Development and application of elemental fingerprinting to track the dispersal of marine invertebrate larvae

The early life history of many marine benthic invertebrate and fish species involves a planktonic larval stage that allows exchange of individuals among separated adult populations. Here, we demonstrate how natural and anthropogenic trace elements can be used to determine larval origins and assess bay-ocean exchange of invertebrate larvae. Trace elements can be effective site markers for estuaries because run-off and pollutant loading often impart distinct elemental signatures to bay habitats relative to nearshore coastal environments. Crab larvae originating from San Diego Bay (SDB) were distinguished from those originating in neighboring embayments and exposed coastal habitats by comparing multiple trace-element concentrations ("fingerprints") in individuals. Discriminant function analysis (DFA) was used to characterize stage I zoeae of the striped shore crab, Pachygrapsus crassipes, of known origin (reference larvae) via trace-elemental composition (i.e., Cu, Zn, Mn, Sr, Ca). Linear discriminant functions were used to identify the origin and characterize the exchange of stage I P. crassipes zoeae between SDB and the nearshore coastal environment during one spring tidal cycle. Elemental fingerprinting revealed that most (87%) of the stage I larvae collected at the bay entrance during the flood tide were larvae of SDB origin that were reentering the bay. Nearly one third of zoeae sampled (32%) at the entrance during ebb tide were coastal larvae leaving the bay and returning to open water. The observed bidirectional exchange contrasts with the unidirectional transport of zoeae out of the bay predicted from stage I vertical migratory behavior. Because P. crassipes zoeal survivorship is lower in SDB than in coastal waters, bay-ocean exchange has significant implications for the dynamics of P. crassipes populations. Trace-elemental fingerprinting of invertebrate larvae promises to facilitate investigations of many previously intractable questions about larval transport and dynamics.