A series of laboratory experiments were conducted to determine the effect of sperm dilution, egg concentration, sperm-egg contact time, and gamete aging on fertilization success in the tropical sea urchin, Echinometra mathaei. The results demonstrated that sperm dilution, sperm age, and sperm-egg contact time were sequentially the most important factors influencing fertilization success, while egg concentration was not significant over the range tested. Sperms retained their potency for more than two hours only in relatively dense sperm suspensions (≥10^-4 dilution of ‘dry’ sperm) whereas they exhibited lower viability with increasing dilutions and age. In egg-sperm contact time experiments more than 80% fertilization was achieved at lower sperm dilutions (10^-3-10^-2) within 10 sec of mixing, while at higher sperm dilutions, longer times of contact were needed to achieve the similar fertilizations. Consequently, eggs remained in good quality for up to 3 h and there was no abnormality or adverse effects in fertilization were observed in a series of sperm dilution tested. These laboratory experiments suggest that sperm dilution and its limited longevity can play an important role in limiting the fertilization of sea urchin eggs in the field during natural spawnings. It follows, therefore, that sea urchin (E. mathaei) are under considerable selective pressures to spawn synchronously in order to generate high sperm concentrations and higher sperm-egg encounters in the water column to maximize the probability of successful fertilization.

View full abstract

The Ryukyu Group, composed of Pleistocene reef-complex deposits that pass laterally into terrigenous sediments, crops out on Kume-jima and its adjacent islet, Ohajima, Ryukyu Islands, southwestern Japan. We propose a major revision of the previous stratigraphic scheme for the Ryukyu Group, based on new investigations, and provide a formal stratigraphic description. These Pleistocene deposits comprise the Nakandakari, Kumejima, and Torishima Formations, in ascending order, on Kume-jima. The lowest of these, the Nakandakari Formation, consists of pumice-bearing detrital limestone (<20m thick); its surface exposure is confined to the type locality, which is on the coast to the northwest of Nakandakari. The unconformably overlying Kumejima Formation exceeds 30m in thickness and is exposed in the northwestern part of Kume-jima at elevations less than ca. 50m. It is divisible into at least three units, each comprising coral limestone and overlying rhodolith, Cycloclypeus-Operculina, and detrital limestones. The Torishima Formation rests unconformably on the Kumejima Formation, and is characteristically thin (<15m thick) and composed of well-sorted detrital and coral limestones that are thought to have been deposited in a shallow lagoon (moat). The Oha Limestone is limited in its distribution to Oha-jima and consists of diagenetically altered, reddish to brownish, coral limestone. The stratigraphic relationship between limestones on Kume-jima and those on Oha-jima remains unknown, as does the geological age of these limestones. It is evident from the stratigraphic succession and configuration of lithofacies that the reefs grew in response to at least three repeated cycles of sea-level change with amplitudes of up to 80m during deposition of the Kume-jima Formation. Subsequently, a relatively small reef now assigned to the Torishima Formation grew to fringe the older reefs.

View full abstract

While spawning and planula release are known to occur during night time in the majority of coral species, a species endemic to high latitude of Japan and Korea, Alveopora japonica, was found to release planulae during daytime. Laboratory experiments were conducted to see how light affects planula release in this species. Under the natural light condition in the laboratory A. japonica released planulae between 07:00 and 12:00 in the morning. When the timing of exposure to light was changed, planulation also shifted and coincided with the time of light exposure irrespective of the actual time of day. When the colonies were exposed to light twice a day, i. e. in the morning and evening, all colonies released planulae only in the morning. Direct observation on planulating polyps showed that polyps elongated before planulation and such polyp elongation occurred only under the light condition. Consequently, it is concluded that light is a proximate factor which governs planula release through polyp elongation in A. japonica.

View full abstract

In order to restore disturbed coral reefs, transplantation of coral fragments or coral heads has been conducted in many parts of the world. We reviewed methods and results of the transplantation experiments described in numerous previous reports in order to determine recommendable methodologies. Problems that should be solved by future studies are discussed. For transplantation, generally, small pieces of coral are taken from the donor colony using hammer, chisel and the like. If the collection site is far from the transplantation site, fragments should be placed in bucket full of seawater. There are various methods to fix the coral fragment onto substrate, e. g. by means of epoxy cement, plastic coated wire and nails. They should be chosen according to size and shape of coral fragments, but we found fixation with nails and cable ties is easy and reliable for staghorn corals. The cost of transplantation with a density of 245, 000 fragments per hectare secured by means of nails and cable ties has been estimated at about 36, 000, 000 JPN Yen. So far, species of the genera Acropora, Porites, Pavona and Galaxea were most frequently used for transplantation as they have relatively high survival rates.

View full abstract

Organic carbon production (photosynthesis-respiration) and inorganic carbon production (calcification-dissolution) of two massive scleractinian corals, Favites sp. and Porites sp., were measured in a mesocosm, at the early and late stages of the 1998 coral bleaching event. Air-sea CO₂ flux was also measured directly on the surface of seawater, and carbon budgets were calculated from these measurements.
Net organic carbon production was 15.8mmol C m^-2 d^-1 at the early stage of coral bleaching and decreased to 9.5mmol C m^-2 d^-1 at the late stage of coral bleaching. Net inorganic carbon production also decreased drastically from 11.2mmol C m^-2 d^-1 at the early stage to -7.0mmol C m^-2 d^-1 at the late stage of coral bleaching, which was caused by the decrease in calcification in the daytime and increase in the dissolution of carbonate at night. Air-sea CO₂ fluxes were -1.8mmol m^-2 d^-1 at the early stage and -1.1mmol m^-2 d^-1 at the late stage, respectively, which indicates absorption of CO₂ from air to seawater.
The gross primary production/respiration ratio (=1.2) at the early and late stages of bleaching was nearly constant, which suggested that the biological activity of zooxanthellae does not alter significantly at the late stage of coral bleaching. The calcification rate was remarkably reduced compared to photosynthesis. Coral bleaching severely damaged on the physiological activities of corals, and had a large effect on the carbon metabolism in coral-algal symbiosis.

View full abstract

Three types of symbiotic dinoflagellates L (large), B (brown) and G (green) found in hermatypic corals from a fringing reef of Sesoko Island (Okinawa, Japan) differed morphologically, physiologically and biochemically. Colonies of the hydrocoral Millepora intricata hosted symbionts of type L only; scleractinian corals containing type B only were Pocillopora damicornis, type G only were Seriatopora caliendrum and S. hystrix, and both types B and G were found living together in Stylophora pistillata and Echinopora lamellosa. The symbiotic dinoflagellates (SD) differed considerably in cell size, shape and structural elements in coccoid state in hospite. SD of these types also differed in photosynthetic capacities, primary production, pigment accumulation and maximum rates of cell division and degradation.
Corals hosting various types of SD significantly differed in light-resistance. Scleractinian corals with symbionts of both types B and G, in the same colony, acclimated to bright light by increasing the relative number of symbionts of type G and acclimated to dim light by increasing the SD number of type B. It was shown that scleractinian corals can photo-acclimate through formation of optimal composition of SD types under various light intensities.

View full abstract