The vast ocean teams with life, from a spectacular diversity of fish through to crustaceans, molluscs, cephalopods, mammals, and the tiny plankton that sustain this marvellous diversity. Even reptiles call this marine oasis home, especially the marine turtles.
Unfortunately, along with crashing fish stocks, critically endangered whales, dolphins, sharks and manatees, marine turtles are also teetering on the brink. Whilst some conservation scientists may view this statement as alarmist, it must always be remembered that the North American passenger pigeon once blocked out the sun with their sheer numbers, but today the best we can present of this species is a handful of stuffed museum specimens. Complacency causes extinctions and all marine turtles are listed as threatened in varying degrees on the IUCN Red Lists.
Worldwide, there are seven living species of marine turtle. Here, we will look at these amazing creatures which are currently facing a myriad combination of threats, from poaching of eggs to collisions with boats, over-exploitation, ingestion of plastic, global climate change and entanglement in fishing nets and plastic items.
Turtles spend most of their lives at sea where foraging and mating take place. A few species will come ashore to bask, and some also enter shallow coral reefs, estuaries and mangroves to feed. It is the mature females that regularly return to shore to lay their eggs.
Females use their hind flippers to carve out a flask-shaped depression in the beach sand to lay eggs. Most nesting takes place at night to avoid excessive temperatures and predators.
Hatching usually occurs at night and the hatchlings dig themselves out of the nest and make their way down to the ocean. In places with coastal development, the light coming from buildings and streetlamps may confuse hatching turtles and attract them to the light source rather than the ocean.
Mortality through predation is extremely high in their first few weeks. Ghost crabs predate on hatchlings and those emerging during the day face the gauntlet of baboons, monitor lizards, seagulls, kites, hawks and other predators. Once in the ocean, many fall prey to various predatory fish. Despite their abundant hatchling numbers, very few from each clutch will make it to adulthood.
The loggerhead sea turtle (Caretta caretta) – IUCN Red List: Vulnerable
A widely distributed turtle found in the Gulf of Mexico, parts of the Atlantic near West Africa, the Mediterranean, the east coast of Africa – mostly off KwaZulu-Natal and Mozambique, Madagascar and around Australasia. The loggerhead nests on beaches in KwaZulu-Natal and in Mozambique, as well as several rookeries in the Atlantic, including Angola. Females lay batches of 23-198 eggs at 11 to 15-day intervals during the nesting season. Incubation takes 47-66 days.
Loggerheads attain a body length of around 1m and weigh between 100-150 kg. Hatchlings and juveniles feed on bluebottles and jellyfish and as they grow, will take a wider variety of prey items including crustaceans and molluscs.
Hawksbill sea turtle
(Eretmochelys imbricata) – IUCN Red List: Critically Endangered
Another widespread turtle found in the Gulf of Mexico and across vast regions of the Atlantic, Indian and Pacific Oceans. Absent in the Mediterranean but found in the Red Sea. A relatively small turtle with an average weight between 35-77 kg. Feeds on sponges, corals, jellyfish, anemones, sea urchins and marine molluscs. Marine plants and seaweed are also consumed.
Kemp’s Ridley Sea Turtle (Lepidochelys kempii) – IUCN Red List: Critically Endangered
This species is restricted to the Gulf of Mexico and up along the east coast of the United States of America. It only nests on beaches in Mexico and Texas.
Olive Ridley Sea Turtle (Lepidochelys olivacea) – IUCN Red List: Vulnerable
Widespread across the Pacific, Atlantic and Indian Oceans but keeps close to land. In Africa it is more common around the Atlantic of central and west Africa. A small turtle with most nesting in Africa occurring in central and west Africa. They feed on oysters, crabs, shrimps, snails and other marine molluscs.
Green turtle (Chelonia mydas) – IUCN Red List: Endangered
Widespread across the Pacific, Atlantic and Indian Ocean. Also found in the Red Sea and the eastern Mediterranean around the Greek islands and Turkey. A few individuals occur in South African waters, but they are more common in the tropical waters off Mozambique, Tanzania and Kenya as well as the islands of Madagascar, Comoros, and the Seychelles.
In some regions they form massive seasonal nesting concentrations. Around 7 000 females nest in Oman and 10 000 in Yemen. Europa Island in the Mozambique Island has one of the largest nesting concentrations with between 10-20 thousand nesting annually. One female was recorded nesting at Rocktail Bay in northern KwaZulu-Natal on 16 January 2014 but nesting in South Africa is generally unknown.
Green turtles have a varied diet which includes seaweeds and sea grasses. They will also swim into shallow mangroves and forage leaves, roots and fruits. Hatchlings and juveniles take a greater variety of jellyfish and similar prey before becoming increasingly vegetarian in their diet.
Flatback sea turtle (Natator depressus) – IUCN Red List: Data Deficient
This turtle is restricted to northern parts of Australia. They feed on soft corals, jellyfish and sea cucumbers.
Leatherback sea turtle (Dermochelys coriacea) – IUCN Red List: Vulnerable, but some populations are Critically Endangered
Found in the Atlantic, Indian and Pacific Oceans. It is more common in the Gulf of Mexico and central and west Africa. Females frequenting the Indian Ocean nest on beaches in northern KwaZulu-Natal and Mozambique. By far the largest and most impressive marine turtle due to their all-black appearance, prehistoric look and size.
Watching a large female measuring over 1,5 metres and weighing over 500 kg hauling herself up a beach to lay her eggs is a sight not easily forgotten.
These turtles differ from the other marine turtles in that they lack scutes or shields, instead having a thick leathery covering. The dorsal surface has seven prominent ridges running across the back and sides. One of the largest leatherbacks recorded measured 291 cm with a weight of 916 kg.
Leatherbacks feed mostly on soft-bodied prey like jellyfish, squid and octopus, but will take sea urchins, crabs and similar items. They have a throat covered in backward-facing spines to prevent soft prey like jellyfish from escaping and may also be used as a sieve.
Most sea turtles are found in the tropics and subtropics as cold waters can incapacitate them. However, the leatherbacks have several anatomical and physiological adaptations that allow them to survive in near-freezing waters. The tough leather-like skin is underlain with fat and they can generate a degree of internal heat through metabolic processes.
Images & Text by Warren Schmidt.
