Whale shark brains: comments on Yopak and Frank (BBE, 74:121-142)

In the fall of 2006 I was at the JB Johnston Club and SFN meetings in Atlanta, Georgia. As usual, I remember feeling overwhelmed by all the information and all the people after a few days, and sought an escape. I ended up at the Georgia Aquarium, which was remarkable for having a gargantuan central tank that housed, among a plethora of smaller fish, several whale sharks. I had only seen those creatures on TV, and I was mesmerized. Although the whale sharks were adolescent, they seemed quite huge and majestic to me. I spent at least two hours watching them, as they circled around the talk again and again, occasionally reverrsing direction or cutting across It was a lot more peaceful than watching stressed-out scientists in a room full of posters.

Several months later I heard that two of the whale sharks I had watched – Norton and Ralph – died shortly after their tank was treated against parasites. Very sad! And it raises questions in my mind about the wisdom of keeping rare and majestic animals in captivity. On the plus side, many people are given a chance to fall in love with them, and hopefully those people will then support efforts to “save nature”; but what about the captive animals themselves? As a comparative neuroscientist, I know they have brains (believe it or not, many people doubt fish have brains). So, how do those creatures feel when we are ogling them or when they bump into the glass? The giant tank at the Georgia Aquarium is probably as good a “cage” as humans can provide for a whale shark, but these animals travel enormous distances in the wild and can dive down to a mile.

Anyway, my perpetual ambivalence about animal welfare aside, I was delighted to read that Kara Yopak and Lawrence Frank managed to study the brains of those two dead whale sharks (http://content.karger.com/ProdukteDB/produkte.asp?Aktion=ShowAbstra...). Cleverly, they put the brains in a 7 Tesla magnet to virtually section them (can you imagine trying to section them with a microtome, even if the tissue quality were good? And who is going to mount those sections, anyway…). The MRI scanner allowed Yopak and Frank to measure not only overall brain size, but also the size of the major brain divisions. I won’t summarize their findings here, but this is what I found most interesting:

Relative brain size is significantly smaller in the planktivorous, pelagic whale sharks than in their relatives. However, this reduction in relative brain size is not as dramatic as in some other filter-feeding sharks (especially if you look at phylogenetically corrected residuals rather than simple encephalization quotients). This suggest that whale sharks might be socially more complex creatures than, say, a basking shark. Indeed, Yopak and Frank mention that large aggregations of whale sharks have been observed. What are those sharks doing? Are they looking for mates? So little is known….

The other thing I found intriguing about the whale shark brains is the large size and complex folding of their cerebellum. Although we are continually learning more about cerebellar function, it remains difficult to say what a large cerebellum is “good for”. I guess it probably depends on species and lifestyle, since cerebellar computations can be useful in many different contexts. Finally, I wonder what controls cerebellar foliation. I have read many papers that try to explain the mechanisms underlying folding of the cerebral cortex, but I’m not aware of any that ask the same mechanistic questions of the cerebellar cortex. Perhaps this merely reflects my ignorance. The beauty of online discussions is that someone may know more…. And won’t be shy…

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Comment by John Irwin Johnson on August 5, 2015 at 7:26pm

Wally Welker was deeply interested in folding of cerebellar cortex, as he was in that of cerebral cortex. His decade or so of pioneering work in fine mapping of somatic sensory projections to cerebellar cortex, with Georgia Shambes and others, was instigated by his hopes of finding some principle underlying the folding patterns of folia, lobules, and lobes. He never found any, but in the search process he definitively proved untenable the ideas embedded in textbooks for a century or so about functional regions of cerebellar cortex. In 2009 Fenna Krienen presented fascinating evidence that segregation of lobules in the posterior lobe of cerebellar cortex was related to the generation of unique gyri and lobules in the frontal and prefrontal cerebral cortex of primates. Further publication of these findings was delayed by the reluctance of her mentor to publish these findings without further experimental verification, and I do not know, but should find out the next time I see Fenna if this ever happened. If so, it will be the first evidence of factors underlying folding patterns of cerebellar cortex other than the long-   and well- known separation of the flocculonodular lobe from the other cerebellar lobes correlated with its function of exclusively mediating functions of the vestibular sensory system .                     

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