Mormyrid Fish Brain

figure 7.11 Longitudinal section through the brain of a mormyrid fish. Note, in particular, the immense size of the cerebellum, which comprises all parts of the brain above the dotted line. The various parts of this hypertrophied structure include the remarkable valvula cerebelli (Va), the central cerebellar lobes (C1 to C4), the crista cerebellaris (Cr), the electrosensory lateral line lobe (ELL), the transitional cerebellar lobe (TL), and the caudal cerebellar lobe (CL). Total length of the brain is about 12 mm. (Reproduced from fig. 7 of Nieuwenhuys and Nicholson (1969; citation is in note 324), with permission of R. Nieuwenhuys. Photograph kindly made available by Professor C. C. Bell, Oregon Health Sciences University.)

figure 7.11 Longitudinal section through the brain of a mormyrid fish. Note, in particular, the immense size of the cerebellum, which comprises all parts of the brain above the dotted line. The various parts of this hypertrophied structure include the remarkable valvula cerebelli (Va), the central cerebellar lobes (C1 to C4), the crista cerebellaris (Cr), the electrosensory lateral line lobe (ELL), the transitional cerebellar lobe (TL), and the caudal cerebellar lobe (CL). Total length of the brain is about 12 mm. (Reproduced from fig. 7 of Nieuwenhuys and Nicholson (1969; citation is in note 324), with permission of R. Nieuwenhuys. Photograph kindly made available by Professor C. C. Bell, Oregon Health Sciences University.)

can be readily seen.'325 In Fig. 7.11, note, in particular, how the cerebellum 'flows' over much of the rest of the brain, in a manner reminiscent of the massive neocortical expansion of the human cerebral lobes. Not only are these lobes enormously enlarged, but in addition to this hypertrophy the total area of the brain is further increased by both a series of deep folds and a series of fine ridges. It is all the more intriguing to note that in relation to its body size the mormyrid brain is comparable to the disproportionately enormous brain within our skulls. Not only that, but more than half the consumption of body oxygen in the mormyrid is accounted for by the brain, a figure three times greater than for humans.326 And this is hardly surprising; as already noted, these fish live in a rich electrical environment, requiring sophisticated neural processing involving recognition of other fish and navigation in near-darkness through a cluttered and potentially lethal environment. And the gymnotids, with their convergent electrical system; what of their brain sizes? Certainly their enlargement is less dramatic than in the mormyrids, but even so they too have - in comparison with most other fish - larger than average brains.327 The enlargement of the gym-notid brain is located in a region known as the torus semicircularis,328 and this appears to be the principal area of electroreception. Even so, the overall similarities of the cerebellum-like structures in both gym-notids and mormyrids are strikingly equivalent in terms of sensory processing and thus reinforce the degree of convergence in these two groups of electrosensory fish.329

If brains can get big independently and provide a neural machine capable of handling a highly complex environment, then perhaps there are other parallels, other convergences that drive some groups towards complexity. Could the story of sensory perception be one clue that, given time, evolution will inevitably lead not only to the emergence of such properties as intelligence, but also to other complexities, such as, say, agriculture and culture, that we tend to regard as the prerogative of the human? We may be unique, but paradoxically those properties that define our uniqueness can still be inherent in the evolutionary process. In other words, if we humans had not evolved then something more-or-less identical would have emerged sooner or later. Such an idea is dismissed out of hand by nearly all evolutionary biologists, but that is exactly what the next two chapters aim to show.

Was this article helpful?

0 0

Post a comment