UC Davis

The timing of vertical migrations by newly-hatched larvae determines the extent of transport away from adult populations and exposure to predatory fishes, but it is largely unknown whether larval swimming behavior is a fixed trait or changes adaptively in response to different ocean conditions that are encountered between habitats. We determined whether larvae of the shore crab, Pachygrapsus crassipes, hatched in the San Francisco Estuary and those hatched nearby on the outer coast undertake tidal and diel vertical migrations. Vertical swimming of larvae that were released by females collected from the two locations were recorded in the laboratory for up to four days in constant darkness without a tidal cycle to detect the presence of endogenous tidal and diel vertical migrations. P. crassipes larvae from the outer coast population did not exhibit rhythmic vertical migrations, remaining near the surface throughout the day, whereas larvae from the estuarine population did undertake complex vertical migrations relative to tidal and diel cycles. Although current patterns differ on the open coast and in the estuary, remaining in surface waters at both locations would favor seaward transport of larvae to offshore nursery areas. However, undertaking tidal vertical migrations in estuaries would expedite seaward transport while increasing the risk of fish predation during the daytime. The differences in behaviors are likely phenotypic, because larvae came from neighboring populations and intermingle in offshore larval nursery areas. This spatial variation in larval swimming behavior among habitats enhances transport to offshore nursery areas. © 2013.