ECOLOGICAL DISTRIBUTION OF EPIPHYTIC HYDROZOA WITH SPECIAL REFERENCE TO SERTULARELLA MIURENSIS

Abstract

Seasonal observations of the hydroid, Sertularella miurensis, associating with Sargassum tortile growing in the sublittoral fringe and the field experiments of transplanting hydroid colonies to various situations showed that the upper extension of the distribution of Ser. miurensis on the shore is delimited by the emersion effect. The time of day when the low tides occur and duration of emersion were important for the mortality of hydroids in the upper margin of their distribution range. Desiccation seemed to be important. By colonizing the proximal part of the algal thalli, however, a hydroid occupies the midlittoral zone. Within the range free from such an adverse emersion effect, as in sublittoral zone, the ecological distribution is largely influenced by the nature of algal preference of settling larvae. Algal selection by the hydroid larvae is most important in determing the ecological distribution, which is established by the continuous association between algae and hydroids. In general Sargasso algae are suitable substrara for hydroids. Some hydroids occupy a definite portion of the algal thallus and it seems to be explained by the nature of larval behaviour and by the interspecies interaction between settling larvae and interspecies competition for space among the epiphytic hydroids in the process of colony growth. A general discussion was given as to 1: delimitation of upper range of distribution on the shore, 2: ecological distribution of epiphytic hydroids and the role of algal selection of settling larvae, 3: microdistribution of hydroids on the thallus of Sargasso algae, and 4: interspecies relationship in the microdistribution.