This paper is the second of a series dealing with the annual cycle of growth andreproduction in British ascidians; a previous paper (Millar, 1952) dealt with the speciesDiplosoma listerianum (Milne Edwards), Ciona intestinalis (Linnaeus),Ascidiella aspersa (Muller), andBotryllus schlosseri (Pallas)The present investigation is concerned with the growth, the course of sexual reproduction and establishment of new generations, and the histological changes in the gonad throughout the year, in the ascidianDendrodoa grossularia (family Styelidae).Samples were examined from two widely separated localities: Fambridge in the River Crouch, Essex; and Farland Point, Isle of Cumbrae, in the Firth of Clyde. The specimens from Essex were dredged from a depth of about 2 m. (below L.W.O.S.T.), and were attached to the empty shells of the European oyster,Ostrea edulis L. and to the shells ofCrepidula fornicata L. The speci-mens from the Firth of Clydewere attached to stones on sheltered parts of the shore. The animals here extended over a zone of the shore from about Chart Datum+ 2.5 ft. (0.76 m.) to about Chart Datum+ 7.5 ft. (2.29 m.): Only in very sheltered positions, such as under large boulders, did Dendrodoa occur in abundance. The two habitats were thus very different in nature, one being sublittoral on the south-east coast of Britain and the other being littoral on the north-west coast. The value of comparisons is limited by the different nature of the conditions to which the sublittoral and the littoral populations were exposed.The observations on growth in the Clyde extended over most of 1951 and 1952, and nineteen samples were taken.
Interactions of larval dynamics and substrate preference have ecological significance for benthic biodiversity and
Ostrea edulis
Linnaeus, 1758 in the presence of
Crepidula fornicata
