Kin‐recognition and the Spatial Distribution of Larvae of the Bryozoan Bugula Neritina (L.)

The distribution of many sessile marine invertebrates is heterogeneous on very small spatial scales and, in many cases, the distributional pattern is produced when larvae settle from the plankton (e.g., Connell, 1961; Caffey, 1982; Keough and Downes, 1982). Thus, some larvae settle near conspecific individuals, while others settle in relative isolation. It is of interest to identify the factors responsible for such variation in settlement, since they may result in spatial variation in a number of agents that may influence an organism's fitness such as the intensity of intraor interspecific competition and the risks of predation. These settlement patterns may result from heterogeneity in the distribution of larvae in the plankton or from variation in the intensity of settlement cues, with larvae settling in proportion to the strength of such cues. Alternatively, larval behavior may vary, and such variation and its sources are the foci of this study. Larvae of a colonial invertebrate, the bryozoan Bugula neritina (L.), were hypothesized to settle preferentially near conspecific larvae or newly attached individuals (hypothesis 1) or conspecific adults (hypothesis 2). This behavior was hypothesized to vary in strength either between (hypothesis 3) or within (hypothesis 4) maternal groups of larvae. Variation in settlement may also be derived from changes in the behavior of individual larvae with time or age (hypothesis 5), since some larvae become less selective with increasing time spent searching unsuccessfully for settlement cues (Knight-Jones, 1953). At the study site in southern California, Bugula neritina colonies live attached to a variety of substrata at depths of 0-20 m and are usually found in clumps of two to eight colonies, with well-established lone colonies being rare. The mode of fertilization is poorly known for this genus, but evidence from a number of species suggests that colonies are composed of protandrously hermaphroditic zooids, with cross-fertilization occurring between colonies (Silen, 1972; Schopf, 1977). Embryos are brooded, and a single colony may release up to 100 large (diameter 300-400 ,um), coronate larvae daily. It is possible that release of both sperm and larvae may be synchronized chemically between neighboring colonies (Str6m, 1969; Silen, 1972). The larvae are initially photopositive, but soon become photonegative/geopositive, and usually settle within a few hours of release (Lynch, 1947).