Abstract
The corallivorous Crown-of-Thorns Starfish (COTS; Acanthaster cf. solaris) has become a major pest across the Indo-Pacific, primarily due to anthropogenic environmental factors that have increased populations beyond sustainable levels. These effects are costly to both the economy and ecology of coral reefs and require human intervention. A better understanding of endogenous reproductive factors could help establish more effective management strategies. Based on information regarding an endogenous spawn-inducing peptide in COTS, known as the relaxin-like gonad-stimulating peptide (RGP), a version of this peptide (called synthetic Aso-RGP; inspired by the sequence from Acanthaster cf. solaris) was chemically synthesized. In this study, we demonstrate that in vivo intra-coelomic injection of reproductively mature COTS with 1 µM synthetic Aso-RGP (100 µl) induced spawning of males and females at 20 ± 6.8 min and 32.3 ± 17.2 min post-injection, respectively. Typical spawning behaviours were also observed, including seeking high-ground, crowning/arching and upwards curling of arms prior to gamete release, leading to >95 % fertilization rate. Subsequent testing of synthetic Aso-RGP at 1 µM on mature COTS ovarian fragments stimulated 100 % oocyte germinal vesicle breakdown (GVBD), similar to that observed for the positive 1-methyladenine (1 and 10 mM) control, confirming activation of the RGP cascade. Synthetic Aso-RGP was not, however, effective in inducing spawning or triggering GVBD outside of the reproductive season, demonstrating it is seasonally functional. Finally, we showed that synthetic Aso-RGP can also stimulate spawning in the COTS sibling species, Acanthaster brevispinus. In summary, this research has established the bioactivity of the synthetic Aso-RGP, which in turn, may prove critical for the development of a novel control approach to manipulate COTS spawning.