Subterranean Fishes of the World

2342

2008

150 pages

University of Zurich

Caves, with their divergent environmental conditions, provide some of the most unusual habitats on earth and harbor a diversity of highly adapted endemic organisms. Many aspects of the ecology and evolution of cave organisms, however, are poorly understood; probably because the inaccessibility of their habitats, the often small population sizes and their conservation status, as well as the lack of closely related epigean species that would allow for comparative studies. For my thesis, I explore how divergent abiotic conditions and correlated biotic conditions affect the ecology of a small livebearing fish occurring in cave as well as in surface habitats. Furthermore, I identify the evolutionary responses to selective pressures imposed by the environment, ultimately with the goal to contribute to the understanding of the processes that lead to ecological and phenotypic diversity and speciation. Chapter 1 provides a short introduction to pertinent concepts in cave biology and a synthesis of my major research questions and results. My research was conducted in Cueva del Azufre system in southern Mexico where the study species (Poecilia mexicana, Poeciliidae) has occurs in four habitat types: non-sulfidic surface, sulfidic surface, non-sulfidic cave, and sulfidic cave. Chapters 2 & 3 provide an introduction and characterization the abiotic and biotic environmental factors in the study system. Specifically, the distribution of toxic hydrogen sulfide (H2S) is investigated in chapter 2, and a newly discovered cave population of P. mexicana is described in chapter 3. The Cueva del Azufre system provides an unparalleled ‘natural experiment’ with two strong selective pressures (the presence or absence of light and H2S) occurring in a fully 2x2 factorial design. For chapter 4, I investigated the genetic and phenotypic differentiation of P. mexicana in different habitat types using molecular genetic and morphological analyses. I document independent and partially heritable morphological variation along each environmental gradient. Molecular genetic analyses using microsatellites as well as cytochrome b gene sequences indicate high population differentiation and very low rates of gene flow among populations from different habitat types despite the spatial proximity and the lack of physical barriers. Altogether, the study provides evidence for parapatric adaptive divergence in response to divergent natural selection by abiotic environmental conditions. For chapter 5, I investigated differences in the tropic ecology of P. mexicana in the different habitat types. Resource use in different habitat types was investigated using gut content analysis. A shift in resource use, from algivory/ detrivory to the incorporation of invertebrate food items, was detected upon colonization of the divergent habitats. P. mexicana in cave habitats further exhibited a higher dietary niche width than conspecifics from surface habitats. Condition of P. mexicana was analyzed using storage lipid extractions, and fish from sulfidic and cave habitats exhibited a very poor condition hinting towards resource limitation or high costs of coping with extreme conditions. Finally, the shift in resource use was accompanied by divergence in viscerocranial morphology. Although the divergent morphological traits investigated were phenotypically plastic to some extent, they appear to have a genetic basis. It is suggested that the morphological diversification is an adaptation to the differential use of resources among populations. Caves are often assumed to be predator-free environment for cave fishes. This has been proposed to be a potential benefit of colonizing these otherwise relatively hostile environments. In chapters 6, I tested this hypothesis by investigating the predator-prey interaction of a belostomatid water-bug (predator) and P. mexicana. I determined feeding rates and size-specific prey preferences of the predator, and estimated the population density of Belostoma using a mark-recapture analysis. Belostomatids were found to heavily prey on cave mollies and to exhibit a prey preference for large bodied fish. The mark-recapture analysis revealed a high population density of the heteropterans in the cave. Although the absence of predators is not a general habitat feature of cavernicolous P. mexicana, this study highlights the fundamental differences in predatory regimes between epigean and cave habitats. In chapter 7, I suggest that extreme environments in general, and cave habitats in particular, may function as refuge from parasite infections, since parasites can become locally extinct either directly, through selection by an extreme environmental parameter on free-living parasite stages, or indirectly, through selection on other host species involved in its life cycle. Populations from such sulfidic and cave habitats are significantly less parasitized by the trematode Uvulifer sp. than populations from a non-sulfidic habitat and it is suggested that reduced parasite prevalence may be a benefit of colonizing otherwise inhospitable habitats. Finally, in chapter 8, I provide some conclusion of my thesis and perspectives for future research.

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