Everything you wanted to know about how whale sharks feed

I’m really excited about a new paper that’s finally out about how whale sharks feed, from the way their filter pads are built to what they eat and how much.  I’m not an author on the paper but I’ve been a witness to a lot of the work and its terrific to see it come to fruition.  So who’s it by and what’s it about?

Phil Motta is the senior author, with 11 co-authors from Georgia Aquarium, Mote marine lab, Project DOMINO and the University of California.  Eleven seems like a lot of co-authors, but it’s a very comprehensive and very broad ranging look at feeding in the worlds largest fish.

First the what.  Many folks are aware that whale sharks are filter feeders, meaning that they swim the worlds oceans sieving tiny food particles from the water.  That much was fairly obvious from their enormous mouths and 20 filter pads that are visible inside.  What wasn’t known was exactly what they eat and how much, especially relative to how much energy they spend, a balancing act we can call the energy budget.  For the first time, Phil and his colleagues were able to measure the size of the whale sharks mouth, how much time they spend with it open and the speed at which they swim, and from that the amount of water that they filter in an average day.  By combining that with measurements of plankton density in the coastal waters of Mexico where whale sharks gather and nutritional analyses of samples taken there, they worked out how much whale sharks eat in that natural setting.  The answer: between 1.5 and 2.7 kg (3-6lbs) an hour, scaling up to between 15 and 30,000 kilocalories a day (up to 125,000 kilojoules).  Not surprisingly, the amount of plankton in the water was higher where whale sharks were eating than where they were not, mostly due to calanoid copepods and sergestid shrimps (one of which, with the cool genus name of Lucifer, is illustrated below).  That could mean whale sharks really like those items, or just that they really like dense patches of food, and those ones just happened to be shrimps and copepods.  Or it could be both.

Some of the coolest stuff in the paper, though, is about HOW whale sharks feed.  They filter, yes, but not like baleen whales and not like other filter-feeding fishes.  A baleen whale takes a huge mouthful of water and then squeezes it out through their baleen combs, which trap the food items, like pasta gets caught in a colander.  Thats a perpendicular or dead-head filter, and the problem with those is that they have to be backflushed from time to time to blow the particles off the screen (left panel below).  In whale sharks, on the other hand (right panel below), water flows mostly parallel to the filter surface, only deviating slightly to dip across the filter surface and siphon out through the gills.  Food particles, which have more momentum, don’t get trapped on the filter but carry on to the back of the mouth, forming an ever more concentrated ball of food.  This is the same principle behind plankton nets and its very efficient because the filter doesn’t clog up with particles the way a baleen plate (or standard kitchen colander) would, and it rarely needs backflushing.

Its an ingenious system, illustrated nicely in the figure above from Elizabeth Brainerd’s 2001 paper in Nature. 

I could go on all day about whale sharks and their feeding, or you can skip the middle man and go get the PDF of Phil’s paper here.  Its well worth a read; there are some great images and a far more interesting and detailed discussion than the precis I have here.  Check it out.  You can learn more about Georgia Aquarium whale shark research from the tag list on the left, or by going here.

Motta, P., Maslanka, M., Hueter, R., Davis, R., de la Parra, R., Mulvany, S., Habegger, M., Strother, J., Mara, K., & Gardiner, J. (2010). Feeding anatomy, filter-feeding rate, and diet of whale sharks Rhincodon typus during surface ram filter feeding off the Yucatan Peninsula, Mexico Zoology DOI: 10.1016/j.zool.2009.12.001

Brainerd, E. (2001). Caught in the Crossflow Nature, 412 (6845), 387-388 DOI: 10.1038/35086666

Just like cows, fish chew their cod

Baddum-tish!  OK, they don't chew their cud, but I can never resist a good pun (although I was seriously considering "Ruminations on the way fish eat" - better?).  I just love this new paper by Gintof et al. about how fish chew, mostly because its an idea that I never would have ever considered.  Basically, they explored whether fish just bolt their food, like lizards and snakes, or whether they engage in "intra-oral prey processing" (= chewing, sometimes sicnetific jargon cracks me up).  After looking at several model fish species, they conclude that yes, fish chew, and they chew about as many times as mammals do.  Its not like mammal chewing (especially herbivores) in that there is little side-to-side motion, but its rhythmic, and thats the most important thing.  This means that the bolting of food by lizards and snakes represents evolutionary loss of chewing, or that the model fish and all mammals both evolved chewing separately (they call this convergent evolution). 

They looked mostly at "basal" fishes like pikes, salmons and arowanas, that is, fish that show the most in common with the ancestors of all fish - I hate to use the term "primitive".  Its significant because it shows that chewing showed up early on in fish evolution.  One theory they put forth for the early appearance of chewing is that the rhythmic pumping of the jaws was necessary to keep fluid moving through the mouth and gills while eating.  Under that view, breathing water through the mouth and gills preadapted all who came after for processing food in their mouth, as opposed to, say, lobsters, whose teeth are in their stomachs.  I would dearly have loved it if they had included a more derived fish like a perch, pufferfish, or the sheepshead (with creepy human-like teeth, shown hereabouts) to show that chewing persisted in other branches of fish evolution, but you can't have everything.

Its a fun paper, you can read it here:

Gintof C, Konow N, Ross CF, and Sanford CP (2010). Rhythmic chewing with oral jaws in teleost fishes: a comparison with amniotes. The Journal of experimental biology, 213 (Pt 11), 1868-75 PMID: 20472774