December 4, 2010

Whale sharks are one of the three living species of filter-feeding shark, along with the basking shark and the megamouth shark.  Despite their size – whale sharks can reach 40 feet in length – they subsist on some of the smallest organisms in the sea.  These animals must eat enough plankton – tiny shrimp, copepods, marine worms and fish eggs – to support their energy needs.  Whale sharks are commonly found at the surface of the water, swimming slowly with their mouths wide open, taking in large volumes of water and straining out the plankton it carries.

How can such large sharks survive while eating such small prey?
Just how much food must a whale shark eat?
A new study by Motta et al in the journal Zoology provides information on how – and how much – whale sharks eat.  The authors studied a large aggregation of whale sharks off the coast of the Yucatan peninsula.  Careful measurements allowed them to calculate the amount of time whale sharks spend feeding (~7.5 hours per day), the amount of water they take into their mouths (600 cubic meters per hour for a 6 meter shark), and the amount of food filtered from the water (2.8 kilograms per hour for this same shark).  Doing the math — a 6 meter whale shark (still a juvenile) eats 21 kilograms, or 46 pounds, of plankton per day!

How are whale sharks able to filter plankton so efficiently from the water?
To answer this question the researchers examined two whale sharks that died at the Georgia Aquarium in 2007.  The unfortunate loss of these animals nevertheless provides an important resource for studying whale shark anatomy.  Examining the mouths of these animals revealed 20 unique filtering pads, filled with a sort of sieving mesh.  Water entering the mouth flows through these mesh pads, with the plankton being caught in the mesh while the water is filtered back out through the gills.  These pads are quite different from the plankton-straining mechanisms of the basking and megamouth sharks; despite their obvious similarities, these species are believed to be only distantly related to whale sharks.

The article can be found at http://dx.doi.org using the identifier

doi:10.1016/j.zool.2009.12.001.