Growth dynamics and mortality of the intertidal encrusting sponge Halichondria okadai (Demospongiae, Halichondrida)

A periodical survey of the growth of an intertidal encrusting sponge Halichondria okadai was conducted on a rocky intertidal shore on the Izu Peninsula, southern Japan. The area covered by each of 16 marked sponges was monitored, and the fusion and fission of specimens were recorded from June 1995 to June 1998. During the investigation, 15 of the original specimens managed to survive accompanying fusions or fissions and only one colony completely disappeared. Ten fusions and 23 fissions were observed in total. The maximum death rate during a semilunar period was 16.7% and cumulative mortality throughout the study period was 46.1%, indicating that the mortality of H. okadai is lower than that of other species previously reported. There was no particular seasonal pattern in the occurrences of fusion, fission and death of sponges. However, fissions were frequently observed in the latter half of the study period and were sometimes followed by fusions or death of specimens. Positive growth rates were often observed during warmer months, and specimens showed little growth or regression in winter. Seasonal patterns in surface area were thought to be related to seasonality in reproductive activity.