Aggregation of pelagic Munida gregaria (Fabricius) (Decapoda, Anomura) by coastal fronts and internal waves

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Abstract
Aerial photographic surveys were used to study the distribution and abundance of the near-surface shoaling juvenile phase of the galatheid crab Munida gregaria in southeastern N.Z. continental shelf waters. On numerous occasions shoals were concentrated into aggregations many metres to a few kilometres long at river plume and headland fronts, and in mid-shelf internal waves. Laboratory experiments established that M. gregaria juveniles showed strong positive phototaxis and that they could sustain swimming speeds of ≡ 16 cm s−1. Field experiments showed that when M. gregaria were displaced to deeper waters, their response was to swim back towards the surface. These results suggest that juveniles swept into convergent zones would resist downward displacement. Since convergent zones occur at coastal front and internal wave sites this mechanism could explain the aggregations of shoals observed and photographed from the air. This mechanism is discussed in relation to literature on physical processes at such sites and to studies of plankton distribution and behaviour.

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