(Last Updated on : 27/08/2014)
Sea Turtle belongs to the families Cheloniidae and Dermochelidae. Sea turtle possess paddle-shaped limbs (flippers) that may span twelve feet in Deemochelys coriacea, the leatherback. They come ashore voluntarily only briefly; to nest and, very rarely, to bask. The nesting female laboriously hauls herself on to a sandy beach where the eggs, usually fifty to two hundred in number, are deposited in a roughly half-metre deep chamber she excavates. Viscous tears that she sheds remove excess salt dried from the turtle's ability to drink sea water - a marine adaptation.
The nest site is levelled or camouflaged by the turtle which then returns to the sea where she may mate again. The female takes no further interest in her eggs or young, which hatch roughly two months later. Up to eight clutches - representing a total of about seven hundred golf-ball-sized eggs - may be laid at two weeks interval between clutches during a nesting year, but most species nest only every two or three years. Incubation temperatures that are too low (24 to 26Ø C) or too high (30 to 32ØC) will, respectively, slow down or speed up hatching, and may result in all-male hatchlings (in the former case) or all-female (in the later). Intermediate temperatures produce both males and females.
The hatching takes two or more days to dig their way out to the sand surface; they emerge in a group usually at night, thus evading avian predators and lethal surface-sand temperatures. Braving ghost crabs and other enemies, the hatchlings rush to the sea, which they usually locate unerringly, even if invisible, by scampering towards the brightest horizon - a tendency which may also draw hatchlings landward to their doom at beaches such as Marina Beach
where disorienting lights exist. Predatory fish take a heavy toll at sea. The list of animals that prey on sea turtle eggs and their young ones are dogs, jackals, monitor lizards, wild and domestic pigs, leopards, hyena, estuarine crocodiles, ghost crabs, hermit crabs, coconut crab, ants, rats, seagulls, crows, sharks and many more. Less than .1 percent of the hatchlings may survive to adulthood in nature. Yet it is mainly man's activities that have decimated many sea turtle population and necessitated urgent international action to reverse the trend towards extinction. The depletory activities include unrestricted egg collection; the killing of adults for commerce in meat or skin ('tortoise shell', the beautiful horny shell material, valuable in the curio industry, is obtained from the Hawksbill sea turtle, Eretmochelys imbricate); commerce in calipee, the cartilaginous tissue which is the main ingredient of widely-relished turtle which is the main ingredient of widely-relished turtle soup; the accidental drowning of sea-turtles in trawl nets; and land development that destroys nesting habitats.
Some species, including Chelonia mydas the Green turtle (named after its greenish fat), navigate with precision across hundreds of miles of open ocean between specific feeding grounds and breeding areas. Sea turtles are believed to nest invariably on, or near, the beach where they hatched. Renesting often occurs at sites lying within yards of nests the turtle had made six or more years earlier. Of the seven surviving species of sea turtles, five representing each of five existing genera, are known from the Indian Ocean
. All except the aggressive Loggerhead turtle Caretta caretta, also nest along the Indian shores, usually on sparsely-inhabited coasts and islands, where predator pressure may be more sustainable. The Olive Ridley sea turtle (Lepidochelys olivacea) nests on most of the mainland shores of India, but exceptionally vast numbers, 'arribadas' (Spanish for 'the Coming') nest each February in the Bhitarkanika Wildlife Sanctuary
in the Indian state of Orissa
. The spectacular yearly arribada is one of the largest in the world, with over 200,000 nesters utilizing a ten kilometres stretch of beach in a period of about two weeks during which the nesting density is such that turtles accidentally dig up each other's eggs while laying their own.
The nesting of the Green Turtle (Chelonia mydas) in the environs of the modern-day Karachi city is another unique spectacle. Despite the throngs of people who enjoy the beaches at weekends, and have constructed concrete beach huts on the shore line, many hundreds still come ashore to lay their eggs between the months of July and November. The fact that competition for nest sites results in one female sometimes digging up a previously completed egg clutch probably indicates that there has not been much diminution in the local breeding population. The Sind Government in collaboration with the WWF has set up a protected turtle-hatching zone and the Sind Wildlife Management Board posts warders along the beaches to protect this endangered species.
Females of at least some sea turtle species have the ability to store viable sperm within their bodies for years. Copulation occurs in the sea near the nesting beach; to discourage males, which are promiscuous; the female may assume a 'refusal position': with body held vertical in the water with the limbs outstretched she faces the male.
Sea turtles being large animals (the leatherback grows to over 2.5 metres and seven hundred kilograms), and capable of forming large populations if given the chance, are valuable as sources of tortoiseshell, oil, skin, eggs and meat (thousands are consumed annually in the state of West Bengal
and Tamil Nadu
). The flesh of the Hawksbill turtle may be seasonally poisonous if consumed, a result of its diet: deaths occur intermittently among the coastal Tamilians who eat it. Turtles have survived and have changed little over the millennia during which many other reptile species have become extinct. It would therefore be a loss, aesthetically and economically speaking, were they to disappear through the agency of man who, since the sixteenth century, has often decimated and, in some instances, and wiped out turtle populations the world over.