Mechanism maintaining dense beds of the sand dollar Scaphechinus mirabilis in northern Japan

The sand dollar Scaphechinus mirabilis, which inhabits the intertidal and subtidal sandy bottom in the Japan Sea and northwest Pacific Ocean near Japan, forms dense beds at some places for several decades. I investigated how this sand dollar maintains long-lasting dense beds by studying the population dynamics over 16 months in a dense bed dominated by adults with a test length of about 40 mm. The density and size structure of the population hardly varied throughout the investigation. The sand dollars reproduced in November, but no juveniles were recruited in the dense bed. In the laboratory, plutei with a well-developed echinus rudiment metamorphosed faster in sand in which an adult was kept for 2 days. This suggests that plutei selectively settle in and around existing habitats. On the other hand, most juveniles died when they were kept for 3 months with 20 adults whose density was equal to that of the dense bed. In contrast, few juveniles died when they were kept with 20 dead tests, or 10 dead tests and 10 adults. Most juveniles died in vessels in which the sand was shaken continuously to simulate bioturbation by adults. I investigated the size structure of the population in 8 habitats. Juveniles were abundant in habitats with few adults, but scarce in habitats with dense adults. This result indicates that the plutei settle in and around existing habitats, but that the bioturbation caused by adults kills most settled juveniles. It seems that the dense beds are maintained by the gradual immigration of juveniles that are able to settle and grow around there, or by the occupancy of juveniles after the mass mortality of adults.