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Notes: Abstract.  Synchronous spawning of many coral species that co-occur on Indo-Pacific reefs raises the possibility that hybridization plays a role in their evolution. Here we use experimental crosses to examine mating compatibilities and breeding barriers in a group of sessile animals whose mating systems are primarily governed by interactions among free-spawned gametes. We found hybridization occurs readily in more than one-third of 42 species pairs from the common genera Acropora, Montipora and Platygyra. Mean fertilization success ranged from 1% to 50% in species crosses, but standard deviations about these means were large and in some cases, fertilization success in individual colony matings was greater than 95%. Cases of high fertility in individual, interspecific matings were found in all three genera. Hybridization occurred most readily between species that were morphologically similar, identifying areas where current taxonomic judgements may require further testing. However, cases of significant hybridization also occurred between species that are morphologically distinct. Evidence of in vitro hybridization combined with the lack of either temporal or spatial barriers to interbreeding among field populations of these species, indicates that natural hybridization may occur commonly between congeneric corals that are currently recognized as distinct species. We also detected mating incompatibilities between some colonies within some species. In some cases, incompatible colonies corresponded to distinct morphotypes, but not in others. Thus some breeding groups in scleractinian corals are potentially larger, but others are smaller, than would be predicted using morphological criteria. Gamete incompatibilities within a morphospecies that readily hybridizes with other species may be the result of a mating system that is governed by gamete-level interactions. Imprecision in the alignment of morphological and breeding boundaries suggests a single species concept may not apply to scleractinian corals and challenges the tacit assumption that currently defined coral species encompass biological, evolutionary and phylogenetic species. Hybridization between supposedly isolated species introduces a reticulate nature to the evolution of corals and has profound implications for present understanding of the population genetics, phylogenetics, and evolutionary biology of scleractinian corals.

Notes: Abstract Following observations of mass spawning of hermatypic corals on the Great Barrier Reef in 1981 and 1982, spawning dates were successfully predicted and documented at five reefs on the Central and Northern Great Barrier Reef in 1983. During the predicted times, 105 species from 36 genera and 11 families were observed to spawn. Of these, 15 species were shown to have an annual gametogenic cycle. All but two of the species observed during mass spawnings shed gametes which underwent external fertilization and development. Synchronous spawning was observed both within and between the five reefs studied, which were separated by as much as 5° of latitude (500 km) or almost a quarter of the length of the Great Barrier Reef. The mass spawning of corals took place on only a few nights of the year, between the full and lastquarter moon in late spring. Maturation of gametes coincided with rapidly rising spring sea temperatures. Lunar and diel cycles may provide cues for the synchronization of gamete release in these species. The hour and night on which the greatest number of species and individuals spawned coincided with low-amplitude tides. Multispecific synchronous spawning, or “mass spawning”, of scleractinian and some alcyonacean corals represents a phenomenon which is, so far, unique in both marine and terrestrial communities.

Notes: Abstract Multispecies assemblages of the coral genus Acropora occur commonly throughout the Indo-Pacific Ocean. Nine species from such an assemblage comprising 41 species of Acropora, at Big Broadhurst Reef on the Great Barrier Reef, were studied during 1981–1983. Similarities and differences in reproductive modes and timing, oocyte dimensions and fecundity, recruitment by larvae and by fragments, and mortality were recorded. All species had an annual gametogenic cycle, were simultaneous hermaphrodites, and had the same arrangement of gonads in polyps. In six species, most colonies released gametes on the same night of the year, in early summer, during a mass spawning event involving many coral genera. A seventh species had colonies spawning at this as well as other times of the year. Another species spawned in late summer, and gametes were not observed to mature in the last species. Eggs were very large (601 to 728 μm geometric mean diameter) and fecundity of polyps low, compared with other corals; no reduction in oocyte numbers occurred during oogenesis. Reef-flat species had slightly bigger and fewer eggs than reef-slope species. All species recruited by larvae, but four also multiplied by fragmentation, either year-round or during occasional rough weather. Yearround fragmenters had few larval recruits; non-fragmenters had many, and a rough-weather fragmenter had an intermediate number of larval recruits. It was concluded that larval recruitment largely determined species composition, and that reduced larval recruitment was responsible for sparse distribution of fragmenting species. Subsequent mortality in some species and increase by fragmentation in others probably determined relative abundances.