Population biology and competitive interactions in subtidal sponge dominated communities of temperate waters

Abstract

The demographic characteristics of subtidal sponge populations were investigated and the characteristics of two basic groups of sponges; those with spreading or encrusting growth form and those with discrete or massive growth forms, was compared. Two encrusting communities dominated by sponges and subject to different intensities of disturbance were studied. The community subject to the highest intensity of disturbance was dominated by encrusting species which covered almost half of the available substrate. The community subject to low levels of disturbance was dominated by discrete species which although more numerous than the encrusting sponges, covered very little of the substratum. Encrusting sponges were hermaphroditic and produced viviparous larvae which settled in 1-4 days after release. Discrete sponges had separate sexes, were apparently oviparous and larvae settled in one to two weeks after release. Some of these discrete species produced buds during the periods of sexual reproduction. Larvae and buds had low powers of dispersal and often settled around the parent sponge. In both sponge groups recruitment was unpredictable and generally low. The only exception to this was one discrete species which produced buds throughout the year. Larvae settled on all surfaces but successful recruitment only occurred on natural surfaces. Encrusting sponges grew at higher rates than discrete sponges and were able to regain space lost through disturbance by a rapid repair mechanism. A few individuals of some discrete species increased in size slightly but in general no measurable rate of growth, even in small sponges, was obtained over the period of study. When damaged, discrete sponges simply repair surface tissues. The initial mortality of newly settled larvae was high, but once recruited the young sponge had a high potential survival rate. Predators very rarely removed established sponges. The species eaten did not relate to toxicity of the sponges. In areas where disturbance was more intense encrusting sponges occupied a greater percentage of the surface than discrete sponges. Discrete sponges cannot cope as well with disturbance as encrusting forms and the abundance of discrete species in the area of low disturbance compared to other habitats where these species also occur, may be due to the peculiar protection offered by the sediment layer in this area from major predators. In the encrusting sponge community spatial patterns were influenced by competition between sponges. The large species were actively aggressive to other sponges in obtaining space. The smaller species usually formed a 'standoff' with other species. Competition may have some small influence among the discrete sponges but this has not been rigorously demonstrated.