Coral Reef Studies of Japan

Japanese coral reef ecosystems are characterized by high biodiversity. Their inherent beauty attracts many tourists, and they provide various kinds of ecological services such as sustenance, educational opportunities, and coastal protection; however, they are easily impacted by human activities. Japanese reefs range from subtropical reefs at the southern tip of the Ryukyu Archipelago to temperate reefs at Tsushima Island in the far north which marks the limit of coral reef distribution in the Pacific Ocean. Reef development in the region is extensive and diverse, and productive fringing reefs, submerged platforms, and mesophotic reefs have accumulated. A long history research on the coral reef ecosystems in Japan has provided important knowledge on basic sciences and conservation including fundamental aspects of the biology of coral reef organisms but also has broadened our understanding of the functioning and survival of coral reef ecosystems and those occurring at environmental extremities. In this chapter, as an introduction for this book, we synthesize some of the latest multidisciplinary information that is available about the coral reef ecosystems of Japan.

Dissolved organic matter (DOM) constitutes the largest organic matter pool in coral reef waters and is released and utilized by various coral reef organisms. In this chapter, we review the distribution and fluctuation of DOM concentrations in coral reefs around the world, with a special focus on Shiraho Reef, Ishigaki Island, Okinawa, Japan, where DOM fluxes have been studied most intensively since the late 1990s. Then, we review the DOM production rates from specific reef organisms and DOM consumption rates by bacteria. Previous studies have shown that both dissolved organic carbon and nitrogen (DOC and DON, respectively) generally have a higher concentration in most coral reefs than in the surrounding ocean. At Shiraho Reef, the average ratio of the net DOC production to the net primary production on the reef flat was 18%, and the C:N ratio of DOM that was produced on the reef flat was estimated to be 9.3. The abundance of heterotrophic bacteria was also higher in most coral reefs than offshore, which indicates that bacterial growth was enhanced by reef-derived DOM. Some of the DOC that was produced in coral reefs was persistent to bacterial decomposition in the long term, which suggests that coral reef ecosystems export some reef-derived DOM to the ambient ocean, irrespective of the water residence time in the reef.

Coral reefs in Japan are threatened by multiple environmental stresses at both the global and the local scales. Declining water quality in coastal reefs has been reported in the Okinawa region due to red soil runoff, agricultural fertilizers, and antifouling chemicals. Many studies on the stress responses of corals have been conducted by Japanese researchers. For example, metabolic changes due to stresses such as high temperature and chemical discharge have been reported in quantitative terms. Antioxidant enzyme activities and mycosporine-like amino acids have been studied as possible defense mechanisms against environmental stress. Moreover, coral bleaching has been frequently reported and actively studied since the 1980s in Japan. The synergistic effects of multiple stressors have also been studied, with several studies reporting accelerated bleaching under conditions of high seawater temperature and low water quality (high nitrate concentration and high bacterial abundance). The importance of water flow has been extensively studied, and it has been suggested to allow more rapid recovery from bleaching and a higher survival rate. To mitigate environmental stresses on coral reefs, it is important to evaluate risks due not only to global warming but also to local stresses.

Increasing anthropogenic CO₂ emissions cause progressive ocean acidification, reducing the calcium carbonate saturation state and coral reef calcification rate. The future uptake of CO₂ by the world ocean is predicted to reduce seawater pH by 0.3–0.5 units over the next few decades, which corresponds to a rate 100 times faster than that observed at any time during the last 20 million years. In this chapter, we discuss the effects of ocean acidification on coral reefs, which have been initially probed by culture experiments at several decreased pH conditions, being subsequently investigated by multiple stress factor experiments and field observations of acidified sites. By considering previous studies, we propose that the evaluation and prediction of future ecosystem dynamics require the development of convenient and inexpensive carbonate chemistry-related field measurement techniques such as pH logging, additionally highlighting the importance of studying two naturally acidified sites in Japan, namely, the Iwotorishima and Shikine Islands.

Corals deposit calcium carbonate (CaCO₃ : aragonite ) skeletons with paired high- and low-density bands that reflect past sea surface conditions during their growth. In addition, the elemental and isotopic compositions of a coral skeleton are affected by environmental factors during deposition, such as sea surface temperature (SST), sea surface salinity (SSS), nutrients, turbidity, pH, and dissolved inorganic carbon (DIC). Therefore, physical and chemical analyses of coral skeletons can provide a record of paleoclimatic and oceanographic changes and coral physiological responses. Because the southwestern part of Japan is at the northern habitable limit of reef-building corals, Japanese corals have distinct biological characteristics, and the area’s climatic and oceanographic setting is unique (e.g., East Asian Monsoon, Pacific Decadal Oscillation, and Kuroshio Current). Studying corals in marginal regions can facilitate understanding of the mechanisms of climatic and oceanographic changes and coral physiological responses and contribute to predicting future changes. This chapter will introduce the use of physical and chemical analyses of coral skeletons as chronological indices, highlight previous coral studies in Japan, and propose future directions for paleoenvironmental research using corals.

In total, 78 genera and 415 species of zooxanthellate scleractinian corals have been recorded in Japanese waters. The waters of Japan can be broadly divided into three latitudinal regions: the coral reef region (24–30°N, coral reefs with high coral species diversity), the non-coral reef region (30–33°N, coral assemblages with moderate coral species diversity and without coral reef structure), and the peripheral region (33–35°N, undeveloped coral assemblages with low coral species diversity). In this chapter, we review the Japanese and English literatures on coral reproduction in these three regions, focusing upon the timing of spawning and fertilization within and between species. In the cases of Acropora and Montipora, the higher the latitude of a given place, the later the spawning occurs in the year. For example, spawning occurs from May to June in the coral reef region, June to August in the non-coral reef region, and August to September in the peripheral region. The date of spawning seems to be determined in part by water temperature and light intensity that decrease with increasing latitude. Conversely, coral species in the family Merulinidae spawn on similar dates (June to August) in all regions. In addition, several fertilization studies have been performed in Japan to understand the basic mechanisms of fertilization in corals, to identify species boundaries or cryptic species, and to establish efficient seeding techniques. Furthermore, hybridization studies have been conducted to clarify the basis for the high level of Acropora species diversity. Summary of these studies in Japan will contribute largely to understanding coral reproduction on other region in the world.

Understanding how coral populations are established and maintained is important to predict how coral reef ecosystems will respond to future conditions. Population genetic analyses using DNA markers have provided useful information on how coral populations are maintained. In this chapter, I briefly introduce the history of using DNA markers in the population genetic analyses of corals. I also explain the merit of population genetic analyses to delineate the species boundaries of corals and infer how reproductive characteristics contribute to connectivity among populations. Based on previous studies on population genetic analyses of corals, I also discuss how population genetic analyses have contributed toward understanding the patterns of connectivity among coral populations and geographic variations in genetic diversity, primarily focusing on examples along the Ryukyu Archipelago. Finally, I propose future directions for the population genetics of corals in Japan, taking several aspects into consideration, including methodological information, such as seascape genetics and the development of novel molecular markers for utilizing next-generation sequencing technologies.

The present chapter reviews the distribution and population outbreak records of the crown-of-thorns starfish, Acanthaster planci sensu lato, in Japan from 1912 to 2015. The literature survey suggests that A. planci sensu lato distribution has been extending northward since 1945 from Amami Oshima (its previous northernmost distribution) to Miyake Island and Goto Island. Genetic homogeneity within Japanese A. planci sensu lato populations indicates that larval dispersal has likely caused this poleward migration. Water temperatures have significantly increased in the temperate area of Japan, implying that global warming is partly responsible for this poleward migration. More frequent and intense population outbreaks in temperate areas were also observed, possibly in relation to increased water temperatures and successive larval dispersal from the south. Overall, complex and persistent patterns were observed for two major successive population outbreaks in Japan: from 1969 to 1991 and from 1995 to now. The evidence suggests that the western Okinawa populations are the most likely origin for secondary outbreaks within Japan. The Amami population is also likely to be an important source for outbreaks in temperate regions. However, no records of population outbreaks were found for least in two regions: Ogasawara and the Osumi Islands. Ogasawara is located approximately 1000 km south of the Kuroshio Current, so infestation via larval dispersal from other populations is more limited than in other Kuroshio regions. The Osumi Islands are, however, located in the middle of the Kuroshio Current, implying that insufficient corals are available for the growth of A. planci sensu lato or that unknown environment factors such as abundant predators of juvenile starfish suppress recruitment and juvenile survival.

In the last years, mesophotic coral ecosystems (MCEs) received increased attention from the research community. MCEs exist at depths between 30–40 m and until 100 m or deeper. While MCEs were already reported from Japan nearly 50 years ago, mesophotic research in Japan did not progress as in other countries like the USA or Australia. Nonetheless, over the years, several interesting studies were conducted on the MCEs found in southern Japan on various fields from geology to biology, and the momentum of Japanese mesophotic research increased in the recent years. In this chapter, we will summarize the knowledge on MCEs in Japan in different domains such as their distribution, their biodiversity, their occurrence in the fossil record, recent findings on the reproduction of mesophotic corals, their potential ecological role in a global change context, threats menacing these ecosystems, and future directions of MCE research in Japan.

Tropical cyclones are extreme geodynamic events that cause serious damage to coral reefs and coral communities, including mechanical destruction of reef builders and changes in reef topography. This chapter reviews hydrodynamic research into the effects of tropical cyclones on coral reefs and coral communities of Japan. Although much research has focused on the effects of changing wind speeds, hydrodynamic parameters such as wave height, wave period, and wave velocity also have a direct influence on reef evolution. More data related to wave activity must be collected if we are to develop a better understanding of the hydrodynamic impacts of tropical cyclones on coral reefs. In Japan, an observation network that records wave height and wave period has been maintained since the 1970s because the region is especially prone to tropical cyclones. In addition, the possible hydrodynamic impacts of tropical cyclones on coral reefs are taken into account assuming that these storms will become more intense in the future.

In this chapter, we discuss the future direction of coral reef studies of Japan to facilitate multidisciplinary approaches for understanding various aspects of coral reefs. In particular, we emphasize that the scientific knowledge of coral reefs of Japan would be applicable to practices for conservation and restoration of coral reef ecosystems in the world.