Ontogeny of Otolith Formation in Two Demersal Estuarine Reef Fishes

Planktonic reef fish larvae locate and orient to reefs during settlement. Consequently, metamorphosis occurs in appropriate juvenile and/or adult habitats. Larval fish use otoliths for hearing (sagittae and asterisci) as well as equilibrium (lapilli) required for directional swimming. Striped blenny (Chasmodes bosquianus) and naked goby (Gobiosoma bosc) larvae, settled individuals, and juveniles were used to describe otolith ontogeny from hatching through settlement, the transition from pelagic to benthic habitats, and metamorphosis. Larvae hatched from nests collected in North Inlet estuary, SC, were cultured from May through July in 2012 and 2013 at ambient temperatures. Sagittae and lapilli were present at hatching in both species. Asterisci were only observed in settlement (gobies and blennies) or metamorphosis (blennies) stage fishes, regardless of age (days post-hatch). Otoliths within a pair were symmetrical. Fish total length increased faster than sagittae otolith length in settlement stage blennies and postflexion gobies. The allometric model explained ∼90% of the variability in sagittae otolith length with total length for both species. Settlement occurred 15–20 days post-hatch in striped blennies and 19–27 days post-hatch in naked gobies. Asterisci were found in 100% of settlement stage striped blennies and 67% of naked gobies. We hypothesize that the presence of asterisci in settlement stage demersal oyster reef fishes facilitates identification of and orientation to suitable settlement habitats thereby enhancing recruitment success.

Distribution and population structure in the naked goby Gobiosoma bosc (Perciformes: Gobiidae) along a salinity gradient in two western Atlantic estuaries

Many species of fish produce larvae that undergo a prolonged dispersal phase. However, evidence from a number of recent studies on demersal fishes suggests that the dispersal of propagules may not be strongly correlated with gene flow. Instead, other factors like larval behavior and the availability of preferred settlement habitat may be more important to maintaining population structure. We used an ecologically important benthic fish species, Gobiosoma bosc (naked goby), to investigate local and regional scale population structure and gene flow along a salinity gradient (∼3 ppt to ∼18 ppt) in two North Carolina estuaries. G. bosc is an abundant and geographically widespread species that requires complex but patchy microhabitat (e.g. oyster reefs, rubble, woody debris) for reproduction and refuge. We sequenced 155 fish from 10 sites, using a common barcoding gene (COI). We also included recent sequence data from GenBank to determine how North Carolina populations fit into the larger biogeographic understanding of this species. In North Carolina, we found a significant amount of gene flow within and between estuaries. Our analysis also showed high predicted genetic diversity based upon a large number of rare haplotypes found within many of our sampled populations. Moreover, we detected a number of new haplotypes in North Carolina that had not yet been observed in prior work. Sampling along a salinity gradient did not reveal any significant positive or negative correlations between salinity and genetic diversity, nor the proportion of singleton haplotypes, with the exception of a positive correlation between salinity standard deviation and genetic diversity. We also found evidence that an introduced European population of naked gobies may have originated from an Atlantic source population. Altogether, this system offers a compelling way to evaluate whether factors other than dispersal per se mediate recruitment in an estuarine-dependent species of fish with a larval dispersal phase. It also demonstrates the importance of exploring both smaller and larger scale population structure in marine organisms to better understand local and regional patterns of population connectivity and gene flow.