Timing of sperm shedding and release of aggregates in the salp Thalia democratica (Urochordata: Thaliacea)
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Many holoplanktonic forms, such as medusae, spawn in response to environmental cues such as light or temperature. Few observations of the time of reproduction have been made in salps. Small groups of mature aggregates of the salp Thalia democratica (Forskal, 1775), collected off the Great Barrier Reef of Australia, were held under constant light in early to mid-December in the years 1990, 1993 and 1994. Sperm were shed between 0300 and 0600 hrs each day with a mean time of about 0450 hrs for all individuals for all years combined. Solitary stages released chains of fertilizable female individuals between 0500 and 0700 hrs each day, with a mean time of 0540 hrs. Field observations suggested that similar timing occurred in nature. Advancing the time of dusk from 1900 to 1700 or 1800 hrs. did not significantly alter the mean time of sperm shedding or chain release. T. democratica aggregates, collected in the Mediterranean Sea in 1995 and maintained in constant light, had a broader distribution of individual spawning times, with a mean time of 0625 hrs, which differed significantly from that of the Australian population. Coordination of the events of sexual reproduction in T. democratica may improve fertilization success and be responsible for recognizable age cohorts within blooms of this species.
KeywordsSmall Group Field Observation Sexual Reproduction Similar Timing Great Barrier Reef
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