Reproduction in Zoanthidea (Anthozoa: Hexacorallia)

Abstract

Summary The state of knowledge concerning the reproductive biology of zoanthids is reviewed. Colonies in the suborder Macrocnemina, with few exceptions, are gonochoric; those in Brachycnemina are probably basically hermaphroditic but accurate sampling in colonies that fragment is difficult and exceptions by species or populations occur. Two deepsea species of Epizoanthus breed continuously but temperate shallow water zoanthids, and those in the tropics outside ∼15° latitude are iteroparous with spawning in summer or when temperatures are rising. Oogenesis occurs over a few months but the spawning event may be very brief. Parazoanthus parasiticusin Bermuda spawns mainly on 1–2 nights in late August or September, apparently at full moon;Protopalythoa sp. 1, and probably Palythoa caesia in the Great Barrier Reef spawn 3–5 nights after full moon in November (early summer). These species with short, precisely defined spawning seasons may undergo a second, smaller-scale spawning event one month after the first. Some intertidal populations seem rarely to reproduce sexually, and there have been no studies on the annual cycles of zoanthids in near equatorial waters. With few exceptions gametogenesis occurs only in non-directive macrosepta; in hermaphroditic colonies oocytes and sperm cysts may be intermingled on the same septa or be separate. Germ cells in Isozoanthus giganteus originate only from the endocoelic face of the septa, but this has not been confirmed in other species. Nutritive structures (trophonemata) are present both in oocytes and sperm cysts, in the latter functioning also for the egress of sperm. Trophonemata may be conspicuous, as in Par-parasiticus, or poorly developed, as in Protopalythoa sp. 1. Live eggs vary from −300 µm (most zoanthids) to ∼500 µm diameter in Palythoa spp.; in the two mass spawning species from the Great Barrier Reef they are shed as compact ovoid egg (or egg/sperm) bundles. Fecundity in zoanthids, from few studies, is variable and dependent on polyp size; estimates of 100–300×103 ova 100 cm−2 of colony have been made for Caribbean reef species. Internal brooding is known definitely only in Isozoanthus giganteus. Amongst zooxanthellate zoanthids, only Protopalythoa sp. 1 is known to transmit zooxanthellae via its oocytes. Several zoanthids are known to produce palytoxin but it is unclear to what extent the toxin accumulates in the ova. Larval development is summarized. Zoanthella-larvae are known to be produced in Palythoa, Protopalythoa and Sphenopus, and zoanthina-larvae in Isaurus and Zoanthus, all Brachycnemina. Larval type in Epizoanthus and Parazoanthus, both Macrocnemina, is unknown. Early metamorphosis of zoanthella and zoanthina larvae has been reported but not studied; older larvae, taken from the plankton, may have lost competency to settle. All zoanthids, except Sphenopus, form colonies, clones, or mixtures of the two, and propagate by budding. Attention is drawn to unusual methods of budding or cloning in Acrozoanthus, Isaurus, Palythoa, Sphenopus, and Parazoanthus axinellae. In reefal Isaurus, Protopalythoa, and Zoanthus actual colony size remains small due to fragmentation, but genet size may be substantial. Population structure depends on the interplay of recruitment, fragmentation, and local dispersal.