Population dynamics of an association between a coral reef sponge and a red macroalga

Abstract

Sponges are often as abundant as corals on tropical coral reefs and many species are symbiotic with algae. These associations may contribute significantly to reef primary productivity. This paper describes the first study ever made of the population biology of any of the known associations between sponges and macroalgae. Populations of the symbiotic association between the sponge, Haliclona cymiformis (Esper), and the red macroalga, Ceratodictyon spongiosum Zanardini, were studied in One Tree Lagoon, southern Great Barrier Reef. The association primarily occurs in areas where the substratum consists of dead coral rubble (rubble banks), at the edge of the lagoon, where it can reach a biomass of up to 270 g wet weight m−2. Fragmentation is the primary mode of reproduction of the Haliclona/Ceratodictyon association at One Tree Reef. Although algal sporangia were frequently encountered during the summer, sperm cysts were infrequently found in the sponge, and oocytes and larvae were never observed. The size-frequency distribution of the clumps at the rubble bank sites was strongly skewed toward small individuals (longest branch length generally ≤80 mm). Haliclona/Ceratodictyon was also abundant at one site in the centre of the lagoon; at this site, which was not affected by strong currents or wave energy, the size-frequency distribution of the association was normal. Populations of the association at the rubble bank sites are very mobile, with intact clumps moved more than 30 cm per day under calm conditions and up to 1 m per day during rough weather. Survivorship of fragments or mobile clumps of the association is very high. At one site, the biomass of the association was not affected by two cyclones and at least one severe storm. At another site, up to 50% of the biomass of the population was lost following each of these storms, but the recovery of the biomass was largely complete within 9 months. The Haliclona/Ceratodictyon association has a rapid growth rate, of 8 mg (g wet weight)−1 day−1, which exceeds that of many other coral reef sponges, but it is within the range of growth rates reported for other species of marine macroalgae.