a1 Department of Zoology, Westfield College, University of London, London, NW3 7ST
The life-history of hydroids from a mixed population of Tubularia indivisa and ‘T. ceratogyne’, from an intertidal site at Felixstowe, Suffolk, has been studied. Tubularia breed throughout the year, but recruitment is greatest in spring with a smaller peak in late summer. The spring-recruited hydroids are the progeny of T. indivisa, but during the summer they mature as ‘T. ceratogyne’ and breed to produce the T. indivisa that dominate the population in spring. It is concluded, on this evidence, that ‘T. ceratogyne’ is a form of T. indivisa.
In this population T. indivisa suffers from predation by the nudibranch Dendronotus frondosus, and their mortality rate varies from 2% per month in winter to 70% per month in summer. The life-span of T. indivisa is approximately one year, but their mean post-larval life expectancy varies from 30 days (spring recruitment) to 160 days (autumn recruitment). There is no evidence that autotomy of the hydranth is an integral part of the life of T. indivisa.