Population Dynamics of the Tropical Chiton Stenoplax alata on a Boulder Shore of Ishigaki Island

Stenoplax alata is a lower intertidal to subtidal chiton that is commonly distributed among coral reefs of South East Asia.  The southern Japanese coast is the northern limit of the chiton.  This species is very rare on the coast of the Japanese mainland; however, it is relatively common on the coral reefs of the Ryukyu Islands (Okinawa).  A dense population of S. alata was discovered in a small habitat range on a boulder shore of the Nagura Bay of Ishigaki Island in the western Ryukyus.  The density and size composition of this local population were investigated from September 1997 to December 2000, and the characteristics of the population were analyzed.  The overall mean density of S. alata during the survey was 3.6 / 0.25m².  The mean density on each sampling date increased in the early half of 1998, peaked in late May (8.0 / 0.25 m²), and then decreased gradually. The fluctuation in density showed no obvious trend.  By cohort analysis of the data on size composition, more than two groups were identified.  Newly recruiting individuals were found in September and formed a distinct size group after November.  The increase in density that occurred in early 1998 was attributed to an increase in the 0+ group.  The minimum newly recruiting chiton was 1.7 mm in the fifth valve width, and total body length was about 5 mm.  Therefore the minimum size ones must settle earlier than I could find.  Assuming that each size group was year class, the estimated longevity of the chiton was about 3 years, the post recruitment growth during a half year was 0.2-0.4 mm / month in 5^th valve width, and the growth seemed to stagnate after the fifth valve width exceeded 6 mm.  Because the mobility of the adult S. alata is very limited, as is that of the other chiton species, and the habitat is primarily sediment with few stones, the maintenance of the population depends on a larval supply from its own population and/or other ones.  The local population would probably be lost if some environmental change such as heavy sedimentation were to occur or if the juveniles were to fail to recruit.  Though it is not yet clear how the population is maintained, it can be assumed that recruitment and survivorship of the chiton in this habitat are better than in the others.  To verify this hypothesis, it would be necessary to investigate the reproductive mode of S. alata and compare the recruiting density and survivorship of the juveniles between the habitats with and without the adult chitons.  Further investigation will provide some information for conservation ecology of marine molluscs.

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