| Summary: | A field survey was conducted of an extensive intertidal population of <i>Pagurus bernhardus</i> at Mumbles Head, S. Wales. Mean densities of up to 80 m<SUP>-2</SUP> were observed in a rock pool and locally, higher densities of up to 800 m<SUP>-2</SUP> occurred. There was no evidence of tidal or seasonal migration, or emigration from the area although there was an influx of large males from the sublittoral during the breeding season. Sublittoral sampling of the species at a depth of 5 metres on a sandy substrate at Oxwich Bay, Gower, revealed a full range of small individuals in that habitat. Comparison of the intertidal and sublittoral samples of hermits occupying <i>Littorina littorea</i> shells revealed differences. Intertidal hermits of given carapace width occupied larger shells and the colour of the integument, the species of epizoites and the pattern of allometry of the major chela were also found to differ. To determine whether these were genetic or phenotypic differences, enzyme electrophoresis of tissue samples from both populations were compared. They proved identical. Laboratory growth experiments confirmed the findings of earlier workers that moult increments are influenced by the size of available gastropod shells. Rearing studies indicated that small hermits observed as late as March must have been recruited the previous year. Growth rate of recruits was followed from field data. Postulated overall growth curves reconstructed from these data indicated a likely longevity of up to three years for intertidal <i>Pagurus bernhardus</i> and five for sublittoral populations. The high reproductive output of the numerous small hermits is facilitated by their precocious puberty. Studies of <i>Diogenes pugilator</i> indicated that this species has stenotopic ecological requirements and can be regarded as a refugial species in its habitat of the lower tidal and shallow sublittoral surf-pounded sands. Unlike <i>P.bernhardus</i> it is tidally and seasonally migratory so does not have separated populations.
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