The spiny crab is characterized by the fact that, when it occurs near the coast, it forms species-specific aggregations, 'heaps'. Since it aggregates also in the laboratory tank, some experiments were made in order to establish whether any regularities prevail in this behaviour. The aggregations of this crab were studied in the laboratory using five groups of twenty specimens. Dominance rank order was correlated with individual positions in the experimental groups. The results of the analysis show that the higher-ranked individuals in the dominance rank order occur predominantly away from the heap, less on its surface and rarely inside it. Conversely, the lower-ranked specimens are found predominantly in the heap. The males are more aggressive than the females and consequently higher-ranked, so that in the heap the females predominate while the males are mostly away from it. The larger crabs are normally higher-ranked and therefore occur more frequently away from the heap. The observed structures of the heaps support the hypothesis that they have a protective function against predators during the time when the cabs are concentrated in shallow water where their predators live. ?? 1971.
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