Microspathodon chrysurus

Lore & Background

Adult and young adult yellowtail damselfish differ significantly in body color, so much so that researchers once believed they were two distinct species. Adults are brown with a yellow caudal fin, while young adults are violet with blue spots on their back and transparent caudal fins. The spawning cycle starts at sunrise and lasts about one hour after males prepare nests on dead coral surfaces. Eggs are demersal and adhere to the substrate, where males guard and aerate them; hatching occurs the morning of the sixth day of incubation. Reproductive activity is highest during the lunar period between full and new moon, when spring tides occur due to high gravitational pull of the sun and moon.

Reader's Guide

The yellowtail damselfish is notable for its dramatic ontogenetic color shift, which historically caused taxonomic confusion. Its reproductive behavior is tied to lunar cycles, with peak spawning during spring tides. The species exhibits clutch cannibalism, where parental males may consume their brood; this behavior is hypothesized to reallocate time and energy for attracting more mates or to improve the parent's growth or survival. Smaller and younger clutches are more likely to disappear. In terms of feeding, adults graze on algae while juveniles are carnivorous; the species is omnivorous with a gradual dietary shift. Gut content analysis shows young adults feed on animal-like nematocysts and zooxanthellae, while adults selectively eat blue-green algae called cyanophytes. Interspecific competition occurs with Stegastes dorsopunicans and Stegastes planifrons, all consuming the same benthic microalgae. Although M. chrysurus is less competent at defending feeding grounds, it uses its larger size to dominate and obtain food aggressively.

Did You Know?

• Adult and young adult yellowtail damselfish were once thought to be two different species due to stark color differences.
• Clutch cannibalism by parental males has been observed directly and through gut content analysis.
• Young adults primarily feed on animal-like nematocysts and zooxanthellae, while adults prefer blue-green algae.