Hexactinellid sponge ecology: growth rates and seasonality in deep water sponges

Abstract

It has been well established that sponges play an important role in benthic ecology as abundant, large, sessile filter-feeders. However, the ecology of one group, the Hexactinellida, whose electrophysiology and cell biology is quite distinct from other Porifera, has received little attention due to the inaccessibility of their preferred deep water habitat. Now, a three year study of a population of the hexactinellid sponge Rhabdocalyptus dawsoni (Lambe, 1892) has been carried out in the fjords of British Columbia, Canada. Rhabdocalyptus was found to have a patchy distribution in Saanich Inlet, British Columbia, with local abundance reaching 5.3 individuals m −2. The mean length of the tube-like sponges in the inlet was 32 cm (an equivalent of 5.8 l volume) although sponges could reach 87 cm in length (36 l volume). The average growth rate of sponges measured over the course of 3 years was 1.98 cm year −1 (min. −0.76 cm year −1, max. 5.7 cm year −1) or 167 ml year −1 (min. −537 ml year −1, max. 856 ml year −1). The rate of tissue regeneration after artificial wounding in the field was 0.05±0.03 cm 2 day −1, some 40 times the rate of growth. No recruitment was observed during the study, but mortality of large individuals was seen. Using the calculated growth rate (average rate of increase in volume), the age of an average-sized sponge was estimated to be 35 years. With the assumption that growth rate is constant, large individuals (1 m in length) were estimated to be 220 years old. All sponges showed seasonal trends in sloughing of the debris-covered outer spicules during winter months (November to February). Increase in outer spicule coat occurred from March to October and sloughing corresponded to the end of seasonal phytoplankton blooms in October or November. These data suggest that hexactinellid sponges have life history strategies and growth rates similar to those of massive tropical and temperate demosponges and that, despite their deep water habitat, they experience seasonality which influences their growth rates and perhaps reproductive period.