Mitsuharu Toba

OBSERVATIONS ON THE MAINTENANCE MECHANISMS OF METAPOPULATIONS, WITH SPECIAL REFERENCE TO THE EARLY REPRODUCTIVE PROCESS OF THE MANILA CLAM RUDITAPES PHILIPPINARUM (ADAMS & REEVE) IN TOKYO BAY

Abstract To obtain basic ecological information on declining Tokyo Bay stocks of the Manila clam Ruditapes philippinarum, life history (including spawning, larval transport and settlement, and growth and survival of benthic clams) was investigated in local habitats, mainly in Sanbanse and Kisarazu. Spawning-stage adult clams were histologically detected from May to October at both sites, with two peak periods in summer and autumn. From May to November, between 13 and 23 larval cohorts appeared annually in each site, even during periods when spawning-stage adults were not detected. Spawning over many months plus larval transport from other habitats may contribute to the continuous appearance of larvae at both sites. This confirms the hypothesis that the clam population is maintained and supported by the reproduction of local populations. High density cohorts of newly settled clams, which later formed adult populations, may originate from larval cohorts spawned during peak spawning periods. A number of summer cohorts of benthic juveniles disappeared within 3–4 mo after settlement, whereas autumn cohorts maintained a relatively high density through the following spring, and subsequently they formed adult populations. Severe mortality resulting from extreme environmental factors may have affected the summer cohorts during summer and autumn. The recent decline of the adult stocks may be related to the unstable recruitment of summer cohorts, which was one of the two major seasonal components contributing to recruitment.

Bedload Transport of Newly-Settled Juveniles of the Manila Clam Ruditapes philippinarum Observed in situ at Banzu Tidal Flat, Tokyo Bay

ABSTRACT There have been numerous attempts to create and/or develop harvestable tidal areas where juveniles of the natural Manila clam Ruditapes philippinarum may be abundantly stocked. To prevent erosion and/or promote the colonization of natural juveniles through improving the physical stability of bottom sediment, it is important to understand the mechanisms that regulate dispersal and recolonization of benthic juveniles. The physical transport of bottom sediment is known to have a substantial effect on the spatial distribution patterns of infaunal bivalves in intertidal soft-bottom habitats. During the summers of 2004 and 2005, we conducted field experiments to identify the physical transport mechanisms for newly-settled postlarval Manila clams until growth to a size of 1 mm shell length at the Banzu tidal flat of Tokyo Bay in Japan. We used sediment traps to collect natural and released hatchery-reared juveniles, in parallel to acquiring measurements of seawater flow and observations of the spatial distribution of newly-settled natural juveniles. There was a sharp increase in the number of clams that were collected in the traps placed on the soft-bottom surface when &tgr;w (wave shear stress) exceeded 0.3 N m-2. Because the &tgr;c (advection shear stress) was far lower than &tgr;w (&tgr;c/&tgr;w =1/60–1/130), the initiation of juvenile transport appeared to depend primarily on wave-generated oscillatory flow. The number of trapped juveniles regressed linearly to the weight of the sediment that was simultaneously collected in the trap (R2 = 0.99, 0.81), which indicated that clams were transported in a similar way to that of sediment grains, despite juveniles and sand particles exhibiting different physical properties (size and specific gravity). Hence bedload transport may have resulted from the biologically induced adhesion of juvenile clams to sediment grains and/or their burrowing behavior. In the release-recovery experiment of marked juveniles, a larger number of clams were recovered from traps that had been placed downstream of the water current from the release point. A denser distribution of the natural Manila clam population settled in mid-July 2004, and subsequently moved several hundreds of meters inshore within a one month period. The concurrent monitoring of bottom flow during a total 4 wk period in the summer of 2004 indicated that &tgr;w frequently exceeded the incipient threshold of bedload transport (assumed to be 0.3 N m-2). Consequently, the bedload movement of Manila clam juveniles in the study area was expected to be initiated at a wave shear stress that was greater than the incipient threshold, and in a downstream direction of the advection current. Because juvenile clams in the summer population appeared to be frequently subjected to hydrodynamic stress, which forces juveniles to move and halt incidentally in the early benthic stages, physical transport is likely to contribute to the changing pattern of juvenile distribution at the Banzu tidal flat of Tokyo Bay in Japan.