Leatherback turtle (Dermochelys coriacea) COSEWIC assessment and status report: chapter 6

BIOLOGY

Reproduction

Mating in leatherbacks was traditionally thought to occur in tropical waters at the time of nesting.  However, Eckert and Eckert (1988) observed rapid colonization of female leatherbacks by pantropical barnacles (Conchoderma virgatum) after their first nesting of the season at St. Croix (U.S.V.I.).  This suggests that gravid turtles do not arrive from temperate latitudes until just prior to nesting (Eckert & Eckert, 1988).  Therefore, mating may take place prior to or during the migration from temperate to tropical waters.  By contrast, two documented observations of copulating leatherbacks suggest that at least some mating occurs in the vicinity of nesting beaches. Carr and Carr (1986) observed a pair of leatherbacks copulating near Culebra Island, Puerto Rico, and Godfrey and Barreto (1998) observed leatherbacks copulating in shallow water off Matapica Beach, Suriname.

Leatherbacks primarily nest in the tropics (see distribution).  They prefer open access beaches, with a minimal amount of coral, rocks, and other abrasive material.  Unfortunately, many of these open beaches offer little shoreline protection and are therefore vulnerable to beach erosion triggered by seasonal changes in wind and wave direction.  Movement on land is slow and laboured; therefore beaches with deep-water approaches are preferred as they allow the female to ride the waves high onto the beach (Mrosovsky, 1983).

When a nest site is chosen, the cavity is carefully excavated with the rear flippers, and 50-166 eggs may be deposited (Ernst et al., 1994). The average clutch size among 13 nesting populations of leatherbacks studied by Van Buskirk and Crowder (1994) was 81.5. Pacific nesters are generally smaller and lay fewer eggs per clutch in comparison to Atlantic females (Van Buskirk & Crowder, 1994).  A large number of yolkless eggs are typically deposited on top of fertile eggs.  Once oviposition is complete, the eggs are covered, and the female returns to sea. Leatherbacks usually nest at 8-12 day intervals (Ernst et al., 1994), but the internesting period may be considerably longer.  Females lay an average of 6 clutches per season (Van Buskirk & Crowder, 1994).  Incubation time is 60-65 days (Ernst et al., 1994).

Developing leatherback embryos are subject to temperature dependent sex determination (TDSD).  Studies on sex ratios of leatherbacks have shown that constant incubation temperatures below 29.25°C produce 100% male hatchlings, whereas constant temperatures above 29.75°C produce 100% females (Chan & Liew, 1995).  The constant temperature at which both sexes are produced (pivotal temperature) is 29.5°C, not necessarily in a 1:1 ratio, (Davenport, 1997).

Growth and Survivorship

Poor nest site selection can result in high egg mortality.  Females will often nest in areas where their eggs are destroyed by tidal inundation.  Hurricane-induced impacts on beaches, including storm-generated waves and wind, can erode nest sites, resulting in total nest loss (NMFS, 1992), and natural beach erosion can also destroy nests.  Debris mats, consisting of large, dense masses of water hyacinth (E. crassipes), sargassum (Sargassum sp.) and piles of washed-up forest debris, will sometimes overlie nests.  These mats can reduce gas exchange, thereby killing developing embryos, or act as a barrier, preventing hatchlings from crawling to the surface (Leslie et al., 1996).

Predation is highest during incubation and emergence.  Ghost crabs feed on eggs and embryos in the nest and attack emergent hatchlings as they scramble to the sea at night.  Ants destroy nests, as do domestic and feral dogs.  Vultures, skunks, raccoons, lizards, opossums, coatis, genet cats and jaguars have all been recorded preying upon nests and hatchlings.  Those hatchlings that reach the water may be eaten by seabirds, including gulls and frigate birds, or sharks.  Adult leatherbacks have few natural predators, only large sharks and killer whales (Caldwell & Caldwell, 1969).

Leatherbacks, which weigh approximately 30 g and measure about 6 cm in carapace length at hatching, can attain adult weights in excess of 650 kg and carapace lengths of up to 180cm.  This represents a nearly 22,000-fold increase in body weight, and, as such, leatherback growth is unparalleled among marine turtles (Rhodin, 1985).  Knowledge of juvenile growth rates in this species derives solely from a few individuals raised in captivity as it has proven difficult to mark hatchling leatherbacks so that they can still be recognized individually as adults (Zug & Parham, 1996).  Captive juveniles exhibit faster growth than that documented in any other reptile, and it has been suggested that leatherbacks may therefore reach maturity in as few as 2 to 6 years (Rhodin, 1985).  Hatchling leatherbacks fare poorly in captivity, and mortality is high (most perish in less than 100 days), hence, the validity of growth estimates based on studies of captive turtles is questionable.  However, even captive hatchling leatherbacks dying from fungal infections, stress and other captivity factors show rapid growth (Zug & Parham, 1996).

Skeletochronological analyses of the sclerotic ossicles -- the ring of bony elements encircling the pupil within the sclera of the eyeball -- from both stranded and captive juvenile leatherbacks have recently been used to estimate age at maturity (Zug & Parham, 1996).  This work suggests that female leatherbacks may mature in as few as 13-14 years, with a minimum age at maturity of 5-6 years.  This translates into juvenile growth rates ranging from 8.6 cm to 39.4 cm per year (Zug & Parham, 1996).  Although life expectancy is not known, the nesting lifetime of one turtle in Tongaland, South Africa, spanned 18 years (Hughes, 1996).

Physiology

Leatherbacks are capable of maintaining body core temperatures as much as 18°C above ambient water temperature (Frair et al., 1972).  This enables them to venture into cool temperate waters and range further than any other species of marine turtle.  The endothermic capability exhibited by leatherbacks is made possible by a number of adaptations.  These include large size and a thick layer of subcutaneous blubber (which favors heat retention from muscular activity), a high volume to surface area ratio (which minimizes heat loss), different compositions of peripheral and central lipids, and countercurrent vascular heat exchangers in the front and rear flippers (Davenport, 1997).

While cheloniid turtle distribution is normally constrained by the 20°C surface isotherm (Davenport, 1997), leatherbacks are routinely found cold temperate waters.  For example, in March, 1984, a leatherback was observed by fishermen in Trinity Bay, Newfoundland swimming vigorously in water approximately 0°C (Goff & Lien, 1988). 

Large, specialized lachrymal glands, designed for excreting salt, enable leatherbacks to maintain osmotic and ionic balance while consuming a diet of jellyfish (which are isotonic to salt water) (Hudson & Lutz, 1986).

Movementand Migration

At the end of the nesting season, leatherbacks follow drifting schools of jellyfish from tropical to temperate waters.  In the course of such migrations, individual turtles may attain speeds of over 9km/h (Keinath & Musick, 1993).

Studies of the distribution of leatherbacks in the Gulf of Mexico (e.g., Fritts et al., 1983), off the Atlantic coast of the United States (e.g., Lazell, 1980; Shoop & Kenney, 1992) and off the East Coast of Canada (James, 2000) suggest that these turtles may preferentially inhabit continental shelf waters.  As fishing activity is often intense in these coastal areas, incidental catch of leatherbacks in fixed and mobile fishing gear is not uncommon, with some accompanying mortality (e.g., Lazell, 1976; Lutcavage & Musick, 1985; Goff & Lien, 1988).

Offshore, leatherbacks are regularly present along thermal fronts, including the edges of oceanic gyre systems (e.g., Collard, 1990; Lutcavage, 1996).  These areas of strong thermal, water colour, or salinity differences are highly productive, concentrating hydromedusae and other soft-bodied invertebrates on which leatherbacks feed.  Since pelagic fishes are found in these same feeding grounds, there is some incidental take of leatherbacks in different pelagic fisheries (Witzell, 1984).

Although flipper tag retention in leatherbacks is poor (McDonald & Dutton, 1996), tagged turtles have been documented far from nesting beaches.  Pritchard (1976) reported on the recovery locations of 6 leatherbacks tagged in Suriname and French Guiana.  Subsequently, one turtle was recorded off West Africa, one in the Gulf of Venezuela, two in the Gulf of Mexico and two on the Atlantic coast of the United States.  Since 1978, an intensive flipper tagging program in French Guiana has yielded several tag returns from remote locales in the north Atlantic.  For example, eight tagged turtles have been captured along the eastern United States, between Florida and South Carolina (Girondot & Fretey, 1996).  Leatherbacks tagged in French Guiana have also been captured in the northeast Atlantic off the coasts of France, Spain and Morocco less than 12 months after nesting (Girondot & Fretey, 1996).  In 1987, a leatherback tagged 128 days previously in French Guiana was discovered entangled in fishing gear in Placentia Bay, Newfoundland (Goff et al., 1994).  The turtle had travelled a minimum straight-line distance of over 5000 km.

While tag return records are too infrequent to establish that post-nesting movements of leatherbacks are directed, rather than random, other evidence suggests that turtles dispersing from equatorial nesting beaches make determined migrations into temperate waters.  Valuable information on the migration and dispersal of marine turtles has been obtained through studies of the barnacles they host.  For example, Zullo and Bleakney (1966) reported platylepadine barnacles (Stomatolepas elegans) on the skin of leatherbacks recovered off Nova Scotia.  As this genus is generally associated with tropical and subtropical conditions, Stomatolepas found on leatherbacks in temperate waters must first settle on these turtles in warmer waters (Zullo & Bleakney 1966).  This finding supports the notion that marine turtles from tropical breeding populations make seasonal journeys into temperate waters.

More direct studies of leatherback migration have involved satellite tracking (e.g., Eckert et al., 1989; Morreale et al., 1996; Hughes et al., 1998).  One study has revealed long-distance movements from tropical nesting beaches to temperate waters of the north Atlantic (Eckert, 1998).  Two leatherbacks tagged on a nesting beach in Trinidad migrated north to waters between 40 and 50 degrees latitude before swimming south to the coast of Mauritania, Africa (Eckert, 1998).  More recently, five leatherbacks satellite-tagged in Eastern Canadian waters have been tracked on their southward migrations to subtropical and tropical waters (James, unpublished data).  Three of these turtles represent the first male leatherbacks to be tracked via satellite telemetry.

Food Habits

Leatherbacks are rarely observed feeding in the wild (e.g., Eisenberg & Frazier, 1983; Grant & Ferrell, 1993), therefore, diet is typically inferred from the stomach contents of dead turtles.  Stomach contents of stranded adult leatherbacks suggest a relatively specialized diet of soft-bodied pelagic invertebrates, including cnidarians (medusae and siphonophores), and tunicates (salps and pyrosomas) (e.g., Davenport & Balazs, 1991; Lutcavage, 1996).  Small fish, crabs, amphipods and other crustaceans also have been documented in the digestive tracts of these turtles (e.g., Hartog & Van Nierop, 1984; Frazier et al., 1985).  However, as many of these organisms are known jellyfish commensals, they are likely ingested incidentally while leatherbacks feed on medusae (Frazier et al., 1985).

Leatherbacks lack the massive jaw construction, crushing plates and musculature found in the cheloniid sea turtles that eat large, hard-bodied prey, such as crustaceans.  Instead, Dermochelys exhibits several adaptations for its diet of buoyant, soft-bodied prey: the edges of the beak are sharp, and the long esophagus features numerous keratinized, backward-pointing spines, or papillae, which likely assist these turtles in swallowing their slippery prey (Bleakney, 1965).  As hydromedusae consist of about 95% sea water and are energy poor, small leatherbacks may have to consume gelatinous prey equal to their biomass each day to maintain a normal metabolic rate (Lutcavage & Lutz, 1986).  Leatherbacks must therefore regularly find dense concentrations of prey.  This may explain the presence of large numbers of leatherbacks in coastal areas and along oceanic frontal systems, where coelenterate productivity is especially high (e.g., Shoop & Kenney, 1992).

There is evidence that leatherbacks do not feed exclusively at the surface.  Limpus (1984) has described a benthic feeding record  (>50m) for a leatherback in Western Australia, and turtles equipped with time-depth recorders have been recorded diving beyond 1000m (Eckert et al., 1989).  This deep diving behaviour may reflect nocturnal foraging on siphonophore and salp colonies and medusae within the deep scattering layer (Eckert et al., 1989).

Behaviour

Leatherbacks will readily consume a variety of edible and inedible slow-moving and buoyant objects.  Though this behaviour is adaptive in exploiting large concentrations of medusae, these turtles regularly mistakenly ingest plastic bags and other floating marine debris (e.g., Mrosovsky, 1981; Fritts, 1982; Hartog & Van Nierop, 1984; Carr, 1987; Lucas, 1992).  Marine debris accumulates at convergence zones, where prey is also naturally concentrated (Carr, 1987; Plotkin & Amos, 1990).  Ingestion of plastics, styrofoam and other waste can be fatal (Plotkin & Amos, 1990).

The leatherback’s insatiable appetite and foraging curiosity also may lead to entanglement in fishing gear.  Front flipper entanglement in ropes and cables is common, and this may result from turtles approaching buoys and biting at them.  Leatherbacks may also become entangled after being attracted to the jellyfish that foul fishing gear.

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