Abstract
In this study, the structure of the polyps and the cell types of the hard coral Seriatopora hystrix and the soft coral Xenia unzbellata were compared, together with the composition of the fatty acids in their tissues. S. hystrix displayed an apparent lack of specialized feeding cells, notably the relatively small number of mucous gland cells and the low percentage of venom containing nematocysts. P-mastigophores accounted for 1.52-5.7% and, B-mastigophores for 1.28% of the nematocysts. Conversely there was a high percentage of holotrichs nematocysts (24.86 - 55.5%) in the tentacles and mesenterial filaments respectively. Zooxanthellae were abundant in the gastrodermis. These charateristics suggest that S. hystrix relies essentially upon autotrophic nutrition. The polyps of X. umbellata were devoid of cnidae, and mucous glands were in abundance, particularly in the lower part of the polyp. The presence of particulate matter in the coelenteron, and low number of zooxanthellae indicate that X. umbellata is a suspension feeder, using mucus to trap the particles on the pinnate tentacles.
Differences were also revealed by a comparative study of their chemical composition. X. umbellata had a high protein and lipid content, whilst S. hystrix was characterized by high calcium carbonate content. In S. hystrix, the fatty acids were found to be predominantly saturated fatty acids (87.3%), the most abundant being 16:0 and 18:0. X. umbellata showed a predominance of unsaturated fatty acids (77.7%), the most abundant being 16:1 and 18:1. The differences in morphology and in biochemical composition suggest that S. hystrix has a greater reliance on autotrophic feeding whilst X. umbellata is a more heterotrophic suspension feeder.