Morphological and Adaptational Evolution Caused by Interaction between Marine Invertebrates, and the Effect of Human Activity on It : Mollusks and Bottom Environment in the Shallow Water of Sagami Bay off Hiratsuka

Abstract

Mollusks and bottom sediment were collected by dredging Sagami Bay off Hiratsuka at depths of 6 to 43m. Five hundred forty specimens belonging to 51 species were obtained. All were warm water species, 45 of which are known only around Japan and 6 of which are distributed from Japan to equatorial regions. Two tropical species, Barbarca tenella (Reeve) and Melanella cf. carchedon (Iredale), have not hitherto been reported in Sagami Bay ; all specimens were juvenile and they are presumably unable to survive the low water temperature in winter. The seafloor at 6 to 13m deep appears to be often turbulent and active burrowing mollusks such as Umbonium costatum (Valenciennes), Glossaulax didyma (Roding) and Olivella japonica Pilsbry live there. The bottom sediment at depths of around 20m is mainly composed of very fine- to fine-grained sand, in which Glycymeris imperialis Kuroda, Nitidotellina hokkaidoensis (Habe) and Lyonsia ventricosa Gould are dominant. The sediment also includes pebbles, on which sessile bivalves like Mytilus galloprovincialis Lamark and Barbarca tenella (Reeve) are attached, indicating a low sedimentation rate. The substrate around 40m deep is very fine-grained sand with 24% mud content, in which Episiphon subroctum Joffreys and Cardiomya gouldiana (Hinds) are dominant