The Neotropical killifish Rivulus marmoratus is widely distributed, but locally rare, throughout much of coastal south and central Florida. Habitat alteration has affected the species throughout the state, especially on the east coast (Indian River Lagoon) where the destruction
of mangroves and impounding of high marsh for mosquito control has altered and fragmented suitable habitat. Within tropical salt-marsh and mangrove forests, R. marmoratus seems best adapted to certain micro-habitats, specifically those precluding the survival/establishment of competing
fishes. On the east coast of Florida, this microhabitat preference is the land crab (Cardisoma guanhumi) burrow. In south Florida and the west coast, R. marmoratus is most often captured in stagnant pools and old mosquito ditches in mangrove forests. Populations of R. marmoratus
in Florida consist of arrays of homozygous clones. Can habitat fragmentation, with its possible effects on clonal diversity, affect the continued success of this species? Are certain clones better-adapted to specific environmental conditions? Isolated populations consisting of only a single
clone could persist indefinitely. Association with habitat type may answer some of these questions. Some degree of plasticity is apparent, as northern clones “transplanted” from crab burrows survive well and achieve high population levels in isolated pools more typical of south
Florida habitats. However, the appearance of male fish in these pools, a phenomenon unknown in natural populations from burrows, suggests aberrant population structure. While general questions remain about the adaptive significance of clonal diversity, the mere presence of this novel fish
in salt marsh/mangrove habitats may indicate that other aspects of biodiversity are in good “order.”
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