The Tortoise Reserve | NC Terrapin Conservation

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Poster text reprinted with permission below:

Mark Outerbridge, Bermuda Biodiversity Project, Bermuda Zoological Society, P.O. Box FL 145, Flatts, FL BX, Bermuda. E-mail: mouterbridge@gov.bm

The diamondback terrapin Malaclemys terrapin is native to the islands of Bermuda, where it is found in the anchialine pond environment. It appears that this Bermudian population is the only wild breeding population outside the USA. Quantitative assessments have been lacking and are limiting conservation efforts for this species, therefore surveys will be undertaken to determine the size and structure of this isolated oceanic population.

Bermuda is situated in a part of the North Atlantic Ocean which regularly receives spin-off eddies from the Gulf Stream (Fig. 4). These eddies have been implicated in the transport of a great diversity of plants and animals from the Caribbean and eastern seaboard of North America to Bermuda, and are most likely responsible for transporting diamondbacks as well (1). Nothing is known about their historical distribution throughout Bermuda, however, diamondbacks are presently only found in three brackish water ponds located on the east end of the Island (Fig. 5). Diamondback terrapins are currently not protected in Bermuda because these pond turtles were regarded as a possible introduced species, until the recent carbon dating of a sub-fossil proved that they are in fact native (2).

Bermuda’s anchialine ponds are isolated, saline, land-locked bodies of water with permanent connections to the ocean. Temperature and salinity are dependent upon the amount of sea water that enters from the ocean and vary from pond to pond, showing predictable seasonal patterns (3). The relative stability and isolation of these ponds has created sanctuaries for the organisms living in them which includes an endemic Bermuda killifish in addition to the diamondback terrapin. Mangrove Lake (Fig. 8) and Trott’s Pond (Fig. 7) are the largest of these ponds (10 Ha and 3 Ha respectively in area), and are believed to have formed during the last 11,000 years (4). They are both simple basins fringed by red mangrove trees Rhizophora mangle and characterized by shallow depths (averaging 2.25 m) with bottoms comprised of deep deposits of highly organic sediment. Annual water temperatures range between 14°C and 31°C, salinities between 26 and 33 ‰. South Pond (Fig. 6) is much smaller (0.2 Ha) and was deliberately dredged to create a golf course water hazard. The salinity in this pond is much lower than in neighboring Mangrove Lake and Trott’s Pond, ranging between 1 and 12 ‰ (5). Mangrove trees are not present at this site, however, there is a small marsh located in the center of the pond. All three ponds are in close proximity to each other and located on the same golf course (Fig. 5).

Little is known about the natural history of Bermuda’s diamondback terrapins; though anecdotal evidence suggests that it is not altogether different from those living in the southern half of the species range in North America. The terrapins in Bermuda’s anchialine ponds are probably feeding upon a variety of mollusks and crustaceans which may include marine snails, crabs, shrimp and mangrove oysters. Both sexes show dimorphism, and body coloration is variable, however, the soft tissue is typically covered in very small dark freckles (Figs 1&2). Mating in Bermuda has been observed in the spring and nesting from April through to at least June. It appears that the sand bunkers on a neighboring golf course are utilized heavily as nesting sites. The average clutch size is 6 eggs, and the average egg size is 34 x 21.5 mm (6). Incubation lasts approximately 70 days. Adult terrapins are active throughout the year. The total population of Bermuda’s diamondbacks is thought to be in the hundreds rather than the thousands.

Habitat availability is probably the single greatest factor limiting Bermuda’s diamondback terrapins from expanding their present range. Anchialine ponds in Bermuda are very small, rare and easily impacted upon both chemically and physically by nearby human activities. All three of the diamondback ponds are adjacent to a golf course, thus the population may be affected to some degree through maintenance and usage activities. Bermuda’s diamondbacks may also be threatened by the encroachment of invasive species, particularly the red-eared slider Trachemys scripta elegans which co-exists with the diamondbacks in South Pond, where local conservationists fear that there is competition for limited nesting sites. Hatchlings and juveniles may be preyed upon by rats (Rattus rattus and R. norvegicus) as well as by a variety of herons which frequent the ponds. Diamondback terrapins are not harvested for food in Bermuda or caught as by-catch in commercial or recreational shellfish pots, and none of the ponds have boating traffic. A small number of terrapins, however, have been caught and kept as pets.

There are no conservation measures currently in place for Bermuda’s diamondback terrapins. However, Mangrove Lake, Trott’s Pond and South Pond are all designated as nature reserves under the 1992 Bermuda Development Plan, which provides some protection for this species. The recent discovery that diamondback terrapins are native to Bermuda was a critical step towards ensuring that appropriate conservation measures ultimately do take place. Genetic material has already been collected for comparative analysis with populations living along the east coast of the US, and initiating a baseline population study will provide data on size, density and structure that will help determine future research and management directions.