 5). The four remaining confirmed host fishes were uncommon and two species
are non-natives. A majority of confirmed host fishes for congeneric mussels were also
infrequent or absent. Chemical and physical variables collected from streams where P.
cataracta were present (N = 17) and absent (N = 104) were similar for most variables
(Figure 19). A majority of occupied sites had nitrate and total nitrogen concentrations of
2-3 mg/L, while none were collected where concentrations >5 mg/L. The scores of
physical habitat variables and biological indices at sites with P. cataracta were similar to
sites where they were absent; however, they were not found at sites with pool glide
quality >7. Mean percent forest and agricultural land cover of catchments with P.
cataracta was higher than catchments where they were absent while urban land cover
was lower at sites where they were present. We typically encountered P. cataracta in
wider streams (>2 m) in larger catchments (1000-5000 ac) with very low gradient
(

Table 5. Frequency of occurrence for fishes collected at sites throughout the range of P.
cataracta (PC) in Maryland. Non-native fishes are indicated with an asterisk (*).
Confirmed hosts
Frequency of occurrence
PC Present
N=23
PC Absent
N=202
Ambloplites rupestris* Rock bass 0.00 0.00
Catostomus commersoni White sucker 0.29 0.33
Cyprinus carpio* Common carp 0.00 0.00
Lepomis gibbosus Pumpkinseed 0.79 0.55
Lepomis macrochirus* Bluegill 0.83 0.58
Perca flavescens Yellow perch 0.13 0.06
Congeneric of confirmed fish host
Lepomis auritus Redbreast sunfish 0.38 0.39
Confirmed fish hosts of congeneric mussels
Amerieus natalis Yellow bullhead 0.08 0.19
Campostoma anomalum Central stoneroller 0.00 0.01
Carassius auratus* Goldfish 0.08 0.01
Fundulus diaphanus Banded killifish 0.21 0.07
Lepomis cyanellus* Green sunfish 0.54 0.31
Luxilus cornutus Common shiner 0.04 0.09
Micropterus salmoides* Largemouth bass 0.79 0.29
Notemigonus chrysoleucas Golden shiner 0.67 0.30
Pomoxis annularis* White crappie 0.00 0.00
Pomoxis nigromaculatus* Black crappie 0.17 0.07
Semotilus atromaculatus Creek chub 0.13 0.27
40

Figure 19. Box-and-whisker plots of environmental variables collected at sites where P.
cataracta was present (P) or absent (A). Conditions at sites with and without P.
cataracta were compared using the Kolmogorov-Smirnov (water chemistry, habitat, and
land use variables) or Mann-Whitney U test (habitat metrics and biological indexes)
A
P
A
P
A
P
5 6 7 8 9
pH
p = 0.02
0 100 200 300 400 500 600
Conductivity (µs/cm)
0 400 800 1200 1600 2000 2400 2800
Acid Neutralizing Capacity (µeq/L)
A
P
A
P
A
P
p = 0.18
18 20 22 24 26 28
Mean Daily Temperature (ºC)
p = 0.59 p = 0.87
0 2 4 6 8 10 12
Dissolved Oxygen (mg/L)
p = 0.29 p = 0.30
41
0 5 10 15 20 25
Dissolved Organic Carbon (mg/L)

Figure 19. continued
A
P
A
P
A
P
p = 0.06
0.0 0.1 0.2 0.3 0.4
Total Phosphorus (mg/L)
p = 0.16
0.00 0.02 0.04 0.06 0.08 0.10
Orthophospate (mg/L)
p = 0.005
0 2 4 6 8 10 12 14
Total Nitrogen (mg/L)
42
A
P
A
P
A
P
0 2 4 6 8 10 12
Nitrate (mg/L)
0.00 0.01 0.02 0.03 0.04 0.05 0.06
Nitirite (mg/L)
p = 0.002
p = 0.001
p = 0.01
0.0 0.1 0.2 0.3 0.4 0.5
Ammonia (mg/L)

Figure 19. continued
A
P
A
P
A
P
0 20 40 60 80 100 120
Chloride (mg/L)
0 10 20 30 40
Sulfate (mg/L)
Discharge (cfs)
p = 0.48
p = 0.93
p = 0.23
0 1 2 3 4 5 6
43
A
P
A
P
A
P
0 2 4 6 8 10 12
Mean Wetted Width (m)
0 1 2 3 4 5
F-IBI
1 2 3 4 5
B-IBI
p < 0.001
p = 0.24
p = 0.08

Figure 19. continued
A
P
A
P
A
P
0 5 10 15 20
Instream Habitat
0 5 10 15 20
Epifaunal Substrate
0 20 40 60 80 100
% Embeddeness
p = 0.28
p = 0.77
p = 0.23
44
A
P
A
P
A
P
p = 0.47
0 5 10 15 20
Velocity Depth Diversity
p = 0.88
0 5 10 15 20
Pool Glide Score
p = 0.93
0 5 10 15 20
Riffle Run Score

Figure 19. continued
A
P
A
P
A
P
0 20 40 60 80 100
% Urban
0 20 40 60 80 100
% Agriculture
0 20 40 60 80 100
% Forest
p = 0.004 p = 0.09
p = 0.002
p = 0.12
45
A
P
A
P
A
P
0 10 20 30 40
% Impervious Surface
p = 0.02
0 5000 10000 15000 20000 25000 30000
Catchment Acreage
p = 0.001
0.0 0.2 0.4 0.6 0.8 1.0
Gradient (% slope)

Strophitus undulatus Say 1817 was previously encountered at four sites in the
Middle Potomac River basin (Figure 19). In 2009, S. undulatus was collected at three
sites (Table 1). Investigating their coincident environmental conditions was not possible
at this time due to the low number of collection records.
Figure 20. Locations where S. undulatus (creeper) was collected.
46

Corbicula fluminea is a non-native bivalve that was introduced in the United
States around the 1920s (Counts 1986) and has been documented throughout much of the
flowing waters of Maryland. In 2009, C. fluminea was collected at 31 sites; seven sites
were where they were previously encountered. They were found in the Gilbert Swamp,
Conocheauge Creek, and Middle Chester River watersheds for the first time. The
statewide distribution of C. fluminea currently represents 293 sites within 69, 8-digit
watersheds of Maryland (Figure 21). There are only a handful of watersheds in the state
where they have not been encountered.
Figure 21. Locations where C. fluminea (Asiatic clam) was collected.
47

DISCUSSION
When combined with previously collected records, the data from 2009 have
produced distributional accounts for 14 species of mussels, four of which are state
endangered. As in previous years, new distributional records or encounters of rare
species continued. Plots of environmental variables provide a valuable context to
describe the conditions of sites and their catchments where mussel species were collected
and potential reasons why they were absent from other sites. Some of the variability
evident in the range of environmental conditions can likely be attributed to the relatively
low sample sizes for A. heterodon and P. cataracta. This further emphasizes the need for
a larger data set of mussel presence and potentially different environmental variables
before more rigorous species level analyses are prudent. Consequently, large sample
sizes can create a situation where statistical significance may not equate to ecological
significance (Strayer and Smith 2003) and is likely evident in the analyses of some
variables for E. complanata. Finally, we caution that the relationships we report are
based on presence-absence data, which typically has weak power to relate a decline in a
species to a condition (Strayer 1999b). Given the short period of time our data analyzes
and lack of historical mussel records at sites with environmental data, it would
inappropriate to use these results to determine population declines without confirmation
of the statistical power to detect a change in rates of mussel encounters at multiple scales.
Chemical variables (pH, specific conductance, and ANC) that relate to water
acidity and ionic concentrations were generally higher at sites where each of the four
species investigated were present compared to sites where they were absent. In the
context of their range (i.e. predominantly Coastal Plain streams), this likely indicates an
avoidance of streams with acidic, blackwater conditions and soft water, which would not
be favorable for shell deposition (Strayer et al. 1981). Another re-occurring pattern
observed was the narrow range of total nitrogen and nitrate concentrations at sites where
mussels were present. It is generally assumed that increased landscape alterations and
nutrient loads impair aquatic organisms, such as unionids (Richter et al. 1997, Strayer et
al. 2004), which are among the more sensitive to ammonia (Augspurger et al. 2003). In
general, nutrient concentrations observed at sites of mussel presence were below median
lethal levels for A. heterodon, L. subviridis, and congenerics of L. cariosa and P.
cataracta (Black 2001, Augspurger et al. 2003, Wang et al. 2007a); however, chronic
ammonia exposure to juvenile mussels may have far more serious consequences on
mussel survival (Black 2001, Wang et al. 2007b). While lower nutrient concentrations
were not correlated with mussel distribution in the Upper Susquehanna and Allegheny
rivers (Strayer and Fetterman 1999, Nicklin and Balas 2007), those studies examined
nutrients at sites within a single watershed versus sites within many watersheds across a
broad region as examined in this study. The narrow range of nutrient concentrations
observed at sites with mussels compared to the broad range at sites without may provide
further evidence for their role as nutrient regulators in stream ecosystems (Vaughn and
Hakenkamp 2001, Strayer 2008, Vaughn et al. 2008). Nutrient deficient waters may also
pose limiting conditions to mussels because many of the items they consume are
primarily and secondarily produced (Strayer et al. 1981, Nichols and Garling 2000,
Raikow and Hamilton 2001). The mean and range of chloride concentrations where A.
heterodon was present in Maryland was also consistent with recent toxicological studies
48

(Black 2001, Wang 2007b). A lack of water quality standards and toxicity studies for
North American freshwater mussels hampers our ability to verify whether our empirical
observations of mussel presence and absence meaningfully relate. Other chemical
variables like dissolved oxygen, total phosphorus, and average stream temperature
produced insignificant differences for some of the mussels analyzed or inconsistent
patterns among species analyzed.
Many of the physical habitat parameters overlapped substantially between sites of
presence and absence. For two more common species, E. complanata and E. fisheriana,
habitat metric scores indicated their presence in streams with higher quality habitat as it
relates to fish and benthic macroinvertebrates. Even in light of the relationship between
various measures of fish communities and freshwater mussels (Watters 1992, Haag and
Warren 1998) and the intuitive association between fish assemblages and stream habitat
quality, it would be somewhat speculative to conclude that superior fish habitat is
conducive to mussels. Metrics with broad overlapping ranges may illustrate inherent
characteristics of habitat variability and a species that traverses multiple physiographic
and ecoregions, rather than an ecological tolerance. The pattern also illustrates the often
confounding nature of mussel-habitat associations as they relates to mussel distribution
(Strayer 2008). For example, Nicklin and Balas (2007) reported mussel density was
positively correlated with physical habitat parameters similar to parameters used in
Maryland; however, their study was conducted in a stream with a more diverse
assemblage of mussels at higher densities. Other studies (Strayer 1981, Strayer and
Ralley 1993) have found weak associations with stream reach and microhabitat scale
variables. Using a density-habitat or richness-habitat relationship as a management tool
or predictive technique should be cautioned against until such relationships have been
confirmed in Maryland. A common theme illustrated in plots for all four species
examined in this study was their presence in streams with increasing width, discharge,
and catchment size. This is not surprising considering the species area relationship that
unionids generally exhibit (Watters 1992). The hydrologic variability in headwater
streams with smaller watersheds also likely plays a role in the absence of mussels from
those habitats. For the most part, there was no clear pattern of mussel species distribution
as it relates to forested and agricultural land use. This is in part due to the prevalence of
records from the predominantly agricultural Coastal Plain. Mussels generally exhibited a
pattern of presence in catchments with very low urban land cover and impervious
surfaces, which is no surprise given the sensitivity freshwater mussels display towards
watershed degradation (Bogan 1993, Brim Box and Mossa 1999, Poole and Downing
2004). Temporal trends between land alteration and mussel abundance should be
examined because of the potential threat it presents to the fauna’s future.
Sites where mussels and stream fishes were collected concurrently provide a first
step towards identifying potential fish hosts or host-limited populations. In general, nonnative
centrarchid spp. were frequently collected with mussels. The low frequency of
occurrence for migratory species is likely an artifact of the MBSS fish sampling period
rather than host availability. Host fish for A. heterodon occurring in Maryland were
previously identified as banded killifish, Blue Ridge sculpin, striped bass, shield darter,
and tessellated darter (Michaelson and Neves 1995, White 2007); however, Blue Ridge
sculpin and A. heterodon do not have overlapping ranges in Maryland. We have rarely
collected banded killifish, shield darter, and striped bass at any site throughout the range
49

of A. heterodon. The apparent rarity of tessellated darter from a few sites where A.
heterodon was collected poses several management implications. Locally, the
populations may not be reproducing due to a lack of a suitable host or because host
abundance is too low to support consistent recruitment (Haag and Warren 1998).
Conversely, the absence of tessellated darter from a site does not entirely discount their
presence at nearby locations given their high site fidelity (McLain and Ross 2005).
Finally, other regionally suitable, yet to be documented hosts may exist. Glochidial
release rates of A. heterodon are highest during April and May (Michaelson and Neves
1995, McLain and Ross 2005); therefore, a comprehensive list of regional hosts would
require sampling when anadromous fishes ascend streams to spawn.
A diverse assemblage of host fishes exists for E. complanata in Maryland (Young
1911, Matteson 1955, Watters et al. 2005, Kneeland and Rhymer 2008); however, recent
studies indicate American eels are superior hosts for E. complanata (Lellis, unpublished
data). The diversity of host species likely allows them to inhabit a broad range of
habitats, though it is increasingly apparent that the absence of eels eventually results in
the absence of E. complanata or depressed populations (Devers 2009). It is unknown if a
density-dependent relationship exists between American eels and E. complanata that
would further explain patterns in their distribution (Haag and Warren 1998), but this is
likely given the high rate of glochidial transformation on eels and reproductive success of
E. complanata at varying densities (Downing et al. 1993, Lellis, unpublished data). Two
glochidial hosts (bluegill and largemouth bass) of E. fisheriana (O’Dee and Watters
2000) were extant throughout much of its range in Maryland, suggesting their absence
from streams may not entirely be related to the lack of a suitable host. These fishes are
not native to Maryland and often found in low abundance, indicating an unknown host
must have existed prior to their introduction. Three fishes native to Maryland that O’Dee
and Watters (2000) found not to transform glochidia were collected at sites with E.
fisheriana and may warrant further investigation. The common practice of stocking nonnative
centrachids into private and public waters has likely played a role in the ubiquitous
distribution of host generalists like P. cataracta and the presence of mussels in reservoirs
(Long 1983).
Investigating fish frequently collected with mussels or hosts of congeneric
mussels for glochidial infestations may answer whether mussel distribution in Maryland
is affected by host availability. For example, American eel was found at a majority of
sites where mussels were present; however, they have been confirmed as a glochidial host
for only a single mussel species in Maryland. Hosts of other Alasmidonta spp., including
fallfish, largemouth bass, and pumpkinseed, were sporadically collected at sites with A.
heterodon. Furthermore, nine species identified as hosts for P. grandis (Trdan and Hoeh
1982, Watters et al. 2005, Cummings and Watters 2009) were present at sites with P.
cataracta. The host fish of lanceolate Elliptios are poorly understood and the taxonomic
uncertainty of these species makes the process even more troublesome. Examining
congeneric fish for glochidia may be another possible strategy to identify glochidial
hosts. The congeneric to the widespread tessellated darter, Etheostoma nigrum (johnny
darter) has been documented as a host for E. fisheriana and the conger to P. cataracta,
yet johnny darter is not sympatric with either species in Maryland. Conversely, the
closely related tessellated darter was collected at most sites (≥80%) with these mussels.
Sculpin (Cottus spp.) serve as hosts for a suite of mussels, yet only for a few species in
50

Maryland (Cummings and Watters 2009). Examining live fish in the field or preserved
fish for glochidia may provide more conclusive answers regarding natural host use and
the recruitment of mussels in Maryland. Laboratory studies of host use and suitability
should also be considered because of the critical information it could provide to
freshwater mussel management mussels and the streams in which they reside.
The exotic Asian clam, C. fluminea, has spread throughout most of Maryland
after its introduction several decades ago (Counts 1986). Their effects upon native
mollusks are poorly understood and undocumented (Strayer 1999a), yet they are often
identified as a cause for declines in unionid populations (Clarke 1986, 1988). Cohen et
al. (1984) found that locally dense populations of C. fluminea removed a majority of the
phytoplankton within a reach of the Potomac River, which suggests their ability to out
compete unionids for food. In at least one study (Ricciardi et al. 1998), C. fluminea coexisted
with dense and diverse unionid assemblages in the Mississippi River basin and
did not appear to significantly affect the abundance or distribution of unionids. Differing
aspects of their life history, diet, and habitat may facilitate this overlap (Strayer 1999a).
At this time, there appears to be no wide-ranging effects that can be attributed to C.
fluminea. They are frequently found in streams with high mussel abundance, diverse
assemblages, or rare species, such as Brown’s Branch, Tuckahoe Creek, and Sideling Hill
Creek, as well as streams that are heavily impaired by poor water quality and land
alteration. The spatial coverage of the MBSS will continue to describe the distribution of
C. fluminea and potentially document finer scale interactions with native unionids.
Several freshwater mussel species continue to remain under-represented in this
report because the MBSS design focuses on 1 st -4 th order, wadeable streams. Significant
mussel resources have been documented in Maryland streams ≥4 th order, yet the
environmental conditions found coincident with these mussel assemblages have received
little attention. The number of sites where mussels could potentially be encountered
during MBSS sampling is initially limited because a majority of streams sampled in a
given year are 1 st order. Recording unionid presence and not species level identification
during MBSS Round 2 (2000-2004) and a vast majority of sites sampled from 2005-2006
further limited the number of freshwater mussel observations by approximately 100 sites.
Preliminary examination of these records found that many exist near locations where
NHP previously documented imperiled mussels. While an earlier version of this report
suggested that NHP mussel records could be combined with MBSS data to increase
sample sizes for rare species, this was not undertaken because of the vast differences
between the sampling methods, apparent detection rates of the two programs, and the
further reliance on presence-absence data. While a handful of sites sampled by NHP
exist in close proximity to MBSS sites where mussels were not encountered, the majority
exist well away from a referable MBSS site. Since mussel data has already been
collected at these sites, MBSS surveys should be conducted to permit these data to
potentially be included with results from MBSS Round 3 (2005-2009). We will
cautiously examine how merging NHP mussel records with MBSS data affects our prior
analyses as we collect targeted data over the coming years. Targeted sampling would
also be required to increase the detection of A. implicata, L. r. radiata, L. ochracea, L.
nasuta, and U. imbecillis, species primarily found in large rivers, lentic systems, or
tidally-influenced waters (Bogan and Proch 1997). Currently, most records for these and
51

other infrequently encountered species were the result of incidental observations made
during other MANTA surveys.
The taxonomic uncertainty of L. cariosa and lanceolate Elliptios (Johnson 1970,
Bogan et al. 2009, Watters unpublished data) further confounds our attempts to define
their distribution and conservation needs. Live and dead specimens exhibiting
characteristics of L. cariosa, L. cardium, and potential intergrades of the two can often be
found at the same location in Potomac River basin streams (Bogan pers. comm.).
Furthermore, given the indiscrete stocking of game fish around the turn of the 20 th
century that undoubtedly resulted in the introduction of L. cardium to Maryland
(Ortmann 1912, Marshall 1917, Marshall 1918, Marshall 1920) it is possible that L. ovata
Say 1817 was also introduced. Layzer (unpublished data) found smallmouth and
largemouth bass (both stocked into the Potomac River) served as glochidial hosts. We
have observed several L. ovata like specimens and Villela (pers. comm.) acknowledged
the likelihood of our suspicion. Bogan et al. (2009) recently concluded that specimens
previously identified as E. angustata Lea 1831, E. producta, and E. fisheriana from
Virginia formed a single clade. The specimens were not related to topotypic E. producta
and thus were recognized as E. fisheriana. He also found that E. lanceolata was a valid
species, but its placement within the genus Elliptio may be incorrect. Currently, E.
producta is listed as a state rare species in need of conservation (MDNR 2007). The
status of E. lanceolata is listed as uncertain due to a lack of records, its cryptic nature,
and confusion over its taxonomic and native status (MDNR 2007). The conservation
status of lanceolate Elliptios will likely need to be addressed in light of recent
phylogenetic evidence (Bogan et al. 2009). Additional survey efforts for E. lanceolata
are warranted to determine the species distribution and conservation status in Maryland.
Examination of previously collected E. lanceolata material and comparison to specimens
analyzed in Bogan et al. (2009) may also shed light on its status. It is imperative that
these cryptic unionids are properly identified. We will collaborate with researchers
working to resolve taxonomic and phylogenetic issues to ensure that consistent, quality
data are reported. If future taxonomic revisions result in changes, we suggest that
freshwater mussel data be accordingly updated in order to ensure proper communication,
management, and conservation of these imperiled resources.
52

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59

APPENDIX I
Key to the 8-digit watersheds of Maryland
60

8-digit watershed Number 8-digit watershed Number
Youghiogheny River 1 Lower Elk River 49
Casselman River 2 Upper Chesapeake Bay 50
Savage River 3 N Branch Upper Potomac River 51
Wills Creek 4 Aberdeen Proving Ground 52
Evitts Creek 5 Lower Gunpowder Falls 53
Town Creek 6 N Branch Upper Potomac River 54
Fifteen Mile Creek 7 Bohemia River 55
Sideling Hill Creek 8 Lower Winters Run 56
Little Tonoloway Creek 9 Little Youghiogheny River 57
Christina River 10 Gwynns Falls 58
Potomac River WA County 11 Jones Falls 59
Tonoloway Creek 12 S Branch Patapsco 60
Big Elk Creek 13 Gunpowder River 61
Licking Creek 14 Sassafras River 62
Little Elk Creek 15 Potomac River FR County 63
Northeast River 16 Bird River 64
Little Conococheague River 17 Back River 65
Conococheague Creek 18 N Brach Lower Patapsco River 66
Octoraro Creek 19 Stillpond-Fairlee 67
Conowingo Dam Susquehanna River 20 Brighton Dam 68
Antietam Creek 21 Upper Chester River 69
Broad Creek 22 Middle River - Browns 70
Conowingo Dam Susquehanna River 23 Aberdeen Proving Ground 71
Deer Creek 24 Baltimore Harbor 72
Loch Raven Reservoir 25 Middle Chester River 73
Prettyboy Reservoir 26 Middle Patuxent River 74
Upper Monocacy River 27 Little Patuxent River 75
Conewago Creek 28 Middle Chesapeake Bay 76
Double Pipe Creek 29 Seneca Creek 77
Potomac River AL County 30 Potomac River MO County 78
Georges Creek 31 Langford Creek 79
North Branch Lower Potomac River 32 Rocky Gorge Dam 80
Furnace Bay 33 Lower Chester River 81
L Susquehanna River 34 Rock Creek 82
North Branch Lower Potomac River 35 Southeast Creek 83
Liberty Reservoir 36 Upper Choptank 84
Marsh Run 37 Bodkin Creek 85
Catoctin Creek 38 Anacostia River 86
Potomac River AL County 39 Tuckahoe Creek 87
Deep Creek Lake 40 Severn River 88
Atkisson Reservoir 41 Magothy River 89
Upper Elk River 42 Upper Patuxent River 90
Little Gunpowder Falls 43 Corsica River 91
Bynum Run 44 Cabin John Creek 92
Swan Creek 45 Lower Chesapeake Bay 93
Back Creek 46 South River 94
Lower Monocacy River 47 Kent Island Bay 95
Bush River 48 Wye River 96
62

8-digit watershed Number 8-digit watershed Number
Eastern Bay 97 Transquaking River 126
Rock Creek 98 Little Choptank 127
Rock Creek 99 Fishing Bay 128
Rock Creek 100 Nanjemoy Creek 129
Anacostia River 101 Gilbert Swamp 130
Kent Narrows 102 Wicomico River 131
Rock Creek 103 Atlantic Ocean 132
Anacostia River 104 Assawoman Bay 133
Potomac River MO County 105 Isle of Wight Bay 134
Rock Creek 106 Wicomico River Head 135
Potomac River MO County 107 Upper Pocomoke River 136
West River 108 Potomac River Lower Tidal 137
Middle Patuxent River 109 Lower Wicomico River 138
Marshyhope Creek 110 St. Clements Bay 139
Miles River 111 Honga River 140
Oxon Creek 112 Breton Bay 141
Oxon Creek 113 Nassawango Creek 142
West Chesapeake Bay 114 Newport Bay 143
Potomac River Upper Tidal 115 Sinepuxent Bay 144
Lower Choptank 116 St. Mary's River 145
Piscataway Creek 117 Wicomico Creek 146
Western Branch 118 Dividing Creek 147
Potomac River Upper Tidal 119 Monie Bay 148
Patuxent River lower 120 Lower Pocomoke River 149
Mattawoman Creek 121 Manokin River 150
Potomac River Middle Tidal 122 Chincoteague Bay 151
Zekiah Swamp 123 Tangier Sound 152
Nanticoke River 124 Pocomoke Sound 153
Port Tobacco River 125 Big Annemessex River 154
63