Larval settlement preference maximizes genetic mixing in an inbreeding population of a simultaneous hermaphrodite (Bugula stolonifera, Bryozoa).

Conspecific aggregations in terrestrial and aquatic organisms can have a significant effect on an individual's survival, growth and reproductive fitness, particularly if these aggregations are composed of closely related individuals. Such aggregations can form passively, as a consequence of dispersal, or actively, as a consequence of kin recognition. In this study, we investigated the genetic composition of individuals in conspecific aggregations in the simultaneous hermaphroditic marine bryozoan Bugula stolonifera. Conspecific larvae routinely metamorphose on adult colonies; the possibility that larvae select or avoid their maternal colony was investigated utilizing 10 newly developed polymorphic microsatellite loci. Adult colonies were collected from Eel Pond, Woods Hole, Massachusetts and inspected for the presence of attached individuals. Adult colonies and their attached individuals were genotyped and compared to assess genetic relatedness within and among these groups relative to the overall genetic variability of the sampling site. Overall, the population of B. stolonifera at this site was found to be outside Hardy-Weinberg equilibrium because of significant levels of inbreeding. No significant genetic differentiation, however, was found between any groups, documenting that a group containing an adult colony and its attached individuals had as much genetic variability as was found for the entire sampling site. Parentage-exclusion analyses showed that the vast majority of attached individuals (>93%) could not have derived from the colony on which they were attached. Kinship analyses showed that the majority of attached individuals (≈63%) shared less than a half-sibling relationship. These results suggest that a colony's nearest neighbours are not composed of siblings, and thus, larval settlement preference can maximize outcrossing in this inbreeding population.

Serotonin and Dopamine Have Opposite Effects on Phototaxis in Larvae of the Bryozoan Bugula neritina.

Adult colonies of the bryozoan Bugula neritina release short-term anenteric larvae that initially are strongly photopositive. Over the course of several hours larvae lose their initial photopositivity and either become photonegative or alternate between positive and negative phototaxis. We report that newly released photopositive larvae rapidly become photonegative upon exposure to 10-6-10-5 M serotonin or its metabolic precursor, 5-hydroxytryptophan. This behavior was not observed in two congeners of B. neritina, nor in larvae of three other species of bryozoans and seven species from four additional phyla. Antibodies to serotonin label cells in the region of the equatorial nerve-muscle ring and in two tracts extending from the apical disc to this ring. In a separate series of experiments, larvae treated with dopamine (10-7-10-5 M) significantly prolonged their photopositive period. This effect was also obtained with the D2 dopamine receptor agonist, quinpirole (10-6-10-5 M). HPLC analysis determined that newly released photopositive larvae contained 0.120 pmol dopamine/ {mu}g protein. These findings implicate serotonin and dopamine as important neurochemical regulators of phototaxis in larvae of B. neritina.

A direct and active influence of gravity on the behavior of a marine invertebrate larva.

Larvae of the bryozoans Bugula neritina and Bugula stolonifera exhibit an apparent negative geotaxis under conditions of darkness and constant temperature. This behavior cannot be accounted for by buoyancy since the larvae are negatively bouyant, nor is it a consequence of gradients in the partial pressures of dissolved gases since the response occurs under conditions where the gradient is reversed or when experiments are conducted in chambers with interfaces of only glass and water. Pressure bomb experiments indicate that the behavior is not a barokinesis. Centrifuge experiments, however, showed that larvae of Bugula stolonifera orient directly and actively to gravity, while those of Bugula neritina have some other measure of geographic up. Since bryozoan larvae lack statocysts, the sensory apparatus mediating the gravity response in Bugula stolonifera is still unknown.

Mycoplasma-like organisms: occurrence with the larvae and adults of a marine bryozoan.

Larvae and adults of the marine bryozoan Watersipora cucullata invariably possess numerous extracellular mycoplasma-like, organisms. Mesodermally encapsulated groups of these atypical bacteria occur in the visceral coeloms of all colony members. In contrast, thousands of the symbionts are externally attached to each larva along a unique superficial groove; the microorganisms are internalized during the complex metamorphosis, thus inoculating the incipient colony. The consequences to the bryozoan of this association are not known.