A Field Guide to Amphibian Larvae and Eggs of Minnesota ...

U.S. Department of the InteriorU.S. Geological SurveyInformation and Technology ReportUSGS/BRD/ITR–2002-0004A Field Guide to Amphibian Larvae and Eggsof Minnesota, Wisconsin, and Iowa

Technical Report SeriesThe Biological Resources Division publishes scientific and technical articles and reports resulting from the researchperformed by our scientists and partners. These articles appear in professional journals around the world. Reports arepublished in two report series: Biological Science Reports and Information and Technology Reports.Series DescriptionsBiological Science Reports ISSN 1081-292XThis series records the significant findings resulting fromsponsored and co-sponsored research programs. They mayinclude extensive data or theoretical analyses. Papers in thisseries are held to the same peer-review and high qualitystandards as their journal counterparts.Information and Technology Reports ISSN 1081-2911These reports are intended for publication of book-lengthmonographs; synthesis documents; compilations of conferenceand workshop papers; important planning and referencematerials such as strategic plans, standard operatingprocedures, protocols, handbooks, and manuals; and datacompilations such as tables and bibliographies. Papers inthis series are held to the same peer-review and high qualitystandards as their journal counterpartsMention of trade names or commercial products does not constitute endorsement or recommendation for use by theU.S. Department of the Interior, U.S. Geological Survey.Front cover photo of Tiger Salamander eggs by Shawn Weick.Back cover photo of Wood Frog by Allen Sheldon, Wood Frog eggs by Melinda Knutson.Suggested citation: Parmelee, J. R., M. G. Knutson, and J. E. Lyon. 2002. A field guide to amphibian larvae and eggsof Minnesota, Wisconsin, and Iowa. U.S. Geological Survey, Biological Resources Division, Information andTechnology Report USGS/BRD/ITR-2002-0004, Washington, D.C. iv + 38 pp.Additional copies of this report may be obtained from the National Technical Information Service, 5285 PortRoyal Road, Springfield, VA 22161 (1-800-553-6847 or 703-487-4650). Also available to registered users from theDefense Technical Information Center, Attn: Help Desk, 8725 Kingman Road, Suite 0944, Fort Belvoir, VA 22060-6218(1-800-225-3842 or 703-767-9050).

Information and Technology Report 2002-0004June 2002A Field Guide to Amphibian Larvae and Eggsof Minnesota, Wisconsin, and IowaBy Jeffrey R. Parmelee, Melinda G. Knutson, and James E. LyonIllustrations by Pearl PodgorniakU.S. Department of the InteriorU.S. Geological SurveyWashington, D. C. 20240

ContentsTitle ............................................................................................................................................ 1How to use the keys ................................................................................................................... 3Preserving amphibian eggs and larvae....................................................................................... 4Raising larvae............................................................................................................................. 4Field key to Amphibian Eggs .................................................................................................... 8Field Key to Larval Salamanders............................................................................................. 10Field Key to Larval Anurans.................................................................................................... 12Species descriptions................................................................................................................. 14Blue-spotted Salamander (Ambystoma laterale)............................................................... 14Spotted Salamander (Ambystoma maculatum).................................................................. 15Small-mouthed Salamander (Ambystoma texanum).......................................................... 16Tiger Salamander (Ambystoma tigrinum) ......................................................................... 17Four-toed Salamander (Hemidactylium scutatum)............................................................ 18Mudpuppy (Necturus maculosus) ..................................................................................... 19Eastern Newt (Notopthalmus viridescens) ........................................................................ 20Eastern Red-backed Salamander (Plethodon cinereus) .................................................... 21Northern Cricket Frog (Acris crepitans) ........................................................................... 22American, Canadian, Great Plains, Woodhouse’s, and Fowler’s Toads(Bufo americanus/B. hemiophrys/B. cognatus/B. woodhousii/B. fowleri) ................. 23Gray and Cope’s Gray Treefrog (Hyla versicolor/H. chrysoscelis) .................................. 25Spring Peeper (Pseudacris crucifer) ................................................................................. 26Western Chorus and Boreal Chorus Frog (Pseudacris triseriata/P. maculata)................. 27Leopard, Pickerel, and Crawfish Frogs(Rana pipiens/R. sphenocephala/R. blairi/R. palustris/R. areolata) .......................... 28American Bullfrog (Rana catesbeiana) ............................................................................ 30Green and Mink Frogs (Rana clamitans/R. septentrionalis)............................................. 32Wood Frog (Rana sylvatica).............................................................................................. 34Plains Spadefoot (Spea bombifrons) ................................................................................. 35Acknowledgments.................................................................................................................... 36References................................................................................................................................ 36

A Field Guide to Amphibian Larvae and Eggsof Minnesota, Wisconsin, and IowaByJeffrey R. ParmeleeSimpson CollegeDepartment of Biology701 North C StreetIndianola, Iowa 50125andMelinda G. Knutson and James E. LyonU.S. Geological SurveyUpper Midwest Environmental Sciences Center2630 Fanta Reed RoadLa Crosse, Wisconsin 54603Apparent worldwide declines in amphibianpopulations (Pechmann and Wake 1997) havestimulated interest in amphibians as bioindicatorsof the health of ecosystems. Because we havelittle information on the population statusof many species, there is interest by publicand private land management agencies inmonitoring amphibian populations. Amphibianegg and larval surveys are established methodsof surveying pond-breeding amphibians. Adultsmay be widely dispersed across the landscape,but eggs and larvae are confined to the breedingsite during a specific season of the year. Also,observations of late-stage larvae or metamorphsare evidence of successful reproduction, whichis an important indicator of the viability of thepopulation. The goal of this guide is to helpstudents, natural resources personnel, and1biologists identify eggs and larval stages ofamphibians in the field without the aid of amicroscope.Anyone undertaking field identificationof amphibians has a responsibility to avoidharming the amphibians or their habitats.Persons planning to sample amphibians shouldwork in cooperation with state or federal wildlifeprofessionals. Lack of knowledge about sensitivehabitats or populations could result in the spreadof diseases, damage to breeding habitats, or localreproductive failure of amphibian populations.State and federal laws protect amphibians fromexploitation. Collection permits are requiredfrom the appropriate state or federal authoritiesbefore capturing, handling, or collectingamphibians. Permission for sampling shouldalso be obtained from the landowner.

Research for this publication began with astudy of farm ponds in southeastern Minnesota.We quickly recognized the need for a way toidentify eggs and larvae of the amphibiansencountered. We used our field knowledge ofmost of the species included here to producethis field guide. Additional information camefrom published keys for Wisconsin amphibians(Vogt 1981; Watermolen 1995; Watermolen andGilbertson 1996) and descriptions from a varietyof sources (e.g., Johnson 2000; Petranka 1998;Harding 1997; Russell and Bauer 1993; Stebbins1985; Wright and Wright 1949).There is no simple way to morphologicallydistinguish between many amphibian speciesduring the egg and tadpole stage. This mightbe expected in closely related species such asthe gray treefrogs Hyla chrysoscelis and Hylaversicolor, which vary in chromosome numberand in subtle morphological traits. It is surprisingthat, to our knowledge, no one has reported away to morphologically distinguish betweenlarvae of several common frog species such asthe Crawfish Frog and Southern Leopard Frog,which bear little resemblance to one another asadults. Even with the technical keys available toherpetologists, amphibian eggs and larvae areoften difficult to identify to species. An excellentkey to all species in the United States (Altig etal. 1998) is available on the internet (http://www.pwrc.usgs.gov/tadpole/). The Altig key(along with earlier versions such as Altig [1970]for tadpoles and Altig and Ireland [1984] forsalamander larvae) requires detailed knowledgeof the anatomy of amphibian larvae. Altig etal. (1998) requires examination of tadpolemouthparts, which are only observable with deadspecimens and the aid of a microscope. Tadpolebiology is summarized by McDiarmid and Altig(1999). Additional research is needed to betterdescribe and understand larval amphibians.2Three major factors make amphibian larvae,especially tadpoles, difficult to identify. First,many species are conservative in their anatomyand members of the true frog (Ranidae) and toad(Bufonidae) families are very similar in larvalform. Second, morphology among tadpoles ofa single species can vary geographically andwith developmental stages. Third, larvae revealdiffering anatomies depending on the physicaland biotic environment where they are found.The proportions, coloration, and anatomy oflarvae change as they grow from hatchlings tometamorphs. Larvae in early stages frequentlydo not show the characteristics necessary fortheir identification.In one description, we summarize closelyrelatedspecies that cannot be distinguishedwithout the aid of a microscope. We believethat the weakness of specific identificationis balanced by ease of use in the field. Inmany instances, identification to species isnot necessary to meet land managers’ goals.However, professional herpetologists workingfor universities, museums, and state and federalagencies can assist with identifying larvae tospecies.No key by itself can help identify everyspecimen; there will always be variantindividuals. In identifying anurans, for example,knowledge of which species were calling at thepond may help sort out identification questions.It also is helpful to note the geographicdistribution of species; you can reasonably ruleout Canadian Toads if the site is in southernIowa. Ruling out a species by geography is noteasy or reliable, however, if your survey site isnear the edge of a species’ range. You couldalso return several times to the breeding site toobserve late-stage metamorphs. Amphibiansoften are easier to identify once they developmature traits. If a specimen cannot be identified

using this key, preserve the specimen and attemptidentification using a more detailed key such asAltig et al. (1998). If still not confident in theidentification, contact the senior author of thiskey or another herpetologist in your area forassistance. Another approach, if adequate timeand facilities are available, is to raise a feweggs or larvae to metamorphosis and identifythe late-stage metamorphs or adults from theirmature characteristics. We anticipate that rapidmolecular DNA techniques suitable for fieldidentification will be available within the nextfew decades. Until then, keys will be needed.How to use the keysMost amphibians lay their eggs in clustersor strings. Four general types of amphibianegg masses are illustrated (Figure 1); globularclusters are large and easier to see than eggs inlong strings, whereas eggs laid singly or in smallclusters are difficult to find, especially in densevegetation.To use the circular keys (Figures 2–4), startin the center and choose among the availableoptions, ending in the outer circle with theidentification of a species or group of species.In some instances, dashed lines are used if aspecies exhibits multiple traits. These traits aredescribed to the left and right of the sectionenclosed by the dashed line. After making anidentification, use the species description toconfirm the identification. If the identificationseems in error (e.g., the collection location isgeographically distant from the range of thatspecies, the specimen looks nothing like theillustration, or the description doesn’t match thespecimen), return to the key and work backwardsto find another species that more closely fits yourspecimen.The jellies of most amphibian eggs are3very small when first laid, but quickly swellwith water. All amphibian eggs in this key arepigmented and have one or several gelatinousenvelopes surrounding them (Figure 2). Thegelatinous capsules are difficult to see, butit helps to use a magnifying lens and adjustthe lighting. The egg key groups most ranidstogether, and mole salamanders also have similareggs.Salamanders (Figure 3) have four limbs andfeathery external gills, whereas tadpoles (Figure4) have four limbs only when they are close tometamorphosis and their gills are internal. Thelocation of the eyes on the tadpole is an importantcharacter separating the treefrogs (hylids) fromthe true frogs (ranids). Eyes dorsal means theeyes do not interrupt the lateral margin of thehead when viewed from above (superior view;Figure 4a). Another early characteristic in thekey is whether the vent is medial or dextral(Figure 4b). This is difficult to see on a squirminglive tadpole; however, hold it with the bottom(ventral) side up, straighten the tail fin, and lookcarefully at the location of the opening of thevent. A magnifying lens will help.The scale bar by each drawing in the speciesdescription indicates 1 cm. Drawings of typicalindividuals are presented, but all specimens willnot look exactly like the drawings because ofnatural variation. The size ranges (TL = totallength from tip of snout to tip of tail, SVL =snout to vent length) may help rule out somelarvae. The range maps were produced frominformation at Minnesota and WisconsinDNR web sites (http://www.dnr.state.mn.us;http://www.dnr.state.wi.us) as well as fromChristoffel et al. (2001) and Casper (1996)for Wisconsin, Oldfield and Moriarty (1994)and J. Moriarity (personal communication) forMinnesota, and Christiansen and Bailey (1991)and J. L. Christiansen (personal communication)

for Iowa. The range maps are color-coded asfollows: dark green indicates the current rangeof the species and red indicates the former rangeof the species (range contraction). Common andscientific names follow Crother (2000).Preserving amphibian eggs and larvaeSamples of amphibian eggs and larvaecan easily be preserved to make a voucher orreference collection or to send to a specialistfor positive identification. Most states requirecollection permits issued by the state Departmentof Natural Resources or other similar agency.Remember to observe all wildlife laws andonly collect where it is legal and where thecollection of a few individuals will not affectthe population.Larvae should be anesthetized according tothe procedure recommended by Green (2001).There is no perfect preservative and techniquesfor preserving specimens are still debated(McDiarmid and Altig 1999). We recommendpreserving amphibian eggs and larvae byplacing them in a small vial filled with a 10%formalin solution. Alcohol is more pleasantto work with and safer than formaldehyde,but tends to dehydrate specimens. Whateverpreservative you use, read the relevant MaterialSafety Data Sheets (MSDS) to learn how tosafely handle and store that chemical. Larvaecan be placed individually, or as a lot of 5-20individuals, in screw top vials. Do not place toomany individuals in one container. Immediatelabeling is a must; use pencil or indelible ink onall submerged tags. Field tags should be linkedto corresponding field notes; labels with detailedinformation must be kept with the specimens.Do not rely on memory as a record of locality,date, and habitat information. The minimuminformation includes date, locality (kilometers4from a crossroad or other landmark, or GPScoordinates), habitat description, and name ofthe collector. We recommend maintaining anumbered log that links to tags on the vials.Other important information includes notes onlive coloration (specimens quickly lose color inpreservative). Specimens should be deposited ina museum or university collection where theycan be appropriately cataloged, maintained, andavailable for researchers worldwide.Raising LarvaeIf a specific identification cannot beaccomplished in the field, raising amphibianeggs or larvae to metamorphosis is onealternative. To appropriately provide for theneeds of captive larvae, consult Mattison (1993).We provide a general description of larval carehere to familiarize the reader with the proceduresinvolved.Raising amphibian larvae in an aquarium isnot difficult, but avoid overcrowding (raise onlyone or two animals per ~4 liters [~1 gallon] ofwater). Water quality is the most important factor.Treat the water as you would for aquarium fishor collect natural pond water or rain water. Tapwater contains chlorine, which will kill larvaeas well as many fish. Water can be effectivelyde-chlorinated by letting it stand uncovered forseveral days. Change the water every few daysor as needed. An aquarium or large, shallowcontainer is preferred; the larger the surface areathe more oxygen will be available. An airstoneis usually not necessary. Do not clean the tankwith soap or other cleansers because residuesmay kill the animals.Salamander larvae are carnivorous and mustbe fed an animal diet. Smaller salamanders canbe fed “zooplankton” (small invertebrates) nettedout of streams or ponds. Larger salamander

larvae can be fed small worms (earthworms ortubifex), tadpoles, feeder fish, and even pieces ofliver if you wriggle it in front of their mouths. Ifyou feed them liver, remember to not leave any inthe container, as it will quickly foul the water.Frog and toad tadpoles can be raised on adiet of fish flakes or rabbit pellets. Boiled lettuceor spinach, naturally collected algae (be carefulnot to introduce predaceous invertebrates suchas dragonfly larvae or diving beetles), and smallamounts of cooked egg yolk can also be fed totadpoles. Offer these foods daily, but be sure toreplace the aquarium water frequently.The length of time required for metamorphosisvaries from several years for American Bullfrogsto a few weeks in the Plains Spadefoot. SomeTiger Salamanders never metamorphose andremain aquatic. Most treefrogs and toads takeabout two months to develop from eggs tometamorphs. Salamander metamorphosis is lessdramatic than frog metamorphosis. Salamandersabsorb their gills and any tail fins when theymetamorphose. Because amphibians areectothermic, the temperature of the environmentaffects their development time. Raise larvae atroom temperature or slightly warmer. Light willhelp maintain the algae that tadpoles feed on;direct sunlight may cause overheating.If your tadpoles are healthy and developingas expected, you will notice the appearance ofhind limbs, then a reshaping of the body, andfinally, front limb emergence. At this stage, thetadpole will begin to absorb its tail and manychanges will occur both internally and externallyas it develops into a juvenile frog. Feeding willcease at some point; the metamorphosingfrog is quite vulnerable at this stage. It can nolonger swim well, but is not yet able to surviveon land. Provide a solid surface area for themetamorphosing animals to climb onto andcover the container to prevent escape. Tilting5the container is an easy way to offer both landand water. You may now be able to identify thefrog from the developing adult characteristics.If you need to continue to raise your larvae foridentification or other purposes, you should feedthe froglet or newly-terrestrial salamander tinyinsects such as pinhead crickets, wingless fruitflies, or bloodworms, which can be purchasedin pet stores.To prevent the spread of disease to nativepopulations, any frogs or salamanders youraise should not be released back into theenvironment. Lab-raised amphibians can beanesthetized and euthanized with benzocaineor tricaine methanesulfonate (MS 222, Green2001). If you anticipate difficulty complyingwith this guidance, you should not undertakeraising larvae in captivity.

abFigure 1. Egg masses of amphibians; (a) a thin raft of eggs (e.g., Green Frog), (b) a globular clusterof eggs (e.g., Leopard Frog), (c) eggs laid singly or in small clusters (e.g., Chorus Frog), (d) eggs laid6

cdin strings (e.g., toads).7

Figure 2. Field key to amphibian eggs.8

Figure 2a. Frog and toad eggs. Dark lines representdistinct egg envelope borders, gray lines representindistinct borders.9

Figure 3. Field key to larval salamanders.10

Figure 3a. Tail fin and costal grooves.Figure 3b. Mole salamander digits (front feet shown).Figure 3c. Tail fin extending onto body (Four-toed Salamander).11

Figure 4. Field key to larval anurans.12

Figure 4a. Eye location (dorsal view).Figure 4b. Vent location (ventral view).Figure 4c. Tadpole anatomy (lateral view).13

Small-mouthed Salamander Ambystoma texanumStatus: Iowa – Declining1 cmSize at hatching, 7 -14 mm; at metamorphosis, 48 – 61 mm total lengthThe Small-mouthed Salamander is foundin the southern third of Iowa. The adult of thisspecies resembles its common name in havinga relatively smaller head and mouth comparedwith other mole salamanders. The eggs are laidsingly or in small loose clusters on vegetationor debris in ponds (rarely in streams). Largermasses may be sausage-shaped. The larvaeare similar to other mole salamanders but havepigmentation on their chin. They are a uniformbrown after metamorphosis.16

Tiger Salamander Ambystoma tigrinumStatus: Wisconsin – CommonMinnesota – CommonIowa – Common1 cmSize at hatching, 13 – 17 mm; at metamorphosis,7.5 – 12.5 cm, sometimes reaching 24 cm total lengthThe Tiger Salamander has the widestdistribution of any salamander in the UnitedStates (Conant and Collins 1991). It tolerateshuman disturbance and survives in agriculturalregions. Adults migrate to breeding pondsvery early in the spring (March), as soon asponds are free of ice at northern latitudes.Eggs are laid singly or in masses of 18–110eggs. Tiger Salamanders use any fish-freebody of water from ponds to cattle tanks.Fresh egg masses are firm (6–7 cm diameter)and older egg masses are flimsy. In older egg masses, you can identifythe developing embryos as salamanders, not frogs, because they haveexternal gills and an elongated shape. The eggs of Tiger Salamandersare large, 2–3 mm in diameter, with three gelatinous envelopes.The digits of larval Tiger Salamanders are quite different than theother three species of mole salamanders. They are flattened and pointedfrom base to tip (Figure 3b). Late-stage Tiger Salamander larvae reach> 100 mm total length versus < 61 mm for the other mole salamanders.17

Four–toed Salamander Hemidactylium scutatumStatus: Wisconsin – Special ConcernMinnesota – Endangered1 cmSize at hatching: 11 – 15 mm. Size at metamorphosis: 17 – 25 mm total lengthAs the name suggests, this salamanderhas only four toes on the hind feet insteadof five as in other salamanders (except theMudpuppy). This species is in the large familyof lungless salamanders, many of which laytheir eggs on land and do not have a freeswimminglarval stage. Four-toed Salamandersusually lay their eggs in moss near the edgesof bogs. The female will stay with the eggs,guarding them. After the eggs hatch, thelarvae drop into the water and spend the nextseveral months as free-swimming larvae beforemetamorphosing into terrestrial adults. The larvae also have four toes,but are much smaller than Mudpuppy larvae and have a dorsal fin thatextends onto their body (Figure 3c).18

Mudpuppy Necturus maculosusStatus: Wisconsin – Locally abundant, but possibly in declineMinnesota – UncommonIowa – Uncommon1 cmSize at hatching, 21 – 25 mm; adult features appear at 13 – 15 cm total lengthThe Mudpuppy is a paedomorphicspecies; it retains larval characteristics in itsadult form. Mudpuppies are only found inlarge lakes and rivers. The eggs are easilyidentified, laid in nests constructed byfemales on the underside of cover objectssuch as rocks, boards, and other sunkendebris. The eggs are suspended individuallyin an area 15–30 cm in diameter; each eggis approximately 5–6.5 mm in diameter. Thelarvae are striped, with large external gills,four toes on the hind feet, and a dorsal fin extending only onto the tail,not onto the body (Figure 3a). Mudpuppies remain in the larval stage forseveral years and never acquire a terrestrial form.19

Eastern Newt Notopthalmus viridescensStatus: Wisconsin – CommonMinnesota – CommonIowa – Endangered1 cmSize at hatching, 7 – 9 mm; at metamorphosis, 21 – 38 mm total lengthThe Eastern Newt is widely distributedover the eastern United States and is foundin ponds in its aquatic form. The female laysthe eggs singly, attaching them to sticks,leaves, and stems in the pond. The eggshave a tough, rubbery jelly, are often oval,and have three envelopes with a sticky outerenvelope that attaches to vegetation. Thelarvae are unique among our salamanders inthat they lack costal grooves (Figure 3a) andhave a dark stripe through the eye. They alsohave yellow spots along the dorsum.20

Eastern Red-backed Salamander Plethodon cinereusStatus: Wisconsin – Locally commonMinnesota – Locally common|_____1 cm_____|(Drawing of a female attending her eggs)Hatchling size, 2 cm total length; adults, 6-10 cm total lengthThis species is our only amphibian thatforegoes the aquatic larval stage, layingterrestrial eggs that develop directly intojuvenile salamanders. The eggs can bedistinguished from the Four-Toed Salamanderbecause there will be an attending femaleEastern Redback Salamander with her eggs.Three to 17 eggs are laid in rotten logs,or in damp soil under rocks or logs. Theeggs are laid individually but in a clusterin subterranean cavities, usually naturallyoccurring cracks and crevices. The femaleremains coiled around the egg cluster until they hatch. The young hatchin about two months and are approximately 2 cm long.21

Northern Cricket Frog Acris crepitansStatus: Wisconsin – EndangeredMinnesota – EndangeredIowa – Declining1 cm2.7 – 4.4 cm total lengthNorthern Cricket Frogs lay eggs singly orin loose clusters of 7–40 eggs near the surfaceof streams and other bodies of water. CricketFrogs have distinctive tadpoles when theirtail tips are black. No other tadpole will havesuch an obvious black tail in contrast to therest of the tail and body. This trait may or maynot be present depending on conditions. Forexample, tadpoles that developed in ponds withpredaceous dragonfly larvae had black tails(directs attacks away from the body), whereasthose developing in lakes or streams with fishhad clear tails (less visible; Caldwell 1982). Tadpoles of this species canalso be identified by their eyes, which are located between dorsal andlateral, and the bands of pigment that lie along the dorsal edge of the tailmusculature.Northern Cricket Frogs are the least arboreal of the tree frog familyin our area. The adults are found along muddy banks of streams, ponds,and lakes. There is considerable concern over the disappearance of thisspecies in the northern parts of its range (indicated in red). A rangecontraction has occurred in the last few decades and the cause is underinvestigation (Lannoo 1998; Hay 1998). There is some evidence that thisspecies is more susceptible to UV radiation than the leopard frogs, toads,and other tree frogs (Van Gorp 2002).22

American, Canadian, Great Plains, Woodhouse's, andFowler's ToadsAmerican Toad Bufo americanusCanadian Toad Bufo hemiophrysGreat Plains Toad Bufo cognatusWoodhouse's Toad Bufo woodhousiiFowler's Toad Bufo fowleriStatus: B. americanus – Common to locally abundant in Wisconsin,Minnesota, and Iowa.B. hemiophrys – Locally abundant in MinnesotaB. cognatus – Locally abundant in Minnesota and IowaB. woodhousii – Locally abundant in IowaB. fowleri – Locally abundant in Iowa1 cm(American toad tadpole)1.8 – 2.4 cm total lengthToad eggs and tadpoles can be found in almost any aquatic situation,from muddy farm ponds to clear swampy areas, and are distinctivelydifferent from those of other frogs. Toads lay their eggs in long stringsthat may be benthic or entwined in (but not attached to) vegetation.You should be able to distinguish the eggs of any species of toad basedon site location and characteristics of the envelope surrounding theeggs. Canadian Toads are found in northwestern Minnesota and have asingle tubular envelope enclosing the eggs. The American Toad has adouble envelope surrounding the eggs, with eggs separated by partitions.Woodhouse’s and Fowler’s Toads, are found in western and southeasternIowa and have a single tubular membrane not separated by partitions.Great Plains Toads are found in western Iowa and Minnesota and have ascalloped egg string.The larvae of different toad species are very similar. Toad tadpolesare small and black or dark brown, often with some bright metallic spots.(continued next page)23

Toad Range MapsB.americanusB. hemiophrysB. cognatus B. woodhousii(western Iowa)B. fowleri(eastern Iowa)They have a clear fin that is rounded at the tip. The eyes are dorsal and thevent is medial. Metamorphosing toads quickly acquire their unique wartyskin and squatty shape.24

Gray and Cope's Gray TreefrogsGray Treefrog Hyla versicolorCope's Gray Treefrog Hyla chrysoscelisStatus: Wisconsin – CommonMinnesota – CommonIowa – Common1 cm3.2 – 3.8 cm total lengthGray Treefrogs and Cope’s Gray Treefrogscannot be distinguished based on morphology;herpetologists use calling rate or chromosomes foridentification (Oberfoell and Christiansen 2001).Differences in eggs and tadpoles are unknown.The adults spend most of the year in trees andvegetation and descend to small ponds to breedin May–June. They lay clumps of 30–40 lightbrown eggs attached to floating vegetation nearthe surface. The outer jelly layer is weak andindistinct. Tree frog tadpoles are striking, withhigh (1.5 times depth of tail musculature) tail fins that often have a red-orangetinge (this color quickly disappears in preservative). Their tails usually endin a distinct flagellum. Eyes are lateral, and older larvae have very heavilypigmented tail fins with the pigment covering both the fins and musculature.Metamorphs have smooth skin and large toe pads.25

Spring Peeper Pseudacris cruciferStatus: Wisconsin – CommonMinnesota – CommonIowa – Common adjacent to Mississippi River1 cm~ 3.4 cm total lengthSpring Peeper adults are associated withforested or brushy areas. Peepers lay eggsin shallow water singly or in small clusters(2-3 eggs) attached to dead leaves, sticks, andgrasses. Spring Peeper tadpoles are highlyvariable. They have lateral eyes and thepigmentation of the tail can be heavy or almostnon-existent. When there is pigmentation onthe tail it is concentrated along the outer edgeand there is a clear area near the musculature.26

Western Chorus and Boreal Chorus FrogsWestern Chorus Frog Pseudacris triseriataBoreal Chorus Frog Pseudacris maculataStatus: Wisconsin – CommonMinnesota – CommonIowa – Common1 cm2.6 – 3.7 cm total lengthThe Western Chorus Frog and the BorealChorus Frog are difficult to distinguish andhave overlapping ranges (Platz 1989). Theexact range of the Boreal Chorus Frog hasnot been determined. Differences in eggsand larvae are unknown and we treat themtogether in this account. The Chorus Frogis often one of the first frogs to call and layeggs in the spring. They breed in ponds andsmaller bodies of water and lay eggs in loose,irregular clumps of 5–20 eggs (occasionallyover 100 eggs) attached to vegetation or debris in shallow water. Thetadpoles can be confused with other tree frogs such as Spring Peepers,which also have lateral eyes and a high tail fin. Chorus Frog tadpoles arebrownish, black, or gray above and bronze or silvery below, and usuallyhave clear fins with a bicolored tail musculature (dark on the dorsal half,light on the ventral half).27

Leopard, Pickerel, and Crawfish FrogsNorthern Leopard Frog Rana pipiensSouthern Leopard Frog Rana sphenocephalaPlains Leopard Frog Rana blairiPickerel Frog Rana palustrisCrawfish Frog Rana areolataStatus: R. pipiens – Common in Wisconsin, Minnesota and IowaR. sphenocephala – Uncommon and declining in IowaR. blairi – Locally abundant but declining in IowaR. palustris – Special Concern in Wisconsin,uncommon in Iowa and MinnesotaR. areolata – Extirpated in Iowa (?)1 cm(Northern Leopard Frog tadpole)4.5 – 8.5 cm total lengthWe group the eggs and larvae of the ranid species above togetherin this account. Geography and observation of adults may be necessaryto help with identification. Southern Leopard Frogs are incidental inextreme southeastern Iowa and the Crawfish Frog has not been seen inextreme southern Iowa in over 60 years (Christiansen 1998). The eggsof these species are laid in globular masses attached to vegetation or onthe bottom in shallow water. The Northern Leopard Frog and PickerelFrog can be distinguished from each other because the NorthernLeopard Frog has eggs that are black on one side and white on theother, whereas the Pickerel Frog has eggs that are brown on one sideand yellowish on the other. The tadpoles are greenish or brownish withdorsal eyes and mottling on the tail. The intestinal coil is at least partlyvisible through the skin on the belly. Pickerel Frog tadpoles have beendescribed as green or olive green with large dark blotches on theirtail musculature and fins. Leopard Frogs are green or yellowish greenwithout the large dark blotches.28

Leopard, Pickerel, and Crawfish Frog Range MapsR. pipiens R.blairiR. sphenocephalaR. palustrisR. areolata29

American Bullfrog Rana catesbeianaStatus: Wisconsin – CommonMinnesota – Range expanding through introductionsIowa – Increasingly common1 cm~16.2 cm total lengthAmerican Bullfrogs are the largest frogsin our area, produce the greatest number ofeggs, and have very large larvae that usuallyoverwinter at the bottom of ponds andmature after a couple of years in the tadpolestage. They are now found statewide inIowa because of introductions with stockedfish. Introductions of American Bullfrogs inthe western United States have caused thedecline of native species (Hayes and Jennings1986) because adult bullfrogs will eat otherfrogs, as well as other large prey. AmericanBullfrogs are at home along permanent bodies of water and are able tosustain populations in the presence of fish predators. American Bullfrogsand Green Frogs have either skin toxins or behavioral defenses againstlarval predation (Kats et al. 1988; Werner and McPeek 1994).American Bullfrogs lay their eggs in large (may exceed 1 mdiameter), flat (about one egg deep) floating rafts in June or July. Theeggs are small (0.12–0.17 mm in diameter) with up to 20,000 eggs in amass. The only other frog with a large floating mass is the Green Frog,but this species has smaller eggs (0.10–0.15 mm diameter) and fewerof them (< 5,000 eggs) in the mass. American Bullfrogs have only(continued next page)30

one weak jelly layer; Green Frog eggs have two jelly layers per egg.American Bullfrog tadpoles get quite large (16 cm or larger) and aregreenish brown with dorsal eyes. Intestines are visible in small tadpoles,but the belly becomes opaque in larger larvae. American Bullfrogs havedistinct black spots on the dorsum of the body and on the tail, these spotsare concentrated on the dorsal half of the tail. The spots are round anddistinct, whereas the Green Frog has fuzzier, less distinct spotting.31

Green and Mink FrogsGreen Frog Rana clamitansMink Frog Rana septentrionalisStatus: R. clamitans – Common in Wisconsin, Minnesota and IowaR. septentrionalis – Locally abundant Wisconsin and Minnesota1 cmGreen Frogs are found near springs, streams, and ponds and MinkFrogs are often found among lily pads in ponds, swamps, and streams.Adult Green Frogs superficially resemble American Bullfrogs as adultsbut are smaller and have dorsolateral folds along the back. Mink Frogadults can be difficult to distinguish from Green Frogs, but matureGreen Frogs have bands, “tiger stripes,” on their hind legs instead of theindefinite patterns of spots and blotches found in Mink Frogs. Also, MinkFrogs emit an odor similar to rotten onions when their skin is rubbedroughly. Mink Frogs are found only in northern Minnesota and Wisconsinwithin our range. Mink Frog and Green Frog tadpoles are similar,although Mink Frogs have been reported to have pink-buff colored spotson their tail.Mink Frogs lay eggs in loose globular masses (500–3,000 eggs in amass), often in deep water (> 1 m depth). Mink Frogs need cold, welloxygenatedwater for egg development because the egg jelly is thick,and they are not laid on the surface of the water as are Green Frogs andAmerican Bullfrogs. Green Frogs lay eggs in a large floating film withup to 5,000 eggs. Green Frog and Mink Frog tadpoles have a musculartail with low fins. The belly is completely opaque and the greenishbodies have numerous dark blotches consisting of fuzzy dots, dashes, andsplotches, but not distinct round dots.32

Green and Mink Frog Range MapsR.clamitansR. septentrionalis33

Wood Frog Rana sylvaticaStatus: Wisconsin – CommonMinnesota – Common1 cm4.2 – 4.8 cm total lengthAdult Wood Frogs have the same generalshape as other members of their genus—butare brown instead of green—with a darkbrown eye mask. They breed early in springand their eggs are often the first ranid eggslaid in the year. The eggs (500–3,000) arelaid in loose, round globular masses (5-12 cmdiameter) attached to submerged vegetation,often near the surface. Many females laytheir eggs in the same vicinity. Egg massesare rather flimsy (although less flimsy thanNorthern Leopard Frog egg masses) and lose their shape when liftedfrom the water. A large amount of clear jelly in the egg mass causes theeggs to be widely separated. Larval Wood Frogs have clear tail fins thatend in a sharp point. The fins are higher than other ranid tadpoles. Themusculature of the body and tail is colored uniformly brownish with agreenish sheen. The belly is usually cream-colored with a cream linealong the edge of the mouth.34

Plains Spadefoot Spea bombifronsStatus: Iowa – Locally abundant4.0 – 4.8 cm total lengthPlains Spadefoots are present in extremewestern Iowa. Adults spend most of theyear underground and breed explosively (allwithin a few days or weeks) in small poolsafter heavy spring rains (even depressions inagricultural fields) and occasionally in morepermanent ponds. The eggs and tadpolesdevelop quickly; eggs hatch in about 20 hoursat 30 o C (Justus et al. 1977), and a populationin Oklahoma required only 13–14 days fromegg to metamorphosis (King 1960). The eggsare laid in a cylindrical or elliptical mass of 10–250 eggs that is attachedto submerged vegetation. Plains Spadefoot tadpoles have a medial vent, acharacteristic they share only with toads in our region. The body is darkbrown or bronze in color, and is large and bulbous (broadest just behindthe eyes) with mostly clear tail fins. Pigmented “veins” are usually foundon the clear fins. The spiracle is found lower on the body than our otherfrogs (Figure 4c). Tadpoles may also be rapidly developing “carnivorousmorphs,” with larger heads and mouths than the typical omnivorousforms (Pfennig 1992). A black “spade” will be observable on the hindfeet well before metamorphosis.35

AcknowledgmentsFunding was provided through theMinnesota Environment and NaturalResources Trust Fund as recommended bythe Legislative Commission on MinnesotaResources (LCMR), the USGS UpperMidwest Environmental Sciences Center, theUSGS Amphibian Research and MonitoringInitiative (ARMI), and Simpson College. Thedrawings, with one exception, are by PearlPodgorniak from specimens in the collectionsof the University of Kansas, Bell Museum ofNatural History, or the personal collection ofJeff Parmelee. The illustration of the EasternRed-backed Salamander is by Sian Marconefrom a photo by Erica Crespi.Bill Duellman and John Simmons at theUniversity of Kansas, and Andrew Simons atthe Bell Museum lent us specimens in theircare. We thank R. W. Van Devender for hisexcellent photographs of a larval Mudpuppy,Four-toed Salamander, Eastern Newt, andSmall-mouthed Salamander, which servedas models for the drawings. We thank JimChristiansen and John Moriarty for helpingus with the distribution maps of amphibians.We borrowed the idea of the circular key fromOldfield and Moriarty (1994) and the generalformat from Christoffel et al. (2001). We alsothank Jerry Cox, Shawn Weick, Josh Kapfer,William Richardson, John Moriarty, CarlKorschgen, Sam Bourassa, Ben Campbell,Joel Jahimiack, and Andy Kimball forlogistical and field assistance. We also thankthe private landowners who participated inthe farm pond research study that led to thedevelopment of this field guide.36ReferencesAltig, R. 1970. A key to the tadpoles of thecontinental United States and Canada.Herpetologica 26(2):180–207.Altig, R. and P. H. Ireland. 1984. A keyto salamander larvae and larviformadults of the United States and Canada.Herpetologica 40(2):212–218.Altig, R., R. W. McDiarmid, K. A. Nichols,and P. C. Ustach. 1998. Tadpoles of theUnited States and Canada: A Tutorialand Key. Contemporary HerpetologyInformation Series 1998 (2). http://www.pwrc.usgs.gov/tadpole/Caldwell, J. P. 1982. Disruptive selection: Atail color polymorphism in Acris tadpolesin response to differential predation.Canadian Journal of Zoology 60:2818–2827.Casper, Gary S. 1996. Geographicdistributions of the amphibians andreptiles of Wisconsin. Milwaukee PublicMuseum, Inc. Milwaukwee, Wisconsin.87 pp.Christiansen, J. L. 1998. Perspectives onIowa’s declining amphibians and reptiles.Journal of the Iowa Academy of Science105(3):109–114.Christiansen, J. L. and R. M. Bailey. 1991.The Salamanders and Frogs of Iowa. IowaDepartment of Natural Resources, DesMoines. 24 pp.Christoffel, R., R. Hay, and M. Wolfgram.2001. Amphibians of Wisconsin. Bureauof Endangered Resources, WisconsinDepartment of Natural Resources.Madison, Wisconsin.Conant, R. and J. T. Collins. 1991. A fieldguide to reptiles and amphibians ofEastern and Central North America. 3 rd

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Upper Midwest Environmental Sciences CenterCENTER DIRECTORLeslie E. Holland-BartelsACTING CHIEF, TERRESTRIAL SCIENCES BRANCHThomas W. CusterREPORT EDITORJerry D. Cox