Subterranean Fishes of the World

3070

2015

AIM 2015

To explore the genetic and ecological factors affecting relative condition factor (length corrected weight) in Astyanax cave fish, we studied the three most accessible caves containing mixed populations of eyed and eyeless fish: Rio Subterráneo (RS), El Sótano de Caballo Moro (CM) and Cueva Chica (CH). We characterized each of the populations in terms of condition factor, eye phenotype, proportion of cave genotype in each individual (PCave), and melanocortin 4 receptor (mc4r) genotypes. PCave was estimated from a STRUCTURE analysis of variation at 26 unlinked microsatellite markers reported earlier (Bradic et al. 2012). Mc4r mutations have been previously identified as major contributors to the adaptation of cave morphs to nutrient-poor conditions (Aspiras et al. 2015). Each of the cave populations is ecologically and genetically unique, varying in energy input and proportion of cave genome (including mc4r alleles). With respect to mc4r, the surface allele predominates in CM, the cave allele predominates in CH, and both are at intermediate levels in RS. In all three populations eye size was negatively correlated with PCave. All four variables are highly correlated in the RS population, reflecting recent migration from the surface and largely representing sorting of the phenotype and genotype into the two main categories, surface and cave. Correlations, while still strong, are significantly reduced in the other populations, suggesting sustained hybridization. Surprisingly, average condition factors of fish from CH, a cave with a high food input, and RS, a cave with a low food input, were identical. While this might reflect atypical conditions the year of the collections, we suggest an alternative ecological possibility: food availability may more affect population size than condition factor because of density dependent regulation through intraspecific competition. That is, more food = more (but not fatter) fish. Counterintuitively, fish from CM are in much better condition than those from RS or CH, in spite of the virtual absence of the mc4r cave allele in CM. We suggest that CM, which has input of light, may have a more extensive ecosystem resulting in the Astyanax population being regulated, in part, by predation rather than direct competition for food.

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