Long-lived sperm and sustained motility in the crown-of-thorns starfish Acanthaster cf. solaris

Outbreaks of the crown-of-thorns starfish (Acanthaster cf. solaris; COTS) pose serious threats to coral reef ecosystems. Because fertilization success links adult reproductive output to larval production, sperm longevity may influence the reproductive potential of COTS populations. However, the duration for which COTS sperm retain fertilizing capacity after activation in seawater remains poorly quantified. We evaluated sperm fertilizing capacity and motility under controlled laboratory conditions at 28°C. Sperm from five males and eggs from one female were used in fertilization assays with sperm aged 6–20 h after activation; an additional 24-h treatment was tested for two males only. For each treatment, fertilization rate was calculated as the percentage of fertilized eggs, identified by fertilization membrane elevation, among 100 fresh mature eggs examined after excluding immature eggs. Additional 0-h observations showed high fertilization rates (>90% in most trials) but were used only as descriptive reference data. Mean fertilization rates ranged from 53% to 74% during 6–12 h after sperm activation but declined to 4.6% at 20 h (one-sided paired Wilcoxon signed-rank test, V=15, P=0.029). Sperm motility measured using a computer-assisted sperm analysis system was generally low but showed a similar decline by 20 h. These results indicate that COTS sperm can retain fertilizing capacity for at least 12 h after activation under laboratory conditions. Although field sperm are rapidly diluted and transported, prolonged fertilizing capacity may extend the temporal window for successful fertilization.