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 o C), 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.

Evolutionary crossroads in developmental biology: amphioxus

“…Nonbiotic variables that could explain these differences, other than the global water temperature, which delimits the spawning season, or day length, have not been found, except for A. lucayanum (Holland, 2011). Individuals of this species seem to spawn predominantly 1 day before the new moon (Holland, 2011). Finally, in all amphioxus species, once the gametes are released and fecundation occurs, embryos remain planktonic (see Glossary, Box 1) until metamorphosis.…”

“…The spawning behaviour of different amphioxus species also varies, from a few contiguous spawning days per year for B. belcheri (Wu et al, 1994), to approximately weekly spawning during the breeding season for B. floridae (Stokes and Holland, 1996). Nonbiotic variables that could explain these differences, other than the global water temperature, which delimits the spawning season, or day length, have not been found, except for A. lucayanum (Holland, 2011). Individuals of this species seem to spawn predominantly 1 day before the new moon (Holland, 2011).…”

“…Thus, the spawning seasons of B. belcheri and B. lanceolatum span 2-3 months in spring (Fuentes et al, 2007;Fuentes et al, 2004;Wu et al, 1994), whereas B. floridae spawns from early May to September (Stokes and Holland, 1996). This 'single spawning season' trait is not a general behaviour; A. lucayanum spawns during two separate 3-month periods during the year (one in spring and one in autumn) (Holland, 2011). The spawning behaviour of different amphioxus species also varies, from a few contiguous spawning days per year for B. belcheri (Wu et al, 1994), to approximately weekly spawning during the breeding season for B. floridae (Stokes and Holland, 1996).…”

“…lucayanum ( Fig. 2C) embryos have been obtained recently from Bimini in The Bahamas (Holland, 2011;Holland and Holland, 2010); however, a spawning induction method has not yet been developed for this species. Embryos are obtained during spawning seasons by keeping ripe adults in a dish that is continuously exposed to artificial light during the late afternoon of collection, and then placing them in the dark at 21:00 h. Although the use of A. lucayanum as a model for evolutionary developmental biology DEVELOPMENT 4822 studies is currently limited, its life cycle, with two spawning seasons per year, and its divergence from the Branchiostoma genus will probably make it an invaluable animal model in the future.…”

Amphioxus photoreceptors - insights into the evolution of vertebrate opsins, vision and circadian rhythmicity

“…All amphioxus genomes examined so far contain a single melanopsin gene, an orthologue of the r-type opsin (melanopsin) expressed in vertebrates in intrinsically photosensitive Retinal Ganglion Cells (ipRGCs) (Provencio et al, 1998). The rest of the putative r-type amphioxus opsins (six opsins) cluster together to form a specific so called "Amphiop6" group (Koyanagi et al, 2002) Holland and Yu, 2004Fuentes et al, 2007Li et al, 2013Holland, 2011 Spawning one day before the new moon b Holland, 2011 TABLE 1…”