Seasonality and lunar periodicity in the sexual reproduction of the coral-killing sponge, Terpios hoshinota

Abstract

Sexual reproduction of the cyanobacteriosponge, Terpios hoshinota, was studied at Lyudao (Green Island), Taiwan, from 2011 to 2013 through histological examinations, electron microscopy, and in situ observations of larval release. Histological examinations identified five reproductive structures: oocytes, sperm cysts, cell masses, early-stage embryos, and mature embryos. These reproductive structures were often observed in the same specimens, indicating that T. hoshinota is a hermaphroditic brooder. No cyanobacteria were seen in the gametes, and transmission of symbiotic cyanobacteria from parental tissues to larvae likely occurred during embryogenesis. The cell mass, a loose aggregate of numerous symbiotic cyanobacteria and maternal sponge cells, appeared to be eventually incorporated and constitutes the inner part of pre-hatching larvae, suggesting that the larval type is pseudoblastula instead of parenchymella as previously suggested. A clear lunar periodicity was seen in the reproductive cycle; larvae were released mostly around the full moon and occasionally around the new moon. Reproductive activity declined during months with low temperatures (January–April). The larvae were characterized by negative buoyancy and limited mobility, suggesting a larval dispersal distance on a scale of meters under calm weather. However, long-distance dispersal of larvae could still be possible through occasional strong currents during bad weather, such as typhoons, if the larvae survived during the dispersal period by accessing extra energy from the symbiotic cyanobacteria through their photosynthates and/or consuming the cyanobacteria as a source of nutrients. This study showed that T. hoshinota has high fecundity, with a monthly release of numerous larvae over a long reproductive season. This high fecundity, along with local larval dispersal and rapid post-settlement growth, enables rapid population expansion of T. hoshinota.