In the past two decades, evolutionary studies of reef corals have greatly advanced owing to the intriguing characteristics of corals, such as hybridization and synchronous mass spawning, and development of molecular techniques. These evolutionary works have mainly focused on the genus Acropora because of its dominance. It has the highest number of species and is a major member of the mass spawning event, which characteristics suggest its unusual evolutionary process called “reticulate evolution.” Although studies using other corals have been very limited, some have demonstrated obscure species boundaries between closely related species, implying hybridization in the present or past. Most of these studies used genetic analysis, because it is very useful for investigation of species and population relationships among and within regions and to infer phylogenetic relationships from the species level to higher taxonomic levels. For anyone wishing to study the evolution and taxonomy of corals, familiarity with these molecular works, even for nongenetic analysts, is essential. This review summarizes the evolutionary studies of reef corals using molecular analyses.

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The distribution of several zooxanthellate zoanthid species (Hexacorallia: Anthozoa) from the genera Palythoa, Zoanthus, and Isaurus in the oceans surrounding Japan are generally now well documented, but no examination of potential environmental factors limiting their distribution has been conducted until now. Here, using distribution data from previous works as well as ocean and air temperature data, we examined the minimum ocean temperature threshold for these zoanthid species' existence, as well as what winter air temperatures limit intertidal distribution of zoanthids. Our results show that in the Japan region zooxanthellate zoanthids are limited to oceans with winter temperatures above approximately 16°C. Additionally, intertidal colonies of zooxanthellate zoanthids were not found in locations where air temperatures dropped below 8°C. Thus, as with zooxanthellate scleractinian corals, zooxanthellate zoanthid species ranges are strictly limited by minimum ocean (overall range) and air (intertidal distribution) temperatures. Utilizing these threshold data, an examination of the potential ranges of Palythoa, Zoanthus and Isaurus on a global scale shows that it is very likely Atlantic and Indo-Pacific species are isolated from each other, as previously seen with zooxanthellate corals. Additionally, the documented northern Japanese ranges of intertidal occurrences as well as overall distribution northern limits will provide valuable baseline data for future surveys to help ascertain whether tropical and sub-tropical marine species are “invading” northwards in Japan due to global warming.

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Two types of injuries (breakage and hand-saw) were inflicted to the corals Porites lutea, P. cylindrica and P. rus. The injured colonies were maintained in outdoor aquaria for a period of 3 months under light intensities in the range from 0.8 to 90% of incident surface photosynthetic radiation (PAR₀). The influence of bright (70-90% PAR₀), moderate (20-30% PAR₀), low (2-5% PAR₀) and extremely low (0.8-1% PAR₀) light on the dynamics, mechanisms and the recovery rates of the injured colonies, on the growth rates of coral fragments, on zooxanthellae population density in coral polyps and on chlorophyll concentration in zooxanthellae were studied. The regeneration of mechanical injuries inflicted on the corals had a similar character in all three species and the regeneration occurred in two stages. During the first stage (two-three weeks), damaged coral tissue recovered. Within the second stage (approximately two months) of the regeneration, living tissue expansion on the dead part of the damages and the formation of polyps by the newly formed tissue occurred. During the first stage, the regeneration rate of the injuries was more than two times higher (in average to 0.15mm per day for all Porites species) than during the second stage (in average to 0.06mm per day for all Porites species. Under low and extremely low light the density of zooxanthellae in the polyps decreased while chlorophyll concentration in zooxanthellae remained high. Light intensity had no significant effect on the regeneration rates. The growth rates of coral fragments were greatest in bright and moderate light, dropping sharply in low light, and the fragments did not grow under extremely low light.

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The concentrations of dissolved cadmium (Cd) were monitored in the surface water of Urasoko Bay and the mouth of the stream that runs into the bay. Urasoko Bay is located on the northern coast of Ishigaki Island, Okinawa, which is surrounded by a fringing reef. Water samplings were carried out from August 2006 to August 2007, and adding to these samplings, freshwater from the upper stream and brackish water that exudates at the beach site were collected from April to June 2007. The concentration of dissolved Cd showed no tendency to decrease from the upper stream to the bay site. The results of Cd behaved non-conservatively in the Cd-salinity plot, which was attributed to daily variations in the Cd concentrations of upper stream water and the irregular input of Cd possibly adsorbed particles into the stream and beach water (the mixture of groundwater and seawater before exudation on the beach).

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To reveal the optimal resource allocation among regeneration, growth, sexual reproduction, and survivorship in a colonial animal, a specimen of the branching coral Acropora muricata (formosa), without oocytes, was separated into fragments of three different sizes (5, 10, and 20cm long) that were then transplanted onto coral pavement. Regardless of the fragment size, a wound was fully closed within 11 days after fragmentation. The number of newly formed polyps on the new tissue layer did not differ significantly among fragment sizes. No significant difference was observed in the monthly growth rate among the three fragment sizes. These results showed that regeneration and growth were independent of fragment size. During the observation period, one 5cm fragment died, the 5cm and 10cm fragments did not spawn, but the 20cm fragments did spawn. The investment to survivorship by the 5cm fragments might have been smaller than that of the 10cm fragments, because the 5cm fragments had smaller energy stocks than did the 10cm fragments. Combined, these results suggest that the energetic investment for regeneration and growth was constant while investment for reproduction was proportional to fragment size. This finding agrees with the theoretical optimal dispersal strategy, in which the allocation to growth by mothers is constant. In conclusion, the colonial coral Acropora muricata generally shows size-independent growth and regeneration, and exhibits a tradeoff among regeneration, growth, survivorship, and reproduction.

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