Sex-biased dispersal and natal philopatry in the diamondback terrapin, Malaclemys terrapin

Nesting ecology and population studies indicate that diamondback terrapins (Malaclemys terrapin) exhibit nest site fidelity and high habitat fidelity. However, genetic studies indicate high levels of gene flow. Because dispersal affects the genetics and population dynamics of a species, we used six highly polymorphic microsatellite markers to investigate sex-biased dispersal and natal philopatry of M. terrapin in Barnegat Bay, NJ. We compared results of spatial autocorrelation analysis, assignment methods and Wright's F(ST) estimators to a mark-recapture analysis. Mark-recapture analysis over a 4-year period indicated that most individuals have relatively small home ranges (<2 km), with mature females displaying greater home ranges than males. Goodness of fit analysis of our mark-recapture study indicated that some juvenile males were likely transient individuals moving through our study location. Mean assignment indices and first-generation migrant tests indicated that mature males were more prone to disperse than mature females, but first-generation migrant tests indicated that per capita there are more female than male dispersers. Thus, the relative importance of males and females on gene flow in terrapin populations may change in relation to population sex ratios. Spatial autocorrelation analysis indicated that mature females exhibited natal philopatry to nesting beaches, but first-generation migrant tests indicated that a small number of females failed to nest on natal beaches. Finally, we discuss the important conservation implications of male-biased dispersal and natal philopatry in the diamondback terrapin.     

Regional differentiation among populations of the Diamondback terrapin (Malaclemys terrapin)

The Diamondback terrapin (Malaclemys terrapin) is a brackish-water turtle species whose populations have been fragmented due to anthropogenic activity such as development of coastal habitat and entrapment in commercial blue crab (Callinectes sapidus) fishing gear. Genetic analyses can improve conservation efforts for the long-term protection of the species. We used microsatellite DNA analysis to investigate levels of gene flow among and genetic variability within 21 geographically separate collections of the species distributed from Massachusetts to Texas. Quantified levels of genetic variability (allelic diversity, genotypic frequencies, and heterozygosity) revealed three zones of genetic discontinuity, resulting in four discrete populations: Northeast Atlantic, Coastal Mid-Atlantic, Florida and Texas/Louisiana. The average number of alleles and expected heterozygosity for the four genetic clusters were N_A = 6.54 and H_E = 0.050, respectively. However, the geographic boundaries of the populations did not correspond to accepted terrapin subspecies limits. Our results illuminate not only the need to sample terrapins in additional sites, specifically in the southeast, but also the necessity for allowing uninterrupted gene flow among population groupings to preserve current levels of genetic diversity.     

Effects of roads and crabbing pressures on diamondback terrapin populations in coastal Georgia

Human activities, including the harvesting of natural resources and land development, place substantial pressure on wildlife. The diamondback terrapin (Malaclemys terrapin) is a small, estuarine species of emydid turtle in decline and at risk due to a suite of human activities. Vehicle-induced mortality from increasing coastal traffic and bycatch mortality in crab pots have been recognized as 2 of the primary conservation concerns for terrapins. We used mark-recapture estimates of terrapin density and sex ratio from repeated seining samples of 29 randomly stratified selected tidal creeks to evaluate the current relationships between road and crabbing pressures and the abundance, sex ratio, and size distribution of terrapin populations along the Georgia coast. We obtained 2005 captures of 1,547 individual terrapins among 29 tidal creeks sampled. Population density estimates ranged from 0 to 1,040 terrapins/km among tidal creeks with a median density of 65 terrapins/km. Among all sites, terrapin density declined with increasing crabbing activity within the creek, but was not related to proximity to roads. Sex ratios did not vary significantly with crabbing activity or proximity to roads; however, we found a significantly larger proportion of smaller-sized terrapins in creeks with no crabbing activity. Although roads may have significant localized effects on terrapin populations, we found no measurable association between proximity to roads and current variation in terrapin density along the Georgia coast. However, we did find that terrapin density and the proportion of smaller sized individuals within the population were negatively associated with crabbing activities. Bycatch from commercial and recreational activities threaten many species. We add to a growing body of research showing crabbing activities are affecting diamondback terrapin populations across much of the species' range. States committed to the conservation of terrapins and coastal species should focus on reducing bycatch risk; for example by regulating soak times and locations, requiring the use of bycatch reduction devices, and removing abandoned or lost crab pots from coastal habitats. © 2011 The Wildlife Society.     

Spatiotemporal analysis of gene flow in Chesapeake Bay Diamondback Terrapins (Malaclemys terrapin)

There is widespread concern regarding the impacts of anthropogenic activities on connectivity among populations of plants and animals, and understanding how contemporary and historical processes shape metapopulation dynamics is crucial for setting appropriate conservation targets. We used genetic data to identify population clusters and quantify gene flow over historical and contemporary time frames in the Diamondback Terrapin (Malaclemys terrapin). This species has a long and complicated history with humans, including commercial overharvesting and subsequent translocation events during the early twentieth century. Today, terrapins face threats from habitat loss and mortality in fisheries bycatch. To evaluate population structure and gene flow among Diamondback Terrapin populations in the Chesapeake Bay region, we sampled 617 individuals from 15 localities and screened individuals at 12 polymorphic microsatellite loci. Our goals were to demarcate metapopulation structure, quantify genetic diversity, estimate effective population sizes, and document temporal changes in gene flow. We found that terrapins in the Chesapeake Bay region harbour high levels of genetic diversity and form four populations. Effective population sizes were variable. Among most population comparisons, estimates of historical and contemporary terrapin gene flow were generally low (m ≈ 0.01). However, we detected a substantial increase in contemporary gene flow into Chesapeake Bay from populations outside the bay, as well as between two populations within Chesapeake Bay, possibly as a consequence of translocations during the early twentieth century. Our study shows that inferences across multiple time scales are needed to evaluate population connectivity, especially as recent changes may identify threats to population persistence.     

Evidence of Eelgrass (Zostera marina) Seed Dispersal by Northern Diamondback Terrapin (Malaclemys terrapin terrapin) in Lower Chesapeake Bay

The initial discovery in May 2009 of eelgrass (Zostera marina) seeds in fecal samples of wild-caught northern diamondback terrapins (Malaclemys terrapin terrapin) was the first field evidence of eelgrass seed ingestion in this species. This finding suggested the potential of terrapins as seed dispersers in eelgrass beds, which we sampled for two additional years (2010 and 2011). Seeds were only found in feces of terrapins captured prior to June 8 in all three years, coinciding with eelgrass seed maturation and release. Numbers of seeds in terrapin feces varied annually and decreased greatly in 2011 after an eelgrass die off in late 2010. The condition of seeds in terrapin feces was viable-mature, germinated, damaged, or immature. Of terrapins captured during time of seed release, 97% were males and juvenile females, both of which had head widths <30 mm. The fraction of individuals with ingested seeds was 33% for males, 35% for small females, and only 6% for large (mature) females. Probability of seed ingestion decreased exponentially with increasing terrapin head width; only males and small females (head width <30 mm) were likely to be vectors of seed dispersal. The characteristic that diamondback terrapins have well-defined home ranges allowed us to estimate the number of terrapins potentially dispersing eelgrass seeds annually. In seagrass beds of the Goodwin Islands region (lower York River, Virginia), there were 559 to 799 terrapins, which could disperse between 1,341 and 1,677 eelgrass seeds annually. These would represent a small proportion of total seed production within a single seagrass bed. However, based on probable home range distances, terrapins can easily traverse eelgrass meadow boundaries, thereby dispersing seeds beyond the bed of origin. Given the relatively short dispersion distance of eelgrass seeds, the diamondback terrapin may be a major source of inter-bed seed dispersal and genetic diversity.     

Microsatellite DNA loci from the Diamondback terrapin (Malaclemys terrapin)

We describe polymerase chain recation (PCR) primers and conditions to amplify one dinucleotide and five tetranucleotide microsatellite DNA loci isolated from the Diamondback terrapin (Malaclemys terrapin). The PCR primers were tested on 21 terrapins from Cape Romain, SC, USA. The microsatellite primers developed yielded a high number of alleles (8–14) and high observed heterozygosities (0.57–1.0).     

Survival rates of two nesting aggregations of diamondback terrapins at neighboring sites in New York City

The diamondback terrapin (Malaclemys terrapin) is a charismatic and long-lived species of the United States eastern coast, and the only North American turtle that exclusively inhabits coastal brackish waters. Many terrapin populations are declining, and these declines have been largely linked to human activities; however, few studies have documented human and natural factors affecting terrapin population demography. Jamaica Bay, an urban estuary located in southern New York City, USA, is home to 2 nesting aggregations of terrapins, located at Ruler's Bar Hassock (RBH) and John F. Kennedy Airport (JFK). We analyzed mark-recapture data for RBH from 2003–2018 and JFK from 2011–2018 to estimate annual survivorship of female diamondback terrapins and test the effects of 3 factors (a hurricane, water quality represented by nitrogen concentration, and injuries) on survival. The initial estimated annual survival was high (0.93) at RBH but declined to 0.89, while survival remained constant at JFK (0.92). Models examining the effect of Hurricane Sandy (2012) indicated a significant decline in survival in the 2 years following the hurricane in the RBH nesting aggregation but no change at JFK. Water quality did not influence trends in either aggregation, but the presence of major injuries (caused by boat collisions, predators, and the hurricane) significantly reduced annual survival at RBH. We propose increasing terrapin survivorship through mitigation of boat traffic, reducing populations of northern raccoons (Procyon lotor) to reduce injury to female terrapins and decrease egg and hatchling mortality, and increased law enforcement to reduce on-going poaching and capture as illegal by-catch.     

Relationships of maternal body size and morphology with egg and clutch size in the diamondback terrapin,Malaclemys terrapin (Testudines: Emydidae)

Because resources are finite, female animals face trade‐offs between the size and number of offspring they are able to produce during a single reproductive event. Optimal egg size (OES) theory predicts that any increase in resources allocated to reproduction should increase clutch size with minimal effects on egg size. Variations of OES predict that egg size should be optimized, although not necessarily constant across a population, because optimality is contingent on maternal phenotypes, such as body size and morphology, and recent environmental conditions. We examined the relationships among body size variables (pelvic aperture width, caudal gap height, and plastron length), clutch size, and egg width of diamondback terrapins from separate but proximate populations at Kiawah Island and Edisto Island, South Carolina. We found that terrapins do not meet some of the predictions of OES theory. Both populations exhibited greater variation in egg size among clutches than within, suggesting an absence of optimization except as it may relate to phenotype/habitat matching. We found that egg size appeared to be constrained by more than just pelvic aperture width in Kiawah terrapins but not in the Edisto population. Terrapins at Edisto appeared to exhibit osteokinesis in the caudal region of their shells, which may aid in the oviposition of large eggs.     

Geographic Variation in Egg Size and Lipid Provisioning in the Diamondback Terrapin Malaclemys terrapin

Abstract Understanding phenotypic differentiation among populations of wide-ranging species remains at the core of life-history research, because adaptation to local environmental conditions is expected. For example, when energy resources influence offspring fitness (as in oviparous ectotherms), the egg and hatchling environments are expected to influence selection by acting on the amount of energy allocated to offspring. Here we identify population variation in egg mass, length, width, and volume from diamondback terrapin Malaclemys terrapin eggs collected in Rhode Island (RI), Maryland (MD), and South Carolina (SC). Egg size (mean volume: 7.6, 8.1, and 9.1 cc in RI, MD, and SC, respectively) and clutch size (mean no. eggs: 16.1, 12.2, and 6.0 in RI, MD, and SC, respectively) differed among populations, which indicated that females produce larger clutches with smaller eggs at high latitudes and smaller clutches of larger eggs at lower latitudes. Lipid analyses indicated that eggs from SC contained yolks with a higher proportion of nonpolar lipids than did eggs from MD or RI (mean percentage of nonpolar lipids: 22.3%, 22.5%, and 31.8% in RI, MD, and SC, respectively). Thus, female terrapins in SC are laying larger eggs with increased lipid content to provide more energy for the developing embryo. Interestingly, total triacylglycerol (energetic lipid) was greater in southern populations but occurred in higher proportions in northern populations (total triacylglycerol: 88.0%, 85.4%, and 81.9% in RI, MD, and SC, respectively). This variation in triacylglycerol levels demonstrates the necessity for quantifying each lipid component. These data indicate a difference in reproductive strategy by which females in northern populations invest in higher fecundity with less energetic resources per offspring, whereas females in southern populations invest in larger eggs with considerably greater energy reserves.     

Behavioural osmotic control in the euryhaline diamondback terrapin Malaclemys terrapin: responses to low salinity and rainfall

Diamond terrapins, Malaclemys terrapin Latreille, inhabit salt marshes and estuaries where they may encounter sustained high salinities for weeks or months. Terrapins can discriminate between salinities. When salt-loaded they avoid drinking high salinities (27.2–34.0%), drink small amounts of salinities which are a little more concentrated than the blood (13.6–20%), and drink copious quantities of lower salinities (0–10.2%). After seven days in full sea water (34%) they can rehydrate themselves in < 15 min if given access to fresh water. Terrapins are capable of drinking from the thinnest of freshwater films (1.6 mm), exploit menisci and have specific postural responses to collect small quantities of fresh water from horizontal and vertical surfaces. Specimens of Malaclemys terrapin respond to the vibration of simulated rainfall by rapid emergence followed by drinking from thin films, either on the exposed substratum or from the surface of the water column. Under simulated conditions of heavy rainfall they collect rain directly from above.     

Egg masses on the move: corixid oviposition on terrapin shells

In a very dry year, when aquatic habitats were scarce, we observed exceptionally massive corixid oviposition on terrapin shells in Doñana National Park, southwestern Spain. Most terrapins inhabiting a large pond devoid of aquatic vegetation exhibited a dense cover of corixid eggs, whilst terrapins from vegetated ponds had no egg cover. Corixid oviposition had also been observed on terrapin shells in previous periods of severe drought. Considering that terrapins frequently alternate between aquatic and terrestrial environments, we experimentally assessed the viability of corixid eggs in varying degrees of dry-exposure. A significant proportion of corixid eggs did not hatch when they were occasionally dried under sun exposure, but egg mortality was lower when they were dried in the shade. The massive oviposition on terrapin shells was probably a consequence of the limited availability of adequate oviposition sites, and it might increase the odds of egg survival when terrapins migrate to other ponds.     

Population genetic analysis of white shrimp,Litopenaeus setiferus,using microsatellite genetic markers

The white shrimp (Litopenaeus setiferus) is a commercially and recreationally valuable species, yet little is known of its population structure or genetic diversity. White shrimp are distributed along the Atlantic coast of the United States and from the west coast of Florida to the Bay of Campeche, Mexico. In this study, shrimp were collected from North Carolina, South Carolina (four separate collections were taken from 1995 to 1999), Georgia, the Atlantic and Gulf coasts of Florida, Louisiana, Texas and Mexico. DNA was isolated from these individuals, and genetic variation was assessed at six microsatellite loci. These loci were, for the most part, highly polymorphic with an average expected heterozygosity of 0.68. Deviations from Hardy-Weinberg proportions were observed over all samples, but experimental results suggested the presence of null alleles, which confounded a biological interpretation of this result. Pairwise tests of the similarity of allele frequency distributions and distance measure analyses showed broad-scale genetic homogeneity superimposed over occasional indications of random geographical and temporal differentiation. FST and RST estimates over all loci and samples were 0.002 or less and indicated little population structure. Weak but significant genetic differentiation was evident only between pooled western Atlantic and pooled Gulf of Mexico samples. Within the Gulf of Mexico or within the western Atlantic, the large-scale genetic homogeneity observed may be a consequence of genetic mixing resulting from pelagic larvae and adult migrations, while the random local genetic differentiation may be a result of genetic sampling or experimental sampling error. The weak differentiation between shrimp from the Gulf of Mexico and the western Atlantic can be explained by a relatively recent separation of these two populations and/or small amounts of ongoing gene flow.     

Relationships among cave swallow populations (Petrochelidon fulva) determined by comparisons of microsatellite and cytochrome b data

The cave swallow (Petrochelidon fulva) is a polytypic species with isolated populations in northwestern South America, southwestern North America, Yucatan, Greater Antilles, and Florida. We compared microsatellite genotypes of at least five individuals each from five populations and cytochrome b sequence data of two individuals each from seven populations plus two outgroups. Microsatellite allelic diversity was substantial, and the North American populations were about equally variable. In contrast, the Ecuadorian population had far less genetic variation. Gene flow was apparent among populations, especially between Texas and Florida. Genetic structure was greater than in widespread North American species but less than that of sedentary Neotropical birds. Microsatellite genetic distances indicated a close affinity between Ecuadorian and northern populations, especially Texas and Florida, but this finding was inconsistent with cytochrome b data, which indicated that the Ecuadorian population is the clear outgroup to the northern populations. Its outgroup status is consistent with recent classifications that designate South American populations as their own species (P. rufocollaris). The cytochrome b data further suggested that the northern populations are divided into two clades: Texas/Yucatan and Florida/Greater Antilles. The microsatellite data incorrectly measured the diversity and affinities of Ecuadorian birds apparently because of an ascertainment bias that results from the use of heterologous PCR primers. Despite these problems in measuring phylogenetic relationships, the microsatellite data appeared to work well as a population genetic marker in indicating population structure and gene flow.     

Mitochondrial DNA (mtDNA) haplotypes reveal maternal population genetic affinities of Sea Island Gullah-speaking African Americans

To better understand the population substructure of African Americans living in coastal South Carolina, we used restriction site polymorphisms and an insertion/deletion in mitochondrial DNA (mtDNA) to construct seven-position haplotypes across 1,395 individuals from Sierra Leone, Africa, from U.S. European Americans, and from the New World African-derived populations of Jamaica, Gullah-speaking African Americans of the South Carolina Sea Islands (Gullahs), African Americans living in Charleston, South Carolina, and West Coast African Americans. Analyses showed a high degree of similarity within the New World African-derived populations, where haplotype frequencies and diversities were similar. Phi-statistics indicated that very little genetic differentiation has occurred within New World African-derived populations, but that there has been significant differentiation of these populations from Sierra Leoneans. Genetic distance estimates indicated a close relationship of Gullahs and Jamaicans with Sierra Leoneans, while African Americans living in Charleston and the West Coast were progressively more distantly related to the Sierra Leoneans. We observed low maternal European American admixture in the Jamaican and Gullah samples (m = 0.020 and 0.064, respectively) that increased sharply in a clinal pattern from Charleston African Americans to West Coast African Americans (m = 0.099 and 0.205, respectively). The appreciably reduced maternal European American admixture noted in the Gullah indicates that the Gullah may be uniquely situated to allow genetic epidemiology studies of complex diseases in African Americans with low European American admixture.     

Connectivity in piping plovers: Do breeding populations have distinct winter distributions?

Resightings of uniquely marked birds from 2001 to 2008 were used to determine winter distributions of 4 breeding populations of a species at risk, the piping plover (Charadrius melodus). Although considerable overlap exists, a distinct pattern in winter distributions was evident. Birds originating from eastern Canada wintered farthest north compared to other populations. Most individuals from the United States Great Lakes were found in South Carolina and Georgia in winter, whereas birds from eastern Canada were found primarily in North Carolina. Although the great majority of birds marked in Prairie Canada were observed wintering in Texas, particularly southern Texas, breeding plovers from the United States Great Plains were more widely distributed on the gulf coast from Florida to Texas. Very few large-scale movements of individuals in winter were reported either within or between years. This study highlights the significance of geographic regions for eastern Canada, the United States Great Lakes, the United States Great Plains, and Prairie Canada populations, and demonstrates relatively high winter site fidelity. This information will help focus conservation efforts for specific breeding populations during the winter. © 2011 The Wildlife Society.     

Mitigating by-catch of diamondback terrapins in crab pots

Chronic by-catch of diamondback terrapins (Malaclemys terrapin) in blue crab (Callinectes sapidus) pots is a concern for terrapin conservation along the United States Atlantic and Gulf of Mexico coasts. Despite the availability of by-catch reduction devices (BRDs) for crab pots, adoption of BRDs has not been mandated and by-catch of terrapins continues. We conducted experimental fishing studies in North Carolina's year-round blue crab fishery from 2000 to 2004 to evaluate the ability of various BRDs to reduce terrapin by-catch without a concomitant reduction in the catch of blue crabs. In 4,822 crab pot days fished, we recorded only 21 terrapin captures. Estimated capture rates were 0.003 terrapins/pot per day in hard crab experimental fishing and 0.008 terrapins/pot per day in peeler experimental fishing. All terrapin captures occurred from April to mid-May within 321.4 m of the shoreline. Longer soak times produced more dead terrapins, with 4 live and 4 dead during hard crab experimental fishing and 11 live and 2 dead during peeler experimental fishing. The 4.0-cm BRDs in fall and 4.5-cm and 5.0-cm BRDs in spring reduced the catch of legal-sized male hard crabs by 26.6%, 21.2%, and 5.7%, respectively. Only the 5.0-cm BRDs did not significantly affect the catch of legal-sized hard male crabs. However, BRDs had no measurable effect on catch of target crabs in the peeler crab fishery. Our results identify 3 complementary and economically feasible tools for blue crab fishery managers to exclude terrapins from commercially fished crab pots in North Carolina: 1) gear modifications (e.g., BRDs); 2) distance-to-shore restrictions; and 3) time-of-year regulations. These measures combined could provide a reduction in terrapin by-catch of up to 95% without a significant reduction in target crab catch. © 2011 The Wildlife Society.     

Recreational Boats and Turtles: Behavioral Mismatches Result in High Rates of Injury

Recreational boats are a dominant feature of estuarine waters in the United States. Boat strike injury and mortality may have a detrimental effect on populations of diamondback terrapins (Malaclemys terrapin), a keystone species in estuarine ecosystems. In Barnegat Bay, New Jersey, 11% of terrapins (n = 2,644) have scars consistent with injuries from boats. Conservative estimates of injury rates from boats increased from 2006 to 2011. When exposed to playback recordings of approaching boat engines of varying sizes and speeds in situ, terrapins did not significantly change their behavior in response to sounds of boat engines of different sizes. The lack of behavioral response of terrapins to boat sounds helps explain high rates of injury and mortality of terrapins and may threaten the viability of terrapin populations. Boater education courses that discuss impacts of boats to wildlife, combined with closure of areas of high terrapin densities to boating, are necessary to protect terrapins and other aquatic species from injury and mortality caused by motorized boats.     

Restricted dispersal in a continuously distributed marine species: common bottlenose dolphins Tursiops truncatus in coastal waters of the western North Atlantic

The marine environment provides an opportunity to examine population structure in species with high dispersal capabilities and often no obvious barriers to genetic exchange. In coastal waters of the western North Atlantic, common bottlenose dolphins, Tursiops truncatus, are a highly mobile species with a continuous distribution from New York to Florida. We examine if the highly mobile nature coupled with no obvious geographic barriers to movement in this region result in a large panmictic population. Mitochondrial control region sequences and 18 microsatellite loci indicate dolphins are partitioning the habitat both latitudinally and longitudinally. A minimum of five genetically differentiated populations were identified among 404 samples collected in the range of New Jersey to northern Florida using both genetic marker types, some inhabiting nearshore coastal waters and others utilizing inshore estuarine waters. The genetic results reject the hypothesis of a single stock of coastal bottlenose dolphins put forth after the 1987–1988 epizootic that caused a large‐scale die‐off of dolphins and suggest instead the disease vector was transferred from one population to the next as a result of seasonal migratory movements of some populations. These coastal Atlantic populations also differ significantly from bottlenose dolphin samples collected in coastal waters of the northern Gulf of Mexico, implying a long‐term barrier to movement between the two basins.     

Population genetic structure of the lemon shark (Negaprion brevirostris) in the western Atlantic: DNA microsatellite variation

DNA microsatellite markers were used to characterize the population genetic structure of the lemon shark, Negaprion brevirostris, in the western Atlantic. This study demonstrates for the first time the usefulness of microsatellites to study population genetic structure and mating systems in the Chondricthyes. Lemon sharks (mostly juveniles) were sampled non-destructively from four locations, Gullivan Bay and Marquesas Key in Florida, Bimini, Bahamas, and Atol das Rocas, Brazil. At least 545 individuals were genotyped at each of four dinucleotide loci. The number of alleles per locus ranged from 19 to 43, and expected heterozygosities ranged from 0.69 to 0.90. Relatively little genetic structure was found in the western Atlantic, with small but significant values for estimators of F(ST) and R(ST) among populations, theta (0.016) and rho (0.026), respectively. No sharp discontinuities were found between the Caribbean sites and Brazil, and most alleles were found at all four sites, indicating that gene flow occurs throughout the western Atlantic with no evidence for distinct stocks.     

Seasonal changes in thermal environment and metabolic enzyme activity in the diamondback terrapin (Malaclemys terrapin)

Diamondback terrapins experience broad fluctuations in temperature on both a daily and seasonal basis in their estuarine environment. We measured metabolic enzyme activity in terrapin muscle tissue to assess thermal dependence and the role of temperature in seasonal metabolic downregulation in this species. Activity of lactate dehydrogenase (LDH), pyruvate kinase (PK), citrate synthase (CS), and cytochrome c oxidase (CCO) was assayed at 10, 20, 30, and 40 °C for tissue collected during summer and winter. The Q(10) for enzyme activity varied between 1.31 and 2.11 within the temperature range at which terrapins were active (20-40 °C). The Q(10) for LDH, CS, and CCO varied between 1.39 and 1.76 and between 10 and 20 °C, but PK exhibited heightened thermal sensitivity within this lower temperature range, with a Q(10) of 2.90 for summer-collected tissue and 5.55 for winter-collected tissue. There was no significant effect of season on activity of LDH or PK, but activity of CS and CCO was significantly lower in winter-collected tissue compared with summer-collected tissue. Results indicate that temperature effects contribute to seasonal metabolic downregulation and dormancy in terrapins, but other environmental factors (i.e. oxygen availability), as well as seasonal shifts in blood biochemistry and circulating hormones may also play an important role.