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How do lunar cycles influence reproduction in fireworms and grunion?

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Diagram comparing reproductive pathways: arrows show sexual reproduction forming a diploid zygote from oocyte and sperm, haploid parthenogenesis forming a haploid individual, and two types of diploid parthenogenesis-automixis and apomixis-both resulting in diploid individuals. Symbols indicate haploid and diploid stages. — Animal reproductive system The process of sexual reproduction and several forms of parthenogenesis.

Lunar cycles play a critical role in synchronizing the reproductive activities of both fireworms and grunion, ensuring that mating occurs under conditions that favor the survival of their offspring.1

Fireworms

Marine fireworms in the West Indies follow a strict lunar rhythm to coordinate their breeding. During spring and summer months, they emerge from sea floor crevices to spawn at the water's surface. This activity is confined to a three-to-four-day period following the full moon. To ensure they can find one another in the dark, they rely on their own bioluminescence, emerging approximately 30 minutes after sunset—after daylight has faded but before the moon rises—to maximize the visibility of their light signals.1

Grunion

The grunion (Leuresthes tenuis) also utilizes lunar cycles to time its spawning. These fish come ashore to mate in the wash of the waves on sandy beaches during the dark of night, specifically during the high tides associated with a full or new moon (spring tides). By laying their eggs in the sand at this peak tide, the grunion ensures the eggs remain buried and protected until they are reached and inundated by the next set of high spring tides, which occurs approximately two weeks later.21