Published online by Cambridge University Press: 11 May 2009
Investigations showed that populations of Cirriformia tentaculata (Montagu) at Plymouth, Coverack and Salcombe produce both free-swimming and demersal larvae from single egg batches. The proportion of free-swimming to demersal larvae varies from one brood to another but is constant in different cultures from a single brood. Control of the proportion in which the two behavioural types are produced appears to be genotypic and not phenotypic. The larval types could not be separated morphologically but additional evidence for the existence of polymorphism in the species was provided by the variation of esterase pattern between adults. The population that existed in Southampton water previous to the winter of 1962/63 and which produced only demersal larvae was probably genetically isolated from other populations of the worm and showed behaviour atypical of the species as a whole.
Since the publication of a note reporting behavioural differences existing between the larval stages of Cirriformia tentaculata (Montagu) from Southampton Water and from Plymouth Sound (George, 1963), continuing investigations have shed more light on the problem. Observation of the Southampton population from 1959 to 1962 showed that demersal larvae were produced throughout the breeding season, from May to September (George, 1964a), whereas Wilson (1936) found that a population from Drake's Island, Plymouth Sound, produced a free-swimming larval stage in July of 1928 and 1933. It is known that some species of polychaete produce pelagic and non-pelagic forms according to the time of year (Mesnil & Caullery, 1917; Thorson, 1946). The Plymouth population was therefore re-examined to determine whether free-swimming larvae were produced during the entire breeding season.