Whale sharks start to give up their secrets

Whale sharks are the largest fish in the oceans; they can grow to 20m in length and weigh many tons, although 7-9m is closer to the common average these days.  Despite their tremendous size, scientists don't know that much about them.  We know that they eat plankton and that they live in the tropical oceans throughout the world and there have been quite a few papers reporting their presence in different waters, but these represent only the most basic foray into the biology of a species.  More recently, there's been a few more including one that explores genetics (Castro et al., see below) and some that have started to explore behaviour (see Brunnschweiler et al.).  Up to this point, the focus has all been external; that is, only the biology that can be observed from the outside.  That's no surprise really; its a logical place to start and there are some huge logistic challenges to working with whale sharks, as you can probably imagine.

There are 4 whale sharks in the collection at Georgia Aquarium in Atlanta and I have been lucky enough to work with these amazing animals since 2006.  Part of that work has involved veterinary examinations, which has allowed us, for the first time, to look at aspects of the internal biology of whale sharks. The first part of that work is now in print: a paper I co-authored with the aquarium's principal clinical vet, Dr. Tonya Clauss, and a colleague from National Aquarium in Baltimore, Jill Arnold (Jill is an expert in medical techniques, especially blood work), which is in the latest issue of Aquatic Biology.  Our paper is a discovery-based one (i.e. not testing a specific hypothesis) about the nature of the blood of whale sharks, both the cells and the chemistry of the blood serum.  Its open access, so you can get it at the journal web page here

In it, we show that whale sharks have blood that is fundamentally similar to that of some other sharks, specifically the bottom dwelling ones like nurse sharks and wobbegongs, but pretty different from the toothy predatory sharks like great whites.  They have very large red cells, actually white cells too, but this is something they share with the bottom dwellers, so it appears to be a feature of the group rather than a function of the size of the whale shark as such.  Whale sharks are the only pelagic members of that group, the order Orectolobiformes.  Why such large cells, then?  Our study didn't answer that question, but my best guess is that they have relatively low metabolism compared to the carcharhinids, which may need the high relative surface area of smaller red cells to improve the movement of oxygen in and out of cells.  This is the first of several hypotheses that we can only begin to pose because of these first discovery-based efforts.

I can't tell you how excited I am that we can begin to share what we've been learning at the Aquarium.  The chance to work with whale sharks is a real gift for a fish nerd like me, and the opportunity afforded by having access to them in the more controlled environment of an aquarium makes it possible to do safely and effectively research that has been prohibitively difficult with free-ranging whale sharks up to this point.  Of course, the ultimate goal is to extend that work to compliment the field research, and I look forward to telling you more about that in future posts.

Brunnschweiler, J., Baensch, H., Pierce, S., & Sims, D. (2009). Deep-diving behaviour of a whale shark during long-distance movement in the western Indian Ocean. Journal of Fish Biology, 74 (3), 706-714 DOI: 10.1111/j.1095-8649.2008.02155.x 

Castro, A., et al. (2007). Population genetic structure of Earth's largest fish, the whale shark ( )
Molecular Ecology, 16 (24), 5183-5192 DOI: 10.1111/j.1365-294X.2007.03597.x 

Dove, A., Arnold, J., & Clauss, T. (2010). Blood cells and serum chemistry in the world’s largest fish: the whale shark Rhincodon typus Aquatic Biology, 9 (2), 177-183 DOI: 10.3354/ab00252

Comments

[Alan]

Very exciting indeed. As a non-biologist, I'm always astonished to discover how little is known about some very famous marine species. I can imagine that the restrictions on fieldwork are immense.

Looking forward to hearing more.

[Al Dove]

Hi Alan,

Its not that field work is restricted so much as just really really hard to do! Its not like you can swim up and put a stethoscope to their chest, or take a liver biopsy, or put ten in one tank and ten in another for a manipulative experiment. Even the simplest jobs take on daunting status when your subject is the size of a school bus.

But that doesn't mean we can't learn anything; we just have to be a bit creative about it, and ask for a little leniency when it comes time to publish. A whale shark paper will rarely have the statistical power of a double-blinded human medical trial with hundreds of subjects, but there is still good science to be done.

Cheers!