The subfamilyMormyrinae contains all but one of the genera of the African freshwater fish familyMormyridae in the order Osteoglossiformes. They are often called elephantfish due to a long protrusion below their mouths used to detect buried invertebrates that is suggestive of a tusk or trunk. They can also be called tapirfish. Fish in this subfamily have a high brain to body mass ratio due to an expanded cerebellum used in their electroperception. Linked to this they are notable for holding the zoological record at around 60% as the brains that consume the most energy as a percentage of the body's metabolic rate of any animal. Previous to this discovery, it was the “human brain, which had been thought to hold the record in this respect”.p. 605The human brain in comparison uses only 20%. Mormyrinae is the largest subfamily in the Osteoglossiformes order with around 170 species.
The range with which the adult brain in all animals regardless of body size consumes energy as a percentage of the body's energy is roughly 2% to 8%. The only exceptions of animal brains using more than 10% are a few primates and humans. However, research published in 1996 in the Journal of Experimental Biology by Göran Nilsson at Uppsala University found that mormyrinae brains utilize roughly 60% of their body O2 consumption. This is due to the combination of large brain size and them being ectothermic. The body energy expenditure of ectothermic animals is about 1/13 of that of endotherms but the energy expenditure of the brains of both ectothermic and endothermic animals are similar. Other high brain percentage animals exist such as bats, swallows, crows and sparrows but these due to their endothermy also have high body energy metabolism. The unusual high brain energy consumption percentage of mormyrinae fish is thus due to them having the unusual combination of a large brain in a low energy consuming body. The actual energy consumption per unit mass of its brain is not in fact particularly high and indeed lower than that in some other fish such as Salmonidae. In comparison, that of rats is 6.02 mg g−1 h−1 and humans 2.61 mg g−1 h−1.Table 1 The oxygen for this in low oxygen conditions comes from gulping air at the water surface.
Large brains
Unlike mammals, the part of the brain enlarged in mormyrinae fish is the cerebellum not the cerebrum and reflecting this is called a gigantocerebellum. This enlarged cerebellum links to their electroreception. They generate weak electrical fields from specialized electric organ muscles. To detect these fields from those created by other mormyrinae fish, their prey animals, and how their nearby environment distorts them, their skin contains three types of electroreceptors. The electroperception they enable is used in hunting prey, electrolocation, and communication. This electroperception, however, requires complex information processing in special neurocircuitry since it is dependent upon the ability to distinguish between self-generated and other generated electric fields, and their self-created aspects and their environment modification. To enable this specialized information processing, with each self-generated electrical discharge, an efference copy of it is made for comparison with the detected electric field it creates. The cerebellum plays a key role in processing such efference copy dependent perception. The muddy waters where they live has resulted in such electroperception playing a key role in their survival and this has resulted in their gigantocerebellum.
Classification
The classification by osteology-based traits of Mormyridae into the two subfamilies of Mormyrinae and Petrocephalinae has been confirmed using molecular phylogeny methods. The classification below comes from FishBase.
