Spawning periodicity of the lancelet, Asymmetron lucayanum (Cephalochordata), in Bimini, Bahamas

Abstract

Developmental genetic studies of cephalochordates, which give evolutionary insights into the origin of the vertebrates from the invertebrates, require a supply of embryos and larvae. To date, such studies have been limited to the genus Branchiostoma. My purpose here is to establish a practical method for obtaining developmental stages of Asymmetron, a second and relatively inaccessible cephalochordate genus. Reproductive periodicity of Asymmetron lucayanum was studied for a total of 1626 specimens in 22 collections made between November 2008 and December 2010 in the lagoon at Bimini, Bahamas. Water temperatures at the collection site were also measured. The lancelets were collected by sieve at early afternoon low tides and transported to the laboratory for determination of sex, body length, gonad index, and capacity to spawn (by dark stimulation on the evening of collection). The sex ratio did not differ significantly from 1:1, and the minimum length at sexual maturity was about 10 mm. During spring and autumn, when water temperatures were moderate (mid to high 20s^oC), lancelets placed in the dark would spawn predominantly 1 day before the date of the new moon. In contrast, during the winter and summer, dark stimulation did not induce spawning, regardless of the moon phase. It is likely that spawning is depressed in months when water temperatures are near their annual maxima and minima. The linkage of temperature and moon phase to spawning raises the possibility that cultures of A. lucayanum maintained in the laboratory under appropriate environmental conditions could provide a year-round, on-demand source of cephalochordate embryos and larvae.

Keywords:

• Amphioxus
• lancelet
• spawning
• reproductive cycle

Acknowledgements

I am indebted to Linda Holland for her helpful criticism, to Greg Rouse for photographing specimens of A. lucayanum alive and to Chuck Messing for his unstinting hospitality in Fort Lauderdale, Florida, the jumping-off place for Bimini. At the Bimini Biological Field Station (‘Sharklab’), the following staff members and volunteers helped in the laboratory and the field: Jimiane Ashe, Jim Barley, Amanda Brown, Erika Cironte, Tyler Clavelle, Christina Comfort, Sheri Connolly, Christopher Crooks, Kat Geldhill, Lindsay Graff, Katie Grudecki, Merika Huhn, Grant Johnson, Emily Marcus, Michael Neuhaus, Steve Pollett, Kristyn Rubertus, Samantha Sherman, Kristine Stump, and Sean Williams. Above all, I am indebted to Sharklab's director, Samuel H. ‘Doc’ Gruber, for his support and inspiring enthusiasm for all things marine biological.