BRYOZOAN GROWTH HABITS: COMPLEXITY AND DIVERSITY IN ECOLOGICAL RECONSTRUCTIONS

Correlation between the occurrence of some bryozoan colony growth habits and the environmental conditions in which they grow has prompted many attempts at paleoenvironmental reconstruction based on this relationship. Most studies, however, have been based on overall colony form and generalized environmental conditions. The goal of this study was to develop and analyze a data set in which each bryozoan species is characterized by its combination of eleven fundamental growth habit characters associated with the colony's relationships to the substrate, construction, feeding surfaces, modular shape, and dimensions of growth. Environmental conditions of water depth, substrate type, relative nutrient level and temperature were known for each sample locality. The data set consists of 35 localities in the near shore (0-60 m), Modern marine setting of the NE Adriatic (within 10 km of Rovinj, Croatia), with distributions of 101 bryozoan species representing 60 different growth habit combinations. Most (43) of the unique growth habit combinations are represented by a single species. Only five of the growth habit combinations are shared by three or more species.

Principal component analysis using sample localities as observations, with individual growth habit characteristics as variables (e.g. tally of erect vs. encrusting species at each), produced a primary axis (33.8% of var.) which segregated localities by water depth. Substrate type and nutrient level failed to correlate with any of the additional PCA axes, suggesting that unidentified microenvironmental heterogeneity is responsible for a great deal of bryozoan growth habit variability among localities. Increased growth habit diversity was associated with increased microenvironmental complexity (presence of key bryozoan taxa generates diversity by providing complex substrates). The analysis revealed problems associated with sampling bias, and patterns were only detectable when only dredge samples were included (scuba collection sites omitted). This emphasizes the need for standardized sampling protocol for fossil analyses.