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Novel Food Habits of Branchiate Mole Salamanders (Ambystoma talpoideum) from Southwestern Arkansas
Renn Tumlison and Brett Serviss

Southeastern Naturalist, Volume 12, Issue 3 (2013): 579–588

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579 R. Tumlison and B. Serviss 22001133 SOUSToHuEthAeSaTstEeRrnN N NaAtuTrUaRlisAtLIST 12V(o3l). :1527,9 N–5o8. 83 Novel Food Habits of Branchiate Mole Salamanders (Ambystoma talpoideum) from Southwestern Arkansas Renn Tumlison1,* and Brett Serviss1 Abstract - Branchiate Ambystoma talpoideum (Mole Salamanders) in fishless ponds can be large enough to act as predators, rather than competitors, during the spring breeding season of other amphibians. Food habits of high-density Mole Salamander populations from 2 proximate woodland ponds in Clark County, AR were examined before and after egg-laying by frogs, with an expectation that the salamanders likely would consume hatching tadpoles. However, salamanders instead commonly fed on the novel item of freshly-laid frog eggs. Results from both ponds indicated that the salamanders, perhaps due to food limitation, consumed smaller prey items than would be expected and heavily consumed frog eggs, a novel item. Introduction Many amphibians reduce risk of predation by breeding in fishless ponds (Semlitsch 1988). Species that either breed early (e.g., Ambystoma opacum Gravenhorst [Marbled Salamander], Boone et al. 2002) or grow quickly (e.g., Ambystoma tigrinum Green [Tiger Salamander], Wilbur 1972) may achieve a body size advantage for predation. In Arkansas, both the metamorphic and paedomorphic forms of Ambystoma talpoideum Holbrook (Mole Salamander) breed in winter, whereas many other amphibians breed in the same ponds in the spring (Trauth et al. 2004). Resulting syntopic occurrence of branchiate Mole Salamanders and larvae of other amphibian species in such ponds is often characterized by differences in size and ontogeny (Nyman 1991), which might allow predation by the larger species (Stenhouse 1985, Stenhouse et al. 1983), facilitated by the larger gape of the mouth (Freda 1983, Taylor et al. 1988). Although fishless ponds provide breeding sites for several potentially competing species of amphibians, “priority effects” can change the relationship from one of competition to one of predation (Blaustein and Mar galit 1996). During the winter of 2002, we located two fishless ponds that supported populations of branchiate Mole Salamander at the periphery of their known distribution (Trauth et al. 1993) in Clark County, AR. Shoreline seine samples in both ponds averaged 20 individuals/m2—a value considered by Semlitsch (1987) to represent a high density population. High population densities of aquatic salamanders can create food limitation in their environments, possibly causing increased predation on other amphibians (Morin 1981, Stenhouse et al. 1983, Taylor et al. 1988). Few studies of food habits of larger branchiate Mole Salamanders exist. Larval Mole Salamanders are known to feed mostly on pond invertebrates (Petranka 1Department of Biology, Henderson State University, Arkadelphia, AR 71999. *Corresponding author - R. Tumlison and B. Serviss 2013 Southeastern Naturalist Vol. 12, No. 3 580 1998) although paedomorphic individuals are known to take occasional conspecific ova (McAllister and Trauth 1996). The Mole Salamander is an aggressive species, a superior competitor to other larval salamanders (Walls and Jaeger 1987), and the facultative paedomorphosis it exhibits may allow the exploitation of productive but transient resources within the pond environment (Semlitsch 1987). We hypothesized that the high density of the populations we found could cause food limitation within the environment and result in shifts in prey selection as new prey become available. The spring breeding season produces a surge in availability of potential amphibian prey as other salamanders and frogs begin to oviposit. Thus, our field situation presented an opportunity to evaluate whether the high-density population of branchiate Mole Salamanders would consume large numbers of hatchling larvae of other amphibians. High occurrences of amphibian prey are undocumented in studies of Mole Salamander food habits. Methods and Materials Laboratory study We conducted laboratory experiments to develop predictions for the field study. Because it is commonly known that feeding in amphibians is often initiated by movement, we placed four branchiate Mole Salamanders (snout–vent lengths [SVL] from 30–35 mm) individually in 1-gallon plastic containers and supplied each with 12 hatchling Lithobates sphenocephalus Cope (Southern Leopard Frog) tadpoles to evaluate feeding attempts on motile amphibian prey. These salamanders were taken from the field the previous day and were selected because they did not have either distended or shrunken stomachs (they were neither full nor starving). A similar trial was conducted with a different set of four Mole Salamanders and egg masses of L. sphenocephalus, in which there would be little or no movement. Further, another set of 4 novice branchiate Mole Salamanders were placed in separate aquaria with egg masses of Ambystoma maculatum Shaw (Spotted Salamander). Though unhatched, the embryos in these egg masses were already elongated, and moved upon stimulation. We closely observed each of these trials for 30 minutes, recording the behavior and positioning of the salamanders in relation to the potential prey, and the frequency and results of each predation attempt. This portion of the study allowed us to examine whether and how the salamanders fed on nonactive, unhatched larvae versus active larvae or motile hatchlings. Field study The field study site was a woodland area located 8 km NW of Gurdon, Clark County, AR (Sec. 27, T9S, R21W). The forest was a pine (Pinus)–hardwood (mostly Carya [hickory] and Quercus [oak]) mix. Two ponds, located about 0.5 km apart, were present within this forest managed by the Ross Foundation, Arkadelphia, AR. Both ponds were permanent: the area of pond 1 was 300 m2 at low 581 R. Tumlison and B. Serviss 2013 Southeastern Naturalist Vol. 12, No. 3 water and 340 m2 at high water; and the area of pond 2 was 340 m2 at low water and 390 m2 at high water. Lithobates palustris LeConte (Pickerel frogs) and Spotted Salamanders were beginning to lay eggs in the ponds by very late February. On 7 March 2002, before many of the available Pickerel Frog eggs hatched (which usually occurs within about 10 days of oviposition, Trauth et al. 2004), a sample of 59 salamanders was collected from pond 1 by use of long-handled dip nets and seines (sample 1), and preserved for stomach analysis. Specimens were taken as randomly as possible, without selection based on body size or degree of distension of the abdomen. We allowed one week for the bulk of the increasing number of frog eggs to hatch, and on 15 March we collected an additional 31 in the same manner (sample 2), and preserved them for comparison analysis of food habits, size, and sex ratio. We also sampled with a benthic net to detect amphibian hatchling s. Just after the first sampling period, pond 2 was discovered and found to support a large population of branchiate Mole Salamanders, and we sampled the pond on 15 March to allow comparison of food habits between ponds on the same date. As with pond 1, we minimized shoreline disturbance in pond 2 while collecting 40 specimens (sample 3) with long-handled dip nets a nd seines. These samples allowed comparison of food habits of branchiate Mole Salamanders from one pond before and after the hatching of the spawn of other amphibians, and a comparison of food habits within 2 local ponds on the same date, and under similar environmental conditions. Other than the qualitative assessment of the increase in amphibian eggs, no food availability data were obtained for potential invertebrate prey items in the ponds. We measured SVL of preserved specimens and determined the sex by internal examination of gonads, aided by a dissecting microscope. Stomachs were removed and opened, and food items were separated, counted, and identified to the lowest taxonomic level possible (Pennak 1989, Thorp and Covich 1991). We summed the number of different taxa consumed by each salamander, then performed ANOVAs to determine whether diversity of foods varied by pond and date. Separate ANOVAs were used to examine differences between dates within pond 1, and between ponds 1 and 2 on the same date, evaluating total numbers of each major food item as the dependent variable. To reduce error with multiple tests, Bonferroni adjustments were made in analyses comparing ponds on the same date, and separately when comparing dates for the same pond (Rice 1989). Further, percent of salamanders ingesting each food item was calculated to evaluate how commonly individuals consumed a prey taxon, and the total number of each item was calculated to rank the overall importance of the item as a food. Results Laboratory study The lab trials with eggs and tadpoles of Southern Leopard Frogs provided different results. When placed in containers with frog eggs, all Mole Salamanders positioned themselves near the eggs, but only occasionally attempted to ingest R. Tumlison and B. Serviss 2013 Southeastern Naturalist Vol. 12, No. 3 582 an egg. Some eggs, including their gelatinous envelopes, were ingested then spit out (2 of the 4 Mole Salamanders attempted to feed on eggs, temporarily ingesting one and two eggs in those two cases). When presented with mobile tadpoles, however, the salamanders consumed 66–100% (8–12) of the prey within 30 minutes in each of the four trials. During the lab trials using Spotted Salamander eggs with elongated embryos, all four Mole Salamanders positioned themselves on top of the egg masses and pushed their heads into the mass. The embryos, which moved within their eggs on disturbance, were consumed from within the gelatin of the egg masses with apparently little or no intake of gelatin. Mole Salamanders consumed 1–3 Spotted Salamander embryos within the four 30-minute trials. A l l s u c c e s s f u l feeding observed in lab trials was in response to motion of prey. Based on these laboratory observations, we anticipated that the large number of branchiate Mole Salamanders in the ponds would commonly consume hatchlings, but not eggs, of other amphibians. Field study Temperature of pond 1, which had a maximum depth of 75 cm, was 17.0° C on 7 March. Elongated larvae were already present in numerous, large Spotted Salamander egg masses, which were green with the symbiotic algae Oophila amblystomatis Lambert Ex Printz. A few scattered masses of Pickerel Frog eggs had recently been laid, and elongation of the embryos was not e vident. On 15 March, the previously deposited eggs of the Pickerel Frogs had disappeared, which we presumed to have hatched, and new eggs had been laid. However, no tadpoles were found during benthic net sampling of the pond. Some of the Spotted Salamander egg masses contained no embryos, but none of their hatched larvae were found during benthic sampling. Primary foods detected during this study included a variety of small crustaceans, insects, and frog eggs (Table 1). Unexpectedly, however, no tadpoles were found in stomachs or in benthic-net-collected samples from the ponds. Instead, many salamanders had distended their stomachs with eggs of Pickerel Frogs, including the gelatinous matrix. Analysis of variance (ANOVA) indicated no differences in any food taxa consumed based on sex (P > 0.05), therefore sexes were combined for further analysis. All but three of the 130 specimens (97.7%) had food items in the stomach. The number of different taxa of food items in a stomach ranged from 0–7 in sample 1 (mean = 2.6), 3–8 (mean = 5.2) in sample 2, and 1–8 (mean = 5.2) in sample 3. A significantly greater diversity of foods was taken by individuals in pond 1 on the latter sample date (sample 1 vs. sample 2: ANOVA, F = 65.54; d.f. = 1, 88; P < 0.0001), yet the diversity of foods taken by individuals on the same date but in different ponds was the same (sample 2 vs. sample 3: F = 0.03; d.f. = 1, 69; P > 0.05). The comparison of foods taken in pond 1 on different dates demonstrated fewer differences than in the comparison between ponds. The only significant 583 R. Tumlison and B. Serviss 2013 Southeastern Naturalist Vol. 12, No. 3 differences detected were increases in the frequency of isopods, zygopterans, chaoborids, and frog eggs (Table 2). In the field, we observed that the number of frog egg masses consumed increased between the sampling periods, and presume that all the increases in consumed taxa were either due to increases in availability due to reproduction of the prey or by increased activity (either of the prey or predator). A higher percentage of salamanders were eating almost every food category on 15 March compared with 7 March. On average, stomachs contained twice as many prey taxa on 15 March. The comparison of foods consumed on the same date (15 March) demonstrated differences between ponds. In pond 1, Mole Salamanders consumed significantly more isopods, amphipods, zygopterans, chaoborids, and frog eggs, Table 1. Food items recovered from stomachs of 130 branchiate Ambystoma talpoideum from southwestern Arkansas, March 2002. Sample 1 = pond 1, 7 March; sample 2 = pond 1, 15 March; sample 3 = pond 2, 15 March. Miscellaneous category includes unidentified Hemiptera. For each sample, percent of salamanders consuming each item is given. Also, the total number of individuals of each prey item (and the percent of the total foods represent ed) is provided per sample. % of salamanders eating item in sample Total # (%) per sample Prey taxon 1 (n = 59) 2 (n = 31) 3 (n = 40) 1 2 3 Annelida Oligochaeta - 3.2 - - 3 (0.4) - Crustacea Cladocera 27.1 32.3 70.0 210 (25.9) 46 (5.7) 1773 (63.2) Copepoda 27.1 67.7 77.5 104 (12.8) 104 (12.9) 366 (13.1) Ostracoda 6.8 9.7 42.5 16 (2.0) 3 (0.4) 61 (2.2) Isopoda 13.6 54.8 22.5 14 (1.7) 24 (3.0) 9 (0.3) Amphipoda 54.2 64.5 17.5 50 (6.2) 41 (5.1) 7 (0.2) Decapoda 1.7 - 2.5 1 (0.1) - 1 (less than 0.1) Insecta Ephemeroptera 13.6 6.5 - 8 (1.0) 2 (0.3) - Odonata Anisoptera 3.4 3.2 - 2 (0.2) 1 (0.1) - Zygoptera - 16.1 - - 5 (0.6) - Heteroptera Corixidae 18.6 25.8 60.0 13 (1.6) 9 (1.1) 74 (2.6) Lepidoptera - - 2.5 - - 1 (less than 0.1) Coleoptera 13.6 35.5 5.0 14 (1.9) 13 (1.6) 28 (1.0) Diptera Chironomidae 8.5 19.4 70.0 8 (1.0) 9 (1.1) 62 (2.2) Chaoboridae 8.5 77.4 47.5 6 (0.7) 76 (9.4) 37 (1.3) Hymenoptera Formicidae - 3.2 - - 1 (0.1) - Gastropoda (limpet) - - 2.5 - - 1 (less than 0.1) Amphibia Ranidae 50.8 96.8 77.5 363 (44.8) 469 (58.1) 383 (13.7) Miscellaneous 1.7 3.2 2.5 1 (0.1) 1 (0.1) 14 (less than 0.1) R. Tumlison and B. Serviss 2013 Southeastern Naturalist Vol. 12, No. 3 584 but in pond 2 significantly more copepods, corixids, and chironomids were consumed (Table 2). Food items most commonly taken from either pond tended to be taken by a higher percentage of Mole Salamanders in that pond (Table 1), with the exception of amphipods, which were taken more often in pond 1 but by a higher percentage of salamanders in pond 2. The presence of enlarged, yolk-filled follicles confirmed that 2 of the 72 females were paedomorphic (Semlitsch 1985). The remaining females in the samples were developing follicles, and we observed some metamorphosis occurring in salamanders in the ponds; thus we could not determine if the population consisted of all paedomorphs, or of a mix of paedomorphic and larval metamorphic individuals. In external appearance, our specimens most closely matched the series of images of paedomorphic individuals provided by Trauth et al. (2004). Table 2. P-values for ANOVAs of the effects of pond and date on food items recovered from stomachs of 130 branchiate Ambystoma talpoideum from southwestern Arkansas, March 2002. * = significant difference (P < 0.05) after Bonferroni adjustment (Rice 1989) was applied separately to analysis by pond and by date. P-values Prey taxon Pond (d.f. = 1, 69) Date (d.f. = 1, 88) Annelida Oligochaeta 0.2589 0.1690 Crustacea Cladocera 0.0306 0.1687 Copepoda 0.0102* 0.1089 Ostracoda 0.0160 0.4959 Isopoda 0.0066* 0.0027* Amphipoda 0.0001* 0.0682 Decapoda 0.3825 0.4716 Insecta Ephemeroptera 0.1061 0.3133 Odonata Anisoptera 0.2589 0.9676 Zygoptera 0.0079* 0.0013* Heteroptera Corixidae 0.0009* 0.5345 Lepidoptera 0.3825 - Coleoptera 0.6794 0.1364 Diptera Chironomidae 0.0001* 0.2869 Chaoboridae 0.0034* 0.0001* Hymenoptera Formicidae 0.2589 0.1690 Gastropoda (limpet) 0.3825 - Amphibia Ranidae 0.0303* 0.0001* Miscellaneous 0.3825 0.4716 585 R. Tumlison and B. Serviss 2013 Southeastern Naturalist Vol. 12, No. 3 Sex ratios were female-biased in pond 1 (sample 1: 0.63 males/female; sample 2: 0.82 males/female) but slightly male-biased in pond 2 (1.11/1). Raymond and Hardy (1990) found a sex ratio bias favoring males in some years, while Petranka (1998) observed no sex ratio bias. The mean SVL of Mole Salamanders from pond 1 was 36.8 mm on both dates sampled (range = 33–41 mm on 7 March and 33–40 mm on 15 March), just large enough to begin metamorphosis (Semlitsch 1987). From pond 2, mean SVL was 34.4 mm (range 28–40 mm), which is slightly less than Semlitsch’s (1987) critical value of 35 mm for metamorphosis. Discussion Based on preliminary laboratory observations, it was expected that salamanders would consume tadpoles in the field. However, the evaluation of stomach contents revealed that many frog eggs never developed to the hatchling stage. Of the 1215 frog eggs found in all stomachs, none was found in which the embryos had begun to elongate, and the gelatinous matrix was present along with the embryo in the stomachs. Likewise, the intact, unwrinkled membranes apparently did not have adequate time to expand via absorption of water, which begins immediately upon egg deposition (Duellman and Trueb 1986), thereby corroborating the conclusion that eggs were eaten during or just after oviposition. To that end, we conjecture that the motion of the eggs being extruded by the frog was enough to elicit a feeding response by the salamanders; otherwise they had learned to take motionless deposited eggs as food. Several field-collected egg masses of frogs hatched successfully after removal to the lab; thus, all eggs were presumed to be fertile and capable of elongation. Although numerous egg masses had been present, no larval Spotted Salamanders or frog tadpoles were found during benthic net sampling of the ponds. Therefore, it is likely that predation by the Mole Salamanders essentially nullified reproductive success of other species of amphibians about the time of oviposition. No tadpoles were found during additional field sampling over succes sive weeks. Interspecific predation in amphibians has been known to cause total spawn failure (Banks and Beebee 1987, Walters 1975), but this level of predation has not been suggested for sub-metamorphic or paedomorphic Mole Salamanders such as we observed. McAllister and Trauth (1996) found that paedomorphic Mole Salamanders consumed a few conspecific ova as well as congeneric larvae. The gelatinous matrix probably is of low nutritional value (Petranka et al. 1998), although the egg itself contains useful nutrients. Regester et al. (2008) noted that energetic importance of amphibian prey is underestimated in studies that measure abundance of prey items; thus, the high levels of occurrence of amphibian prey reported herein were likely of even greater energetic importance to the salamanders. Considering the abundance of salamanders in the ponds, food limitation (Morin 1981, Stenhouse et al. 1983, Taylor et al. 1988) may have forced the ingestion of some of the foods documented during our study. Under the stress R. Tumlison and B. Serviss 2013 Southeastern Naturalist Vol. 12, No. 3 586 of food limitation, tadpoles of Lithobates sylvaticus LeConte (Wood Frog) prey upon embryos of the Spotted Salamander (Petranka et al. 1998). Furthermore, hard-bodied prey in the diet (e. g., Coleopteran adults and corixids) require more energy to digest (Secor and Boehm 2006), which should make those prey less desirable if softer-bodied prey were available, further supporting the hypothesis of food limitation. If food limitation were occurring, predators would be expected to adjust search images toward whatever prey was available and to consume multiple individuals of sub-optimally sized prey. Our samples included individual salamanders that had consumed up to 52 frog eggs, 641 cladocerans, 37 copepods, 10 corixids, 16 ostracods, or 10 amphipods. Cladocerans, copepods, and ostracods are particularly small compared to the gape and size of the salamanders studied. Cladocerans are so small that they would seem to be of little value as food for the larger submetamorphic or paedomorphic salamanders that we examined. The fact that 641 individuals of this sub-optimally-sized prey were found in one stomach (several other salamanders also had consumed large numbers of cladocerans) indicates that their availability, coupled with a probable search image, made them an often-taken food for a hungry salamander. Taylor et al. (1988) also reported consumption of large numbers of cladocerans by a few, but much smaller, larval Mole Salamanders. Copepods, even when abundant, are thought to occur infrequently among the foods of larger aquatic salamanders due to the copepods’ rapid darting abilities (Taylor et al. 1988); however, they were common in our samples. Most of the copepods we found also possessed enlarged egg sacs, which may have made them larger as well as slower, thereby increasing both their caloric value and susceptibility to predation. Copepods and cladocerans were taken more commonly in pond 2, in which salamanders were smaller on average. Common foods of smaller Mole Salamanders include copepods and cladocerans (Branch and Altig 1981, Taylor et al. 1988) but older larvae shift their diets to include larger prey such as chironomids (Taylor et al. 1988). Ostracods also are common foods for smaller larval salamanders (Taylor et al. 1988), and under experimental conditions, they seem to be more common in the absence of salamanders (Holomuzki et al. 1994). Ostracods were not common foods in the present study, but, like cladocerans and copepods, they were taken more commonly in pond 2. No food availability data were obtained in this study, but the higher presence of ostracods, copepods, and cladocerans as food in pond 2 could reflect take by smaller food-limited salamanders. At the beginning of our study, we anticipated that the high density of salamanders would limit food resources, thereby resulting in a shift toward hatchling amphibian prey. Instead, we discovered that consumption of amphibian eggs became prevalent, which apparently led to general spawn failure for the Pickerel Frog. In a prey-depleted environment, the nutrient flush appearing with the hatch of other amphibians could provide the energy necessary to fuel metamorphosis. Ryan and Semlitsch (2003) noted that Mole Salamanders were more likely 587 R. Tumlison and B. Serviss 2013 Southeastern Naturalist Vol. 12, No. 3 to metamorphose when growth rates were high later in development. Under stressful conditions, larger larvae metamorphose to escape unfavorable aquatic habitats (Doyle and Whiteman 2008). Consumption of eggs by branchiate Mole Salamanders could supply the requisite energy (Regester et al. 2008) for metamorphosis, but eggs had been reported previously only once for the Mole Salamander (McAllister and Trauth 1996). Additional research is needed to evaluate the effect of food limitation followed by a nutrient flush of amphibian prey on metamorphosis of high-density populations of larval ambystomatid salamanders. Acknowledgments We thank M. Karnes and the Ross Foundation for access to the study site. The Arkansas Game and Fish Commission provided collecting permits. Literature Cited Banks, B., and T.J.C. Beebee. 1987. 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