Caballes CF and Pratchett MS (2017), Environmental and biological cues for spawning ...

ECB-ART-45388

PLoS One 2017 Mar 28;123:e0173964. doi: 10.1371/journal.pone.0173964.

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Environmental and biological cues for spawning in the crown-of-thorns starfish.

Caballes CF , Pratchett MS .

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Sporadic outbreaks of the coral-eating crown-of-thorns starfish are likely to be due, at least in part, to spatial and temporal variation in reproductive and settlement success. For gonochoric and broadcast spawning species such as crown-of-thorns starfish, spawning synchrony is fundamental for achieving high rates of fertilization. Highly synchronized gamete release within and among distinct populations is typically the result of the entrainment of neurohormonal endogenous rhythms by cues from the environment. In this study, we conducted multiple spawning assays to test the effects of temperature change, reduced salinity and nutrient enrichment of seawater, phytoplankton, gametes (sperm and eggs), and the combined effect of sperm and phytoplankton on the likelihood of spawning in male and female crown-of-thorns starfish. We also investigated sex-specific responses to each of these potential spawning cues. We found that (1) abrupt temperature change (an increase of 4°C) induced spawning in males, but less so in females; (2) males often spawned in response to the presence of phytoplankton, but none of the females spawned in response to these cues; (3) the presence of sperm in the water column induced males and females to spawn, although additive and synergistic effects of sperm and phytoplankton were not significant; and (4) males are more sensitive to the spawning cues tested and most likely spawn prior to females. We propose that environmental cues act as spawning ''inducers'' by causing the release of hormones (gonad stimulating substance) in sensitive males, while biological cues (pheromones) from released sperm, in turn, act as spawning ''synchronizers'' by triggering a hormonal cascade resulting in gamete shedding by conspecifics. Given the immediate temporal linkage between the timing of spawning and fertilization events, variability in the extent and synchronicity of gamete release will significantly influence reproductive success and may account for fluctuations in the abundance of crown-of-thorns starfish.

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Genes referenced: LOC100893907 LOC585694 LOC590297

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	Fig 1. Proportion of starfish that spawned in response to cues.(a) seawater temperature, (b) water quality, (c) phytoplankton, (d) conspecific gametes, (e) sperm and phytoplankton. FSW = 0.2-μm filtered seawater; LS-FSW = low salinity filtered seawater; NE-FSW = nutrient-enriched filtered seawater; PP = combination of three phytoplankton species. Bars traversing the dashed lines represent spawning of more than 50% of individuals exposed to a given treatment.
	Fig 2. Response time and cumulative probability of spawning in male and female crown-of-thorns starfish after exposure to environmental and biological cues.(a) seawater temperature, (b) water quality, (c) phytoplankton, (d) conspecific gametes, (e) sperm and phytoplankton. Solid circles are individual spawning events and different letters indicate significant differences based on multiple comparisons (Holm-Šídák) after Log-rank analyses. FSW = 0.2-μm filtered seawater; LS-FSW = low salinity filtered seawater; NE-FSW = nutrient-enriched filtered seawater; PP = combination of three phytoplankton species.
	Fig 3. Schematic diagram of proposed cascade model for spawning induction and synchrony in response to environmental and biological cues.Grey arrows are responses to environmental cues and black arrows refer to biological cues. Neurohormonal mechanisms are based on Giese and Kanatani [14] and Mita et al. [15]. GSS = gonad-stimulating substance (relaxin-like gonad stimulating peptide); MIH = maturation-inducing hormone; MPF = maturation-promoting factor; FEBD = follicular envelop breakdown; GVBD = germinal vesicle breakdown.

References [+] :

Babcock, Synchronous multispecific spawning on coral reefs: potential for hybridization and roles of gamete recognition. 1995, Pubmed