Annual and Lunar-Related Breeding Rhythmicity in the Acorn Barnacle Tetraclita kuroshioensis (Chan, Tsang & Chu, 2007) in Okinawa, Japan

Abstract Barnacles, a class of marine crustaceans, have adapted to various environments. This study investigated the breeding activity of Tetraclita kuroshioensis, a species prevalent on the coast of Okinawa Island, Japan, with a focus on gonadal development, the retention of eggs and larvae at different levels of attachment sites, and the role of environmental factors in their breeding rhythmicity. Monthly collections of T. kuroshioensis were conducted around the full moon from September 2019 to October 2020. Histological observations showed varying oocyte developmental stages in ovaries collected from September to October 2019 and March to September 2020. Developed testes were obtained from April to October. The breeding season of this species lasts for at least 7 months and is influenced by water temperature. Samples collected in April, August, and October displayed little variation in ovarian development across attachment sites; however, there was a noticeable vertical difference in egg and larval retention, with a preference for the new moon phase in the upper and middle parts of attachment sites in August and October. The findings indicate that T. kuroshioensis larval release exhibits seasonal variation, influenced by the lunar and tidal cycles of their habitat.