Ecology and behaviour of postlarvae and juveniles of the velvet swimming crab Necora puber (L.)

Abstract

Ecological studies of early benthic stages of brachyurans are rare for most of the species, leaving a gap in the understanding of processes regulating the recruitment to adult populations. The velvet swimming crab Necora puber (L. ) is a valuable commercial species where most of studies have been focused on the adult population. Little is known about the supply-side ecology and the post-settlement processes that affect the dynamics of the adult populations. The aims of the present study were to investigate the physical processes affecting the supply of recruits to the nearshore, to examine the processes associated with habitat selection during settlement, and to describe the biology and ecology of the juveniles of N. puber. The study was carried out in the shallow waters and on the rocky shores of Plymouth Sound, on the southwest coast of the UK. Distribution of the pelagic postlarvae was patchy, and the abundance varied spatially in tens and thousands of metres. In temporal scales, the annual pattern was dominated by low occurrence of megalopae, punctuated by episodic peaks of high abundance. In the water column, most of the megalopae were collected at the surface and their abundance appears to be regulated by the tidal cycle, as megalopae were more abundant during flood than ebb tides. This behaviour could produce a net shoreward transport of megalopae. Laboratory experiments indicated that flow conditions set initial patterns of distribution of settlers on substrata of different tri-dimensional structure. However, active habitat selection occurred and the settlers were actively modifying the distribution patterns set by the hydrodynamics. Ontogenetic shift in habitat use occurred early in the juvenile phase and first juvenile instars were less habitat specific than megalopae. During the benthic phase, juvenile growth was markedly seasonal, and virtually no growth occurred in winter. Results from the present study indicated that juvenile growth rate is slower than previously described for N. puber. Magnitudes of juvenile recruitment were variable between years, but level off at the end of the 1+ year class on most of the shores studied. This suggests that mortality is high for early juveniles and appears to be density dependent on some shores. High density-dependent mortality can obscure the connectivity between larval and adult populations, so for the population of N. puber studied, better correlations may be obtained from juvenile-to-adult relationships.