You would think that having a dedicated set of neurons that triggered super-fast escape responses to get away from fast predator attacks and other sudden events in your area would be something that you’d want to keep around. This is usually so, but it turns out, not always. This is a problem I’ve been struggling with for some time now, and I’m thrilled to bits to find another example.
Fish have a group of neurons that trigger escape responses called C-starts, so called because the fish bends in a C shape away from the source of the stimulus. The largest of these neurons – large enough to be called giant neurons – are Mauthner cells, named after their discoverer. Bigger neurons send faster signals, so if Mauthner neurons are inactivated experimentally, fish can do C-starts, but they’re slower at it.
Anna Greenwood and colleagues have now found an interesting case of a natural experiment: two fairly similar fishes that differ dramatically in their responses to startling stimuli. They’re two different pufferfish: the green puffer (Tetraodon nigroviridis; below left) and the long-spine porcupinefish (Diodon holocanthus; below right).
Give these two animals the same sudden sound, and both will respond... but the green puffer is much faster, performing a classic C-start (start of movement marked with asterisk in first row of silhouettes). The porcupinefish takes twice as long to react (second row of silhouettes), and doesn’t bend nearly as much.
The neural anatomy correlates with these differences in behaviour. Large Mauthner cells in green puffers, no large Mauthner cells in porcupinefish. The position of the Mauthner cells is consistent enough that Greenwood and colleagues were able to provide argue that the Mauthner cells are missing in porcupinefish, not just tiny.
Greenwood and company suggest two scenarios where the Mauthner cells might be lost. There might be relaxed selection on the trait, and it’s lost sort of by happenstance, a little like how cave dwelling organisms tend to lose pigments or eyes.
Alternately, there might be some active disadvantage to having the Mauthner cells. For the porcupinefish, that disadvantage might be that Mauthner cells and C-starts interfere with a dramatically different anti-predator behaviour: inflation. Greenwood and company don’t provide hard numbers, but report that porcupinefish are much more likely than green puffers to do the classic behaviour that gives puffers their names.
The result of all of this detailed behavioural analysis and anatomical work is... to send researchers back to the field. To understand what drove their neurobehavioural changes, we’ve got to get a better handle of the differing ecology of these species.
Greenwood, A., Peichel, C., & Zottoli, S. (2010). Distinct startle responses are associated with neuroanatomical differences in pufferfishes Journal of Experimental Biology, 213 (4), 613-620 DOI: 10.1242/jeb.037085