Dispersal and Population Variation in the Bryozoan Bugula Neritina

Abstract

The scale of spatial and temporal fluctuations is described for populations of the arborescent bryozoan Bugula neritina, and observational and experimental data were used to explain these fluctuations. Colonies were found to be abundant on seagrasses in the northern Gulf of Mexico from mid to late fall through winter. Populations increase in mid to late fall, roughly with the onset of cooler weather. They decline between midwinter and early summer, apparently when cold fronts pass. We found no colonies on seagrass in summer, but some large colonies attached to polychaete tubes lie dormant through summer, regenerating in fall. Spatial variation occurs on very small scales. Bugula were absent from some seagrass patches, even when substantial populations were <100 m away. Sites with Bugula populations in one season had populations in subsequent seasons, while seagrass patches without Bugula lacked Bugula at other times. This suggests that there is little interaction between spatial and temporal variation. The absence of Bugula neritina from patches of seagrass is not the result of poor survival of post—settlement individuals, but rather is likely due to poor dispersal of the lecithotropic larvae. Newly metamorphosed juveniles were transplanted to sites with (university marine lab) and without (Lanark) natural Bugula populations. Mortality was uniformly high (°70%) for the 1st wk after settlement, but in the 2nd wk transplanted juveniles had higher survivorship at the Lanark site. In weeks 3 and 4 there were no significant differences and °80% survival at both sites. For weeks 4—8, mean survivorship decreased to 50%/wk at the marine lab site, while remaining >80% at Lanark. Growth rates were also higher at the site without natural Bugula colonies, and colonies there began to reproduce 1 wk sooner than those at the marine lab site. Juveniles transplanted to Lanark reproduced, and their offspring grew and reproduced successfully, producing a second generation of Bugula. The newly established population persisted through summer and was still present in 1986. Local extinctions of Bugula populations are probably caused by storms; one such extinction was observed. The spatial pattern is maintained by restricted dispersal and population bottlenecks. Unoccupied seagrass patches are unlikely to be colonized by larvae, but rafting of mature colonies on seagrass occurs frequently, with the potential of seeding new habitats.