Behavioral adaptations in larvae of brachyuran crabs: A review

• Brachyuran larvae are strong swimmers compared to other zooplankton.
• Swimming behavior occurs in response to gravity, light, and hydrostatic pressure.
• Larvae undergo diel vertical migration with attendant predator/prey advantages.
• Vertical swimming allows larvae to take advantage of vertical shear in horizontal currents.
• Chemical cues for metamorphosis are undetectable beyond a few meters from a settlement site.

Larval development in brachyuran crabs includes a number of zoeal stages followed by a single megalopal stage. Zoeae and megalopae are relatively strong swimmers, and movement up or down in the water column takes advantage of vertical shear in ambient currents with consequent transport in the horizontal dimension. For some species, this process is important in controlling the dispersal of early-stage larvae away from spawning sites and in maintaining the supply of late-stage larvae to juvenile nursery habitat. For other species it enables retention near spawning sites throughout the larval period. Vertical position in the water column also modulates predator-prey interactions, which impact growth and survival of larvae. Swimming behavior in larval crabs is regulated by both external cues detected in the water column and endogenous rhythms entrained by external oscillators. Gravity, hydrostatic pressure, and light are the primary external cues because of their predictability in the environment. Light is also the most common external oscillator entraining swimming rhythms. Secondary cues include salinity, temperature, turbulent kinetic energy, and feeding state. Crab larvae also respond to chemical and tactile cues that facilitate settlement in juvenile habitat. This paper presents a review of the physical and chemical characteristics of these cues, the behavioral responses of crab larvae to the cues, and the patterns of larval transport that emanate from these responses.