Intraspecific Variation in Life History Traits among Two Forms of Ambystoma Barbouri Larvae
Venesky, Matthew D., Parris, Matthew J., The American Midland Naturalist
Variation in available resources (e.g., environment or food) can influence life history variation. Populations of the streamside salamander (Ambystoma barbouri) breed in both streams and ponds and larvae exhibit diverse life history patterns related to the environment within which they develop. Previous experiments demonstrated that environmental factors are largely responsible for differences in larval period, mass and size at metamorphosis between the two forms of A. barbouri; however, it is unclear which environmental factors influence the variation. Under controlled laboratory conditions, we tested if hydrological differences between lotic and lentic habitats contribute to life history divergence in A. barbouri. We reared laboratory born larvae from stream- and pond-collected eggs in laboratory environments with hydrological patterns similar to their natal environments. Ambystoma barbouri stream larvae metamorphosed faster and were smaller in mass and size compared to pond larvae. Although we did not test whether genetic or environmental factors influence larval life history traits, our data provide support that hydrological variation alone may contribute to the life history variation in the two forms of larval A. barbouri.
Many amphibians have complex life cycles consisting of an aquatic larval and terrestrial metamorphic stage (Duellman and Trueb, 1986). In the aquatic larval stage, amphibians express intra- and interspecific differences in growth rates, which are likely evolutionary responses to the variety of environmental conditions in which they develop (Wilbur and Collins, 1973). For example, larval period and growth rate can vary with differences in hydroperiod and food availability (Wilbur, 1980). In ephemeral environments, a shorter larval period can reduce the risk of desiccation (Wilbur and Collins, 1973). However, a reduced larval period may carry costs, including an inadequate body mass to survive the first winter and decreased reproductive success (Semlitsch et al., 1988; Morin, 1983). An understanding of the mechanisms driving life history variation is a fundamental goal in evolutionary ecology; however, distinguishing which, if any, environmental factors influence life history traits may be equally important.
Ambystomatid salamanders have a broad geographic range and express variation in their life history traits (Petranka, 1998), specifically inter- and intraspecific variation in larval growdi and developmental rates (Bruce, 1982; Petranka, 1984; Whiteman, 1994). Accordingly, they provide an excellent model for understanding variation in life history traits. Unlike most pond-breeding ambystomatid salamanders, individuals of the streamside salamander (Ambystoma barbouri) breed in bodi first-order streams and ponds (Petranka, 1984). Larvae in these two habitats differ in their behavior, morphology and life history traits, such as larval period and size at metamorphosis (Petranka, 1998). It appears that life history variation between larvae in these two habitats is due largely to environmental factors. In nature, pond A. barbouri grew faster and were larger at metamorphosis compared to stream A. barbouri; however, when larvae from bodi habitats were reared in the laboratory in still water, no differences were observed in their size at metamorphosis or duration of their larval period (Petranka, 1984). In ponds and streams, many biotic and abiotic factors (e.g., dissolved oxygen, food availability) may contribute to these life history differences. It is unclear if water flow alone may contribute to this life history variation.
Under controlled laboratory conditions, we mimicked the hydrological differences between streams and ponds and tested if this specific environmental difference contributes to life history variation in Ambystoma barbouri. By placing stream and pond larvae in laboratory environments with hydrological patterns similar to their natal environments, we tested wheuier differences in hydrological conditions alone influenced developmental rate, size and lengdi of larval period. If either lotie or lentie habitat conditions could elicit different life history responses between pond and stream A. barbouri, we expected to detect differences in the growth and developmental rates between larvae from the two habitat types.
MATERIALS AND METHODS
Study sites. - On 11 Mar. 2007, we collected Ambystoma barbouri eggs from ponds and streams and transported them to die laboratory. We collected four distinct egg masses from two ponds (Campbell and Boone Counties, KY, USA; 39°01'59''N; 84°22'S6''W and 38°59'59''N; 84°48'22''W) and five masses from two streams (Owen Co., KY, USA 38°26'07''N; 84°57'14''W). An egg "mass" was either a monolayer of eggs beneath a stream rock or a discrete cluster of adjoining eggs attached to a single twig or piece of pond vegetation, and eggs were Harrison stage 9-12.
Laboratory techniques. - Egg masses were held separately in 4.2 L plastic containers filled with aged tap water until hatching (24-26 Mar. 2007; median date 25 Mar.). Larvae from both environments were then held separately by habitat type and clutch in 4.2 L plastic containers at a density of 5 larvae/L for approximately 2 wk. During this time, larvae were fed brine shrimp (Artemia espp.) ad libidum every three days. After this period, we removed all larvae that had not begun foraging regularly, randomly selected free-swimming larvae from each clutch, and placed them individually in 4.2 L plastic containers of two types. To test for differences in length of larval period and size at metamorphosis (mass and SVL) of Ambystoma barbouri reared in different environments, we placed stream-collected larvae in containers (n = 16) simulating a stream environment and pond-collected larvae in similarly sized containers (n = 8) simulating a pond environment. Our limited sample sizes precluded an experiment where all larvae were fully crossed between the two container types.
Each stream container had an inflow and outflow tube connected to a 7.6 L plastic bucket filled with water, acting as a water reservoir. Throughout the experiment, water was pumped from the bucket into the plastic container in circulation. The inflow and outflow holes were positioned so that the water level in the plastic container remained at 2 L. Pond containers were maintained with 2 L of aged tap water at an identical depth as the stream containers. Throughout the experiment, we changed water weekly by siphoning approximately 70% of the water from the container and replaced with the same volume of aged tap water. On 7 May 2007 (Day 42), we began feeding larvae a mixture of brine shrimp and blackworms (Lumbriculus variegates). After 6 d, all larvae successfully ate blackworms and were fed only blackworms every 3 d throughout the remainder of the experiment
Upon metamorphosis (complete absorption of the gills) , we recorded the date of metamorphosis, and obtained the mass and SVL of each metamorph. We used multivariate univariate analysis of variance (MANOVA) to test for effects of the independent factor (stream and pond) on the dependent variables (length of larval period, body mass and body size (SVL) at metamorphosis). We used univariate analysis of variance (ANOVA) contrasts on each response variable to determine significant contributors to the multivariate effects. Our data met the assumptions of the parametric statistical analyses used.
RESULTS AND DISCUSSION
We detected an effect of environment type on the life history traits we measured (MANOVA: Wilk's ? = 0.261; F^sub 3,19^ = 17.93; P < 0.0001). On average, stream larvae reached metamorphosis 8 d faster than pond larvae (ANOVA F^sub 1,21^ = 6.60; P = 0.018; Fig. IA). In addition, metamorphs of stream larvae weighed approximately 18% less (ANOVA Fiju = 35.75; P < 0.0001; Fig. IB) and metamorphosed approximately 10% smaller (ANOVA: Fii2i = 15.21; P = 0.0008; Fig. IC) than metamorphs of pond larvae.
In a previous study examining the developmental rates of stream and pond larval Ambystoma barbouri, field observations revealed that pond larvae had a shorter larval period and were larger at metamorphosis compared to stream larvae (Petranka, 1984). When larvae from both habitats were reared in a common laboratory pond environment, they did not express life history differences (Petranka, 1984), suggesting that environmental conditions may be responsible for die different life history traits. Our data provide support that one environmental factor, water velocity, can contribute to the life history variation in the two forms of larval A. barbouri In our experiment, we found intraspecific life history differences of larval A. barbouri in response to hydrological conditions. However, unlike Petranka's (1984) field observations, stream larvae in our experiments had a shorter larval period and metamorphosed smaller than pond larvae. These differences are likely attributed to temperature differences between our experimental conditions and die streams in which Petranka's (1984) observations were made.
Sih (1987) suggested that there are a broad set of life history traits, such as shorter larval period, that larvae in ephemeral habitats possess to maximize survival rates in unstable habitats. Portions of the streams in which larval Ambystoma barbouri develop are highly ephemeral, which may select individuals who possess shorter larval periods. Maurer and Sih (1996) provided additional support for this suite of traits in two closely related species of Ambystoma. They documented high activity levels that were positively correlated with feeding rates and proposed that the mechanism driving faster developmental rates is activity: to survive stream habitats, individuals must develop rapidly, which requires increased foraging effort and thereby activity levels (Maurer and Sih, 1996). Additional support for water velocity as a selective force in rapid developmental rates in stream-dwelling organisms also has been documented in some non-amphibian taxa, such as giant scallops, Placopecten magellanicus (Wildish and Saulnier, 1992) and blackflys, SimuUum lundstromi (Zhang and Malmqvist, 1997). A faster developmental rate has been correlated with obtaining sufficient food in streams (Wildish and Saulnier, 1992; Maurer and Sih, 1996; Zhang and Malmqvist, 1997), suggesting that either food, water velocity or a combination of those forces may contribute to faster developmental rates. In our experiment, food availability and quantity was controlled, suggesting that water velocity alone may influence developmental rates in A. barbouri larvae.
It is important to note that our design did not fully cross each form of Ambystoma barbouri in the two environments, and therefore we were unable to fully discern genetic and environmental components of life history variation. Petranka (1984) suggested that environmental factors are the primary factors driving the differences in life history traits; thus, it was our goal to discern whether hydrological differences between lotie and lentie habitats could elicit different life history responses. The possibility exists that genetic factors influence intraspecific variation in life history of A. barbouri larvae from bodi habitat types. In an attempt to control for genetic factors, the genetic diversity used in our study was comparable to Petranka's (1984) study. By controlling other environmental factors, such as food availability and water temperature, our data suggest that hydrological differences between lentie and lotie environments can contribute to the life history variation.
Acknowledgments. - We thank J. MacGregor, J. Niedzwiecki, J. Rohr and A Storfer for graciously providing locality data. L. Venesky assisted in collecting the animals used in this study. J. Grubaugh donated some of the equipment used in this study. We thank M. Takahashi for providing assistance wiui statistical analyses. M. Rensel, M. Takahashi, T. Wilcoxen and two anonymous reviewers improved its quality and clarity of this manuscript. Salamanders were collected with permission from the Kentucky Department of Fish and Wildlife Resources, and laboratory experiments were conducted under approved Institutional Animal Care and Use Committee protocols at The University of Memphis.
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MATTHEW D. VENESKY1ANd MATTHEWJ. PARRIS, The University of Memphis, Department of Biology, Tennessee 38152. Submitted 31 March 2008; accepted 30 June 2008.
1 Corresponding author: Telephone: (901) 678-1648; e-mail: email@example.com…
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Publication information: Article title: Intraspecific Variation in Life History Traits among Two Forms of Ambystoma Barbouri Larvae. Contributors: Venesky, Matthew D. - Author, Parris, Matthew J. - Author. Journal title: The American Midland Naturalist. Volume: 162. Issue: 1 Publication date: July 2009. Page number: 195+. © 1995 University of Notre Dame, Department of Biological Sciences. Provided by ProQuest LLC. All Rights Reserved.
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