Beach sediments in the Ryukyu Archipelago are mainly composed of skeletal fragments and shells produced by calcifying organisms. However, the production age, transport time and depositional age of beach sediments have not yet been fully understood. Here we show the age, transport and depositional process of beach sediments around Sesoko Island (Okinawa, Japan), based on the abrasion grades and radiocarbon (14C) ages of empty tests of Baculogypsina sphaerulata (Foraminifera, known as star sand). Results of abrasion-grade analysis showed that well-preserved tests of B. sphaerulata (a pristine test with most spines remained) became fewer from a reef flat to a beach, where the abraded tests with no spines were found abundantly. Results of 14C dating showed that all B. sphaerulata test ages were after ca. 700cal yr BP (calibrated year before 1950 AD). B. sphaerulata test ages generally became older from a reef flat toward a beach, although relatively old abraded tests were found in reef sediments as well. B. sphaerulata test ages in beach sediments were generally younger in the north side than the central and southern parts. Most 14C ages of coral fragments and mollusk shells in beach sediments were younger (Modern: after 1950 yr AD) than those of B. sphaerulata tests. These results suggest that the production of B. sphaerulata tests increased after ca. 700cal yr BP. This is likely caused by the development of a reef flat and the increase of B. sphaerulata habitats associated with a relative sea-level fall at late Holocene. After being transported from a reef flat to a beach by wave-induced currents within tens to hundreds of years, B. sphaerulata tests were gradually accumulated into the central part of a beach mainly from the north side by nearshore tidal currents. The presence of modern coral fragments brought by strong waves and typhoons suggest that beach progradation continues to the present.
The Coral Reef Preservation and Rehabilitation Project by Okinawa Prefectural Government, Japan, attempted to validate the possibility of large-scale reef restoration with asexually-derived corals and to establish refined methodologies of active coral reef restoration in Okinawa. Coral fragments taken from donor corals raised in donor farms were used. During 3-years of consecutive out-plantings from 2012 to 2014, about 30,000 non-nursery-farmed and nursery-farmed corals were out-planted in degraded reefs off the Onna Villege. Various outcomes were obtained through refinement of the reef restoration technique. The present study shows achievement of scientific research and technique based on the work until May 2017. The average annual growth rates of the 15 coral species out-planted in 2012 and 12 species in 2013 were 3.7cm and 7.5cm GMD, respectively; the rate out-planted has increased to 12.0cm GMD for the 6 selected species in 2014. The survival rate of the 2014 out-planted corals was more than 63% at 26 months after. Proper choice of species with respect to the restoration site, size of fragments, and the processes of nursery farming and out-planting are decisive factors for the success of coral out-planting. Genomic DNA analysis of Acropora tenuis indicated that substantial portions of clonal individuals existed in the farmed donor colonies. However, no clonal colonies were found in A. tenuis collected from neighboring waters, suggesting that the propagation of the species mainly takes place sexually in nature. The methodologies flexibly developed in the project comprises the following: 1) farming of donor corals; 2) finding suitable species and locations for out-planting; 3) genomic DNA analysis of donor colonies; 4) farming of seedling colonies; 5) out-planting design to advance fertilization and genetic diversity; and 6) long-term monitoring. Nearly 2,000 Yen (18 US$) per colony was required for the asexually-derived fragment farming, out-planting, and monitoring.
The blue coral (Heliopora coerulea, Pallas 1766) is a reef-building coral, which is recently been classified as a threatened species. Recent population genetic analyses have found two genetically distinct, cryptic lineages of this coral (types A and B). However, the ecological differences between them, as well as the drivers of this speciation, are still unclear. The aim of this study was to examine the possible differentiation in the reproductive timing of the two lineages, as a plausible cause of speciation. We subjected both formalin- and ethanol-preserved samples of the coral, collected from sympatric H. coerulea populations, for tissue and genetic analyses. Samples were collected every one or two months, from June 2014 to September 2017, around Sekisei Lagoon, southwest Okinawa, Japan. We also conducted field observations of the coral’s reproductive behavior (larval brooding) in mid-June and July. As a result we could observe larval brooding behavior from mid- to late-July in type B; however no such behavior was observed in type A during the observation and sampling period. After examining a total of 468 histological samples, we found that the diameters of the oocytes and spermaries peaked from May to June in type A, whereas in type B they peaked from June to July, after which the spermaries disappeared or diminished. Our study demonstrates that the two H. coerulea lineages, under the same environmental conditions, exhibit different reproductive timings. This gap in reproductive timing, which possibly stemmed from endogenous differences, might have promoted their genetic differentiation and speciation into two distinct lineages.