Changes
in cavernicolous bivalve assemblages and environments within a submarine cave
in Okinawa Islands during the last 5,000 years
Abstract. Although many
workers have investigated the ecological and evolutionary significances of cryptic
organisms and short-term (seasonal to annuals) fluctuations in environments and
fauna within submarine caves, no studies have examined millennia scale
variations in the organisms and environments. In this study, we analyzed the
sedimentary characters and species composition of bivalves from surface and
cored sediments within the Daidokutsu submarine limestone cave on the fore-reef
slope of Ie Island, off the island
of Okinawa,
Japan.
The sediments in the central and innermost area of the cave consist of
carbonate mud. Their deposition indicates that a still-water environment
prevailed over the past 5,000 years. Analysis of the spatial and temporal
distribution of the bivalves shows that species living in the innermost area of
the cave became dominant over at least the past 5,000 yrs, while species living
near the entrance of the cave declined in abundance. This indicates that the
environmental conditions of the innermost cave area gradually spread to the
entrance of the cave. We believe that this phenomenon is explained by the
spreading nutritional deficiency within Daidokutsu cave.
Key words: cavernicolous
bivalves, environmental changes, Ie Island, submarine cave deposits
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