Reproductive morphology and biometry of the non-zooxanthellate papillose cup coral Paracyathus pulchellus (Scleractinia: Caryophylliidae)

Basic information on the reproductive biology of many scleractinian corals species is limited or entirely lacking, particularly from temperate zones, though it is essential for a better understanding of their ecology. This study describes the morphological aspects of gametogenesis and biometric parameters of the papillose cup coral Paracyathus pulchellus collected at Palinuro (Italy, Southern Tyrrhenian Sea), filling a knowledge gap about the reproductive biology and growth of a widespread Mediterranean and Northern Atlantic coral, which is currently labelled as Data Deficient by IUCN. Samples of P. pulchellus were collected by SCUBA diving between 5- and 10-meters depth during 18 monthly collections from June 2010 to December 2011. Polyps have been analyzed through histological techniques. All 57 polyps displayed either oocytes or spermaries, indicating that P. pulchellus was gonochoric. Gametogenesis began with undifferentiated germ cells arose in the gastrodermis that migrated towards the mesoglea of the mesentery where they completed the development. During spermatogenesis, spermary diameter increased from 25 to 83 μm. Oocyte diameter ranged from 9 to 146 μm and during oogenesis the nucleus/cytoplasm ratio decreased due to the accumulation of yolk. The nucleus migrated to the periphery of the oocyte adhering closely to the cell membrane. No embryo was observed in the coelenteric cavity of the polyps, suggesting a broadcast spawning reproductive mode with a possible planktotrophic larval development due to the small-sized mature oocytes. The analysis of the main biometric parameters (polyp width, height, dry skeletal mass, volume, surface/volume ratio and bulk skeletal density) showed a negative correlation between size and skeletal density, and no sexual dimorphism.