Spawning phenology and larval dispersal potential of the razor clam Solen gordonis in Sasebo Bay and adjacent waters, northwestern Kyushu, Japan

Abstract

Timing of larval release is a key determinant of larval dispersal in coastal benthic invertebrates. This study aimed to characterize the spatial variation in spawning phenology and to evaluate its potential consequences for population connectivity in the razor clam Solen gordonis, a species that exhibits a single annual mass spawning event restricted to a short period. Contrary to this aim, our monitoring results indicate that the timing of mass spawning was largely synchronized across sites. Mass spawning in S. gordonis could be triggered by a sharp temperature drop rather than by reaching a specific temperature threshold, resulting in interannual variation in the water temperature experienced by larvae. Larval rearing experiments conducted at 25°C, 20°C, and 15°C revealed that larval growth and survival in S. gordonis varied with water temperature within the range experienced during the spawning period and that S. gordonis has a comparatively short planktonic larval duration (PLD; 3.50 days after hatching in the 25°C tank, 6.25 days in the 20°C tank, and 15.88 days in the 15°C tank). Consequently, larval dispersal potential might vary among years due to temperature-dependent larval growth and survival. These results highlight that a robust assessment of population connectivity requires accounting for environmental variability across sites and years, as well as the phenological traits of focal populations, especially in species like S. gordonis with a short PLD and highly synchronized, seasonally restricted spawning.