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Recovery Strategy for the Leatherback Turtle

2.4 General Biology and Description [1]

2.4.1 Phylogeny

One of only seven species of marine turtle, the leatherback (Dermochelys coriacea) is the sole member of the family Dermochelyidae, which diverged from other turtles 100-150 million years ago (Zangerl, 1980).  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 sub-species 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 little divergence 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 nesting events (e.g., Hughes, 1996) may enable gene flow between these ocean basins (Binckley et al, 1998).

In Canadian waters, leatherbacks are derived from multiple nesting assemblages and may be considered a single population for management purposes.  Canadian recovery efforts focus on two groups based on ocean basin: (1) the Pacific leatherback turtle and (2) the Atlantic leatherback turtle.

2.4.2 Appearance

Leatherback turtles lack a bony shell, and are the only soft-shelled species among all seven marine turtles.  They may attain a carapace (or shell) length of nearly two metres.  The tapered carapace has a four-centimetre-thick covering of tough, oil-saturated connective tissue covering a mosaic of thousands of small dermal bones (Pritchard, 1971).  The body mass of the leatherback typically does not exceed 500kg (Zug & Parham, 1996) and the immense paddle-shaped front flippers often equal or exceed half the carapace length.

Leatherbacks lack the hard mandible structure of hard shelled turtles.  Instead, the upper jaw has two tooth-shaped projections, flanked by deep cusps for cutting soft tissue.  Their oesophagus is also lined with backward pointing spines to aid them in swallowing their jellyfish prey.  The carapace of the turtle is black, or bluish-black, with scattered white and pink blotches, while the plastron is predominantly white.  Each adult leatherback has a uniquely patterned “pink spot” on the top of the head (McDonald & Dutton, 1996).

The only visual way to distinguish male from female adult leatherbacks is by examining the tail length.  The male’s tail typically extends beyond the length of the rear flippers, while the female’s tail is shorter than the flippers (Pritchard 1971).

Figure 1.  Schematic depicting a mature adult leatherback turtle and key morphological features. 

Figure 1.  Schematic depicting a mature adult leatherback turtle and key morphological features. 

2.4.3 Foraging Ecology

Leatherbacks feed primarily on jellyfish (medusae) and other soft-bodied pelagic invertebrates (e.g., Lazell, 1980; Lutcavage & Lutz, 1986, Grant et al., 1996).  Necropsies have identified many small fish, crabs, amphipods and other crustaceans in the digestive tracts of leatherbacks (Hartop & Van Nierop, 1984; Frazier et al., 1985).  These may be jellyfish prey or commensal to jellyfish and are likely ingested incidentally by leatherbacks (Frazier et al., 1985).

The leatherback exhibits several adaptations for its diet of soft-bodied prey including a sharp-edged beak and backward-pointing spines in the throat, which likely assist in swallowing slippery prey (Bleakney, 1965).  Since these soft-bodied prey are energy poor, consisting of about 95% sea water, small leatherbacks may have to consume an amount equal to their weight daily in order to maintain a normal metabolic rate (Lutcavage & Lutz, 1986).  Therefore, leatherbacks must locate dense patches of food, which might explain why these turtles are numerous in coastal areas and along oceanic frontal systems where prey productivity is high (Shoop & Kenney, 1992).

Leatherbacks also exhibit deep diving behaviour at night in tropical waters, which reflects their foraging on medusae (Eckert et al, 1989).  In eastern Canada, the distribution and movements of leatherback are thought to be closely associated with seasonally abundant prey, particularly Cyanea sp., their principal jellyfish prey (Bleakney, 1965; Goff & Lien, 1988; Shoop & Kenney, 1992; James, & Herman 2001).