The reproductive cycle of the sea urchin Psammechinus miliaris (Echinodermata : Echinoidea) in a Scottish sea loch

The sea urchin Psammechinus miliaris was sampled at monthly intervals from two replicate sites at contrasting locations (littoral and subtidal habitat s) on the west coast of Scotland. Samples were collected from November 1995 to October 1997. A study of gonad development showed a clearly defined ann ual cycle of gametogenesis with a single spawning period. Gonad indices (GI s) varied between locations, between sites at the same location and between study years. Gonad indices were maximal prior to the onset of the spawning period in June and July. Ripe gametes were shed by dissected urchins from June to September in 1996 and June to August in 1997 coinciding with the ti me of year when the gonad colour was best in terms of what is desired in th e marketplace. Over winter the GIs were very low and gonads became very dar k in colour. Higher summer GIs in the urchins from one littoral site sugges ted that the urchins there had a seasonal influx of a more nutritious or mo re abundant food resource. Histological determination of the reproductive s tate showed that for these food-limited populations high GIs do equate with the spawning period. The gametogenic cycle fits the six stage pattern desc ribed for other echinoid species. In females oocytes of various development al stages were present throughout the annual cycle. Recruitment to the litt oral location was monitored over two seasons by quantifying the number of u rchins with a test diameter <5 mm appearing in the population. Recruitment appeared variable between sites and between seasons suggesting that the col lection of juveniles resulting from natural settlement may not be a reliabl e alternative to hatchery produced spat. Although P miliaris has commercial potential when in cultivation, this study provided no evidence of an oppor tunity for a commercial fishery.