Mass spawning events, seasonality and reproductive features in Milleporids (Cnidaria, Hydrozoa) from Reunion Island

Abstract

AbstractThree 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.