Notes on the induced spawning, embryonic and larval development of the window-pane shell, Placuna placenta (Linnaeus, 1758), in the laboratory
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Intragonadal injection of 0.5 ml of a 2-mM serotonin solution or seawater irradiated by ultraviolet (UV) light (925–1395 mW h/l) induced spawning in male and female window-pane shell, Placuna placenta (Linnaeus, 1758). Generally, mature shells spawned 15–30 min after serotonin injection or 30–60 min after exposure to UV light-irradiated seawater. The average number of eggs released per female were 1.57 × 106 and 1.24 × 106, for serotonin and UV light-irradiated seawater, respectively. The method using UV light-irradiated seawater is simpler to perform than serotonin injection into the gonads and can be applied to both individual- and mass-spawning experiments. Spawned eggs averaged 56 ± 5 μm in diameter and fertilization was 100% successful. After fertilization, the polar body formed after 15 min and trochophores were observed actively moving in 325 min. Embryonic development was completed in less than 6 h. Straight-hinged veligers with mean shell length (SL) of 84 ± 18 μm appeared in the cultures 18–20 h after fertilization. Fed daily with microalgae Isoschrysis galbana (Parke), the larvae developed to early umbo on the second day of culture when SL was 103 ± 17 μm, while late-umbonal veligers of SL 145 ± 21 μm appeared on the seventh day. Crawling pediveligers were observed on day 9 when SL reached 205 ± 15 μm. Newly metamorphosed larvae appeared on the 14th day when SL of 238 ± 9 μm was reached. The survival rate at metamorphosis was 13%. The duration of the planktonic larval stages of P. placenta was about 14 d.