There’s no way I’m going to have time today to post any of the promised articles – sorry. Once more, all I can do to combat the frustration is post a picture of the day: this one depicts the head of a Green turtle Chelonia mydas and was provided by Dave Hone, thanks Dave. Chelonia is a wide-ranging oceanic cheloniid (or hard-shelled sea turtle). Its common name comes from the colour of its body fat, not from the colour of its scutes or skin. The fact that its generic name is the same as that sometimes used for the entire turtle clade explains why turtle workers mostly prefer to use the name Testudines for the turtle clade these days (Joyce 2004, Joyce et al. 2004). Chelonia is omnivorous: its eats algae, the roots of mangrove plants, crustaceans, sponges and sea jellies. It eats more plant material than any other sea turtle, and the serrated lower tomium (viz, the beaked margin of the lower jaw) is probably an adaptation for cropping sea grasses and other plants. Chelonia is also unique in that it is the only sea turtle that comes out on land to bask. Males have a long, prehensile tail quite different from the short organ of females, and males also differ from females in that the single curved claw on the forelimb is larger (Ernst & Barbour 1989).
There are six living species of hard-shelled sea turtle (most people miss the Flatback turtle Natator depressus, a species first named in 1880 but then mostly ignored for about a century). I’m particularly fond of the Hawksbill Eretmochelys imbricata [shown in adjacent pic]: the only tetrapod, so far as we know, that is a specialised spongivore. That’s a big deal, because, firstly, sponges are toxic: they contain alkaloids*, terpenes and brominated compounds, and some species produce neurotoxins. Indeed, ‘sporadic mass mortalities’ of people who have eaten hawksbill meat have been reported, and these deaths might have resulted from secondary poisoning. Secondly, the sponge skeleton is made up of silica spicules, meaning that hawksbills go around with a gut-load of about 500 g of glass-like shards. Anne Meylan (1988) found siliceous spicules embedded in the epithelia of hawksbill guts, but couldn’t find any morphological adaptations that allowed hawksbills to cope with this. A sponge-eating tetrapod is pretty amazing: elsewhere among vertebrates there are a few sponge-eating fish, but I think that’s it.
* I don’t think alkaloids are toxic to reptiles, but they’re presumably toxic to something that might otherwise eat sponges.
As usual I’ve written too much; would like to say more but don’t have the time. I think, however, that this is the first Tet Zoo article on sea turtles. Woo-hoo.
Refs – –
Ernst, C. H. & Barbour, R. W. 1989. Turtles of the World. Smithsonian Institution Press, Washington, D. C. & London.
Joyce, W. G. 2004. Phylogeny, Nomenclature, and Ecology of Mesozoic Turtles. Unpublished D. Phil. thesis, Yale University, pp. 470.
– ., Parham, J. F. & Gauthier, J. A. 2004. Developing a protocol for the conversion of rank-based taxon names to phylogenetically defined clade names, as exemplified by turtles. Journal of Paleontology 78, 989-1013.
Meylan, A. 1988. Spongivory in hawksbill turtles: a diet of glass. Science 239, 393-395.