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Due to their pluripotent nature and unlimited cell renewal, stem cells have been proposed as an ideal material for establishing long-term cnidarian cell cultures. However, the lack of unifying principles associated with “stemness” across the phylum complicates stem cells’ identification and isolation. Here, we for the first time report gene expression profiles for cultured coral cells, focusing on regulatory gene networks underlying pluripotency and differentiation. Cultures were initiated from Acropora digitifera tip fragments, the fastest growing tissue in Acropora. Overall, in vitro transcription resembled early larvae, overexpressing orthologs of premetazoan and Hydra stem cell markers, and transcripts with roles in cell division, migration, and differentiation. Our results suggest the presence of pluripotent cell types in cultures and indicate the existence of ancestral genome regulatory modules underlying pluripotency and cell differentiation in cnidaria. Cultured cells appear to be synthesizing protein, differentiating, and proliferating.

Growth anomalies (GAs) in corals are characterized by morphological abnormalities of the skeleton as well as polyps and coenosarcs. GAs commonly appear as protuberances with fewer polyps and are paler in color due to decreased zooxanthellae density. To test the hypothesis that morphological anomalies in GAs may be caused by unregulated cellular kinetics, the relative abundances of apoptotic cells and proliferating cells were compared between GAs and apparently healthy regions in 2 corals, Porites australiensis and Montipora informis. Apoptotic cells and pro- liferating cells were detected using TUNEL assays and BrdU incorporation assays, respectively. The labeling indices for apoptotic nuclei and BrdU-labeled nuclei were measured in the epider- mis, oral gastrodermis, aboral gastrodermis, and calicodermis. The labeling index for apoptotic nuclei in the oral gastrodermis and the calicodermis was significantly lower in GAs than in healthy regions in both coral species. The index for BrdU-labeled cells in the calicodermis was signifi- cantly higher in GAs than in healthy regions in both coral species. When GA regions partially died, the GA tissues directly adjacent to the dead areas exhibited signs of necrosis, although some apoptotic cells were also present. Healthy oral gastrodermis adjacent to the border between the healthy and GA regions exhibited higher frequencies of apoptotic cells. These results suggest that apoptotic pathways were suppressed and cell proliferation was promoted in GA regions, although cells in GAs may die through both necrosis and apoptosis.

Coral bleaching is caused by environmental stress and susceptibility to bleaching stress varies among types of coral. The physiological properties of the algal symbionts (Symbiodinium spp.), especially extent of damage to PSII and its repair capacity, contribute importantly to this variability in stress susceptibility. The objective of the present study was to investigate the relationship between the growth rates and photosynthetic activities of six cultured strains of Symbiodinium spp. (clades A, B, C, D, and F) at elevated temperature (33 °C). We also observed the recovery of photodamaged-PSII in the presence or absence of a chloroplast protein synthesis inhibitor (lincomycin). The growth rates and photochemical efficiencies of PSII (Fv/Fm) decreased in parallel at high temperature in thermally sensitive strains, B-K100 (clade B followed by culture name) and A-Y106, but not in thermally tolerant strains, F-K102 and D-K111. In strains A-KB8 and C-Y103, growth declined markedly at high temperature, but Fv/Fm decreased only slightly. These strains may reallocate energy from growth to the repair of damaged photosynthetic machineries or protection pathways. Alternatively, since recoveries of photo-damaged PSII at 33 °C were modest in strains A-KB8 and C-Y103, thermal stressing of other metabolic pathways may have reduced growth rates in these two strains. This possibility should be explored in future research efforts.

This study examined the importance of mycosporine-glycine (Myc-Gly) as a functional antioxidant in the thermal-stress susceptibility of two scleractinian corals, Platygyra ryukyuensis and Stylophora pistillata . Photochemical efficiency of PSII ( F v / F m ), activity of antioxidant enzymes, superoxide dismutase (SOD) and catalase (CAT), and composition and abundance of mycosporine-like amino acids (MAAs) in the coral tissue and in symbiotic zooxanthellae were analyzed during 12-h exposure to high temperature (33 °C). After 6- and 12-h exposures at 33 °C, S. pistillata showed a significantly more pronounced decline in F v / F m compared to P. ryukyuensis . A 6-h exposure at 33 °C induced a significant increase in the activities of SOD and CAT in both host and zooxanthellae components of S. pistillata while in P. ryukyuensis a significant increase was observed only in the CAT activity of zooxanthellae. After 12-h exposure, the SOD activity of P. ryukyuensis was unaffected in the coral tissue but slightly increased in zooxanthellae, whereas the CAT activity in the coral tissue showed a 2.5-fold increase. The total activity of antioxidant enzymes was significantly higher in S. pistillata than in P. ryukyuensis , suggesting that P. ryukyuensis is less sensitive to oxidative stress than S. pistillata . This differential susceptibility of the corals is consistent with a 20-fold higher initial concentration of Myc-Gly in P. ryukyuensis compared to S. pistillata . In the coral tissue and zooxanthellae of both species investigated, the first 6 h of exposure to thermal stress induced a pronounced reduction in the abundance of Myc-Gly but not in other MAAs. When exposure was prolonged to 12 h, the Myc-Gly pool continued to decrease in P. ryukyuensis and was completely depleted in S. pistillata . The delay in the onset of oxidative stress in P. ryukyuensis and the dramatic increase in the activities of the antioxidant enzymes in S. pistillata , which contains low concentrations of Myc-Gly suggest that Myc-Gly provides rapid protection against oxidative stress before the antioxidant enzymes are induced. These findings strongly suggest that Myc-Gly is functioning as a biological antioxidant in the coral tissue and zooxanthellae and demonstrate its importance in the survival of reef-building corals under thermal stress.

Coral species in a similar habitat often show different bleaching susceptibilities. It is not understood which partner of coral–zooxanthellae complexes is responsible for differential stress susceptibility. Stress susceptibilities of in hospite and isolated zooxanthellae from five species of corals collected from shallow water in Okinawa were compared. To estimate stress susceptibility, we measured the maximum quantum yields (Fv/Fm) of in hospite and isolated zooxanthellae after 3-h exposure to either 28 or 34 °C at various light intensities and their recovery after 12 h under dim light at 26 °C. Significant reduction in photochemical efficiency (Fv/Fm) of photosystem II (PSII) was observed in in hospite zooxanthellae exposed to high light intensity (1000 μmol quanta m−2 s−1), while PSII activity of isolated zooxanthellae decreased significantly even at a lower light intensity (70 μmol quanta m−2 s−1). The recovery of the PSII activity after 12 h was incomplete in both in hospite and isolated zooxanthellae, indicating the presence of chronic photoinhibition. The stress susceptibility of isolated zooxanthellae was more variable among species than in hospite zooxanthellae. The order of stress susceptibility among the five coral species was different between in hospite and isolated zooxanthellae. The present results suggest that the host plays a significant role in determining bleaching susceptibility of corals, though zooxanthellae from different host have different stress susceptibilities.

Time course of enhancement of calcification of the coral Galaxea fascicularis in light and decay of light-enhanced calcification in darkness were studied. The hourly rates of calcification and of photosynthesis/respiration of isolated polyps of G. fascicularis were measured successively in light or in darkness using the alkalinity anomaly technique. Corals were kept in darkness for 15-19h before experiments. Calcification rate during 50min incubations in light was about four times higher than that during previous incubation in darkness. Light-enhanced calcification did not decrease abruptly when the coral was returned to darkness but decayed gradually to about 46%, 3h after illumination was terminated. The rate of light-enhanced calcification increased gradually when corals were incubated in light. The photosynthetic uptake of CO2 by corals held in light was 3.7 times faster than CO2 uptake due to calcification. Dark respiration increased after 50min incubation in light. The slow decay of light-enhanced calcification in darkness suggests that there is an internal pool of substances which enhance calcification, and that accumulation of the substances might sustain high rates of calcification in darkness.

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.

Cnidarians possess unique intracellular organelles, cnidae, which discharge by evaginating their tubular contents following certain appropriate stimuli. Every cnida consists of a capsule, a tubule or shaft, or combination of the two, and intracapsular fluid and is contained in a cell called a cnidocyte. Cnidae are divided into three major categories: nematocysts, spirocysts, and ptychocysts (Mariscal 1984). The cell containing a nematocyst, spirocyst, or ptychocyst is called a nematocyte, spirocyte, or ptychocyte, respectively.

The catch apparatus (CA) of the sea-urchin spine has been known to have a muscle-like holding property, though it is composed mainly of extracellular collagen fibres. An electron microscopic study has been made on the CA of the sea urchin, Anthocidaris crassispina, with special reference to its content of muscle cells and to structural changes of the collagen components on elongation of the CA. The stretch resistance of the CA in a highly extensible state and in a very inextensible state was also measured. Although very thin smooth muscle cells were found scattered among the collagen fibres in the CA, the difference in the passive tension was greater than the estimated stress which could be generated by the muscle cells in the CA by three orders of magnitude. The collagen fibrils remained undeformed but slid along one another during the length change of the CA. The present results suggest that the cohesive force between the collagen fibrils rather than the contractile activity of the muscle cells plays a significant role in determining the mechanical properties of the CA.