Millepora tenera

Abstract: Three species of fire corals are present on the fringing coral reefs of Reunion Island (SW Indian Ocean): Millepora exaesa, Millepora platyphylla and Millepora tenera. Compared with scleractinian corals, reproduction studies on milleporids are scarce and this is the first study to be conducted in the Indian Ocean and on M. exaesa. We used a combination of weekly in situ observations and laboratory studies to document reproductive stages and phenology. The sexual reproductive period was the austral summer (Nov-Jan), the maturation of the colonies being correlated to the seasonal increasing water temperature. We describe both ampullae and short-lived medusae (medusoids), and report differences in the size and density of ampullae and the distribution of nematocysts in the medusoids of the three species. The absence of radial and circular canals in all medusoids agrees with the original description but not with the current diagnosis of the family Milleporidae. We observed the first mass spawning event in M. exaesa and M. platyphylla, and provide the first description of a zooxanthellate Millepora larva. Spawning always began before dark but was not correlated with the lunar or tidal cycles. Millepora tenera colonies released several batches of medusoids during the reproductive period, suggesting that this species may be a serial spawner.

Abstract: Looking at taxa that could dominate Anthropocene reefs, the fire corals, Millepora spp., represent compelling alternative because of their similar role than scleractinian corals. However, species identification has been the subject of much debate over the past 150 years. As they may turn as among the last refuges for reef biodiversity, it is critical to accurately delimit their species boundaries to appreciate better their resilience to future environmental conditions. Combining morphological and molecular approaches, we investigated Millepora species delimitation in Reunion Island where three species co-occur: Millepora exaesa, Millepora platyphylla and Millepora tenera. Our results showed that both pore characters and polyp morphologies successfully distinguished species. Gastropore and dactylopore numbers and diameters should be used as standard traits in Millepora. Regarding molecular delineation, the mitochondrial gene clearly segregated the species, whereas the nuclear gene showed shared haplotypes between species, likely influenced by ancestral polymorphism. Confirming the results of a recent study on Red Sea fire corals showing their distinction from the Indo-Pacific ones, we emphasize that formal re-descriptions of the Indo-Pacific M. cf. platyphylla and M. cf. exaesa are needed. Our study highlights the importance of a trait-based qualitative and quantitative approach coupled to molecular delineation, especially for structurally important reef species.