The self-fertilizing mangrove rivulus, Kryptolebias marmoratus, is an amphibious fish capable of reversible gill remodelling when moving between aquatic and terrestrial environments. In this thesis I determined how plastic morphological and physiological respiratory traits were integrated during transitions between environments. In two isogenic lineages, I found that behaviour (increased emersion) of individual fish caused gill morphological changes (enlargement of the interlamellar cell mass (ILCM)) that reduced gill surface area. I also found that large ILCMs that formed after 7 d of air exposure increased both gill ventilation and critical oxygen tension (Pcrit) when fish returned to water. These results indicate that large ILCMs reduce aquatic respiratory function, and increased gill ventilation was unable to maintain oxygen uptake at extreme levels of hypoxia. Ultimately, this study highlights the trade-offs in gill structure and function during the transition between air and water, and demonstrates that differences in behaviour can generate morphological variation.

The hermaphroditic fish, Rivulus marmoratus, is the only vertebrate known to reproduce by internal self-fertilization; this process results in populations of homozygous clones. Most natural populations consist entirely of hermaphrodites, but phenotypically distinct, fertile males occur at frequencies up to 24% on some islands off the coast of Belize. The presence of large numbers of males in natural populations prompted this study to determine if males are involved in the mating system. The occurrence of cross-fertilization between males and hermaphrodites was determined by surveying progeny of field-caught hermaphrodites for non-segregation or segregation of DNA fingerprint markers as an indication of the homozygosity or heterozygosity of the parent. DNA fingerprinting revealed no segregation of markers among the offspring in 12 of 12 Florida and Brazil laboratory lines and in 5 of 30 Belize Cay broods. These data indicate that the hermaphrodite parents were homozygous; thus, no detectable outcrossing has occurred in these populations. However, DNA fingerprinting revealed segregation of markers among the offspring in 25 of 30 Belize Cay broods. Twenty-four of these broods were from the island of Twin Cays. An average of 30% of the parental bands were segregating among the offspring; values ranged from 0.09 to 0.50. Offspring were, on average, 8% dissimilar to one another; values ranged from 2.08% to 15.09%. These data suggest that the 25 hermaphrodite parents were heterozygous; thus, males are involved in the mating system in some Belize Cay populations. These data are the first evidence of outcrossing in this species. / Master of Science

The utility of hypervariable DNA polymorphisms as a general population genetics method was studied in three fish species by the use of multilocus DNA fingerprinting. Laboratory lines and field caught specimens from Belize and Florida of the clonal species <i>Rivulus marmoratus</i> were examined to determine the relative contributions of mutation and migration to genetic variation in the species. Specimens of <i>Poecilia latipinna</i>, the sailfin molly, from Florida and Georgia were used to explore the properties of hypervariable markers in the context of an outbred and abundant species that exhibits typical levels of genetic variability at nuclear loci. The results were compared to those of a previous allozyme survey of the same populations. Samples of <i>Morone saxatilis</i> from the Chesapeake Bay system were used to investigate the utility of hypervariable markers in the description of genetic variation of an outbred species depauperate in other measures of genetic variation. The results of this study indicate that variation observed among among <i>R. marmoratus</i> clones characterized by hypervariable loci may be the result of natural selection; based on the analyses of mutation rates and population structure. Results from the work with <i>P.latipinna</i> showed that hypervariable loci could have general utility as a method for studying population structure. This utility was demonstrated in the examination of Chesapeake Bay populations of <i>M. saxatilis</i>. Large degrees of interindividual variation at hypervariable loci permitted the characterization of population structure within Chesapeake Bay populations of this species. / Ph. D.