Comparative study of the brain and olfactory organ of the troglobitic catfish, Pimelodella kronei (Ribeiro 1907), and its putative ancestor, P. transitoria (Ribeiro 1912) (Siluriformes Pimelodidae)

Abstract

The relative size of the encephalon and its parts were studied in samples of the eyed catfish, Pimelodella transitoria (Ribeiro 1912), and of two populations of the troglobitic P. kronei (Ribeiro 1907), from a karstic area in Southeastern Brazil. The size of the olfactory organ and number of olfactory lamellae were also compared. General structure of the brain and olfactory organ were similar to that of other catfish that have been studied and probably correspond to a “Siluriform pattern”.

In general, brain measurements were larger in the putative ancestor, P. transitoria, than in the two troglobitic populations, from the Areias and Bombas caves. The anterior region of the telencephalon of P. kronei from Areias cave was, however, larger than in both P. transitoria and P. kronei from Bombas. The gross morphology of the olfactory organ did not differ in the three populations, except for the number of lamellae that was higher in P. transitoria.

Behavioural and sensory preadaptation of P. transitoria to cave life could account for the relatively low degree of mesencephalon reduction and the absence of conspicuous compensatory development of mete- and myelencephalon in the blind catfish.

The enlargement of the telencephalon in P. kronei from Areias cave may be due to an improvement of olfactory and integrative centres involved in social communication, related to food scarcity and the low density of conspecifics in the cave habitat. Other hypotheses are the development of topographical memory in the active, highly exploratory blind catfishes, and of non-olfactory senses also represented in the telencephalon: gustation, lateral-line mechanoreception, and audition.

Key Words:

• cave fish
• brain morphology
• olfactory organ
• fish behaviour
• Pimelodidae
• Brazilian caves