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
Shoaling preferences in decapod crustaceaThe aggregation behaviour of fish has been extensively studied, but little is known about the shoaling ofmarine invertebrates. We investigated aggregation behaviour in two species of a decapod crustacean: thebrown 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 toaggregate. A field survey revealed that prawns had a strongly clumped distribution, whereas shrimpsonly showed a tendency towards aggregating. However, size segregation was found to be strong in bothspecies. Choice experiments in the laboratory confirmed the field results on the differences in the aggregativetendencies for the two species, as prawns showed a strong preference for shoaling with a group offive 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 inthe field might be a consequence of indirect or passive factors rather than individual preferences
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