Our knowledge of the reproductive biology and ecology of most deep-sea animals is very limited due to the logistic difficulties of deep-sea investigations. In this study, spawning by males and females of the vesicomyid bivalve Calyptogena soyoae and C. okutanii complex were observed in situ over 1 yr using the long-term deep-sea observatory located at cold seeps in Sagami Bay, Japan. Egg-and sperm-spawning events by clam aggregations occurred roughly 90 and 213 times m -2 for 1 yr, respectively, and the local population of the C. soyoae and C. okutanii complex yielded 5.8 × 10 8 eggs m -2 yr -1 into the water column over the seep location studied. Males displayed a 'sprinkle siphon' behavior, waving their siphons left and right to sprinkle sperm into the water. Female spawning by egg release into the water column was always preceded by male spawning and decreasing near-bottom current speeds. Two hypotheses are proposed for cues to stimulate egg release by C. soyoae/okutanii females. First, a 'double trigger' hypothesis requires the presence of sperm in the water column and decreasing current speed before eggs are released. Second, a 'single trigger' hypothesis requires that egg release is induced when compounds in or released with sperm exceed a threshold concentration. These results demonstrate the utility of long-term observatories for studies of deep-sea reproductive biology, particularly for species characterized by rare, episodic spawning events.