MEPS 272:191-202 (2004)  -  doi:10.3354/meps272191

ABSTRACT: Settlement is a critical period in the life cycle of crabs and can be a major factor influencing the structure of local populations. Under natural conditions, tidal stream velocities commonly exceed swimming capabilities of postlarvae and can affect settlement onto favourable habitats. In an annular flume simulating natural flow velocities, the diel variability of megalopal distribution, and the ontogenetic change in habitat use of young juveniles of the velvet swimming crab Necora puber were examined simultaneously on 4 substrata of different structural complexity (a red macroalga, pebbles, artificial grass and sand). The distribution of dead animals was non-even and higher numbers occurred on complex substrata. Live animals showed a positive response to the macroalga and pebbles, causing an increase in settlement compared to the passive distribution of dead animals. In contrast, numbers of settlers on artificial grass were significantly lower than for dead animals. Differences in the pattern of distribution between day and night suggested that megalopae were less selective during the day, and avoidance of Astroturf tended to be stronger at night. Although megalopae and early juveniles show capability of active selection of substrata, the differential distribution on natural substrata during settlement was most likely set by hydrodynamic conditions; active movement may only occur when a substratum of lower suitability is encountered.

KEY WORDS: Substratum · Postlarvae · Settlement behaviour · Habitat selection · Flume