food habits of sympatric larval ambystoma tigrinum and notophthalmus viridescens

Society for the Study of Amphibians and Reptiles

Food Habits of Sympatric Larval Ambystoma tigrinum and Notophthalmus viridescensAuthor(s): Timothy E. BrophySource: Journal of Herpetology, Vol. 14, No. 1 (Mar. 31, 1980), pp. 1-6Published by: Society for the Study of Amphibians and ReptilesStable URL: http://www.jstor.org/stable/1563867 .

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Volume 14 March 31, 1980 Number 1

Food Habits of Sympatric Larval Ambystoma tigrinum and Notophthalmus viridescens

Timothy E. Brophy

Department of Zoology, Southern Illinois University Carbondale, Illinois 62901*

ABSTRACT-The large overlap in diet between these species suggests that partitioning of food resources was minimal. All larvae of both species ate ostracods; ostracods were the most important prey for both, by weight and numerically. As the larval period progressed, there was an increase in the percentage of the diet, by weight, consisting of pouch snails and decrease in the percentage of the diet, by weight, consisting of ostracods and cyclopoid copepods in the diets of both A. tigrinum and N. viridescens. It may be more efficient for larger larvae to invest energy in capturing a few larger prey (snails) rather than many smaller prey (ostracods and copepods). Differences in food habits were probably due to difference in size between larvae of the two salamander species. In samples containing both species, large prey were eaten more frequently (chaoborid larvae and fingernail clams) or exclusively (corixids) by A. tigrinum whereas some small prey (cylclopoid copepods and aphids) were eaten more frequently by N. viridescens. There was no predation between species nor was there cannibalism within either species.

INTRODUCTION

There have been some studies of food habits of larval salamanders, but few that compare food habits of larval salamander species where they occur together (e.g., Anderson, 1968). The larvae of Ambystoma tigrinum tigrinum and Notophthalmus viridescens louisianensis often share habitats (woodland ponds, ditches) in midwest and south central United States but food habits in sympatric populations have not been compared. There have been detailed studies of food habits of larval A. tigrinum (Dineen, 1955; Dodson and Dodson, 1971) and N. viridescens (Hamilton, 1940; Burton, 1977) where they occur separately.

In this study, the food habits of A. tigrinum tigrinum and N. viridescens louisianensis in a pond in southern Illinois are compared. The purposes are to (a) determine food habits of each species throughout the larval period and (b) determine if there is partitioning of food resources between these sympatric species.

MATERIALS AND METHODS

The study site was a small pond in McGuire's Orchard, 11.3 km south of Carbondale, Jackson County, Illinois. It was built to serve as a water reservoir for spraying operations in surrounding

*Present address: 201 W. Adams, Nashville, Illinois 62263 USA

1980 JOURNAL OF HERPETOLOGY 14(1):1-6 (1)



TIMOTHY E. BROPHY

orchards, but no longer serves this function. The maximum surface area of the pond during the course of this study was approximately 300 m2, the maximum depth 1.7 m. Although the water level fluctuated seasonally, the pond did not completely dry up during the course of this study. There were no fish in the pond. Larval A. tigrinum and larval and adult N. viridescens were the only abundant aquatic vertebrates.

A total of 110 larval A. tigrinum (13 samples, 23 March-28 August, 3-10 larvae per sample) and 68 larval N. viridescens (8 samples, 21 May-18 September, 4-10 larvae per sample) were collected in 1976 and their gut contents identified. Seven samples containing both species collected together were obtained between 21 May and 28 August, 1976. Larvae of both species were collected with an aquarium net, dip net or 3 x 10 foot seine.

Larval A. tigrinum were found throughout the pond, larval N. viridescens most frequently in masses of Potamogeton foliosus. All samples but two were collected between 1000 and 1400 hours. Two samples, one of larval A. tigrinum only (16 April) and one of both species (21 May), were collected between 1900 and 2100 hours.

Larvae were simultaneously killed and fixed in 10% formalin immediately after capture. Twenty-four hours later they were briefly rinsed in water and transferred to 70% ethanol. Snout-vent lengths, measured to the nearest millimeter, were recorded for all larvae.

The entire digestive tract of each larva was removed and its contents were examined, sorted, identified and counted. Plant material, usually pieces of Salix sp. leaves, occasionally was found in guts but was probably incidentally ingested and, thus, not recorded.

Whole prey items were dried to constant weight and weighed to the nearest 0.1 mg with a digital analytical balance. Average weights of uniformly small items (cladocerans, copepods and ostracods) were calculated for 100 to 300 individuals. For larger uniform-sized prey (turbellarians, nematodes, water mites, corixids, notonectids, aphids and dipteran larvae), all from all samples were weighed and average weights calculated. For prey items of variable size (pouch snails and fingernail clams), all from each sample were weighed and the total recorded. Infrequent, small items and oligochaetes were not weighed. Oligochaetes were not weighed because no whole ones were found.

For each species of salamander, Pearson product moment correlation coefficients and least squares regressions (Roscoe, 1969) between snout-vent lengths of larvae and three dependent variables were calculated. These variables were: (1) percentage of total calculated weight of ingested prey consisting of pouch snails, (2) percentage of total calculated weight of ingested prey consisting of ostracods and copepods, and (3) number of prey taxa. The number of prey taxa was used as index of variety of the diet. In all cases except two (the class Turbellaria and the phylum Nematoda), family level taxa were counted. For this calculation the class Turbellaria and the phylum Nematoda were each counted as a family.

Samples of invertebrates and tadpoles (when present) were collected from McGuire's pond at the same time larval salamanders were collected. Leaf litter and debris were collected with a dip net and sorted with a 595 micron sieve to remove invertebrates. Plankton samples were taken with a plankton net. Invertebrates and tadpoles were preserved in 70% ethanol and identified. Samples of invertebrates were taken to document potential salamander prey.

RESULTS

Diet of Larvae.-Table 1 summarizes overall gut contents of both Ambystoma tigrinum and Notophthalmus viridescens during their larval periods. The most important prey for both species in terms of number eaten, weight and frequency of occurrence (number and percentage of guts containing prey) were ostracods of two species in the family Cypridae (Physocypria dentifera and Cypridopsis okeechobei). To determine whether or not the two species or ostracods were eaten equally frequently by the two species of salamander, those from all salamanders collected 21 May and 28 August were individually identified. Of the 3202 ostracods eaten by 15 A. tigrinum, 97.6% were P. dentifera, 2.4% C. okeechobei. Of 951 eaten by 20 newts, 99.2% were P. dentifera, 0.8%

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FOOD HABITS OF SYMPATRIC SALAMANDER LARVAE

TABLE 1. Summary of gut contents of 110 larval Ambystoma tigrinum (A) and 68 larval Notophthalmus viridescens (B) collected from McGuire's pond, Jackson Co., Illinois, between 23 March and 18 September, 1976.

Number of Prey Weight of Prey Guts Containing Prey

Percentage Percentage mg of total N of total

Prey A B A B A B A B A B

Turbellaria 29 10 5.8 2.0 0.1 0.2 15 3 13.6 4.4 Nematoda 548 3 98.8 0.5 1.2 * 50 2 45.4 2.9 Naididae (Oligochaeta) 68 57 ** 7 5 6.4 7.4 Daphnia ambigua (Cladocera) 317 15.8 0.2 20 18.2 Macrocyclops albidus (Copepoda) 1537 141 107.6 9.9 1.3 0.8 62 40 56.3 58.8

Diaptomus ashlandi (Copepoda) 7 0.5 *2 1.8 Cypridae (Ostracoda) 38424 6557 4227.0 721.3 49.3 61.3 110 68 100.0 100.0 Eylais sp. (Hydracarina) 1 41 0.1 4.1 * 0.3 1 1 0.9 1.5 Arrenurus expansus (Hydracarina) 1 0.1 *1 0.9 Piona sp. (Hydracarina) 5 3 0.5 0.3 *4 3 3.6 4.4 Lestes sp. (Odonata) 10 * ** 3 4.4 Libellulidae (Odonata) 1 * ** 1 1.5 Trichocorixa calva (Hemiptera) 32 688.3 8.0 18 16.4 Notonecta sp. (Hemiptera) 2 70.0 0.8 2 1.8 Gerris sp. (Hemiptera) 1 ** * 1 0.9 Unidentified Hemiptera 1 * ** 1 1.5 Cercopidae (Homoptera) 1 . . 1 0.9 Aphididae (Homoptera) 8 50 1.6 10.5 * 0.9 6 17 5.4 25.0 Chaoborus sp. (Diptera) 436 5 863.3 9.9 10.1 0.8 57 4 51.8 5.9 Culicidae (Diptera) 3 * ** 3 2.7 Ceratopogonidae (Diptera) 26 26.3 0.3 14 12.7 Chironomidae (Diptera) 25 24 20.7 19.9 0.2 1.7 13 10 11.8 14.7 Ephydridae (Diptera) 2 1 ** * ** 2 1 1.8 1.5 Unidentified Insect 2 1 .. * ** 2 1 1.8 1.5 Physa sp. (Gastropoda) 200 324 2261.0 392.8 26.4 33.4 23 39 20.9 57.4 Snail eggs 16 8.2 0.1 2 1.8 Sphaerium sp. (Pelecypoda) 94 6 173.7 4.9 2.0 0.4 30 6 27.3 8.8

*Less than 0.1%. **Not weighed.

C. okeechobei. Thus, there appeared to be no significant difference between salamanders in species of ostracods eaten. Numerically, ostracods formed over 90% of the diet of both species and were found in every larva examined. By weight, ostracods accounted for 49.3% of the diet of A. tigrinum and 61.3% of the diet of N. viridescens.

Other invertebrates found in 10% or more of A. tigrinum guts were (in decreasing order of frequency of occurrence) cyclopoid copepods (Macrocyclops albidus), chaoborid larvae (Chao- borus sp.), non-parasitic nematodes, fingernail clams (Sphaerium sp.), pouch snails (Physa sp.), cladocerans (Daphnia ambigua), corixids (Trichocorixa calva), turbellarians, ceratopogonid larvae and chironomid larvae. Of the two species of corixids found in McGuire's pond (Hesperocorixa nitida and Trichocorixa calva) only the smaller species, T. calva, was eaten.

Other invertebrates which occurred in 10% or more of N. viridescens guts were (in decreasing order of frequency of occurrence) cyclopoid copepods (Macrocyclops albidus), pouch snails (Physa sp.), aphids and chironomid larvae. Snails in the guts of N. viridescens averaged smaller than those eaten by A. tigrinum. The mean individual weight of snails eaten by A. tigrinum was 11.3 mg, for N. viridescens 1.2 mg. Aphids, which were eaten by 25% of newt larvae, are terrestrial insects and probably fell into the pond from surrounding vegetation.

Several types of invertebrates found in McGuire's pond were not found in guts of either salamander species. Beetles of four families, Dytiscidae (Acilius sp., Hydroporus sp., Celina sp.

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TIMOTHY E. BROPHY

and Laccophilus sp.), Gyrinidae (Dineutus sp.), Haliplidae (Peltodytes sp.) and Hydrophilidae (Berosus sp., Tropisternus blatchleyi, T. lateralis and T. mexicanus), were commonly collected, but none was found in larval salamanders. Leeches (Placobdella rugosa), fishing spiders (Dolomedes triton), water scorpions (Ranatra buenoi and R. fusca) and pond snails (Lymnaea sp.) were also present but not eaten.

Tadpoles of three species (Hyla crucifer, Rana catesbeiana and R. clamitans) were commonly collected in the pond from May until September, but were not found in any salamander gut.

There was no predation by larval A. tigrinum on N. viridescens nor by larval N. viridescens on A. tigrinum. There was no evidence of cannibalism within either species.

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FIGURE 1. Correlations between snout-vent length of 68 larval Notophthalmus viridescens and percentage of prey, by weight, consisting of pouch snails (top) and ostracods and copepods (bottom). Newts were collected from McGuire's pond, Jackson Co., Illinois, between 21 May and 18 Sep- tember, 1976. Plotted are mean snout-vent lengths of larvae collected in each sample. Regression equations are Y = 9.65X - 126.5, r = 0.86, P < 0.01 (top) and Y = -11.6X -252.3, r = -0.91, P < 0.01.

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FIGURE 2. Correlation between snout-vent length of 110 larval Ambystoma tigrinum and number of prey taxa. Sala- manders were collected from McGuire's pond, Jackson Co., Illinois, between 23 March and 28 August, 1976. Plotted are mean snout-vent lengths of larvae collected in each sample. Regression equation is Y = 0.19X + 2.9, r = 0.78, P < 0.01.

The diets of larval salamanders from McGuire's pond and those of larvae from other populations are similar. Both Dineen (1955) and Dodson and Dodson (1971) found that copepods, cladocerans, chaoborid and chironomid larvae and fingernail clams were frequent prey for A. tigrinum larvae in ponds in Cass Co., Michigan and Gunnison Co., Colorado, re- spectively. Dodson and Dodson (1971) also found dragonfly naiads and amphipods to be important prey items. In McGuire's pond only one dragonfly naiad and no amphipods were found in guts of tiger salamanders. Hamilton (1940) found that N. viridescens from a pond near Ithaca, New York, fed frequently on ostracods, copepods, chironomid larvae, small snails and fingernail clams. Burton (1977) found that newt larvae from a pond in Grafton Co., New Hampshire, fed frequently on ostracods, cladocerans, chironomid larvae and amphipods. No cladocerans nor amphipods were found in guts of newt larvae from McGuire's pond.

Seasonal Changes of Diet.-The diets of both species changed as the larval period progressed. For both N. viridescens (r = 0.86, P < 0.01) (Fig. 1) and A. tigrinum (least squares regression: Y = 0.90X - 18.8, r = 0.66, P < 0.02) there were significant positive correlations between snout-vent length and percentage of the diet, by weight, consisting of snails of the genus Physa. Conversely, there were significant negative correlations between snout-vent length and percentage of the diet, by weight, consisting of ostracods and copepods for N. viridescens (r = -0.91, P < 0.01) (Fig. 1) and A. tigrinum (least squares regression: Y = -0.73X + 84.9, r = -0.61, P < 0.05).

Larval A. tigrinum ate a wider variety of prey as the season progressed. For A. tigrinum, but not for N. viridescens, there was a positive significant correlation between snout-vent length and the number of prey taxa (r = 0.78, P < 0.01) (Fig. 2). Small larvae (< 18 mm SVL) fed mostly on zooplankton in March and early April, even though larger prey (corixids, snails and

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FOOD HABITS OF SYMPATRIC SALAMANDER LARVAE

fingernail clams) were present in the pond. From mid-April until the end of the larval period, larger larvae (18-65 mm svl) still ate zooplankton, but also included larger prey in their diets.

Larval A. tigrinum grew more rapidly and reached a larger size before transforming than did larval N. viridescens. Average snout-vent lengths of A. tigrinum from each sample ranged from 12.3 mm in March to 60.0 mm in August; the range for N. viridescens was from 12.3 mm in May to 19.2 mm in September. Most larvae of both species had transformed and left the pond by 18 September.

DISCUSSION

There was a large overlap in food habits of sympatric larval Ambystoma tigrinum and Notophthalmus viridescens. Both species ate a wide variety of invertebrates, but two items, ostracods (Physocypria dentifera and Cypridopsis okeechobei) and snails of the genus Physa, accounted for most of the diet, by weight, of both. However, there were large differences in the frequency of occurrence of other, less numerous prey in salamander guts. Turbellarians, nematodes, chaoborid larvae (Chaoborus sp.) and fingernail clams (Sphaerium sp.) were more frequently eaten by A. tigrinum, but cyclopoid copepods (Macrocyclops albidus) and aphids were more frequently eaten by N. viridescens. Corixids (Trichocorixa calva) and ceratopogonid larvae were eaten only by A. tigrinum.

Some of these differences in food habits were probably due to the difference in size between larvae of the two salamanders. The smaller body size and, therefore, smaller mouth of N. viridescens prevented them from feeding on some prey that were available to A. tigrinum. Dodson and Dodson (1971) concluded that the maximum size of prey eaten by A. tigrinum depended on the size of the mouth gape, which increased as larvae grew. In McGuire's pond, large prey were eaten more frequently (chaoborid larvae and fingernail clams) or exclusively (corixids) by A. tigrinum whereas some small prey (cyclopoid copepods and aphids) were eaten more frequently by N. viridescens. Within a prey group of highly variable sized individuals (snails), those eaten by N. viridescens were considerably smaller.

Habitat differences within the pond may have been responsible for some differences noted in feeding habits of larval species. Ambystoma tigrinum were collected fairly uniformly throughout the pond, most N. viridescens from masses of Potamogeton foliosus adjacent to the shoreline.

As larval A. tigrinum and N. viridescens grew as the season progressed, they both ate more snails and fewer ostracods and copepods. It may be more efficient for larger larvae to invest energy in capturing a few larger prey (snails) rather than many smaller prey (ostracods and copepods). An alternative explanation for the shift from ostracods and copepods to snails as the season progressed is that the snail population may have bloomed as ostracod and copepod populations declined in late summer, but this possibility was not investigated. Hamilton (1932) concluded that seasonal variation in prey abundance determined what adult newts ate, and that food and feeding habits of aquatic vertebrates changed as prey organisms reached their peak of density, then declined.

Larval A. tigrinum ate a wider variety of prey as the season progressed. This may have been because the larger mouth of older, larger larvae enabled them to eat both large prey (corixids, dipteran larvae, snails and fingernail clams) and small prey (zooplankton), whereas younger, smaller larvae were restricted to small prey (zooplankton). Dodson and Dodson (1971) also found that younger, smaller A. tigrinum larvae (< 20 mm SVL) ate zooplankton nearly exclusively, but older, larger larvae had a more diverse diet that included both zooplankton and larger prey such as amphipods, dipteran larvae, dragonfly naiads, snails and fingernail clams.

The positive correlation between snout-vent length of A. tigrinum larvae and the number of prey taxa is partly due to the fact that larger larvae had more prey items per gut than did smaller larvae. In McGuire's pond, a wide variety of invertebrates were eaten by larvae. Therefore, it would be expected that the guts of larger larvae, which contained more prey items, would also contain more different types of prey items.

Larval A. tigrinum did not eat larval N. viridescens though they were large enough that they probably could have. It is possible that newt larvae were unpalatable and thus avoided. Several

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TIMOTHY E. BROPHY

workers (Webster, 1960; Brodie, 1968; Hurlbert, 1970) have demonstrated unpalatability or toxicity of adult and eft N. viridescens. Twitty (1966) has shown that embryos and newly hatched larvae of related species of Taricha are toxic.

Water scorpions, corixids of the species Hesperocorixa nitida, fishing spiders and tadpoles were probably too large to have been eaten by larval salamanders of either species. Notonectids were abundant in the pond throughout the summer, but were eaten by only the largest A. tigrinum larvae. The irritant, perhaps even poisonous, bite of notonectids probably prevented most larvae from eating them.

Powers (1907), Burger (1950), and Rose and Armentrout (1976) found cannibalism in populations of larval A. tigrinum where larvae were densely crowded and other food items were scarce. The abundance of other prey in McGuire's pond may have precluded cannibalism. Also, tiger salamanders in McGuire's pond may not have been as crowded as in populations where cannibalism was found. Cannibalism has not been reported for larval N. viridescens, though cannibalism of adults on eggs and larvae has been (Smallwood, 1928; Morgan and Grierson, 1932; Burton, 1977).

Anderson (1968) found major differences in feeding habits between sympatric larval Amby- stoma tigrinum and A. macrodactylum in a pond in Santa Cruz Co., California. There, A. tigrinum fed mostly on tadpoles of Hyla regilla and Rana aurora, whereas A. macrodactylum ate mainly chironomid larvae, ostracods and oligochaetes. The large overlap in diet between A. tigrinum and N. viridescens larvae in McGuire's pond suggests that partitioning of food resources was minimal there. If there was competition for food between species of salamander larvae in McGuire's pond, it did not result in widely divergent feeding habits.

ACKNOWLEDGMENTS

This paper is from a Master's thesis prepared under the guidance of Ronald. A. Brandon at Southern Illinois University at Carbondale. I wish to thank Dr. Brandon for offering valuable suggestions throughout the course of the study. Joseph A. Beatty aided in the identification of some invertebrates. I thank Nancy Brophy, Daniel Gates and Stephen Sarikas for assistance in the field.

LITERATURE CITED

Anderson, J. D. 1968. A comparison of the food habits of Ambystoma macrodactylum sigillatum, Ambystoma macrodactylum croceum and Ambystoma tigrinum californiense. Herpetologica 24:273-284.

Brodie, E. D., Jr. 1968. Investigations on the skin toxin of the red-spotted newt, Notophthalmus viridescens. Amer. Midi. Natural. 80:276-280.

Burger, W. L. 1950. Novel aspects of the life history of two Ambystomas. J. Tenn. Acad. Sci. 25:252-257. Burton, T. M. 1977. Population estimates, feeding habits and nutrient and energy relationships of Notophthalmus v. viridescens

in Mirror Lake, New Hampshire. Copeia 1977:139-143. Dineen, C. F. 1955. Food habits of the larval tiger salamander, Ambystoma tigrinum. Proc. Indiana Acad. Sci. 65:231-233. Dodson, S. and V. Dodson. 1971. The diet of Ambystoma tigrinum larvae from Western Colorado. Copeia 1971:614-624. Hamilton, W. J., Jr. 1932. The food and feeding habits of some eastern salamanders. Copeia 1932:83-86.

.1940. The feeding habits of larval newts with reference to availability and predilection of food items. Ecology 21:351-356.

Hurlbert, S. H. 1970. Predator responses to the vermilion-spotted newt (Notophthalmus viridescens). J. Herpetol. 4:47-55. Morgan, A. H. and M. C. Grierson. 1932. Winter habits and yearly food consumption of adult spotted newts, Triturus viridescens.

Ecology 13:54-62. Powers, J. H. 1907. Morphological variation and its causes in Amblystoma tigrinum. Univ. Nebraska Studies 7:197-272. Roscoe, J. T. 1969. Fundamental research statistics for the behavioral sciences. Holt, Rinehart and Winston, Chicago. 336 pp. Rose, F. L. and D. D. Armentrout. 1976. Adaptive strategies of Ambystoma tigrinum Green inhabiting the Llano Estacado of

West Texas. J. Anim. Ecol. 45:713-729. Smallwood, W. M. 1928. Notes on the food of some Onondaga Urodela. Copeia 169:89-98. Twitty, V. C. 1966. Of scientists and salamanders. W. H. Freeman and Company, San Francisco. 198 pp. Webster, D. A. 1960. Toxicity of the spotted newt Notophthalmus viridescens to trout. Copeia 1960:74-75.

Accepted 28 Sept 1979 Copyright 1980 Society for the Study of Amphibians and Reptiles

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food habits of sympatric larval ambystoma tigrinum and notophthalmus viridescens

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