Fire corals Millepora spp. are cousins of the predominant reef corals, the Anthozoan Scleractinians. Scleractinian reproductive biology has attracted attention partly because of mass spawning and multispecies mass spawning events. Such spawning behavior---synchronized broadcast of eggs and sperm into the "water column" is common in marine species, particularly species that are attached to the sea bottom. Among coral reef species, broadcast spawning is a prevalent reproductive strategy, partly because the reef is a bulwark of hungry animals, so it is believed that by broadcasting eggs and sperm can facilitate their escape until they are competent to make their way to a reef---the same or even another reef at great distance---and claim their own living spaces. The literature of coral reef invertebrate and fish reproduction is vast, presenting a panoply of fascinating and diverse life patterns.
Millepora species hold a special interest in regard to their life histories. They hold a large amount of space on coral reefs, wherever they occur. They live a complex lifestyle, as both consumering---meeting some proportion of their nutritional needs by capturing plankton; and produccers---through their partnership with photosynthesizing zooxanthellae, which provide them with a preponderance of their needs. They, like other reef-building corals---which likewise could not thrive and build massive reef frameworks without their zooxanthella partners---take on characteristics of both plants and animals. Like plants, they obtain energy from the sun, and they are immobile, living in the shallow waters where sufficient sunlight exists to support photosynthesis; and at the same time, they possess all the characteristics of animals: their polyps can snag prey, their cells are of the animal type, without cell walls or photosynthesizing plastics. Early naturalists noticed these peculiarities, and they were classified as Zoophytes---a living hybrid between animal and plant. Their animal nature demanded them to be classified among the animal kingdom, however, and eventually, it was recognized that the comprised a unique kind of amalgam, as animals with single-celled dinoflagellates living within them. This relationship is more than a friendship---the coral cannot survive without it's partner, and the partner is specially adapted for life within animals and animal cells. Such organisms are now identified as Holobionts, similar to Lichens, amalgams of fungus and alga partners.
Millepora holds a special interest among "corals." Their general nature is so similar that one might be forgiven for classifying them together with Scleractinia, but a closer leads to a completely different conclusion: these are Hydrozoans, where Scleractinians are Anthozoans, both belonging to the Phylum Cnidaria---known for their stinging cells, their cnidae, or nematocysts. Moreover, the life cycles of these Hydrozoans is very unlike that of their cousins, as Millepora species exhibits the same Alternation of Generations that is characteristic of many comon hydroids, between polyp and medusa. Like butterflies and caterpillars, these alternate stages seem completely unlike each other. The reproductive cycle of Millepora spp. invokes a medusoid to carry the gametes---egg and sperm---away from the colony, into the water column where they spawn within a matter of hours, and die (like ephemeral insects).
Millepora spp., like Scleractinians, create massive Calcium Carbonae skeletons, although the size of each polyp is less than 1mm across. The name "Millepora" means, literally, thousand pores: the colony is peppered with numerous minute pores, ordinarily of two kinds: the gastropores---from which the larger of two types of polyps, the gastrozoids, the eating polyps, can access the exterior; and the dactylopores---home to the smaller dactylozoids, the stinging zooids. These two types are arranged in cyclosystems, a central gastropore encircled by five or six dactylopores. The colony, then, consists of one large organism comprised of a multitude of smaller specialized individuals, the zooids. Like other hydrozoans, Millepora spp. also have a third kind of zooid: a medusoid. This zooid develops only at certain times, and it does not have access to the exterior: this zooid is the host to developing gametes; it has two tasks: to swim away from the colony at the appropriate moment, and to spawn. Pretty obviously, this must be the adult, like the butterfly, where the colony serves as the relatively giant host of these minute adults.
The medusoid develops within a protective cocoon---the ampulla---a pit within the stony skeleton covered by a thin layer of calcium carbonate, isolated from the external world. The medusoid cannot access the outside environment; but at the time of its liberation, it is essential that it escape. This is made possible because the cover of the ampulla dissolves in concert with the development of the gametes and the maturation of the medusoid itself, as the moment for the medusoid to carry the gametes away. Remarkably, this concert is synchronized among other medusoids on this colony, as well as on other, nearby colonies, at the appropriate moment.
My study focused on this unique character of Millepora spp., that as the egg and sperm have matured toward the point of readiness, the ampulla dissolves, making possible the escape of the medusoid. By examining skeletal fragments, collected over a period of months, it should be possible to determine the time of reproductive activity.
A very important question arises: how are these developments all synchronized on the reef? Because Millepora spp. are gonochoristic---each colony bearing either male or female gametes---unless this process is tightly orchestrated, fertilization will fail. Not only that, but synchronized timing is necessary to ensure maximal efficiency in success of spawning.
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