Invertebrate associates of the sea urchin,Echinus esculentusL., from the Scottish West Coast

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Abstract
A total of 21 invertebrate species associated with Echinus esculentus were collected from 28 monthly samples of the urchin in 1980–1982. Four of the associates were ecto-parasitic or commensals, and one was an endoparasite; the remainder are regarded as casuals. The endoparasitic nematode Echinomermella grayi was found in < 0.8% of urchins, had a curiously discontinuous size distribution, and showed no evidence of seasonality in either size or egg maturity. Of the external associates, the polychaete Adyte assimilis showed a clear annual life history, with settlement beginning in July, and the worms maturing in the following May. The amphipod Euonyx chelatus occurred in 0.4-3% of Echinus from shallow or deep water respectively. The evidence suggests that reproduction is mainly restricted to the period May-July, and that this species is also an annual. The copepod Pseudanthessius liber occurred throughout the year on all Echinus populations, but most prolifically on urchins from within the Laminaria. In this situation population densities peaked late in the year, but otherwise remained low. Elsewhere, populations remained low, showing little seasonality. Reproduction occurred throughout the year. Finally, the caprellid Pariambus typicus inermis was found on Echinus from one location only, the only known record of its occurrence on an echinoid. Large populations built up during the autumn and winter, reducing rapidly to very low levels throughout the remainder of the year. Breeding was continuous throughout the year. Whilst the females were similar to those described in the literature, the males were much smaller, and apparently precocious.