The ontogeny of larval swimming behavior in the crab Hemigrapsus sanguineus: Implications for larval transport

• We studied behavior over larval development in the crab Hemigrapsus sanguineus.
• Swimming rhythms and responses to gravity and light in Z1 larvae fit with export.
• Behavioral responses change in Z3 and Z5 stages to favor diel vertical migration.
• Larval behavior in H. sanguineus matches an export-return model.

Many estuarine crab species are exported from this habitat at spawning and are physically transported to coastal waters where they undergo zoeal development. Later, they recruit back to the adult habitat as megalopae. Larval behavior within the context of wind-driven coastal circulation contributes to this process. The present study tested whether the crab Hemigrapsus sanguineus, an invasive species to the mid-Atlantic coast of the United States as well as in Europe, possesses behavioral responses to exogenous cues (gravity and light) and endogenous rhythms that would facilitate this scenario. Throughout larval development, H. sanguineus zoeae were negatively buoyant. In a laboratory test chamber, early stage (Z1) larvae displayed a strong negative geotaxis in darkness which resulted in surface swimming that was not offset by behavioral responses to light exposure. These Z1 stage larvae also possessed an endogenous tidal rhythm in swimming activity that was in phase with ebb tides in the spawning habitat and was independent of light exposure. These behaviors are consistent with export of larvae from estuarine to coastal waters where larval development occurs. Intermediate (Z3) and late (Z5) stage larvae exhibited a predominant negative geotaxis and surface swimming, with some zoeae showing positive geotaxis and/or sinking in the Z5 stage. Visual spectral sensitivity of eyes in Z3 larvae peaked between 450 and 510 nm, with a secondary peak evident at short-wavelengths (~ 370 nm). In both the Z3 and Z5 stages, larvae descended in response to light exposure, as expected under a negative feedback model of depth regulation common among crab larvae. Photoresponse thresholds of 5.8 × 1012 and 1.2 × 1014 photons m− 2 s− 1 for Z3 and Z5 stage larvae, respectively, predict both stages to be in the lower water column during the day. After molting to megalopae, H. sanguineus display positive geotaxis and/or sinking in darkness and lack a phototactic response, consistent with settlement in the adult benthic habitat. Thus, larvae of the invasive crab H. sanguineus possess endogenous rhythms and behavioral responses to environmental cues that are similar to those observed in successful native species with a similar coastal larval development pattern.