ABSTRACT
The ecological conditions in the subterranean environment are characterized by two main factors: permanent darkness, thus absence of green plants and of photoperiods, and a more or less constant temperature. Subterranean habitats in different parts of the world have been colonized successfully by teleostean fi shes from 20 families (Proudlove, this volume). In the majority of these families, epigean (surface) members are known to be active at dusk and/or night, so it is not surprising that those species with a preference for activity in darkness colonize subterranean habitats such as caves. But among the ancestors of troglobitic fi sh we also note members of the families Characidae (tetras), Cobitidae (loaches), Balitoridae (loaches), Cyprinidae (carp-fi sh) and Poeciliidae (toothcarps), which include many species with clear diurnal activity. Mainly for these species the question arises what enables certain families to survive and to reproduce in caves. One possibility would be that members of these families show behavioral preadaptations to life in darkness. Preadaptations (or exaptations) may be defi ned as character states conferring performance advantage in a given selective regime, but which have been selected in another, independent previous regime (Trajano 2005). In addition, or associated with the predominantly nocturnal activity, preadaptations in epigean fi shes include enhanced mechano
and/or chemosensorial systems, as in the amblyopsids and siluriforms respectively, and an opportunistic, generalist feeding. Nevertheless, in order to distinguish preadaptations, which are plesiomorphic states for troglobites, from autopormorphies resulting from the differentiation in the subterranean habitats, it is necessary to establish the direction of transformation series into cladograms, by applying the comparative method. For behavior, both the outgroup comparison and the ontogenetic methods may be used, but they require phylogenetic trees and ontogenetic series respectively. Unfortunately, there are few ontogenetic series available for cavefi sh, because relatively few species reproduce in laboratory and collection of juveniles directly in the natural habitat is rare, and even fewer behavioral data for the younger stages. As well, phylogenies including troglobitic fi sh species are scanty. The evolutionary interpretation of character states observed in subterranean organisms is particularly diffi cult for the so-called phylogenetic relicts, i.e., species without known epigean close relatives, as is the case with the Brazilian phreatic characiform, Stygichthys typhlops. At fi rst considered a characid, recent taxonomic studies pointed to a more basal position within the characiforms, but its exact familial position was not established yet. As well, the phylogenetic positions of the four recognized Brazilian species of troglobitic Rhamdiopsis (see Trajano and Bichuette, this volume) within the genus and that of the phreatic African catfi sh, Uegitglanis zamaranoi, within the Clariidae, were not established. Consequently, hypotheses on direction of evolutionary change for character states observed in this highly modifi ed species are questionable. At the moment, the most parsimonious solution is to compare the studied troglobitic species with the geographically closest epigean congeneric species, in the assumption that this would be a less modifi ed descendant of a common ancestor with the differentiated cave species (the hypothesis of extinction of the epigean sister-species followed by dispersion of less related species to a closer geographic area requires more evolutionary steps). For the outgroup comparison method, a third species is required. In the absence of phylogenies, available data on any other epigean congeners has been used (e.g. Trajano and Bockmann 1999). In this chapter, we compare behavior patterns of troglobitic fi shes with their closest epigean relatives, aiming to detect apomorphic character states resulting from specialization to the subterranean way of life, and discuss common patterns among troglobitic species, in search of homoplastic traits indicating convergent adaptation to this peculiar life due to similar selective regimes. We examine whether differences observed in relation to epigean relatives are phenotypically plastic responses to cave life, or whether the observed behavioral changes are genetically fi xed.
