|Sheltered Bay Environments of Amami-Oshima Island: Wonderful Biodiversity Hidden by Turbid Waters
|Date: 13rd March 2021 13:00 - 17:00
Place: On line (Zoom)
Amami-Oshima Island, Kagoshima, is a popular island for marine activities in Japan, with by coral reefs with transparent water and beautiful tropical fishes. On the other hand, the complex coastline of Amami-Oshima Island forms various sheltered bay environments where the effects of waves and wind are few, and these bays are comparatively easily influenced by terrestrial environments. Recently, many unique marine benthic organisms have been found in these naturally turbid waters in sheltered bay environments around Amami-Oshima Island. For example, the discovery of a new puffer fish that makes an “underwater crop circle” was reported in 2014. Among others, a new hermit crab that lives with a colony of living coral on its back was recently discovered, and various huge zooxanthellate coral assemblages consisting of species adapted to shaded and turbid waters exist, although they have yet to scientifically examined.
This symposium focuses on the sheltered bay environments of Amami-Oshima Island, which are often misunderstood as “dirty”, and the various interesting organisms found in these environments. We hope to reaffirm the values of these unique environment with the symposium participants.
13:00 Opening Address
13:10 「Biodiversity in Sheltered Bays of Amami-Oshima Island, with a Focus on Benthoses and Symbioses in Tidal Flats」
●GOTO Ryutaro (Seto Marine Biological Laboratory, Field Science Education and Research Center, Kyoto University)
Amami-Oshima Island is a subtropical island located in the mid-northern part of the Ryukyu Archipelago and well known for its rich flora and fauna including many endemic taxa. The coastal areas of this island with convoluted coastlines contain many estuaries with different type of tidal-flat habitats, such as fine-sand flats, mud flats, reef flats, seagrass beds, and mangroves. The sedimental bottom of these tidal flats are inhabited by various burrowing benthic animals, such as crustaceans, annelids, and echinoderms. Interestingly, the burrows or body surfaces of these animals are often utilized as habitats by other smaller symbiotic animals (e.g., commensal bivalves, snails, shrimps, and scale worms). Such symbiotic fauna are ones of major components of the under-ground biodiversity of tidal flats, although their diversity remains poorly investigated. In this presentation, I will introduce the diversity of the symbiotic relationships between burrowing animals and their associated fauna in the tidal flats of Amami-Oshima Island, including recent new findings.
14:10 「Diversity of the Benthic Organisms Found from the Sheltered Bay Environments around Amami-Oshima Island and Their Features Adapted to Turbid and/or Unstable Sea Floor」
●FUJII Takuma (International Center for Island Studies, Kagoshima University)
Under water of the sheltered bay, the physically unstable sand or mud seafloor often dominated environments, is easily turbid in nature due to the influence of surrounding forests and rivers. Many new findings on marine biodiversity have been obtained from such environments, which is often perceived as less inhabitable for sessile organisms than the coral reef environment facing the open sea.
It was just a few years ago that a surprising new species of puffer fish which produces a mysterious spawning nest known as a crop circle reported worldwide. Some aggregations of unique free-living coral only habit on sandy and muddy bottoms, such as so called “walking dendro corals” Heteropsammia cochlea or Heterocyathus aequicostatus, can be found as well. Not only the sites where they can be observed in situ by diving are rare in worldwide are somewhat rare, a new species of hermit crab considered to be specific symbiont of the two coral species above has been discovered. In addition, new discoveries of various organisms have been made in the last few years, including the northern limit of algal corals, which are rarely reported in the world, and the first record in Japan. These organisms, which can be said to be unique to the inner bay environment, show a great variety of shapes and ecology.
Some of the creatures newly discovered in the inner bay environment along the coast of Amami-Oshima Island are potentially to be a local resource, such as a famous target of leisure diving. In this presentation, the fascination of these creatures found in the subtidal zone of the inner bay of Amami-Oshima Island will be introduced, and their value and the importance of further research will be discussed.
15:20 「Changes in the Diversity of Corals in Nakagusuku Bay, Okinawa Island, between the 1970s and 2018」
●*REIMER James Davis (Graduate School of Engineering and Science, University of the Ryukyus/Tropical Biosphere Research Center, University of the Ryukyus) and YAMAGIWA Hirotaka (Graduate School of Engineering and Science, University of the Ryukyus) (with collaborators from University of the Ryukyus and Leeds University)
In recent years, coral reef degradation has become a problem around the world due to coral bleaching and disease caused by rising seawater temperatures associated with global warming, coastal development, and water pollution associated with human activities. In Nakagusuku Bay, Okinawa, there are many human disturbances such as landfill, increasing turbidity, and water pollution associated with the development of Nakagusuku Bay and Awase Port area. However, coral diversity surveys in Nakagusuku Bay have not been conducted since 1975-1976.
Therefore, we aimed to clarify how coral diversity has changed over 43 years by conducting a coral diversity survey at the same points as the 1975-76 survey. From June to October 2018, we investigated coral diversity by scuba diving and snorkeling at 16 sites in Nakagusuku Bay. Comparing survey results from 1975-76 and 2018, the composition of corals by colony shape and numbers of colonies showed a decrease in branching corals and increases in encrusting and massive corals. The corals in Nakagusuku Bay appear to have changed by responding to environmental changes. Comparing average water depth of each coral genus between 1975-76 and 2018, genera that were at shallow water depths transitioned to deeper depths, and genera that were at deeper depths transitioned to shallower water depths, indicating ‘reef compression’. Corals are known to transition to deeper depths when exposed to high sea surface temperatures or strong UV radiation, and to shallow waters in search of light for photosynthesis when turbidity increases.
Therefore, in Nakagusuku Bay, environmental changes such as increases in sea surface temperatures and an increase in turbidity appear to have occurred over the past 45 years, dramatically changing the composition of the coral reef ecosystems in the bay. These results can provide important lessons for the future of more pristine inner bay locations in the Ryukyu Archipelago.
17:00 Closing Remarks