The role of habitat in determining the distribution of a sponge-red alga symbiosis on a coral reef

At One Tree Reef, populations of the symbiotic association between the sponge, Haliclona cymiformis (Esper, 1794), and the red macroalga, Ceratodictyon spongiosum Zanardini (1878), occur predominantly on the rubble banks inside the northeastern side of the reef crest surrounding One Tree Lagoon, immediately behind the most exposed section of the reef crest. There is only one population in the centre of One Tree Lagoon, where the substratum is sand rather than coral rubble. When clumps of the association were transplanted from the edge to the sandy centre of the lagoon, to areas where it did not occur naturally, some clumps survived for at least 349 days and more than tripled their original biomass, while control clumps left at the rubble bank did not grow noticeably larger. Fusion experiments between individuals collected from different sites showed some histocompatibility, suggesting that all the existing populations of Haliclona/Ceratodictyon may have originated, via fragmentation, from the same parent population. These experiments suggest that the lack of Haliclona/Ceratodictyon in the centre of the lagoon may be due to an inability of fragments or propagules of the association to colonize these sites, because of a lack of solid substrata for attachment, rather than to differences in the physical environment.