WANG, WEN-XIONG, AND NICHOLAS S. FISHER ... - ASLO

LIMNOLOGY
AND
OCEANOGRAPHY
Limnol. Oceanogr., 41(2), 1996, 197-207
0 1996, by the American Society of Limnology and Oceanography, Inc.
Assimilation of trace elements and carbon by the mussel
Mytilus edulis: Effects of food composition
Wen-Xiong Wang and Nicholas S. Fisher’
Marine Sciences Research Center, State University of New York, Stony Brook 11794-5000
Abstract
March 1996
Volume 41
Number 2
Assimilation efficiency (AE) is an important physiological parameter in determining trace element influx
from food sources into aquatic animals. We used radiotracer techniques to examine the influence of diet
[seven species of algae (two diatoms, two chlorophytes, a prasinophyte, and two dinoflagellates) and glass
beads] on the assimilation of seven trace elements (Ag, Am, Cd, Co, Cr, Se, Zn) and C in the mussel Mytilus
edulis. Trace element assimilation was related to C assimilation and cytoplasmic distribution in the algae
and to gut passage time in the mussels. Mussels displayed different C AEs for the different algal diets; the
chlorophytes, which had highly refractory cell walls, were the least digestible food. Assimilation of Cd, Se,
and Zn was directly correlated with C assimilation; for Am, Ag, and Cr, no relationship with C assimilation
was apparent. For each species except the chlorophytes, AEs of all elements significantly correlated with their
cytoplasmic distribution within each algal cell. Among all species, AEs of Am, Co, and Se also increased
with elemental penetration into the cytoplasm; however, this relationship was not statistically significant for
Ag, Cd, or Zn. With the exception of Cr, AEs of elements increased with gut passage time, implying more
efficient digestion-absorption when the element was retained longer in the digestive tract. In waters containing
large mussel populations, unassimilated particle-reactive elements should be removed from suspension and
enriched in biodeposits in sediments, whereas assimilated metals should be enriched in mussel tissues.
For bivalve molluscs, both dissolved and particulate
sources can contribute to metal accumulation in tissues.
Uptake of metals from the dissolved phase occurs pri-
marily across the gills, after which metals are transported
into various organs. Accumulation of metals from par-
ticulate sources occurs by ingesting and assimilating met-
als from particulate sources. Metal influx from particles
is a direct function of the assimilation efficiency (AE) of
metals, the metal concentration in the ingested particles,
and the ingestion activity of the animal (Thomann 198 1;
Luoma et al. 1992). Assimilation represents a first-order
physiological process that can be compared among dif-
ferent metals, food types, and environmental conditions,
and AE can be used to measure metal bioavailability from
specific foods.
l Corresponding author.
Acknowledgments
We thank S. Luoma, M. Bricelj, and two anonymous review-
ers for many helpful comments.
This study was supported by grants from the U.S. EPA Office
of Exploratory Research (R8 194720 l), the New York Sea Grant
Institute (NA90AADSG078), and the National Association of
Photographic Manufacturers.
This is MSRC Contribution 1004.
197
Assimilation also determines the degree of trophic
transfer of materials (nutrients-energy) in the food web.
Many factors can influence assimilation in suspension-
and deposit-feeders, including characteristics of the ani-
mal (e.g. ingestion rate, gut volume-gut passage time, di-
gestive enzyme activity, and extracellular and intracel-
lular digestive partitioning) and the food (e.g. food quan-
tity and cell characteristics such as size, cell wall structure,
and biochemical composition) (Decho and Luoma 199 1;
Reinfelder and Fisher 199 1; Bayne 1993).
Bivalves are dominant suspension-feeders in many
coastal and estuarine environments, and calculations in-
dicate that they are capable of maintaining phytoplankton
biomass at a low level and controlling eutrophication in
coastal waters (Cloern 1982; Officer et al. 1982). Deple-
tion of phytoplankton standing stocks due to bivalve feed-
ing has been reported in many ecosystems (e.g. Nichols
1985; Dame 1993). Additionally, bivalves can be instru-
mental in regenerating nutrients for phytoplankton pro-
duction in coastal waters (Kelly et al. 1985). Thus, marine
bivalves can effectively remove particulate matter from
the water column and, through biodeposition, profoundly
affect the geochemical properties of sediments (Asmus
and Asmus 1993). Because suspended particles (including
phytoplankton) can scavenge and concentrate trace ele-
ments very appreciably from seawater (Fisher 1986), the

198 Wang and Fisher
Table 1. Algae used as diets for mussels in this study. Cell dry weights were measured by
the ammonium formate-rinsing technique.
Algae
Alexandrium tamarense
Chlorella autotrophica
Nannochloris atomus
Phaeodactylum tricornutum
Prorocentrum micans
Tetraselmis maculata
Thalassiosira pseudonana
Clone
GtL12 1
CCMP243
CCMP509
CCMP630
CCMP689
CCMP897
3H
removal of organic matter due to bivalve feeding may
have a significant impact on the geochemical cycling of
trace elements in coastal waters.
Mussels, particularly Mytilus edulis, have been used
worldwide as bioindicators to monitor coastal contami-
nation (Goldberg et al. 1978; O’Connor 1992). Metal AEs
should be tested rigorously in this bioindicator species
and bioaccumulation models should be validated by com-
paring model predictions of metal concentrations in an-
imals with metal concentrations in field-collected ani-
mals. Relatively few studies have measured metal assim-
ilation in bivalves (Decho and Luoma 199 1; Absil et al.
1994; Wang et al. 1995), and the factors that control it
are not well known. We used radiotracer techniques to
determine the influence of food quality on metal assim-
ilation by marine mussels (M. edulis). Seven species of
algal food were uniformly radiolabeled with l lomAg, 241Am,
lo9Cd, 57Co, 51Cr, 75Se, and 65Zn and fed to mussels for
30 min, after which the retention of radiotracers in in-
dividual mussels was followed for 4 d. We selected these
elements because they are of considerable geochemical
interest in marine systems, are of environmental concern
in certain coastal waters, and allow comparisons of con-
trasting affinities for important ligand types (e.g. S vs. 0)
and of essential vs. nonessential elements. Carbon assim-
ilation from these algal diets also was measured. The
overall objectives of this study were to determine the AEs
of trace elements in mussels fed different algal foods, to
compare trace element with C assimilation, and to iden-
tify the physiological processes that affect assimilation.
Materials and methods
Mussels (M. edulis, 3.0-cm shell length) were collected
from Long Island Sound between February and April and
kept in a recirculating aerated flowthrough seawater (28%0)
system at 15°C for -2 weeks before experiments; details
of the experimental apparatus are described elsewhere
(Wang et al. 1995). During this acclimation period, mus-
sels fed only on natural seston in unfiltered seawater.
Before starting the radioactive feeding experiments, mus-
sels were continuously fed an algal diet of the same species
for 2-3 d. Food concentrations typically were above the
maintenance requirements of the mussels (2% of body
dry wt d-l).
Division
Dinophyceae
Chlorophyceae
Chlorophyceae
Bacillariophy :eae
Dinophyceae
Prasinophyce ae
Bacillariophy ceae
Dry wt
(pg cell- I)
7,268 f2,046
18+1
lo+0
32+ 1
2,310f84
115f13
23+1
Radiolabeling algal cells - Seven algal species (Table
I), representing four classes and having variable digestibility
to mussels, were kept in unialgal, clonal cultures
in 0.2~pm sterile-filtered, 35%~~ surface seawater (collected
8 km off Southampton, New York) enriched with f/2
nutrients (Guill ard and Ryther 1962). Cultures in late log
phase were resuspended from polycarbonate filters into
0.2~pm filtered seawater (200-600 ml). The culture medium
was enriched with f/2 nutrients (N, P, Si, vitamins)
and f/20 trace metals minus EDTA, Cu, and Zn to minimize
metal toxicity and artificial chelation with metal
radiotracers. In experiments assessing carbon assimilation,
cells were cultured in complete f/2. The initial cell
density was be,tween 4 x 1 O4 and lo5 cells ml-l for all
cultures.
Three cultures were double- or triple labeled with l lomAg,
lo9Cd and 57Co or 241Am 75Se and 65Zn or 14C and
51Cr. The gamma emission of each nuclide was measured
and analyzed with software to account for spillover from
one isotope’s energy window to another’s. Radionuclide
additions were 1 l-l 8 kBq (corresponding to 19-3 1 nM)
of l lomAg (in 0 1 N HN03), 18.5-37 kBq (12-24 nM) of
241Am (in 3 N HN03), 370 kBq of 14C (NaH14C03, in
distilled water), 18-37 kBq (2.1-4.3 nM) of lo9Cd (in 0.1
N HCl), 18-37 kBq (1.8-3.6 PM) of 57Co (in 0.1 N HCl),
185-370 kBq (280-560 PM) of 51Cr (in 0.1 N HCl), 27-
37 kBq (300-400 PM) of 75Se (as SeOs2-, in distilled
water), and 27-54 kBq (7.4-14.8 nM) of 65Zn (in 0.1 N
HCl). Radioisotope additions typically were in microliter
amounts. Beta-lse most isotopes were dissolved in dilute
acid, the pH of the culture was kept at 8.0 by adding
microliter amclunts of 0.5 N Suprapur NaOH immediately
before isotope additions.
In experime:lts involving glass beads, two groups of
beads (5-l O-pm diam) were radiolabeled for 24 h (just
before mussel feeding) with lo9Cd and 57Co (llomAg did .
not appreciably adsorb onto the glass beads) or with 24*Am,
75Se, and 65Zn in 50 ml of 0.2-pm-filtered seawater (300
mg glass beads liter- I). Glass beads were used to evaluate
the assimilation of ingested trace elements bound to purely
inorganic substrate.
Feeding and depuration experiments-Cells were ex-
posed to isoto;Des for 4-8 d and kept on a 14 : 10 L/D
cycle at 15°C. After the cells (in log phase) had undergone
several divisions (typically > 4), they were considered to

e uniformly labeled. Algal cells were collected by filtra-
tion onto polycarbonate membranes and resuspended into
unlabeled 0.2-pm-filtered seawater. The radioactivity of
algal cells used in feeding experiments varied with the
algal species and ranged from 5 (for Chlorella autotro-
phica) to 1,500 (Tetraselmis maculata) PBq cell-’ for
llOmAg, 0.028 (C. autotrophica) to 7.5 (Alexandrium ta-
marense) mBq cell- 1 for 241Am, 1 (Nannochloris atomus)
to 1,000 (A. tamarense) PBq cell-* for lo9Cd, 4 (N. ato-
mus) to 2,600 (Prorocentrum micans) PBq cell-l for 57Co,
1 (C. autotrophica) to 1,500 (A. tamarense) PBq cell-l for
75Se, and 0.03 (N. atomus) to 7.5 (T. maculata) mBq cell-l
for 65Zn. Aliquots of these cells were analyzed for cellular
distribution of accumulated radioisotopes by means of a
differential centrifugation scheme (Fisher et al. 1983b;
Reinfelder and Fisher 199 1). The radiolabeled glass beads
were resuspended twice to remove weakly bound metals.
Radiolabeled food particles were then added to 500 ml
of glass-fiber-filtered seawater held in a l-liter polypro-
pylene beaker. Before adding the mussels, the resuspend-
ed radiolabeled cells were allowed to equilibrate for - 15
min, after which the fractions associated with the partic-
ulate phase were determined (Fisher et al. 1983a). The
cell density in the mussel feeding suspensions was ad-
justed to -0.4 mg dry wt liter-l; no pseudofeces was
produced at this cell density.
Mussels that had been preadapted to the same type of
algal diet for 2-3 d were placed individually into each
beaker and allowed to feed on this food suspension for
0.5 h, during which > 90% of the food particles were
ingested by the mussels. There were 5 replicate individ-
uals in each experimental group. A control beaker without
food, but with the same amount of dissolved radiotracers
was used to monitor mussel uptake of dissolved radioi-
sotopes that had desorbed from algal cells during the
pulse-feeding period. Immediately after the pulse feeding,
mussels were rinsed with filtered seawater and the radio-
activity of each individual mussel was counted; gamma
detection is nondestructive, so the radioactivity in the
same individual bivalve can be assessed over time (see
below). Individual mussels were then transferred into a
240-ml polypropylene chamber in an enclosed recircu-
lating seawater system (18 liter) containing the same type
of food (not radioactive) (Wang et al. 1995). In the glass-
bead experiments, the diatom Thalassiosira pseudonana
(clone 3H) was used to purge mussel guts during depu-
ration. The daily feeding rate of mussels was kept rela-
tively constant, equivalent to -3% of tissue dry wt d-l,
by dosing algal food continuously into the recirculating
aquarium with a peristaltic pump. The seawater flow rate
through the mussel chamber was 1.8 liter h-l.
The radioactivity of each mussel was measured peri-
odically (every 3-l 2 h) and noninvasively by a large-well
NaI(T1) crystal gamma detector interfaced to a multi-
channel analyzer (Canberra Series 35 plus). All counts
were corrected for decay and spillover and related to stan-
dards. Because mussels complete their digestion and as-
similation of food within 3 d (Wang et al. 1995), they
were allowed to evacuate their guts for 4 d, after which
they were dissected and the radioactivity associated with
Trace element assimilation in mussels 199
the shell was determined. Because ingested food was the
principal source of the radionuclides for animals, radioactivity
in the shells generally represented a small fraction
of total body activity and was subtracted from the total
whole body count before the calculation of metal retention
in mussel tissues.
During the depuration period, fecal pellets were collected
every l-3 h for the first 24 h and every 4-12 h
thereafter. Radioactivity of fecal pellets was measured
with an LKB Compugamma NaI (Tl) detector that was
intercalibrated with the other gamma counter. Gamma
emissions were detected at 65 8 keV for 1 lomAg, 60 keV
for 241Am, 88 keV for lo9Cd, 122 keV for 57Co, 230 keV
.for 51 Cr, 264 keV for 75Se, and 1,115 keV for 65Zn. Count-
.ing times were adjusted so that the propagated counting
errors were generally < 5%, except for some samples whose
counts were not significantly above background levels.
AE was defined as the proportion of ingested metal
retained after completion of digestion and gut evacuation
(at 72 h). C assimilation was determined either by the
mass balance method (14C retained divided by 14C ingested,
using both feces and mussel tissue data) or the
i4C : 51 Cr ratio method (Calow and Fletcher 1972) by assuming
that 51Cr was inert to the mussels. C AE therefore
was calculated as
(14c~51wfeccs
1
x 1oo
(14C/51Cr),,, ’
(14c/5’wreces is the ratio of 14C radioactivity to 51Cr radioactivity
in mussel cumulative feces (72 h), and (14C/
51Cr)f00d is the ratio of 14C radioactivity to 51 Cr radioactivity
in algal cells during the radioactive feeding.
Fecal pellets and mussel soft tissues were solubilized
(Solvable, NEN Research Product) and scintillant (Ultima
Gold XR, Packard) was added to these samples to
measure 14C radioactivity. Because 51 Cr emissions can
interfere with 14C counting, all 14C samples were counted
after four half-lives of 51Cr (t,/, = 27.7 d) to minimize
interference by 51Cr. The 14C activity was measured with
an LKB Rack Beta liquid scintillation counter. Quenching
was corrected with the external standard ratio method.
Results
Elemental retention in mussel soft tissues-The retention
of trace elements and C from different algal diets and
glass beads in mussel soft tissues during the96-h depuration
period is shown in Fig. 1. Generally, depuration
was characterized by an initial rapid egestion of radiolabeled
materials in the first 24 h, followed by a period
of more gradual loss. Elements differed greatly in their
egestion patterns. Most unassimilated 241Am, 14C, lo9Cd,
5 1 Cr, and 75Se was egested within the first 24 h, after which
very little radioactivity was lost (as confirmed by fecal
pellet data). By contrast, the second phase of digestion
(after 24 h) played a significant role in determining metal
assimilation of 1 lomA g, 57Co, and 65Zn. These metals were
lost continuously from the mussel tissues throughout this
period. Our results confirmed that 51Cr was highly inert

B
100
0 20 40 80 80 100
60 80 100 0 20 40 60 80 100
+----7do
0 20 40 60
10-1 I 10-1
0 20 40 60 80 100 (i) 20 40 60 80 100
Time
Fig. 1. Trace element and C retention (%) in mussels fed on
different algal diets during the 96-h depuration period. In glass-
bead experiments. Thalassiosira pseudonana cells were used to
purge mussel guts during depuration. Only means (based on 5
replicate individuals) are presented, but the standard deviations
were generally small (Table 2). T. pseudonana-m; Phaeodac-
tylum tricornutum- 0; Chlorella autotrophica-0; Nanno-
chloris atomus-0; Alexandrium tamarense-A; Prorocentrum
micans - A; Tetraselmis maculata - +; glass beads- x .
Wang and Fisher
to the mussels, as > 98% of the ingested 5 * Cr was defecated
by 24 h. In all experiments, uptake of all radioisotopes
from the dissolved phase was negligible (~5% of that
ingested by the mussels) during the radioactive feeding
period.
Element AEs were calculated as the % of the amount
of radioactivity retained at 72 h divided by the amount
of radioactivity ingested (Table 2). Mussels assimilated
the essential trace elements (57Co, 75Se, and 65Zn) with a
higher efficiency than the nonessential elements (* *OrnAg,
241Am, lo9Cd, and 51Cr); among all trace elements 75Se
was assimilated most efficiently. lo9Cd 57Co and ‘j5Zn
were assimilated moderately (1 O-50%): AEs ‘for 241Am
were consistently < 7%, indicating that this element was
relatively inert to the mussels and was a tracer of food
passage through the digestive process; however, for the
other elements AEs varied widely among different food
types (Table 2). Generally, lo9Cd, 75Se, and 65Zn from the
chlorophytes C’. autotrophica and N. atomus were con-
sistently assimilated with a lower efficiency than from
other algae, but this trend was not apparent for * *OrnAg,
241Am, 57Co, o:- 51Cr The effect of food type on AE was
greatest for * lorr Ag, for which there was a difference of up
to lo-fold among algal species (Fig. 1, Table 2).
C AEs were calculated by the mass balance method and
the 14C : 51Cr ratio method (Table 3). AEs calculated by
these two methods were comparable for each algal diet,
except for Phaeodactylum tricornutum and T. maculata,
for which the ratio method gave a significantly lower AE
than the mass balance method. C from T. pseudonana
was assimilated by mussels most efficiently (85%), where-
as C from another diatom, P. tricornutum, was only modi-
erately assimilzted (54%), comparable to the assimilation
from two dinollagellates (P. micans and A. tamarense).
Again, AEs of C from the two green algae were the lowest,
measuring only 2 1% for C. autotrophica and 8% for N.
atomus.
Assimilation of the trace elements adsorbed to glass
beads was comparable to assimilation from algal diets,
except for 57Ccj for which assimilation (43%) was signif-
icantly higher l;han that from algal food (20-38%; Table
2). AEs of Se and Zn from glass beads were intermediate
between the low value for N. atomus and values for other
algal species.
For all algal species, there was a significant correlation
between C AE (calculated by the mass balance method)
Table 2. Trace element assimilation efficiencies in mussels (O/o, mean + SE; n = 5) for each food type. (Not determined--d.)
Food type Ae Am Cd co Cr Se Zn
Alexandrium tamarense 33.8k2.5 1.5kO.l 24.12 1.2 24.8-c- 1.3 0.6-t-0.0 60.724.3 37.8k4.0
Chlorella autotrophica 12.4-t 1.4 4.9+ 1.0 13.4+ 1.6 20.5 k13.6 1.1 f0.7 48.0f4.1 35.8f2.1
Nannochloris atomus 22.4+ 1.7 1.7kO.4 11.3k2.4 24.0+ 1.1 0.6kO.O 13.7+ 1.2 16.0f2.0
Phaeodactylum tricornutum 9.3k2.3 6.220.7 15.3k2.9 19.5+ 2.8 0.9ao. 1 55.1k1.9 48.3k2.7
Prorocentrum micans 10.6k 1.2 3.320.4 28.3k2.6 26.1k1.6 0.5ao.o 55.8k4.6 44.4+. 1 .o
Tetraselmis maculata 3.8kO.4 0.7&O. 1 23.5k3.0 28.6k 3.2 1.3kO.5 71.9+ 1.8 31.9f2.7
Thalassiosira pseudonana 16.8k2.3 4.6kO.7 34.3k4.7 37.7f 3.5 0.2fO.O 71.5k2.0 43.8+ 1.9
Glass beads nd 3.020.3 21.023.2 43.2+ 2.0 nd 27.9k3.5 30.2+ 1 .O

Trace element assimilation in mussels
Table 3. Comparison of C assimilation efficiency (AE, %, mean k SE; n = 5) measured
by the mass balance method and the 14C : 51Cr ratio method. Both 24- and 72-h AEs ‘are
shown.
Algae
Alexandrium tamarense
Chlorella autotrophica
Nannochloris atomus
Phaeodactylum tricornutum
Prorocentrum micans
Tetraselmis maculata
Thalassiosira pseudonana
24 h
72 h
24 h
72 h
24 h
72 h
24 h
72 h
24 h
72 h
24 h
72 h
24 h
72 h
and AE for lo9Cd, 75Se, and 65Zn (Fig. 2). These relation-
ships are best described as
lo9Cd: AE, = 7.60 + 0.30 AEc
(r = 0.838, n = 7, P < O.Ol),
75Se: AE, = -33.4 + 54.8 x log AEc
(r = 0.915, n = 7, P < O.Ol),
65Zn: AE, = - 10.4 + 29.7 x log AE,
(r = 0.907, n = 7, P < 0.01).
AE, is trace element assimilation efficiency (O/o) at 72 h
and AEc is C assimilation efficiency (%) at 72 h. By contrast,
assimilation of llomAg, 241Am and 51Cr was not
correlated (P > 0.05) with C assimilation, and only a
weak correlation was detected for 57Co at 24 h (Fig. 2)-
57co: AE, = 27.00 + 0.28AEc
(r = 0.708, n = 7, P < 0.05).
Assimilation in relation to element cytoplasmic distribution
in algal cells-For all algal diets, the AE of each
trace element was assessed in relation to its cytoplasmic
distribution in the algal cells used as food (Fig. 3, Table
4). Statistically significant correlations were found between
AEs and cytoplasmic distributions for 241Am, 57Co,
and 75Se (with P < 0.05), whereas for llomAg, ro9Cd, and
65Zn, correlations were not statistically significant. The
slope of the linear regression was - 1 for 241Am, but for
other elements it was < 1, indicating that element cytoplasmic
distribution had the greatest effect on the assimilation
of 241Am. For each algal species, AEs for the seven
trace elements were also assessed in relation to their cytoplasmic
distribution in each algal species (Fig. 4, Table
Mass
balance
AE
58.8k6.3
56.6k6.1
22.0+ 3.9
21.Ok4.0
11.8k2.2
8.1f1.5
57.6k2.6
54.2k2.6
56.3k4.1
52.3k3.6
43.1 k3.0
40.3f3.0
86.2k4.3
85.1k4.1
60
14C : 51Cr ratio
14C : 51Cr in
AE labeled food
6O.OIk6.0 0.904
58.Ok5.7
19.423.3 0.095
18.6k3.3
9.4+ 1.6 1.018
6.8k 1.1
38.2k4.4 0.171
33.3k4.3
53.2k4.0 0.67 1
48.7k4.4
25.Ozk5.4 0.46 1
21.5k5.3
87.Ok3.9 0.124
86.2k3.8
OJ ’ ’ - . m -
0 20 40 60 80 1 3
Cd
0-I 0 20 40 60 60 100
-0 20 40 60 80 100
60
OJ ’ ’ ’ . * -
0 20 40 60 80 1 0
0 20 40 60 80 100
Zn B
F
F
c
E
G
D
0 20 40 60 80 100
% carbon assimilated
Fig. 2. Trace element assimilation efficiencies (mean f SD;
n = 5) in relation to C assimilation efficiencies (mean zk SD; n
= 5) in mussels fed on different algal species. C assimilation
efficiency was calculated by the mass balance method. Thal-
assiosira pseudonana - A; Phaeodactylum tricornutum -B;
Chlorella autotrophica-CC; Nannochloris atomus--D; Alexan-
drium tamarense- E; Prorocentrum micans - F; Tetraselmis
maculata -G.
A

202 Wang and Fisher
40-
Ag
E
30- t
20-
B
A
!
Ff f c
‘O: 1
+G
--
a ,Cd
80
60
D
1
Am
% in algal cytoSO1
1
0’ 0 10 20 30
Fig. 3. Trace element assimilation efficiencies (mean k SD;
n = 5) in relation to cytoplasmic distributions in different algal
species. Symbols as in Fig. 2.
4). Relationships were not statistically significant for the
two chlorophytes, C. autotrophica and N. atomus, but
were statistically significant for the other species (P <
0.05).
Assimilation ,‘n relation to elemental gut passage times-
Gut passage time (GPT), defined as the time at which
90% of the cumulative defecation of an element is re-
covered (assuming 100% recovery at 96 h), was calculated
for all elements and all food sources. 75Se was not included
in these calculations because of a detectable loss of 75Se
into the dissolved phase (presumably by excretion). For
all other elements, defecation accounted for > 85% of the
loss from mussels during the 96-h depuration period. AE
was then fitted to GPT with an Ivlev curve, y = a[ 1 -
exp( -bx)] (Fig. 5), but for l lomAg and 57Co, this rela-
tionship was best described by a linear regression. Over-
all, AE increased with GPT, suggesting more efficient
assimilation when an element was retained longer in the
digestive tract ItFig. 5). Mussels thus were able to differ-
entiate diets and elements during their gut passage. The
GPT for 14C varied 7-fold (from 5 to 35 h) among the
different algal species. The GPTs of the green algae (C.
autotrophica and N. atomus) were the shortest (4.6 and
11.5 h), coinciding with the least retention of C from these
species. By contrast, the diatom T. pseudonana was re-
tained in the gut >35 h, concomitant with the highest C
assimilation for this alga. Among the different elements,
the GPT of 51Cr was the shortest (< 4.5 h), followed by
241Am and lo9Cd (< 30 h). The essential elements (14C
and 65Zn) generally were retained longer by the mussels.
No relationship between GPT and AE was found for 51Cr,
presumably because mussels could not retain this element
(< 2% AE) and most 51 Cr was defecated within the first
few hours of depuration (Fig. 6).
Discussion
Metal and carbon assimilation -Assimilation in aquat-
ic animals is generally measured either by the mass bal-
ancc method (i.e. gravimetric and radiotracer) or by the
ratio method 1:i.e. radiotracer and ash ratio) (Conover
1966; Calow and Fletcher 1972; Bricelj et al. 1984). Each
method has advantages and disadvantages. The mass bal-
Table 4. Relationships of trace element assimilation efficiency (AE, %) in mussels with
cytoplasmic distribution (M, %) of trace elements in phytoplanklon cells used as diet.
Ag
Am
Cd
co
Se
Zn
Equation r P
Within trace elements, among algal species
AE = 5.10 + 0.53 x IV[ 0.500 > 0.05
AE = -3.59 + 0.96 x N[ 0.943 co.01
AE = 14.72 + 0.28 x M 0.680 > 0.05
AE = 13.00 + 0.8 1 x M 0.826 0.05
Nannochloris atomus AE = 5.50 + 0.53 x M 0.707 BO.05
Phaeodactylum tricornutum AE = -5.92 + 2.22 x M 0.943 co.01
Prorocentrum micans AE = 7.00 + 0.91 x M 0.784 co.05
Tetraselmis maculata AE = 2.33 + 0.84 x M 0.814 co.01
Thalassiosira pseudonana AE = 5.92 + 0.82 x M 0.857 co.01

-0 20 40 60
80-
Tetraselmis
0 .’
se,*’
60.
.’ 2
.’ #’
/ /
40. al ,/’ ,’
20.
Cr’;Ag
l
“,I
I’ /
“$m .
0 /
/’
l
cd
I
/’
20 40 60 60
% in algal cytosol
\
Fig. 4. Assimilation efficiencies (mean + SD; n = 5) of trace
elements in relation to cytoplasmic distributions in each algal
species. Dotted line indicates a 1 : 1 relationship.
ante method requires quantification of total ingestion and
egestion (including feces and dissolved excretion). The
radiotracer ratio method assumes that the element under
investigation passes through the digestive tract at a rate
similar to an inert tracer (e.g. Cr or Am) and that loss
rates from feces into the dissolved phase are also com-
parable. We used both the mass balance and the ratio
(14C : 51Cr) methods to measure C assimilation. C AEs
calculated by the two methods were comparable, sug-
gesting that both can be applied to study C assimilation
in M. edulis. Our mass balance method did not take the
respiratory loss of 14C into account, but this component
is generally considered to be small in the short-term en-
ergetic budget of ingested 14C (< 8% of total ingestion C,
Bricelj et al. 1984; Kreeger 1993).
Application of noninvasive gamma-emitting radio-
tracer techniques to study trace element assimilation in
Trace element assimilation in mussels 203
30-
Ag
0-I - - - . * - - - . 1
0 10203040506070
‘-20 30 40 50 60 70
:1’-‘:. . , . , , . _ 1
0 5 10 15 20 25 30 35
.-
. Cd
GPT (h)
30- t
c
+
IO- f D
l
D
B
A
+Q
20 40 ( 3
Fig. 5. Element assimilation efficiencies (mean k SD; n =
5) in relation to gut passage times (GPT: mean k SD; n = 5)
in mussels fed on different algal species. Calculation of GPT
given in text. Symbols as in Fig. 2. Equations describing rela-
tionship of element AEs (%) with GPT (h) follow. Ag: AE =
0.44 x GPT + 3.94; Am: AE = 5.04 [l - exp(-0.155 x GPT)];
C: AE = 85.0 [l - exp(-0.047 x GPT)]; Cd: AE = 28.8 [I -
exp(-0.171 x GPT)]; Co: AE = 0.28 x GPT + 10.56; Zn: AE
= 41.6 [l - exp(-0.282 x GPT)].
individual mussels allows AEs to be directly computed,
assuming that digestion and assimilation are completed
at 72 h. Furthermore, the pulse-chase feeding technique
ensures that the radioactive feeding period is shorter than
the gut passage of ingested food materials (i.e. the first
appearance of labeled materials in the feces). This tech-
nique therefore helps avoid the experimental artifacts as-
sociated with metal recycling, physiological turnover, and
pervious contamination history typically associated with
long-term experiments and may represent a more realistic
measure of elemental assimilation.
C AEs of the different algal diets measured were con-
sistent with previous studies of mussels and other marine
bivalves (i.e. P. tricornutum: 52% at 2 x lo4 cells ml-l,
Kiorboe et al. 1980; Tetraselmis sp.: 60% at lo4 cells
ml-l, Thompson and Bayne 1974; Alexandrium fun-
dyense: 62%, Bricelj et al. 1990). Our observation that
mussels assimilated C from the chlorophytes (C. auto-
trophica and N. atomus) at a lower efficiency (8-2 1%)

204 Wang and Fisher
2.0
. Cr
T
1
G
4
B
1 “A
Ic
3 D -
2 6
GPT (h)
Fig. 6. Assimilation efficiencies of Cr, a tracer for the gut
passage of refractory materials (mean k SD; n = 5), in relation
to its gut passage time (GPT; mean f SD; n = 5) in mussels
fed on different algal species. Calculation of GPT given in text.
Symbols as in Fig. 2.
than they did from other algal species is consistent with
other studies in which green algae were assimilated at a
lower efficiency by clams and bay scallops (17-24%, Bass
et al. 1990; Pierson 1983). Typically, the cell walls of
green algae are rigid and contain sporopollenin (Atkinson
et al. 1972), a highly refractory compound resistant to
enzymatic digestion and physical breakdown. There also
was a marked difference in the C assimilation of the two
diatom diets. C in T. pseudonana was highly available to
the mussels (85%), whereas P. tricornutum C was assim-
ilated at a moderate efficiency (54%). A possible expla-
nation of this difference is that P. tricornutum has a high
inorganic content (m 25%, Widdows 1978). Consequent-
ly, mussels exhibit differential assimilation of algal food,
probably due to the difference in algal biochemical com-
position.
Decho and Luoma (1991) showed that in two marine
clams (Macoma balthica and Potamocorbula amurensis)
Cr could not be assimilated during extracellular digestion
but was assimilated efficiently during intracellular inges-
tion. They observed a distinct biphasic digestion in these
two bivalves. This biphasic digestion, as reflected by the
defecation pattern of the inert tracer llomAg (from radio-
labeled T. pseudonana and Isochrysis galbana cells), was
also evident in A4. edulis (Wang et al. 1995). The first
phase, presumably dominated by extracellular digestion,
occurred within the first 17 h of food ingestion; the second
phase occurred between 17 and 72 h and may have been
associated with intracellular digestion in the digestive
gland, in which the digestive cells phagocytized the fine
materials sent from the stomach. Among the eight ele-
ments studied, the highest proportion of ingested Am and
F
Cr was lost during the first 12 h, suggesting that these
elements were .not assimilated but passed through the
digestive tract directly, with little or none subjected to a
second phase of’ digestion. For C, Cd, and Se, there was
very little loss after 17 h, indicating that these elements
were retained longer than 17 h and were assimilated by
the mussels; however, Ag, Co, and Zn were continuously
lost through deft=cation during this period, implying that
the second phase of digestion was not efficient in absorb-
ing these metal;. Thus, for some elements (Am, C, Cr,
Cd, Se) extracellular digestion seems to be responsible for
the difference in assimilation among all food types. For
other elements (Ag, Co, Zn), both digestive phases are
important for the difference observed among food types.
A significant c:orrelation between C and essential trace
element (Se and Zn) assimilation (Fig. 2) indicates that
these elements Mlow the same digestive pathway as or-
ganic C. These elements generally penetrate to a greater
extent into the cytoplasm of algal cells than do the non-
essential elemerts, which adsorb mostly onto cell surfaces
(Reinfelder and Fisher 199 1). For example, Se may co-
valently bind to protein or nonprotein seleno-amino acids
and exists in soluble forms in the cytoplasm (Wrench
1978; Fisher and Reinfelder 199 1). Zn is an essential
enzyme cofactor in organisms. Consequently, the similar
phase associations (e.g. soluble or organic fractions) of C
and other essential elements may be largely responsible
for their correlation in assimilation. No relationship in
assimilation of Ag and Am with C was detected, sug-
gesting that the digestive behavior of Ag and Am may be
different from that of C, presumably due to their different
phase associations in the algal cells. Lee and Fisher (1993a)
found that the release of essential elements (Se and Zn)
from decomposing planktonic debris followed C release,
whereas the release of the nonessential elements (e.g. Am,
Ag) was independent of C and protein.
A weak relationship with C assimilation was measured
for Co at 24 h--the only element examined here that is
essential and potentially displays redox chemistry in bi-
valve guts. Nolan et al. (1992) reported that only cobal-
amine was assimilated by diatoms, whereas inorganic Co
was less available. Inorganic Co taken up by algae is large-
ly bound to cell surfaces (> 80%, Reinfelder and Fisher
199 1; this stud.y) and may not behave as an essential
element. Furthermore, once associated with biodetritus,
Co can be microbially oxidized and strongly retained in
the particulate phase (Lee and Fisher 1993b).
Cd and Zn exhibited similar relationships with C in
assimilation by mussels. Borchardt ( 198 5) demonstrated
a direct relatio:lship between Cd and C assimilation in
M. edulis fed different food rations and found that greater
C incorporation at high food densities resulted in greater
Cd accumulation from both food and the dissolved phase.
Cytoplasmic distribution of elements- Element distri-
bution in algal cytoplasm has been shown to account for
differences in the AE of many different elements in marine
copepods and bivalve larvae (Reinfelder and Fisher 199 1,
1994; Hutchins et al. 1995). These grazers, which have
very short gut transit times (I 30 min), assimilate only

the cytoplasmic fraction of elements, whereas the surface-
bound elements get rapidly packaged into fecal pellets. In
adult bivalves, the involvement of both extracellular and
intracellular digestion and the longer gut residence time
and large gut volume allow mussels to process food ma-
terials in a more complicated way than do copepods and
bivalve larvae. In adult bivalves, the cellular distribution
of elements may affect their AE through the partitioning
of extracellular and intracellular digestion. Elements that
are mostly associate with the cell surface would be sub-
jected primarily to extracellular digestion, which is not
efficient in absorbing metal (Decho and Luoma 199 1).
Nevertheless, significant relationships were observed for
mussel AEs of ingested elements vs. cytoplasmic distri-
butions in all algal cells except the chlorophytes (Fig. 4).
In our study, >98% of the Cr and > 92% of the Am
were associated with algal cell surfaces, and these metals
were passed directly through the intestine with little chan-
neled into the digestive diverticula (intracellular diges-
tion). With increased elemental fractionation in the algal
cytoplasm (such as for C and Se), the fraction directed
into the digestive gland for intracellular digestion would
be higher and result in a higher AE. This relationship is
generally consistent with our observations. We found a
significant correlation between the cytoplasmic distri-
bution of elements in the algal food and their AEs in
mussels for Am, Co, and Se but not for Ag, Cd, and Zn
(Fig. 3). The latter metals typically have a strong affinity
for sulfur ligands and proteins, including metallothi-
oneins (Roesijadi 1992). Thus, these metals may directly
bind with metallothioneins once they are ingested by the
mussels, resulting in a more complex digestive behavior
than that of Am, Co, and Se.
This study also compared trace element assimilation
from phytoplankton with that from glass beads (used to
represent an extreme end member of inorganic particles).
AEs of trace elements measured in these particles were
comparable (generally at the lower range) to the values
recorded for algal diets with various food quality. Trace
element assimilation from glass beads presumably results
from desorption of the trace element in the acidic gut (pH
- 5.5) of mussels (Owen 1974). Different ligands on glass
beads and algal cell surfaces may result in different de-
sorption rates for trace elements from these particles in
the acidic gut. Also, ingested beads may pass through the
digestive tracts rapidly without further channeling into
the digestive gland for additional intensive digestion and
assimilation.
Many studies have demonstrated that accumulation of
dissolved metal in aquatic organisms is proportional to
free-ion activity (see Campbell 1995). Our measurements
also highlight the importance of trace-element phase as-
sociation in influencing metal bioavailability from in-
gested particles. As with copepods and bivalve larvae,
metal in the cytoplasm of algal cells is most easily assim-
ilated by adult mussels and may be considered analogous
to the free-ion metal in the dissolved phase.
Gut passage time-Several studies have shown food
selectivity in the stomach and differential egestion of un-
Trace element assimilation in mussels 205
assimilable particles after ingestion (Purchon 1977; Bri-
celj et al. 1984; Shumway et al. 1985). Shumway et al.
(1985), for example, demonstrated that a cryptomond
flagellate was preferentially absorbed in the bivalve gut
compared with a dinoflagellate and a diatom of similar
size, Consequently, selective mechanisms in the stomach
may affect organic enrichment in the particles entering
the digestive tubules (Bayne 1993). Such a sorting mech-
anism often can be indicated by the GPT of food mate-
rials. This parameter is critical in affecting AE because it
determines the time period that food particles can be
retained within an animal’s gut. Longer retention may
enable food materials to be subjected to more rigorous
digestion (i.e. intracellular digestion) and more efficient
absorption. Bricelj et al. (1984) found that in the clam
Mercenaria mercenaria an algal diet of N. atomus passed
rapidly through its digestive tract (< 7 h) and was poorly
assimilated, compared with the algal species Pseudoiso-
chrysis paradoxa, which was retained significantly longer
(27 h) and was absorbed with a higher efficiency.
Mussels are capable of selective feeding on particles of
different sizes and on different particles of similar size
(Newell et al. 1989). However, particle sorting in the di-
gestive tract based on particle size (Purchon 1977) was
not apparent in our study. The size of the algal cells in
our study ranged from 2 to 40 pm, and we found little
evidence to suggest that AE was directly related to the
particle size of each algal species. A recent endoscopic
study showed that food particles were ingested primarily
in the form of mucus strings that formed in the ventral
groove (Ward et al. 1993). Thus, algal size may not be
important during the digestive period and may not sig-
nificantly affect AE.
The AE of each element was highly dependent on GPT.
This finding is consistent with many empirical studies
and with an optimal digestion model for A4. edulis show-
ing that C AE increases with GPT until it reaches a max-
imum beyond which AE remains relatively constant
(Bayne et al. 1987, 1989; Willows 1992). The GPT of
51Cr (~5 h) probably reflects the passage of refractory
materials because this element is bound almost exclu-
sively to algal cell walls (> 98%). For other nonessential
elements (e.g. Ag) which could penetrate into the cyto-
plasm, intracellular digestion also may be responsible for
the longer GPT compared to Am and Cr. Overall, mussels
are able to modify their GPT in response to changing
food concentration (Bayne et al. 1989; Wang et al. 1995)
and food quality (Bayne et al. 1987) and exhibit consid-
erable digestive flexibility (e.g. sorting and GPT). How-
ever, regulation of feeding rate to optimize energy balance
may prove to be a better strategy for mussels compared
with the regulation of digestion and assimilation, which
uses significantly more energy (17% of total feeding cost)
compared to the metabolic cost of mechanical pumping
(< 3% of total feeding cost) (Widdows and Hawkins 1989).
Fisher and Teyssie (1986) compared the kinetics of
bioaccumulation and release of 241Am and 65Zn in mus-
sels fed different food types (including a diatom, a green
alga, glass beads, and egg albumin particles) for 5 d. They
found no appreciable influence of food type on the assim-

206 Wang and Fisher
ilability of 241Am and 65Zn. The main source of Am influx
into mussels is probably the dissolved phase (adsorption)
(Bjerregaard et al. 1985) because AEs for this element are
typically low and food type may have little influence on
bioaccumulation of this element. Mussels are able to reg-
ulate Zn uptake and show a less pronounced response to
a change in environmental Zn concentrations (Amiard-
Triquet et al. 1986). One of the possible physiological
mechanisms underlying Zn regulation is the change in Zn
AE (Wang et al. 1995). Zn influx from the particulate
phase (controlled by a combination of Zn concentration
in food particles, Zn AE, and mussel feeding activity) can
be regulated when mussels are feeding on different types
of food particles. Therefore, the difference in Zn influx
from different food sources could be insignificant, as shown
by Fisher and Teyssie (1986). The influx rate of Zn from
the dissolved phase increases directly with ambient Zn
concentration, suggesting that dissolved Zn uptake is pri-
marily a passive process and does not significantly affect
the regulation of Zn uptake in mussels (Wang and Fisher
unpubl.).
Conclusions
Trace element assimilation in marine mussels seems
to be determined by cytological distributions in ingested
algal cells and GPT in the mussels. Differences in C and
trace element assimilation in mussels ingesting diverse
algal diets suggest a food sorting mechanism in the di-
gestive system of the mussels. In addition to the different
AEs noted for different algal species, the effects of food
composition on metal influx rate from the particulate
phase also depend on the feeding activity of the mussels
on the specific food item and the metal concentration in
the ingested particles. Because mussels selectively ingest
organic-rich particles (Ward and Targett 1989), the higher
AE of some trace elements (e.g. Cd, Se, Zn) associated
with these particles may further increase their overall
influx into mussels. AEs from different food sources must
be considered in quantitative modeling of metal accu-
mulation in mussels.
Additionally, the assimilation of trace elements and
biodeposition of unassimilated elements (in feces and
pseudofeces) may have a pronounced impact on sus-
pended particle loads and the cycling of trace elements
in coastal waters, particularly in waters containing large
bivalve populations (Kelly et al. 1985; Dame 1993). El-
ements that are not efficiently assimilated should be readi-
ly packaged into feces and deposited in sediments, thereby
enriching surficial sediments, as observed by Brown
(1986). Thus, the transfer of trace elements from particles
suspended in the water column to sediments should be
aided by the production of feces (or pseudofeces) by bi-
valves. The retention of trace elements in bivalve fecal
material, which has been relatively little studied (Bjer-
regaard et al. 1985), should influence the fate of these
biodepositcd elements; elements with long retention times
may be buried in the sediments or reingested by benthic
fauna, whereas metals that desorb from fecal deposits
may be remineralizcd back into the dissolved phase. Those
elements that are efficiently assimilated should be con-
centrated in mussel tissues.
References
ABSIL, M. C. P., J J. KROON, AND H. T. WOLTERBEEK. 1994.
Availability of copper from phytoplankton and water for
the bivalve Macoma balthica. 2. Uptake and elimination
from 64Cu-labelled diatoms and water. Mar. Biol. 118: 129-
135.
AMIIARD-TRIQUET, C., B. BERTHET, C. METAYER, AND J.-C.
AMIARD. 1986. Contribution to the ecotoxicological study
of cadmium, lead, copper and zinc in the mussel, Mytilus
edulis. 2. Experimental study. Mar. Biol. 92: 7-13.
ASMUS, H., AND IL M. ASMUS. 1993. Phytoplankton-mussel
bed interactions in intertidal ecosystem, p. 57-84. In R. F.
Dame [ed.], Elivalve filter feeders in estuarine and coastal
ecosystem process. NATO AS1 Ser. V. G33. Springer.
ATKINSON, A. W., B. E. S. GUNNING, AND P. C. L. JOHN. 1972.
Sporopollenin in the cell wall of Chlorella and other algae:
Ultrastructure, chemistry, and incorporation of 14C-acetate,
studied in synchronous cultures. Planta 107: l-32.
BASS, A.E.,R.E. MALOUF, ANDS. E. SHUMWAY. 1990. Growth
of northern quahogs (Mercenaria mercenaria, Linnaeus
1758) fed on picoplankton. J. Shellfish Res. 9: 299-307.
BAYNJZ, B. L. 19’23. Feeding physiology of bivalves: Timedependence
and compensation for changes in food availability,
p. l-24. In R. F. Dame [ed.], Bivalve filter feeders
in estuarine and coastal ecosystem process. NATO
V. G33. Springer.
ASI Ser.
-, A. J. S. HAWKINS, AND E. NAVARRO. 1987. Feeding
and digestion by the mussel Mytilus edulis L. (Bivalvia:
Molluscs) in mixtures of silt and algal cells at low concen-
-
tration.
-
J. Exp. Mar. Biol. Ecol. 111: l-22.
-- AND I. P. IGLESIAS. 1989. Effects of
seston cancer tration on feeding, digestion and growth in
the mussel A4ytiIu.s edulis. Mar. Ecol. Prog. Ser. 55: 47-54.
BJERREGAARD, P., S. TOPCUOGLU, N. S. FISHER, AND S. W.
FOWLER. 198 5. Biokinetics of americium and plutonium
in the mussel Mytilus edulis. Mar. Ecol. Prog. Ser. 21: 99-
111.
BORCHARDT, T. 1985. Relationship between carbon and cadmium
uptake in Mytilus edulis. Mar. Biol. 85: 233-244.
BRICELJ, V. M., A. E. BASS, AND G. R. LOPEZ. 1984. Absorption
and gut passage time of microalgae in a suspension feeder:
An evaluation of the 5LCr : 14C twin tracer technique. Mar.
-,
Ecol. Prog. Ser. 17: 57-63.
J. H. LEE., A. D. C EMBELLA, AND D. M. ANDERSON.
1990. Uptake kinetics of paralytic shellfish toxins from the
dinoflagellate Alexandrium fundyense in the mussel Mytilus
edufis. Mar. E:col. Prog. Ser. 63: 177-188.
BRO'WN, S. L. 1986. Feces of intertidal benthic invertebrates:
Influence of particle selection in feeding on trace element
concentration. Mar. Ecol. Prog. Ser. 28: 2 19-23 1.
CALOW, P., AND C. R. FLETCHER. 1972. A new radiotracer
technique involving 14C and 51Cr for estimating the assimilation
efficiencies of aquatic, primary consumers. Oecologia
9: 155-l 70.
CAMPBELL, P. G. 12. 1995. A critique of the free-ion activity
model, p. 45-102. In A. Tessier and D. R. Turner [eds.],
Metal
Wiley.
special ion and bioavailability in aquatic systems.
CLOERN, J. E. 19;32. Does the benthos control phytoplankton
biomass in south San Francisco Bay? Mar. Ecol. Prog. Ser.
9: 191-202.

Trace element assimilation in mussels 207
CONOVER, R. J. 1966. Assimilation of organic matter by zoo- Francisco Bay during the 1976-1977 drought. Estuarine
plankton.
DAME, R. F.
Limnol. Oceanogr. 11: 338-354.
1993. The role of bivalve filter feeder material
Coastal Shelf Sci. 21: 379-388.
NOLAN, C., S. W. FOWLER, AND J.-L. TEYSSIB. 1992. Cobalt
fluxes in estuarine ecosystems, p. 245-269. In R. F. Dame
speciation and bioavailability in marine organisms. Mar.
[ed.], Bivalve filter feeders in estuarine and coastal ecosys- Ecol. Prog. Ser. 88: 105-l 16.
tem process. NATO AS1 Ser. V. G33. Springer.
DECHO, A. W., AND S. N. LUOMA. 199 1. Time-courses in the
retention of food materials in the bivalves Potamocorbula
amurensis and .Macoma balthica: Significance to the absorption
of carbon and chromium. Mar. Ecol. Prog. Ser.
O’CONNOR, T. P. 1992. Recent trends in coastal environmental
quality: Results from the first five years of NOAA mussel
watch project. U.S. Dep. Comm. NOAA Natl. Ocean Serv.
46 p.
OFFICER, C. B., T. J. SMAYDA, AND R. MANN. 1982. Benthic
78: 303-3 14.
FISHER, N. S. 1986. On the reactivity of metals for marine
phytoplankton. Limnol. Oceanogr. 31: 443-449.
filter feeding: A natural eutrophication control. Mar. Ecol.
Prog. Ser. 9: 203-210.
OWEN, G. 1974. Feeding and digestion in the bivalvia. Adv.
-, P. BJERREGAARD, AND S. W. FOWLER. 1983a. Interactions
of marine plankton with transuranic elements. 1.
Comp. Physiol. Biochem. 5: l-35.
PIERSON, W. M. 1983. Utilization of eight algal species by the
Biokinetics of neptunium, plutonium, americium, and cal- bay scallop. Argopecten irradians concentricus (Say). J. Exp.
ifornium in phytoplankton. Limnol. Oceanogr. 28: 432-
447.
-,K.S.BURNS, R.D. CHERRY,AND M. HEYRAUD. 1983b.
Accumulation and cellular distribution of 24LAm, 2LoPo, and
Mar. Biol. Ecol. 68: l-l 1.
PURCHON, R. D. 1977. The biology of the Molluscs, 2nd ed.
Pergamon.
REINFELDER, J. R., AND N. S. FISHER. 1991. The assimilation
*lOPb in two marine algae. Mar. Ecol. Prog. Ser. 11: 233- ofelcments ingested by marine copepods. Science 251: 794-
-,
237.
AND J. R. REINFELDER. 199 1. Assimilation of selenium
in the marine copepod Acartia tonsa studied with a radi-
-,
796.
AND -. 1994. The assimilation of elements ingested
by marine planktonic bivalve larvae. Limnol. Oceanotracer
ratio method. Mar. Ecol. Prog. Ser. 70: 157-164.
AND J.-L. TEYSSIB. 1986. Influence of food composiogr.
39: 12-20.
ROESIJADI, G. 1992. Metallothioneins in metal regulation and
tion on the biokinetics and tissue distribution of zinc and
americium in mussels. Mar. Ecol. Frog. Ser. 28: 197-207.
toxicity in aquatic animals. Aquat. Toxicol. 22: 8 l-l 14.
SHUMWAY, S. E., AND OTHERS. 1985. Particle selection, inges-
GOLDBERG, E. D., AND OTHERS. 1978. The mussel watch. En- tion and absorption in filter-feeding bivalves. J. Exp. Mar.
viron. Conserv. 5: 101-125.
GUILLARD, R. R. L., AND J. H. RYTHER. 1962. Studies of
Biol. Ecol. 91: 77-92.
THOMANN, R. V. 198 1. Equilibrium model of fate of micromarine
planktonic diatoms. 1. Cyclotella nana Hustedt and
Detonula confervacea (Cleve) Gran. Can. J. Microbial. 8:
229-239.
contaminants in diverse aquatic food chains. Can. J. Fish.
Aquat. Sci. 38: 280-296.
THOMPSON, R. J., AND B. L. BAYNE. 1974. Some relationships
HUTCHINS, D. A., W.-X. WANG, ANDN.S. FISHER. 1995. Co- between growth, metabolism and food on the mussel Mypcpod
grazing and the biogeochemical fate of phytoplankton
iron. Limnol. Oceanogr. 40: 989-994.
KELLY, J.R.,V.M. BEROUNSKY,~. W. NIXON,ANDC. A. OVIATT.
tilus edulis. Mar. Biol. 27: 317-326.
WANG, W.-X.,N. S. FISHER, AND S.N. LUOMA. 1995. Assimilation
of trace elements ingested by the mussel, Mytilus
1985. Benthic pelagic coupling and nutrient cycling across edulis: Effects of algal food abundance. Mar. Ecol. Prog.
an experimental eutrophication gradient. Mar. Ecol. Prog. Ser. 129: 165-176.
Ser. 26: 207-219.
KIBRBOE, T.,F. MBHLENBERG,AND 0. NBHR. 1980. Feeding,
particle selection and carbon absorption in Mytilus edulis
WARD, J. E., B. A. MACDONALD, R.J. THOMPSON, AND P. G.
BENNINGER. 1993. Mechanisms of suspension feeding in
bivalves: Resolution of current controversies by means of
in different mixtures of algae and resuspended bottom material.
Ophelia 19: 193-205.
KREEGER, D. A. 1993. Seasonal patterns in utilization of dietary
protein by the mussel Mytilus trossulus. Mar. Ecol.
Prog. Ser. 95: 215-232.
LEE, B.-G., AND N. S. FISHER. 1993a. Release rates of trace
endoscopy. Limnol. Oceanogr. 38: 265-272.
-, AND N. M. TARGETT. 1989. Influence of marine microalgal
metabolites on the feeding behavior of the blue
mussel Mytilus edulis. Mar. Biol. 101: 3 13-32 1.
WIDDOWS, J. 1978. Combined effects of body size, food concentration
and season on the physiology of Mytilus edulis.
elements and protein from decomposing planktonic debris.
J. Mar. Biol. Assoc. U.K. 58: 109-l 24.
1. Phytoplankton debris. J. Mar. Res. 51: 39 1-421.
-,AND- . 199 3 b. Microbically mediated cobalt ox-
AND A. J. S. HAWKINS. 1989. Partitioning of rate of
heat dissipation by Mytilus edulis into maintenance, fcedidation
in seawater revealed by radiotraccr
Limnol. Oceanogr. 38: 1593-l 602.
LUOMA, S. N., AND OTHERS. 1992. Determination
experiments.
of selenium
ing, and growth components. Physiol. Zool. 62: 764-784.
WILLOWS, R. I. 1992. Optimal digestive investment: A model
for filter feeders experiencing variable diets. Limnol.
bioavailability to a benthic bivalve from particulate and Oceanogr. 37: 829-847.
solute pathways. Environ. Sci. Technol. 26: 485-491.
NEWELL, C. R., AND OTHERS. 1989. The effects ofnatural seston
WRENCH, J. J. 1978. Selenium metabolism in the marine phytoplankters
Tetraselmis tetrathele and Dunaliella minuta.
particle size and type on feeding rates, feeding selectivity
and food resource availability for the mussel Mytilus edulis
Mar. Biol. 49: 231-236.
Linnaeus at bottom culture sites in Maine. J. Shellfish Res.
8: 187-196.
NICHOLS, F. H. 1985. Increased benthic grazing: An alternative
Submitted: 12 May 1995
Accepted: 11 September 1995
explanation for low phytoplankton biomass in northern San
Amended: 24 October 1995