The small brain of the biggest shark in the world

“We’re going to have some problems getting this under the microscope...”

There are just times you’d like to be a fly on the wall when certain science projects are being planned. I can’t quite imagine the conversations that led up to this paper. “Let’s look at the brain of the biggest fish in the world.” (I suppose the fish start small and have to grow up big. But still.)

The brains of sharks are interesting, in part because they are much larger than people would think. People tend to think of sharks as primitive (how many shark documentaries have used the phrase, “unchanged for millions of years”?), and primitive means small brains. But compared to body size, shark brains are often as big as birds’ and mammals’.

And when thinking about evolution of brains, extremes are often very informative. The whale shark (Rhincodon typus) is not only extreme in its size (as noted, they’re bigger than any other fish in the world), but extreme in its diet: it’s a filter feeder, living off tiny plankton. This is not the first thing that comes to mind when people hear the words, “giant shark.”

This paper is not only interesting because the species is unusual for neurobiology, it’s interesting because it applies a technique that is used a lot for humans, but quire rarely for other beasties: magnetic resonance imaging (MRI). Now, this is not fMRI, which is constantly in the science headlines: this is purely anatomical data, not imaging the brain of a live shark.

Although the whale shark has a massive brain in absolute terms, it turns out that it isn’t very large relative to its body mass compared to other sharks. In fact, it’s small.

In a situation like this, there are two hypotheses that come to mind. The first is that the feature was inherited from a common ancestor, in which case, you’d predict that the whale shark’s relatives also have small brains. The second is that the feature may be an adaptation to the particular ecology of the species, and the prediction there would be that species with the most similar lifestyle would have small brains.

In this case, the whale shark has a small brain in common with other large filter feeding sharks, like the basking shark (compared using previously published data). It’s easy to think that filter feeders can afford to have small brains, but the authors caution that social behaviours in sharks and allies is another factor that is often strongly correlated with brain size.

When looking at individual regions of the brain, the whale sharks also had something in common with other oceanic, pelagic sharks, but not their relatives: a very large cerebellum. Cerebellum is usually described as being involved in motor coordination. Why would these open ocean sharks need such a large cerebellum? The authors suggest that perhaps the use of that open ocean is more complex than you might expect. The sharks are not just lazing around at the top of the water, but making significant vertical migrations and travel for very long distances.

These possibilities seem a bit foggy, however, based on the traditional notions of cerebellar function. Usually, the cerebellum is involved in coordinating fine movements, not long range navigation. There may be some other undiscovered ecological or behavioural force in play shaping the brains of these massive animals.

Reference

Yopak K, Frank L. 2009. Brain size and brain organization of the whale shark, Rhincodon typus, using magnetic resonance imaging. Brain, Behavior and Evolution 74(2): 121-142. DOI: 10.1159/000235962

Photo by user TANAKA Juuyoh (田中十洋) on Flickr, used under a Creative Commons license.