Diamondback Terrapins and the ‘Lost Years’

By KENNETH W. ABLE | Nov 28, 2018
Photo by: Ryan Morrill A female diamondback terrapin prepares to lay eggs.

The diamondback terrapin is the only turtle on the U.S. East Coast that lives in estuarine waters. The larger juveniles and adults are known to spend the summer in marsh creeks and adjacent bays, but seldom in marsh surface ponds. In the winter, they bury in the mud at the bottom of marsh creeks and open water areas adjacent to marshes.

There are only two periods in the life history of these terrapins when they live on land. During June and July in the Mullica Valley, adult females can be found on the marsh surface, especially during daytime. They are particularly common on high tide in areas of higher elevations, such as in dunes and along roadsides, where they lay their eggs in nests buried in the sand, typically with about 10 eggs per nest.

The females return to the water, but then leave for land later in the summer to deposit more eggs. In general, egg size decreases through the reproductive season. The developing eggs remain in the nests during the summer until they hatch in the fall. However, some hatch but stay in the nests over the winter and then leave the nest in the spring.

Adult nest site selection, movements and road mortality have been extensively studied along Great Bay Boulevard. Tracking of adult females indicated that they crossed the road multiple times, but they also demonstrated evidence of nest site fidelity when they reproduced more than once.

Other tag-recapture studies in the Mullica Valley (watershed) and nearby Barnegat Bay found these females had relatively small home ranges, an average of approximately 1.2 miles. Related studies demonstrated a roadkill mortality rate of 8.8 percent, with the highest mortality on the part of the road with the highest automobile traffic. Based on our studies, another source of mortality, mostly for the smaller males, is predation by bald eagles as indicated by the occurrence of empty shells under the nests in the watershed.  

Once the eggs are laid, they may be exposed to a variety of egg and hatchling predators including raccoons, foxes, gulls and crows. After the hatchlings leave the nest they can also be preyed upon by ghost crabs. However, the hatchlings and small juveniles have been little studied because they have, historically, been difficult to find.

The absence of information on the early life history stage of the diamondback terrapin has been referred to as the “lost years.” Accumulating evidence from several sources in other estuaries supports a near-terrestrial existence of hatchlings and small juveniles at higher elevations in salt marshes.

Recent studies, including our own in the laboratory and in marshes along Great Bay Boulevard, have shown that these small terrapins may not spend any time in water. Rather, they are on the marsh surface in and under wrack that collects on the higher elevations, where they feed on small invertebrates that can be found there.

More specifically, recently hatched terrapins have been found along Great Bay Boulevard where they are most abundant near the bridges over the creeks, apparently because these approaches to the bridges are the highest elevation in the Sheepshead Meadows marshes. This is where the females prefer to lay their eggs or, perhaps, more eggs survive there. Most hatchlings are found during fall (September) and the following spring (April-May).

Subsequent observations in the laboratory indicate a reduced preference for open water and a distinct preference for saltmarsh cordgrass wrack. Further, they have the ability to burrow into marsh mud.

Terrapins’ preferences for the higher elevations of estuarine marsh systems, both for incubation in nests and hatchling habitat, suggest that these life history stages are especially vulnerable to flooding, as will occur with sea level rise, a prediction for the Mullica Valley. Thus, enhancing marsh elevation for nesting and juvenile diamondback terrapin nurseries would help to increase the resilience of these critical habitats for this species.

Kenneth Able is a distinguished professor in the Rutgers University Department of Marine and Coastal Sciences and director of the university’s Marine Field Station in Little Egg Harbor. This essay is part of an upcoming book by the author on the underwater natural history of the Mullica Valley to be published by Rutgers University Press.






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