Leatherback turtle (Dermochelys coriacea) COSEWIC assessment and status report: chapter 3

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SPECIES INFORMATION

Common name of species: Atlantic leatherback, leatherback, leatherback turtle, trunkback turtle, leathery turtle, tortue luth

One of only seven species of marine turtle, the leatherback (Dermochelys coriacea) (Fig.1) is the sole member of the family Dermochelyidae, a lineage that diverged from other turtles during the Cretaceous or Jurassic Period, 100-150 million years ago (Zangerl, 1980).  Leatherbacks may attain a straight carapace length of nearly 2 metres.  Body mass is typically less than 500 kg (Zug & Parham, 1996), however, there is a record of a male turtle weighing 916 kg (Eckert & Luginbuhl, 1988).  Unlike all other sea turtles, leatherbacks do not have scales, nor do they possess claws.  Lacking shell scutes, the leatherback’s carapace is composed of four centimetres of tough, slightly flexible, cartilagenous, oil-saturated connective tissue.  The carapace is conspicuously elongate and tapers to a supracaudal point.  A mosaic of thousands of small dermal bones underlies the leathery outer skin of the carapace and seven longitudinal ridges run along it.  The immense paddle-shaped front flippers often equal or exceed half the carapace length.  The dorsum of the turtle is black, or bluish-black, with scattered white and pink blotches, while the ventrum is predominantly white.  One external characteristic unique to each adult leatherback is the size, shape, colour and pattern of the pineal spot or “pink spot” on the top of the head (McDonald & Dutton, 1996).

Figure 1.  Adult Dermochelys coriacea photographed off Nova Scotia, 1998.

Photo: L. Hatcher, Nova Scotia Leatherback Turtle Working Group.

Two subspecies have been described: Dermochelys coriacea coriacea (Linnaeus, 1766), the Atlantic leatherback, and Dermochelys coriacea schlegelii (Garman, 1884), the Pacific leatherback.  However, these supposed subspecies are poorly differentiated, and distinctions based on colouration and differences in forelimb and head length are questionable (Pritchard, 1979).  Therefore, one species is now generally recognized. Genetic analyses, revealing low mtDNA sequence divergence (0.0081) between Pacific and Atlantic populations (Dutton et al., 1996), have corroborated this view.  Low genetic variation between leatherbacks occupying Pacific and Atlantic waters may be a product of recent evolutionary separation between these populations.  Alternatively, the leatherback’s extraordinary migratory ability (e.g., Hughes et al., 1998) and two to three year intervals between nestings (e.g., Hughes, 1996) may enable gene flow between these ocean basins (Binckley et al., 1998).

Leatherbacks feed principally on jellyfish and other soft-bodied pelagic invertebrates (e.g., Lazell, 1980; Lutcavage & Lutz, 1986, Grant et al., 1996).  Bleakney (1965) was the first to document scientifically the occurrence of leatherbacks in Eastern Canada.  His analysis of 26 records of leatherbacks in this region (1889-1964) suggested a seasonal, rather than accidental, intrusion of the species into the cold waters of the northwest Atlantic.  Examination of the digestive tracts of five of these animals yielded remains of Cyanea capillata arctica, a large, temperate species of jellyfish.  Bleakney therefore concluded that leatherbacks venture to waters off Canada’s Atlantic coast to forage on seasonally abundant populations of jellyfish.  After collecting 20 records of free-swimming and entangled leatherbacks reported by fishers in Newfoundland waters (1976 to 1985), Goff and Lien (1988) also suggested that leatherbacks are regular migrants to waters off Atlantic Canada.

Recently, a fishermen-scientist collaborative venture –The Nova Scotia Leatherback Turtle Working Group - was initiated in Atlantic Canada to investigate the distribution of leatherback turtles in the northwest Atlantic (James, 2000).  Over 300 leatherback sightings were reported through this program in 1998 and 1999.  These results demonstrate that waters off the Atlantic provinces are within the normal range of this species.  In an earlier status report on the leatherback in Canada, Cook (1981) stated that “There is no proof to date that leatherbacks which come this far north actually find their way south again to breed”.  A recent satellite telemetry study suggests that mature male and female leatherbacks do indeed successfully migrate to southern latitudes after foraging in Canadian waters (James, unpublished data).