Thursday, March 31, 2016

Leatherback Sea Turtle, Dermochelys coriacea (Family Dermochelyidae)

By any standard leatherbacks are spectacular animals. They are the largest living turtles; most adults have carapace lengths between 1.5 and 1.8 m and body masses of 209–480 kg. The record specimen, taken near Monterey, California in 1961 was considerably larger than average, weighing 867 kg. Hatchling Leatherbacks are 50-77 mm in length, and weigh 40-50 grams. Thus average-sized turtles increase their length about 34 times and their mass 9700 times– remarkable given their diet of scyphozoan jellyfish. 
Fossil evidence suggests dermochelyid turtles have been around since the Eocene, about 43 million years ago (MYA) but the DNA clock suggests a slightly earlier date for their origin, about 50 MYA. This is roughly the date when the two families of living sea turtles: the leathery sea turtles - Dermochelyidae and the hard-shelled sea turtles – Cheloniidae. While leatherbacks and hard shelled sea turtles share some morphology, ecology, and an ancestor (are marine, have legs modified into flippers, and only leave the water to lay eggs) the two families are quite distinct. The Cheloniidae (Green Turtles, Loggerheads, Ridleys, and Hawksbills) will make migratory ocean crossings, but they are primarily coastal turtles and stay over the continental shelf. However, Leatherbacks are pelagic, swimming and diving in truly deep water is what they do. Dives to depths of 200 m or less are common, and there are documented dives to 1230 m. Leatherbacks are also world travelers, and have been reported to swim straight line distances of 7000 km. One radio tracked individual moved 18,994 km over 8.1 months, swimming an average of almost 80 km per day.
Large body size is one way to avoid predation, but in the oceans, there are still large predators capable of using adult leatherbacks as food. Ovideo et al. (2008) have reported orcas (Oricinus orca) harassing leatherbacks off the northeast of Venezuela and summarize literature that suggests orcas do prey on leatherbacks. 
A suite of morphological and physiological adaptations adapt leatherbacks for their pelagic, globe-trotting lifestyle. They have been found within the Arctic Circle during the summer but some spend the summer in the tropics. Being active and surviving cold water is made possible by a mass of specialized tissue known as brown fat which generates heat combined with a counter current heat exchange system. Veins carrying cold blood from the limbs are parallel and adjacent to arteries carrying warmer blood from the body, and allow for heat exchange. 
Leatherbacks are probably making those deep dives to survey the water column for jellyfish, during the day large schools of jellies submerge in deep water, and rise to the surface to feed on plankton at night. Finding a school of jellyfish, may cause a Leatherback to stay in a location to feed. Their esophagus is lined with flesh, finger-like projections that trap prey so that the turtle can continue to feed without losing the jellyfish already captured and increasing the volume of prey that can be taken during a single feeding episode. 
Leatherbacks make excellent candidates for ecotourism, their large size and interesting natural history give them the charismatic status found in few other reptiles. Grand Riviere is a tiny fishing village on Trinidad’s north coast, and an unlikely stop for tourists. However, it has an exceptionally dense population of nesting female Leatherbacks. From March to August female turtles are depositing eggs every night and often in the later afternoon and early morning. The beach is relatively small, about 970 m long and 28 to 60 m wide and as pointed out, very accessible to people. A few female leatherbacks seem to nest early or late almost every day during the nesting season (May to October) and these were easily photographed. The night tours with guides have rules about not using electronic flashes because they tended to confuse the turtles when it came time to returning to the water. For this same reason residences and businesses located on turtle nesting beaches are often required to keep their lights off on the beach side of the buildings.

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