MEPS 550:147-161 (2016)  -  DOI: https://doi.org/10.3354/meps11678

ABSTRACT: The role of environmental variables in the triggering and synchronization of spawning in natural populations of broadcast invertebrates is not fully understood. Here, we report the results obtained from a fine-scale temporal study of the timing of spawning of the sea urchin Strongylocentrotus intermedius and the associated environmental conditions in the northwestern Sea of Japan. Sea urchins were sampled in August-September of 2011 (3 sites) and 2012 (4 sites) at intervals of 3-12 d (4.2 d on average). Sixteen spawning events were revealed by a significant increase in the number of spawned females and a decrease in the gonad index and/or in the portion of individuals ready for spawning, determined as the percentage of sea urchins releasing gametes after dissection from the gonopores. Depth, temperature, salinity, dissolved oxygen and chl a concentrations were measured by sounding of the water column at the sampling sites daily and at 26 other sites in the bay every 5 d in 2011 and every 3 d in 2012. At one of the sampling sites, environmental parameters were recorded every 15 min by a data logger. No apparent relationships were found between temperature, salinity, dissolved oxygen or tidal activity and the timing of spawning, but there was a significant positive relationship between chl a concentration and the spawning events. Our results strongly suggest that phytoplankton concentration is the main factor for initiation of S. intermedius spawning and that the lunar cycle may serve as an additional factor which is able to affect this process.

KEY WORDS: Broadcast spawning · Echinoderms · Reproductive success · Phytoplankton bloom · Temperature · Upwelling · Downwelling · Moon cycle