The echinoids Paracentrotus lividus (Lamarck) and Psammechinus miliaris (Gmelin) in the intertidal zone of the French atlantic coast Introduction to studies on growth and boring activity

This chapter draws attention to morphological features which are useful in differentiating between Paracentrotus lividus and Psammechinus miliaris. The population dynamics of both species are investigated in context of their associated biota and biotopes (i.e. intertidal pools) near Douarnenez (South Finistere) and at Lerat (Loire-Atlantique). At Douarnenez, P. lividus is very abundant in mid-intertidal pools where it bores into siliceous rock. Both echinoids are known from the coast near Lerat since 1857, when they were first observed. The tidal pools have been variously settled by, or empty of, urchins. In 1882, the young of both species were observed under rocks. Recolonisation and bioerosion by the urchins are given particular attention.