Preadaptation, evolution, and convergence in cave fishes
Adaptation to the cave environment often involves two contrasting trends in the development of structures. Organs that may have been useful to surface ancestors but are of limited use in the cave, such as eyes and pigment, are gradually lost, a process known as regressive evolution. They are replaced by hypertrophied (“overdeveloped”) structures, such as widely distributed and enlarged lateral line and chemosensory receptors and their neural correlates. The mechanisms and agents of selection leading to regressive evolution – namely the relative importance of neutral or directional selection, pleiotropy, energy economy, population size, time since isolation, and gene flow – remain a matter of active debate (Culver 1982).
Some groups possess preadaptations that may have made the transition to cave life quicker. Surface-dwelling Mexican characins show reduced eye development when raised in the dark, and blinded surface fish are as effective at avoiding obstacles as are cave-adapted fish. At least 10 cave families commonly contain nocturnal species; nocturnality and its attendant emphasis on non-visual sensory modes would be an important preadapation for cave living. Some cave-dwelling characins develop taste buds outside the mouth. This pattern also exists in surface-dwelling ictalurid catfishes; in fact, taste buds are more numerous on the barbels and general body surface than in the mouth of ictalurids, which could make transition to a cave environment easier. An elongate body and other eel-like features occur in nearly one-third of cave forms, such as the synbranchid swamp eels, cusk-eels, clariid catfishes, loaches, trichomycterid catfishes, and arguably the amblyopsid cavefishes themselves. Seven acanthopterygian species (i.e., non-anguilliforms) are eel-like. Anguilliform swimming may be advantageous in the narrow confines of many caves (see Locomotory types). Evolution of eellike bodies has occurred in several dozen non-anguilliform fishes, another case of convergent evolution worth studying in its own right (see Habitat use and choice).
Several authors have noted the similarities in traits between cave fishes and bathypelagic deepsea forms, referring to the similarities as the deepsea syndrome. Similar adaptations in the two habitat types include losses of pigmentation, squamation, and light receptors, expanded lateral line and chemosensory receptors, and attendant modifications in the brain. In the blind catfishes, which live deeper than most other cave fishes (400–500 m), additional convergences occur in terms of reduced body size, gas bladder regression, large lipid deposits, and reduction of body musculature and skeletal ossifi cation. These changes can be viewed as adaptations to overcome problems associated with energy conservation in an environment with limited food availability (Langecker & Longley 1993). These parallels underscore once again the descriptive power of the Principle of Convergence: if selection pressures and processes are strong and analogous, convergence can occur not just among species within a habitat but also between habitats.
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