MEPS 752:95-116 (2025)  -  DOI: https://doi.org/10.3354/meps14760

ABSTRACT: Despite considerable research interest in various aspects of the biology of the Japanese sea cucumber Apostichopus japonicus, a species of high economic value, its diurnal and seasonal behavior in the field is not fully understood. Using high-resolution (1 min) round-the-clock video recording in parallel with measurements of environmental variables (sea level, temperature, salinity), we studied the behavior of A. japonicus on an artificial reef populated with the bivalves Crenomytilus grayanus and Crassostrea gigas during the breeding season (June-September). A total of 12 adult A. japonicus were placed on the reef and this number did not change during the study period. A significant number (approximately 23 ind. m^-2) of 1 yr old juvenile A. japonicus were found on the reef. The reef was also colonized by other invertebrates such as sea urchins and brittle stars. Adult A. japonicus were predominantly nocturnal, spending 5 times more time outside shelters at night than during daytime. Juveniles were exclusively nocturnal. Extremely high temperatures (up to 28.2°C) and strong temperature variation over approximately 20 d reduced the time spent by adult A. japonicus outside shelters by approximately 30% but did not affect the activity of juveniles, which increased the time spent outside shelters with growth and with seasonal lengthening of the night. Spawning in A. japonicus occurred exclusively at dusk and night and coincided with the spawning of sea urchins, brittle stars and bivalves. Our results may have implications for the development of co-culture with bivalve mollusks and restocking technologies for A. japonicus.

KEY WORDS: Diel rhythms · Holothurian · Light · Seawater temperature · Sheltering behavior · Spawning behavior · Spawning timing · Video recording