Basin-wide morphology and metabarcoding-based comparison of ichthyoplankton diversity and community structure in the Gulf of Mexico

Abstract

Comprehensive surveys of marine fish egg and larval assemblages provide insight into spawning distribution, dispersal, transport processes, connectivity, and recruitment. We report on a rare basin-wide synoptic survey of fish eggs and larvae sampled throughout the Gulf of Mexico (GoM), including the Loop Current (LC) and Bahamas Channel in the Atlantic. Based on paired sample collections, we identified fish larvae through morphology-based identification at the family level and with cytochrome oxidase I metabarcoding to genera or species; eggs were identified by metabarcoding. Sampling within one-month during summer 2017 included on-shelf (<200 m depth) and off-shelf stations (depths >1000 m). We found general agreement between identification approaches at the family level, but the higher taxonomic resolution achieved with metabarcoding and the inclusion of ecological information such as adult habitat indicated three predominant assemblages: oceanic, shelf, and LC-associated. In larval samples, 72 and 77 families were identified based on morphology and metabarcoding, respectively and 62 families (70 %) were identified with both methods. Thirty-nine families were identified in fish egg samples, likely due to their shorter stage duration. The greater taxonomic resolution of metabarcoding yielded 265 and 75 species in larvae and egg samples, respectively. While family-level diversity indices did not differ between on and off-shelf assemblages, species-level analysis indicated significantly higher egg and larval diversity at on-shelf stations. Hence, family-level analyses may be too coarse to elucidate patterns in alpha diversity. Family-level cluster analysis indicated on– and off-shelf stations grouped separately, except for an off-shelf station off the Mississippi River that reflected local offshore transport of neritic taxa. Metabarcoding detected an additional cluster within the LC’s path of travel that included Caribbean reef taxa. All off-shelf stations included larvae of neritic families, reflecting offshore transport throughout the GoM. In contrast, only some on-shelf stations included oceanic families, indicating less prevalent oceanic-to-shelf transport. Amplicon sequence variants, roughly indicative of abundance, showed a positive association with mean water column temperature and dissolved oxygen, known to influence spawning and larval growth and development, as well as surface chlorophyll, a proxy for larval food availability. Off-shelf stations were positively correlated with indicators of water masses and mesoscale eddies. Similar results were found for the egg stage, but the correlation with chlorophyll a was insignificant, consistent with endogenous feeding during the embryonic stage. Our synoptic view of the GoM’s early life stages of fishes can serve as a reference for examining the impacts of climate change and other anthropogenic disturbances.