An investigation of the nutritional condition of low-latitude fish larvae: Growth, transport, and implications for population connectivity

Coral reef fishes are typically characterized by a protracted pelagic larval dispersal period, which creates the potential for connectivity of geographically discrete populations. The degree to which connectivity occurs is affected not only by whether larvae are transported from one reef to another, but also by whether they reach a settlement site in adequate nutritional condition to survive the juvenile period and beyond. It is possible that larvae with different dispersal trajectories (i.e. those that are retained close to shore as compared to those that travel great distances) may differ in their condition levels, and thereby, the extent to which they serve to replenish local populations. Condition levels during larval life, and their relationship to environmental factors, are thus important determinants of regional demography and patterns and scales of population connectivity. In the work presented here, larval fish condition was measured using two different indices: RNA/DNA ratios (R/Ds) and otolith-derived growth measurements. R/Ds are utilized frequently in studies of temperate larval fish ecology, but have only rarely been applied to investigations of low-latitude taxa. The sensitivity of the R/D to variations in prey availability in a tropical/subtropical context was assessed in a laboratory feeding experiment in which larval cobia were subjected to full and reduced (20%) rations. R/Ds were found to respond to reductions of prey availability, and this response was on par with analogous decreases in larval otolith growth. Having established that the R/D can reflect changes in larval food supply in warm water species, the index was used in concert with otolith size and growth to assess the condition of coral reef fish larvae collected in and around the Florida Keys Reef Tract. When nearshore (likely locally retained) and offshore (broadly dispersing) larvae were compared, it was found that, for three of four species examined, nearshore larvae exhibited faster growth and higher R/Ds as compared to their offshore counterparts. An examination of the changes in the distributions of individual condition levels with age (coupled with measurements of larval fish prey availability) indicated that the observed differences in mean condition were likely due to predation-related selective loss of the lowest condition larvae in nearshore waters. To identify possible molecular correlates of larval survival and condition, single nucleotide polymorphisms (SNPs) were genotyped in nearshore and offshore-collected larvae of a common Caribbean reef fish, the bluehead wrasse. Results revealed multiple loci that were likely under selection due to association with condition-related traits, and these loci may therefore be relevant to future investigations into gene-mediated physiological determinants of condition. As a whole this dissertation sheds light on both environmental and genetic components of larval coral reef fish condition, and it thereby contributes to our understanding of the processes that govern population connectivity, as well as our ability to manage and protect coral reef resources in a rapidly changing environment.