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The role of hybridization through multi-specific synchronous spawning in the evolution of reef-building corals has been discussed since the 1990s, particularly for the genus Acropora. However, F1 hybrids have been reported as common in only one case in the Caribbean, with no evidence of mechanisms that would allow continuous reproduction of the hybrids. In this study, we report for the first time the fecundity of two F1 hybrid colonies produced experimentally from two Indo-Pacific species, A. intermedia and A. florida. These F1 hybrids spawned at the same time as the parental corals. Backcrossing and F1 hybrid crossing were successful in both directions. Furthermore, more than 90% self-fertilization was achieved in an F1 hybrid, although it was negligible in the parental corals. While it is possible that the F1 hybrid was a chimera, these results suggest that some products of interspecific hybridization may persist as the offspring of self-fertilizing F1 hybrids.

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.

Background The coral genus Acropora contains more than 150 species with very high morphological diversity. This high diversity may have been caused by repeated hybridization via mass spawning. However, we have little information whether hybrids are formed in these corals. Identifying morphological differences between hybrids and their parental species would provide an opportunity to find wild hybrids in the field and to understand how colony shapes of Acropora have become highly diversified throughout evolutionary history. In the two morphologically distinctive coral species Acropora florida and A. intermedia in the Indo-Pacific, their gametes show high rates of bi-directional intercrossing in vitro, and thus these two species are ideal species to investigate the morphological traits of the hybrids. Methods We examined morphological characters of F1 hybrids from A. florida to A. intermedia, which were produced from in vitro crossing experiments. To compare morphological differences, we grew juveniles and mature colonies of reciprocal F1 hybrids (FLOint: A. florida eggs × A. intermedia sperm, and INTflo: A. intermedia eggs × A. florida sperm) and of the parental species (purebreds of A. intermedia and A. florida). We analyzed skeletal morphology such as colony size, branch length, and branching number, and compared them with those of a putative F1 hybrid between A. florida and A. intermedia found in the field. We also confirmed the molecular phylogenetic position of F1 hybrids, parental species, and a putative F1 hybrid using the mitochondrial non-coding region. Results Our morphological analysis revealed that branching number of the F1 hybrids was intermediate relative to the parental species. Moreover, the FLOint hybrids were morphologically more closely related to the maternal species A. florida, and the INTflo hybrids were to A. intermedia. Molecular data showed that A. florida and A. intermedia were clearly divided into two clades, and that F1 hybrids grouped in the clade based on their maternal parent. A very similar pattern to the INTflo hybrids was obtained for the putative F1 hybrid in nature. Discussion Our results revealed that F1 hybrids between two Indo-Pacific species A. florida and A. intermedia had intermediate morphology relative to their parent species but reflected the maternal parent more. Similarity to maternal species in hybrids is opposite to the Caribbean Acropora species that had more paternal morphological characters in hybrids. These results further suggest that some genetic factor in eggs is likely to affect determination of colony shape in the Indo-Pacific. At present, we have considered colonies with intermediate morphs between different species to be intra-specific morphological variation, but they may be real F1 hybrids. Indeed, a putative F1 hybrid represented similar morphological and molecular features to the F1 hybrids, and thus it is plausible to be attributed as a “real” F1 hybrid in nature.

Reproductive success via sexual reproduction is a key factor in restoring and maintaining populations of branching corals. It is crucial to consider the relationship between genetic similarity and reproductive success when restoring branching acroporids because these corals have characteristics such as fragmentation and localized recruitment. In this study, we examined the relationship between genetic similarity (as measured by amplified fragment length polymorphisms) and reproductive success of Acropora intermedia in 2009 and 2010, at Okinawa, southern Japan. There was a significant negative relationship between fertilization rate and genetic similarity. However, there was no significant relationship between genetic similarity and planula survivorship. Based on these findings and the characteristics of A. intermedia, asexual reproduction may decrease genetic diversity and promote inbreeding in the population.

Favites flexuosa is one of the abundant coral species in the temperate regions of Japan. Although the species ranges widely, from tropical to temperate regions of the Indo-Pacific, colonies of this species living in temperate areas exhibit specific morphological features (in terms of color and skeletal attributes). In addition, the temperate-specific morphotype is limited in distribution to the temperate regions of Japan and has not been recorded in subtropical Okinawa. In the present study, we recorded the temperate morphotype in subtropical Lyudao (Green Island), Taiwan, suggesting that the morphotype is more widely distributed than previously thought and may have colonized temperate Japan directly from Lyudao. We describe the first records of spawning at Lyudao, Taiwan, and also at Wakayama in temperate Japan. In laboratories located in either region, spawning was observed between 30 min and 1 h after sunset. Intraspecific crossing of morphotype samples from either region revealed that the fertilizatio...

The distribution of sea anemones on the rocky intertidal zone at 12 stations in Mutsu Bay and at 7 stations at a 5-km scale was investigated. Seven species of sea anemone were noted at sampling stations: Anthopleura japonica, A. fuscoviridis, A. pacifica, A. atodai, Cnidopus japonicus, Haliplanella lineata, and Actinia equina. In Anthopleura, the appearance frequencies of A. japonica (spawner) were higher than those of other anemones that occurred at each station, while A. atodai (brooder) showed a limited distribution. C. japonicus, C. japonicus, and A. equina were very rare. Sea anemones and sympatric benthic organisms occurred at the same stations, and no significant relationship between the number of sea anemones and the coverages of mussels which might be strong competitors for space was found. We show that the reproductive mode and sympatric organisms of sea anemones may be included among the factors that determine the distribution of sea anemones in Mutsu Bay, Japan. However, we need to further examine the environmental factors in this area, as well as intra- and interspecific relationships among anemones.

We investigated the seasonal prevalence of reproductive activities and of the development of brooded propagules in an intertidal sea anemone, Anthopleura sp., on the rocky shore of Mutsu Bay, in northern Japan. A monthly examination of anemones, by dissection and histological techniques, revealed no sign of gonad development, but did reveal that they produce and internally brood propagules throughout the year. Release of propagules was observed in the field. This anemone population appears to be entirely asexual and agametic, and may persist solely through clonal propagation.

Coral populations are in decline due to environmental changes and biological attacks by predators and infectious diseases. Here, we report a localized bloom of the benthic filamentous cyanobacterium Moorea bouillonii (formerly Lyngbya bouillonii) observed exclusively on the gorgonian (sea fan) coral Annella reticulata at around 20 m depth in Japan. The degree of infection has reached 26% among different sizes of Annella colonies. Thick and continuous growth of Moorea may be sustained partly by symbiotic alpheid shrimp, which affix Moorea filaments to gorgonian corals for use as food and shelter. Most filaments get entangled on the coral colony, some penetrate into the stem of the coral with a swollen end like a root hair, which appears to function as an anchor in Annella. In addition to the cyanobacterium–shrimp interaction, the new trait of anchoring by the cyanobacterium into gorgonian coral may contribute to persistence of this bloom.

Dispersal of propagules plays an important role in the distribution of corals. Pocillopora damicornis, Seriatopora hystrix, and Stylophora pistillata are all brooders and release planulae having symbiotic zooxanthellae. Planulae showed a great size variation, especially at peaks of planulation, and we found negative correlations between zooxanthella density and planula size in S. hystrix and S. pistillata. Studies of the larval life of planulae under both light and dark conditions have revealed that larger planulae have a longer lifetime. When planulae of the same size were compared, it was found that they lived longer under light conditions than under dark conditions. These findings suggest that planulae utilize energy from photosynthetic products of zooxanthellae and that these corals enjoy long-distance dispersal by producing larger planulae with greater dispersal potential. It is conceivable that variation in the dispersal potential of planulae is a means of adaptation by which planulae can increase their chances of finding a suitable habitat.

Most corals undergo spawning after a particular moon phase, but how moon-related spawning is endogenously regulated in corals remains unknown. The objective of the present study was to evaluate whether dopamine (DA) affects spawning in Acropora tenuis. When pieces of four A. tenuis colonies were reared under a natural photoperiod and water temperature, spawning was observed after the predicted moon phase. After exposure to water containing DA at 0.1 μM, pieces of the same colonies only released 5 to 10 bundles. Co-treatment with DA and pimozide (D1 and D2 receptors antagonist), but not domperidone (D2 receptor antagonist), induced mass release of bundles from the colonies. A cross-experiment revealed high fertilization rates between the control colonies (95%) and between the control and DA-treated colonies (90%), suggesting that gametes developed normally in coral tissue. Therefore, DA appears to have an inhibitory effect on the spawning of A. tenuis.

Multi-specific synchronous spawning risks both sperm limitation, which reduces fertilization success, and hybridization with other species. If available sperm of conspecifics are limited, hybridization with heterospecific sperm could be an alternative. Some species of the reef-building coral Acropora produce hybrid offspring in vitro , and therefore hybridization between such species does sometimes occur in nature. Here, we report that the interbreeding species Acropora florida and A. intermedia preferentially bred with conspecifics at optimal gamete concentrations (10 6 cells ml −1 ), but when sperm concentration was low (10 4 cells ml −1 ), A . florida eggs displayed an increased incidence of fertilization by sperm of A . intermedia . However, A . intermedia eggs never crossed with heterospecific sperm, regardless of gamete concentrations. It appears that A . florida eggs conditionally hybridize with heterospecific sperm; in nature, this would allow A . florida to cross with later-spawning species such as A . intermedia . These results indicate that hybridization between some Acropora species could occur in nature according to the number of available sperm, and the choice of heterospecific sperm for fertilization could be one of the fertilization strategies in the sperm-limited condition.

We report the isolation and characterization of eight microsatellite loci from the scleractinian coral, Acropora nobilis. The microsatellite loci were obtained using compound SSR primers or an enrichment protocol. All the loci were polymorphic with four to eight alleles per locus and observed heterozygosities ranging from 0.22 to 0.76. Some of the primers developed for the two congeners, Acropora palmata and Acropora millepora were applicable to A. nobilis. These loci are useful for studying the connectivity among A. nobilis populations in Okinawa, southern Japan.

Worldwide decline has been observed in coral populations due to environmental changes at both regional and global levels. Since corals have a typical metapopulation structure, the recovery of severely disturbed local populations depends on larval recruitment from outside the disturbed region. Therefore, the connectivity among local populations may play a key role in the disturbance/ recovery dynamics of coral populations. On the basis of the simulation of surface currents, derived from ocean currents, we hypothesized that the genetic connectivity of coral populations is weak between the northern and southern regions of Sekisei Reef, Japan. To test this hypothesis, the connectivity of the population of the broadcast-spawning coral Acropora digitifera, one of the dominant corals in the Sekisei Reef, was analyzed at six sampling sites that were 5-25 km apart by using six microsatellite markers. The degree of genetic differentiation of A. digitifera was very low among all the sites (F(ST) from -0.006 to 0.015), and no evidence was found in support of our hypothesis. The relative lack of genetic differentiation of A. digitifera among the sites may be attributable to the large effect of wind-driven surface currents, which highly vary on an annual basis, on coral larvae or to the time gap between simulation based on ocean current patterns and accumulation of genetic differences. Considering the pattern of the genetic connectivity of A. digitifera in the Sekisei Reef, the sites that are located within less-disturbed areas may act as larval sources and should, therefore, be preserved.

The polyp (scyphistoma) of the jellyfish Cassiopea sp. can be maintained in culture for a long time, as polyps repeatedly reproduce asexually via formation of vegetative buds or propagules. The medusa, which is the sexually reproducing stage, typically has a relatively short life span. As a first step to understand the difference in life spans of the polyp and medusa stages of Cassiopea sp., we measured telomerase activity in different life cycle stages. We found telomerase activity in tissues of aposymbiotic polyps and propagules and symbiotic ephyrae (newly budded medusae) and adult medusae. No significant difference in telomerase activity was found between polyps and the bell region of the medusae. The cloned elongation products of the stretch PCR contained the TTAGGG repeats suggesting that the jellyfish has the ‘vertebrate’ telomere motif (TTAGGG) n . This is the first study to show that somatic tissues of both polyp and medusa stages of a cnidarian had telomerase activity. Telomerase activity in somatic tissues may be related to the presence of multipotent interstitial cells and high regenerative capacity of cnidarians.

We examined gametogenesis and the periodicity of planula release in the brooding coral Acropora (Isopora) brueggemanni (Brook, 1893) on Akajima Island, Okinawa, Japan. We captured the moment when A. brueggemanni would be in the process of self-fertilization. Whole colonies of this species were cultured separately or together with other colonies in plastic containers. We observed no apparent periodicity of planula release in the collected colonies. A few planulae were released intermittently during the observation period, regardless of whether the colonies were cultured individually or with other colonies. Serial paraffin sections of A. brueggemanni showed follicle-like cells surrounding the oocyte during vitellogenesis. In September and October, some spermaries looked half-broken and some ova were surrounded by sperm or spermaries instead of follicle-like cells. Such ova were heart-shaped and may have been cells at the first cleavage stage. These observations suggest that the migration of spermaries and/or oocytes resulted in the close proximity of oocytes and sperm, which would allow self-fertilization. This possibility, together with the production of planula larvae by isolated colonies, suggests that this brooding coral engages in self-fertilization.