Takeshi Hayashibara

An Ancient Chemosensory Mechanism Brings New Life to Coral Reefs

The first scleractinians, progenitors of modern corals, began to appear 240 million years ago; by the late Jurassic (150 Ma) most families of modern corals had evolved and begun forming reefs (1, 2). Mechanisms controlling the recruitment of new corals to sustain these structures are, however, poorly understood (3). Corals, like many marine invertebrates, begin life as soft-bodied larvae that are dispersed in the plankton (3, 4). As the first step in developing a calcified coral colony, the larva must settle out of the plankton onto a suitable substratum and metamorphose to the single calcified polyp stage cemented to the reef (3, 5). Our analyses of the metamorphic requirements of larvae in divergent coral families surprised us by revealing the existence of a common chemosensory mechanism that is required to bring larvae out of the plankton and onto the reef. This mechanism appears to be quite old, predating both the phylogenetic divergence of these coral families and the development of different modes of coral reproduction.

Isolation and characterization of a mini-collagen gene encoding a nematocyst capsule protein from a reef-building coral, acropora donei

Genomic and cDNA clones of a mcol gene encoding mini-collagen (MCOL), a nematocyst capsule protein, have been isolated from a reef-building coral, Acropora donei (Anthozoa). The gene and its flanking regions, comprising 5382 bp and covering three exons and two introns, were sequenced. Exons 2 and 3 together have an open reading frame which can encode a MCOL of 176 amino acids (aa). The coral MCOL has all the characteristic regions present in the four hydra MCOL specified by the four mcol cDNA clones previously isolated from Hydra magnipapillata (Hydrozoa) by Kurz et al. [J. Cell Biol. 115 (1991) 1159-1169], including a central Gly-Xaa-Yaa region and flanking Pro-rich and Cys-repeat regions. This observation suggests that a mcol family is highly conserved in Anthozoa and Hydrozoa, and also that the characteristic regions present in MCOL are essential for the structure and function of these peptides.

Induction and control of spawning in Okinawan staghorn corals

The genus Acropora is the most abundant and diversified genus on coral reefs in the Indo-Pacific. As such, these corals have been the focus of many different types of studies, such as reproductive biology and phylogenetic studies. They have been chosen for genetic studies and have been found to be a good model for the theory of reticulate evolution (Grigg 1995; Veron 1995; Kenyon 1997); the large number of species within this genus has been used in cross-hybridization experiments among species (Willis et al. 1997; Hatta et al. 1999). Members of this genus also seem to be particularly amenable to genetic analysis as evidenced by the number of studies using coral genes isolated from gametes released at the time of mass-spawning, the reproductive strategy adopted by members of this genus (Odorico and Miller 1997; Hatta et al. 1999; van Oppen et al. 2001). Many species of staghorn corals participate in mass spawning events that occur annually either in early summer or early fall (Wallace 1999). It is, however, difficult to predict the exact date of spawning. For example, synchronous spawning of Acropora spp. has been recorded at Okinawa, Japan, over a wide time frame, -3 days to +7 days relative to the full moon (Hayashibara et al. 1993). Opportunities for studying reproductive biology and cross-fertilization experiments are quite limited, often only once a year. The ability to control the induction of spawning of acroporid species is a very desirable tool not only for reproductive and phylogenetic studies, but also for reef restoration. During recent worldwide bleaching events, staghorn corals were subject to severe damage and death (Baird and Marshall 1998; Wilkinson et al. 1999; Fujioka 2002). Additionally, the corals in the genus Acropora are particularly susceptible to predation (Fujioka and Yamazato 1983, De’ath and Moran 1998). At the same time, their recovery is helped by the fact that these corals grow more rapidly than most others (Veron 1995). This latter property has made them target species for reef restoration projects that are dependent on a supply of reproductive products (gametes) to provide seed for restoration and remediation (Petersen and Tollrian 2001, Heyward et al. 2002, Iwao et al. 2002). Control of reproduction has been achieved with other invertebrates, particularly mollusks. Morse et al. (1977) successfully induced spawning of the red abalone with the use of hydrogen peroxide (H2O2). They found that H2O2 activated the prostaglandin-dependent spawning reaction in abalone and other mollusks (Morse et al. 1977; Morse 1984). A number of prostaglandins have been found in some soft coral species, and one of them (15-epiacetoxy-PGA2) was surmised to play a role in egg release (Coll et al. 1989). As far as we know, however, there has been no attempt to induce spawning with H2O2 in cnidarians. Therefore we were prompted to try to induce spawning (gamete release) of corals with the same chemical.

Establishment of coral-algal symbiosis requires attraction and selection.

Coral reef ecosystems are based on coral–zooxanthellae symbiosis. During the initiation of symbiosis, majority of corals acquire their own zooxanthellae (specifically from the dinoflagellate genus Symbiodinium) from surrounding environments. The mechanisms underlying the initial establishment of symbiosis have attracted much interest, and numerous field and laboratory experiments have been conducted to elucidate this establishment. However, it is still unclear whether the host corals selectively or randomly acquire their symbionts from surrounding environments. To address this issue, we initially compared genetic compositions of Symbiodinium within naturally settled about 2-week-old Acropora coral juveniles (recruits) and those in the adjacent seawater as the potential symbiont source. We then performed infection tests using several types of Symbiodinium culture strains and apo-symbiotic (does not have Symbiodinium cells yet) Acropora coral larvae. Our field observations indicated apparent preference toward specific Symbiodinium genotypes (A1 and D1-4) within the recruits, despite a rich abundance of other Symbiodinium in the environmental population pool. Laboratory experiments were in accordance with this field observation: Symbiodinium strains of type A1 and D1-4 showed higher infection rates for Acropora larvae than other genotype strains, even when supplied at lower cell densities. Subsequent attraction tests revealed that three Symbiodinium strains were attracted toward Acropora larvae, and within them, only A1 and D1-4 strains were acquired by the larvae. Another three strains did not intrinsically approach to the larvae. These findings suggest the initial establishment of corals–Symbiodinium symbiosis is not random, and the infection mechanism appeared to comprise two steps: initial attraction step and subsequent selective uptake by the coral.

Narrower grid structure of artificial reef enhances initial survival of in situ settled coral.

The initial factors that cause a decline in the survival of in situ settled corals remain poorly understood. In this study, we demonstrated through field experiments that the design of artificial grid plates may influence the initial survival of Acropora corals, with narrower grids being the most effective. In fact, grid plates with a 2.5-cm mesh presented the highest recorded survival rate (14%) at 6 months after settlement (representing approximately 50 corals per 0.25 m(2) of plate). This is the first study where such high survival rates, matching those of cultures under aquarium conditions, were obtained in the field without using additional protective measures, such as guard nets against fish grazing after seeding. Therefore, our results provide a foundation for establishing new and effective coral restoration techniques for larval seeding, in parallel to clarifying the details of the early life stages of reef-building corals.

Habitat differentiation in the early life stages of simultaneously mass-spawning corals

The settlement process of coral larvae following simultaneous mass-spawning remains poorly understood, particularly in terms of population and community parameters. Here, the larval settlement patterns of Acropora corals, which are the most diverse genera of scleractinian corals at the species (haplotype) level, were investigated within a single subtropical reef. Across a 4-year period (2007–2010), the mitochondrial and nuclear molecular markers of 1,073 larval settlers were analyzed. Of the 11 dominant haplotypes of recruited populations, nine exhibited non-random patterns of settlement distribution. This result suggests that the actual habitat segregation starts during the early swimming larval stages of their life history, rather than by natural selection after random settlement. In addition, the presence of a depth-related settlement pattern supports that species-specific vertical zonation of coral larvae may play a role in the establishment of habitat segregation. Moreover, in some species that showed a preference toward the shoreward area of the bay, the settlement pattern was consistent with that of the adult distribution. This result indicates that the gametes were not mixed between fore and back reefs in the period from fertilization to settlement during the mass-spawning event, even within a single small reef. Another compatible hypothesis of this pattern is that the larvae are able to recognize various types of environmental information, facilitating the selection of optimal micro-habitats. Overall, Acropora coral larvae that are produced from a simultaneous mass-spawning event may have adapted to complex reef topography by means of multi-step habitat selection at settlement, corresponding to different spatial scales.

Genetic evidence of peripheral isolation and low diversity in marginal populations of the Acropora hyacinthus complex

Zooxanthellate corals are found throughout the tropics, but also extend into subtropical and marginal locations due to the presence of warm ocean currents. The population history of corals in marginal locations is of great interest in relation to changing global climatic conditions, as species edge zones might play an important role in evolutionary innovation. Here, we examine the genetic structure of a widely distributed coral species complex, Acroporahyacinthus, from tropical to high subtropical regions along the Kuroshio Current in Taiwan and Japan. Population genetic analysis of 307 specimens from 18 locations (7 reefal and 11 marginal) identified at least four genetic lineages within the A. hyacinthus complex: HyaA, HyaB, HyaC (dominating reefal locations) and HyaD dominating marginal locations in mainland Japan and Taiwan, except the upper Penghu Islands, which were dominated by HyaC. Crossing experiments suggested semi-incompatibility and hybridization between HyaC and D from reefal locations, implying that the existence of hybridization partners enhances diversification and genetic diversity. An incomplete barrier between the HyaC and HyaD dominations was found along the two straits in the Ryukyu Islands, where Kuroshio Current flows constantly. Despite geographical distance, the genetic composition of populations in mainland Japan was comparable to that in mainland Taiwan, which may reflect a region-specific connectivity around the northern limit of A. hyacinthus in the Pacific. In contrast, populations in the Ryukyu Islands were not significantly different from those of Palau and the Great Barrier Reef. While the precise taxonomic nature of the lineages found around the Kuroshio Current remains to be elucidated, these results indicate that, despite the presence of four lineages in the Kuroshio triangle, low genetic diversity populations of the two main lines might be isolating and differentiating in the marginal region.