Cave Colonization by Fish: Role of Bat Predation

Abstract

Freshwater fishes found in a tropical pool regularly enter a subterranean source of water. They do so mostly when fishing bats are active in the evening. If fish are experimentally attracted to the pool during this period, fishing bat activity increases. Laboratory studies demonstrate that avoiding open areas in the evening is characteristic of these fishes. Predation by fishing bats can be a selective pressure favoring cave dwelling, an alternative hypothesis on the origin of cave colonization to entrapment and directional evolution. INTRODUCTION Although many examples exist of obligate (troglobitic) and facultative (troglophilic) cavernicoles for which a surface (epigean) ancestor can be presumed, the origin of cave colonization remains puzzling. Some theories of the evolution of cave populations assume accidental entry into caves followed by permanent entrapment of the organisms. Others propose some directional (regressive) evolution on the assumption that cavernicolous animals represent dying phylogenetic lines which seek refuge in caves (see Barr, 1968, for a review of these theories). Neither assumption has experimental confirmation. A ubiquitous freshwater teleost, Astyanax fasciatus mexicanus (Characidae), is frequently cited as an example of cave adaptation from an epigean ancestor. It is found as an eyed and pigmented surface form and as a blind and depigmented cave one. Although Hubbs and Innes (1936) described the cave form as a new genus and species (Anoptichthysjordani), breeding experiments (Peters and Peters, 1973; Sadoglu, 1957), as well as cytogenetic (Kirby et al., 1977) and electrophoretic studies (Avise and Selander, 1972), indicate that these two forms are conspecific. These forms differ not only in their morphology but also in their behavior. In contrast to the behavior of the eyed form, the blind one does not school, lacks periodic activity cycles (i.e., does not have rest periods like the epigean one), is not aggressive and, although it produces an alarm substance, does not react to it (Breder, 1943; Pfeiffer, 1966; Schemmel, 1980). Several species of tropical bats capture fish such as Astyanaxfasciatus from surface waters (Bloedel, 1955; Reeder and Norris, 1954). The echo-locating Noctilio leporinus, for example, is known to prey on A. fasciatus and on the cichlid Cichlasoma urophthalmus (Simmons et al., 1979; Suthers, 1967; Villa-R., 1966). The distributions of N. leporinus and A. fasciatus are very similar, the former ranging from central W Mexico to Argentina, the latter from southwestern USA to Argentina. At the beginning of the wet season (May) of 1982, I studied a two-species assemblage of about 120 fishes, ca. 60% Astyanax fasciatus and 40% the poeciliid, Brachyraphis rhabdophora. This assemblage colonized a shaded pool, fed by subterranean waters, that was constructed in 1976 close to La Hacienda de Palo Verde, Guanacaste Province, Costa Rica. Diurnal observations on these fishes suggested that both species show an affinity for the subterranean habitat, evidenced by their behavior and distribution. Both species carry food into the subterranean cavity prior to eating it (Romero, 1984). Casual observations indicated that fishes move into the subterranean source of water at dusk the time that fishing bats begin foraging (Bloedel, 1955)and that they reappear in the pool after the bats cease flying over it. METHODS AND RESULTS To examine whether the subterranean cavity serves to protect fish during the time