Hard-shelled sea turtles and a diet of glass

i-d002a4bbbf9e18fd188849cc5d8f238f-Green turtle head 10-10-2007.jpg

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.

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Shouldn't that be Natator depressus?

[from Darren: yes, thanks. Now corrected]

Also, there are so many different alkaloids out there that some of them have to be toxic to "reptiles".

By David Marjanović (not verified) on 10 Oct 2007 #permalink

Do you know the book "Fire in the Turtle House: The Green Sea Turtle and the Fate of the Ocean" by Osha Gray? It is a really highly interesting, but also shoking book, because it deals also a lot with this tumor-causing virus. When I´ve seen for the first time those male green turtles with their long tails, I had to think of plesiosaurs. I fact I asked me how flexible their tails were, and if the males perhaps used them to hold the females at mating. I asked Adam Stuart Smith if he knows something about it, but it seems that nobody ever really looked at the tail-flexibility and supposed mating behavior of plesiosaurs. It is interesting, that the tail of the green sea turtles are highly flexible, but when I look at the reconstructions of plesio- and pliosaurs, I have always the impressio that they should have had a comparably stiff tail.

Didn't Swain implicate angiosperm alkaloids to explain K/T archosaur extinctions. Obviously that's probably not a consensus opinion these days, but I share David's skepticism that all 'reptiles' are immune to all alkaloids. Any reason to think spongivorous fish or inverts would be more susceptible?

With a lazy google search I did track down one record of an alkaloid being used as an emetic for a Saltwater Croc:


Although, "the expulsion of vomitus was not particularly forceful and was accompanied by a lateral shaking of the head." You gotta love experimental biology...

Somewhere in a still-packed box I have a copy of Tony Swain's paper. As I recall, he invoked the relative immunity of tortoises to alkaloid poisoning to explain their survival of the angiosperm radiation, as opposed to herbivorous dinosaurs.

By Sven DiMilo (not verified) on 12 Oct 2007 #permalink