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キーワード:
Porifera; Pione lampa; taxonomy; reproduction; gemmules; buds; worm reef
要旨:
A bioeroding sponge was found in a sabellariid worm reef in Florida, USA. The sponge was investigated with standard techniques for species identification (tissue digestion, microscopy techniques, examination of spicules) and was identified as the clionid Pione lampa (De Laubenfels, 1950). This species is extremely similar to Pione vastifica (Hancock, 1849). Morphological and ecological similarities and possible dissimilarities are discussed. Arguments for conspecifity were found to be weaker than evidence of P. lampa and P. vastifica being separate species. The sampled sponge contained asexual reproductive elements: superficial buds and internal gemmules. Buds were very delicate, consisting mainly of a radially- oriented mesh of collageneous threads. They further comprised abundant granular material and acanthose microrhabds. Numerous choanosomal gemmules were found in the sponge body attached to erosion chamber walls. They were subspherical to lenticular and had an aspicular, unstructured, smooth and rigid coat. It enclosed dense cell material and various spicule types. Whereas buds are interpreted to function as dispersal elements, gemmules will primarily ensure survival under adverse conditions such as smothering, exposure to air and high temperatures. Gemmules from this site might occasionally be freed and scattered, since the Florida reef can suffer heavy damage during periods of increased wave activity such as that created during hurricane season. Bioerosion activity of the sponge increases the chance to free gemmules, as the sponge not only etches into calcareous particles cemented into the matrix produced by the worms, but also into the matrix itself. This ability enables the sponge to utilise the reef as substrate. Within the Clionidae, Cliona annulifera and three species of the genus Pione are the only species known to produce typical gemmules. Possible reasons are: (1) reproduction of bioeroding sponges is understudied and asexual bodies may have been overlooked in other species, and (2) clionid gemmules are an adaptation to survive life in risky environments. Sponges of the genus Pione are comparatively successful in environments in which they are close to their physical limits or in potentially unstable or mobile substrates.
