Systematics, variation, and developmental instability: analysis of spine patterns in ancestrulae of a common bryozoan

Abstract

Since the species was originally described in 1960, the cheilostomate bryozoan Bugula stolonifera is known to be widely distributed in temperate and sub‐tropical regions. Evidence also exists that it is an invasive species that can spread rapidly. Yet there have been no studies of its variability at the level of individual populations. One of the taxonomic characters cited in the diagnoses of Bugula spp. is the spination pattern surrounding the frontal membrane of the ancestrula, the founding member of the colony that arises from the metamorphosis of a larva. Although some authorities have recognized that the number and distribution of ancestrular spines can vary, there has been no systematic study of the extent of this variation. We examined the spination patterns of 11,162 ancestrulae derived from larvae released from 54 colonies collected at Woods Hole, Massachusetts, USA. The formula cited in descriptions of the species was present in only 53% of the ancestrulae. In total, 34 distinct patterns were recorded. Two trends in spine appearance were statistically significant. First, those individuals possessing a spine in the proximal position were more likely to gain an additional spine. Second, those individuals lacking a proximal spine were more likely to lose an additional spine. The significance of these trends remains to be explored. Fourteen second‐generation ancestrulae possessing five variant (other than the most common) spination patterns were placed in the field for 29 days. Colonies grown from these ancestrualae were returned to the laboratory and 225 larvae produced by these colonies were collected. The spine patterns of ancestrulae resulting from metamorphosis of these larvae were recorded. Twelve patterns not present among those of the second‐generation parents were observed in addition to the five original ones. Although all colonies grown in the field were from variant ancestrulae, 51% of the second‐generation ancestrulae possessed the basic pattern. Finally, there were no significant differences between left and right side asymmetries in either the first‐ or second‐generation ancestrulae. From these studies we conclude that spine pattern of ancestrulae is not a reliable taxonomic character, at least in the Woods Hole population of B. stolonifera. Second, there exists considerable variation in spine patterns yet there is no bias to left or right asymmetry. Third, possession of a particular spine pattern is not a heritable feature in one‐generation studies. The production of ancestrular spines appears to be strongly influenced by random events occurring in the developmental process. Present address: Paula J. Rodgers, Department of Biology, University of Maryland, College Park, MD 20742, USA.