EFFECT OF COLONY SIZE ON THE COMPETITIVE OUTCOME OF ENCRUSTING COLONIAL ORGANISMS

The relative importance of colony size ratio of interacting species wa s studied in Tomioka Bay, Japan. Six encrusting colonial species belon ging to the following three different taxonomic groups were tested: As cidia (three species), Bryozoa (two) and Porifera (one). Colonies of t hese organisms were grown in the community of sessile organisms develo ped on plastic panels. Logistic regression analysis was carried our to determine the effect of size ratio on the competitive outcome of inte racting colonies. The results between all possible combinations among these six species did not show a significant size effect in competitiv e outcome (i.e. a larger colony size did not always prove important in the success of a competitive interaction with smaller colonies of oth er species). On the contrary, competitive success depends on the types of species interacting. Certain species such as Didemnum moseleyi (as cidian) and Haliclona sp. (sponge), in spite of being smaller in colon y size, won in competitive interactions with larger colonies of other species such as Diplosoma mitsukurii (ascidian) and Watersipora subovo idea (bryozoan). These results contradict the one reported earlier: th at the larger the colony size, the more chance the colony will have to win in competitive interactions.