Shoaling preferences in decapod crustacea

The aggregation behaviour of fish has been extensively studied, but little is known about the shoaling of marine invertebrates. We investigated aggregation behaviour in two species of a decapod crustacean: the brown shrimp, Crangon crangon, a cryptic species that should not rely on aggregations to avoid predation, and the rockpool prawn, Palaemon elegans, a species that lives in areas without shelter and we expect to aggregate. A field survey revealed that prawns had a strongly clumped distribution, whereas shrimps only showed a tendency towards aggregating. However, size segregation was found to be strong in both species. Choice experiments in the laboratory confirmed the field results on the differences in the aggregative tendencies for the two species, as prawns showed a strong preference for shoaling with a group of five conspecifics versus a single conspecific, while shrimps only showed a trend in that direction. Surprisingly, we found no evidence for a size-assortative preference in prawns, suggesting that size segregation in the field might be a consequence of indirect or passive factors rather than individual preferences.