Larval release pattern in Antarctic bryozoans

Most marine bryozoans in temperate shallow waters incubate their larvae and release them following a circadian rhythm regulated by daylight. Antarctic bryozoans have been reported to sexually reproduce mainly in the summer months, which prompts the question whether larval release in such bryozoans follows a similar circadian pattern to those from lower latitudes. The present study aims at answering such a query. In January 1999, a total of 35 species of bryozoans were collected by scuba diving between 15 and 36 m depth at Doumer Island, (Base Yelcho, Instituto Antàrtico Chiteno, at 64°52’24”S, 63°36’00”W) and kept in running seawater. The cycle of larval release was studied for 2-3 days after collection using larval release chambers. The dominant species, Nematoflustra flagellata (Waters 1904) and Himantozoum antarcticum (Calvet 1909), and their associated species, released larvae almost exclusively in daytime when photon flux density was above ~0.5 pE m-2 s-1. Larval release was almost absent from 23 h at night until 05 h in the morning. Therefore, larval release in shallow waters Antarctic bryozoans follows a similar circadian pattern to bryozoans in temperate and tropical waters. Since shallow water Antarctic bryozoans restrict their larval release to summer and autumn, seasons at which sunlight is present, it can be concluded that the process of larval release in shallow water bryozoans is controlled world-wide by the daylight cycle. The adaptive selective advantage of releasing larvae in response to light is discussed. The results of the present study also provide evidence that fecundity is low in Antarctic bryozoans.