Reproductive ecology of the ascidiansMolgula CitrinaAlder & Hancock 1848 andAplidium Glabrum(verrill 1871) from the gulf of maine, U.S.A.

Abstract

Abstract We examined long-term patterns of sexual reproduction in the solitary ascidian Molgula citrina, and both sexual reproduction and colony growth in the colonial ascidian Aplidium glabrum from the shallow subtidal zone in the Gulf of Maine, U.S.A.. Ascidians of each species were collected regularly from two sites over a three-year period, then examined in the laboratory to determine size and fecundity, and, for the colonial species, density of individuals (zooids) in colonies. Reproductive characteristics were compared between ascidian species and between sites. In both species, sexual reproduction was highly seasonal, exhibiting marked peaks from late spring throughout summer. Periods of peak asexual reproduction in A. glabrum generally occurred when ascidians were not reproducing sexually. Fecundity of M. citrina ( # larvae per gram of parent ascidian) during peak periods of sexual reproduction differed between sites. Colonies of A. glabrum also differed between sites in several characteristics: #zooids per gram, #larvae per zooid, #larvae per colony, and #larvae per gram. To determine whether size constrained brood capacity, we examined the allometric relationship between fecundity (also zooid number for the colonial species) and weight, and compared this relationship between sites within species. Fecundity of M. citrina exhibited an isometric relationship with body weight, and this relationship did not differ between two populations. Colony-wide fecundity in A. glabrum increased isometrically with colony weight, but fecundity of individual zooids was independent of colony weight, Bothtypes of fecundity relationships in A. glabrum differed between sites. Zooid number increased allometrically with colony weight, with larger colonies containing disproportionately-fewer zooids than smaller colonies.