Population structure and breeding biology of Pisidia longicornis and Porcellana platycheles

Quantitative sampling of littoral and sublittoral populations of Pisidia longicornis (L.) and a littoral population of Porcellana platycheles (Pennant) has been carried out over a 2 year period (1970/1972). On the shore, P. longicornis were most abundant at mean low water spring (MLWS), whereas P. platycheles were found mostly at lower mid-shore level. Differences were found between littoral and sublittoral P. longicornis. Sublittoral females are gravid for a slightly shorter period (April to September), carry more eggs, and are smaller than littoral females. It is suggested that their lifespan may also be shorter than that of littoral individuals. Young of both species settle in autumn and grow until December, when growth ceases until the following spring. Numbers decline in the summer months due to the mortality of older individuals. Many sublittoral and some littoral P. longicornis are sexually mature towards the end of their first year on the shore, and may copulate after the early spring moult and carry eggs in the subsequent summer months. P. platycheles appear to follow a similar basic pattern, but results are less clear.