Phenological variation in the life histories of amphidromous gobies endemic to the Hawaiian islands

Despite the benefits that can come from coordinated movement, many diadromous fishes have evolved partial migration strategies, where a portion of a population does not migrate. Co-occurring life-history variants might nonetheless gain some advantage by exhibiting congruent phenologies, with key events timed to follow the same cue. Here, we assess this possibility by examining otolith microchemistry and structure to quantify the lunar periodicity of hatching and metamorphosis according to migration strategy in a facultative amphidromous goby endemic to Hawai'i. Our analyses corroborated prior evidence that Awaous stamineus exhibits partial migration and that residential A. stamineus are often proportionately more abundant than the migratory form. Contrary to expectation, we found a significant difference in hatching time between migratory and residential A. stamineus. A higher proportion of A. stamineus migrants hatched at the full moon and metamorphosed at the new moon, whereas residents exhibited the opposite phenology. Comparisons to Sicyopterus stimpsoni intended to provide a broader context revealed that the obligately amphidromous species tends to hatch at the new moon and metamorphose at the full moon, mirroring the phenology of residential A. stamineus. Evidence of convergent lunar phenologies suggests that synchronising life-history events can confer benefits that extend beyond species boundaries. Notably, phenological discordance could be acting as a prezygotic isolating barrier between sympatric life-history variants, pointing to the presence of heretofore unrecognised evolutionary diversity within partially migratory species like A. stamineus.