Reproductive isolation and morphological divergence between cryptic lineages of the copepod Acartia tonsa in Chesapeake Bay

Recent advances in molecular technologies have revealed cryptic species across many marine zooplankton taxa. However, the patterns and drivers of cryptic divergence are complex, and few studies have examined reproductive status among lineages through crosses. In this study, we performed pair crosses within and between 2 deeply divergent (cryptic) lineages (named 'fresh' [F] and 'salt' [S]) of the estuarine copepod Acartia tonsa from upper Chesapeake Bay, USA, to examine egg production and hatching rate. We also examined differences in morphology (prosome length) and chemical composition of the 2 lineages. Crossing experiments revealed that egg production did not differ among cross types but hatching rate was significantly lower for the between-lineage crosses (mean hatching rate of 0.02 for FxS vs. 0.46 and 0.52 for FxF and SxS, respectively). The nearly complete lack of nauplii production for between-lineage crosses suggests strong reproductive isolation, which supports previous molecular data. Significant differences between the lineages in size (F lineage is 13-14% shorter) and chemical com position (F lineages have 70% less carbon per copepod) may indicate pre-zygotic barriers to reproduction (e.g. morphological or gametic incompatibility). Overall, based on the crossing, mor phological, and chemical data reported here, and synthesizing previous biological data on the F and S lineages, we suggest that these cryptic lineages are likely to be separate, reproductively isolated species. Further work examining how divergent lineages of A. tonsa respond to environmental change and how they differ in their quality as prey items will be important for understanding trophic dynamics in estuarine environments like Chesapeake Bay.