Under the Microscope: Recent filming of a live giant squid for the first time drew attention to spectacular large sea creatures. We have such a giant on our doorstep - the leatherback turtle, Latin name Dermochelys coriacea.
Normally we associate sea turtles with warm tropical seas and sandy beaches. For most types of turtle this is true, but the leatherback, the largest of all, belongs in Irish waters just as much as do the migratory birds that visit our estuaries each year.
The biggest leatherback ever recorded drowned in lobster pot lines at Harlech in north Wales. He was 916kg (about the weight of a small family hatchback), and nearly 2.8 metres (9.2ft) across the flippers. Stuffed, he is now in his own gallery at the National Museum Cardiff. The leatherback head is massive and some people believe that leatherbacks may have contributed to sea serpent myths as they surfaced to breathe.
Although it nests on tropical beaches, the leatherback forages widely in temperate waters, feeding almost entirely on huge quantities of jellyfish. Leatherbacks are regular summer or autumn visitors to coastal waters around Japan, Peru, Newfoundland, Norway, Iceland, the UK and Ireland. They are reported off southern Ireland most years, especially in warm periods when jellyfish are abundant. Records of sightings in European waters date back to the 17th century.
Leatherbacks have an ancient history - identifiable ancestors date back to the Tertiary, 60 million years ago. Instead of the rigid shell of other sea turtles they have a highly streamlined, ridged, thick, semi-rigid, leathery skin. They have a thick layer of blubber beneath the skin and possess an amazing gullet lining made up of hundreds of cones covered in fingernail-like material designed to shred jellyfish on their way to the turtle's stomach.
During the 1980s researchers working on marine mammals such as elephant seals developed satellite tags that could record geographical position, time and depth. Turtle researchers soon transferred this technology to their subjects, fitting such tags to female turtles when they climbed beaches to lay their eggs. Before this work was done, the life of turtles at sea was almost totally unknown.
We now know that leatherbacks are deep divers, capable of reaching 3,900ft (1,200 metres), though they rarely dive deeper than 200-400 metres (660-1,310ft). At depth, leatherbacks inevitably encounter cold water. Cold immobilises all other sea turtles, which become comatose below 18-20 degrees Celsius. Leatherbacks, unlike all other living reptiles, are warm-blooded, maintaining a core temperature of about 25 degrees (compared with 37 degrees in seals). Their warm bloodedness is possible because of their large size and the fact they are insulated by blubber. They have "counter current heat exchangers" at the root of each flipper, preventing cold blood entering the turtle's core.
Leatherbacks are endangered. During the 1970s, aerial surveys of tropical beaches for nesting tracks suggested numbers for females in excess of 100,000, but the last 30 years has seen dramatic declines in most leatherback colonies, some to extinction, partly because of collection of eggs (for their supposed aphrodisiac qualities), but also because of incidental catching of adult turtles in fishing gear, particularly trawls and long-lines. A single female probably produces thousands of eggs in her lifetime, each with a low likelihood of resulting in an adult leatherback.
Loss of any adult is therefore a serious setback to the species. It is easy to criticise egg poachers in the tropics, but many leatherbacks die in the northern waters of Europe, either hit by ships or suffocated in nets. Encouragingly, education programmes have resulted in fishermen around Ireland releasing live entangled animals, but worldwide the most important current problem for leatherbacks is mortality caused by long-lining.
For the past two years Prof John Davenport and Tom Doyle from University College Cork, together with Prof Graeme Hays and Dr Jon Houghton of the University of Swansea have collaborated in an EU-funded study of leatherbacks around Irish and Welsh coasts. Aerial surveys have spotted individual turtles and the aggregations of jellyfish in the Irish Sea to which leatherbacks are drawn. However, at present the source of "our" leatherbacks is obscure. It is likely - but not definite - that they breed in the Caribbean; certainly, their migration routes are unknown. No leatherback had ever been satellite-tagged in European waters before. Tags are expensive (€5,000), but early in the programme two tags were purchased to be attached (under permit from the National Parks and Wildlife Service) should turtles become accidentally entangled in fishing nets anywhere around Ireland.
On August 31sta female leatherback (about 500kg), successfully disentangled from a lobster pot line by a local fisherman, was brought back to Dingle, Co Kerry, and held briefly in Dingle Oceanworld Aquarium instead of being immediately released. A satellite tag with a three-year battery was attached and she was released. Perhaps understandably unappreciative of Irish hospitality, she swam due south for several days at about 40km per day. The tag tells the Cork-Swansea research team where she is, when she has dived and to what depth, and what temperatures she encounters. She started diving and feeding in the Bay of Biscay, and is currently west of Lisbon, occasionally diving to 160 metres (525ft). Over the next three years her tag should tell us where, how far and in what direction she travels, which risky fishing areas she traverses, and possibly where she breeds. This will add to basic knowledge of leatherback biology and help in the species' conservation.
• You can follow the leatherback's progress by visiting www.turtle.ie
• William Reville is associate professor of biochemistry and public awareness of science officer at UCC (http://understandingscience.ucc.ie)