1985; Fadlallah et al., 1984; Ryland and Babcock, 1991). Colonies in the wild have been found to be composed of just male, just female, male and female, or hermaphroditic polyps, depending on the species and the location (see Cooke, 1976; Karlson, 1981; Kimura et al., 1971; Muirhead et al., 1986; Ryland and Babcock, 1991;
Yamazato et al., 1973). These observations raise the possibility that some zoanthid species may be protogynous hermaphrodites.
To date the species studied have been found to be external broadcast spawners, releasing sperm and eggs into the water, where fertilization then occurs. In a population of Protopalythoa (formerly Palythoa) studied on the Great Barrier Reef, ovaries were found to develop along the mesenteries. Development occurred several months before spawning and spermaries developed later in the reproductive cycle, also along the mesenteries (Ryland and Babcock, 1991). Studies in Jamaica have shown that the average diameter of the ovaries can vary from 274 urn to 1885 um (Karlson, 1981). Cooke (1976) found that zoanthids in Kaneohe Bay, Hawaii, had egg diameters ranging from 75 um to 280 um, while spermaries were 50 um to 390 um in diameter. Sperm were observed to be bell-shaped, with a rounded apical end and a bulge basally, with a 50 um long tail. Egg counts in Protopalythoa sp. from the Great Barrier Reef ranged from 800 to 2400 eggs per full grown female polyp (Ryland and Babcock, 1991). In the Jamaican zoanthids, once fertilization occurs the larvae eventually settle out in areas of coralline algae growth and crawl along the surface till they find a suitable site, usually along the edges of shaded undersurfaces (Karlson, 1981). Although fecundity is very high in zoanthids, settlement rates appear to be low. Sexual reproduction is therefore thought to allow for dispersal and colonization over large distances, while asexual reproduction via growth and fragmentation, is more important for local colonization.
Spawning generally occurs in the spring in the Caribbean (March-April) and on the Great Barrier Reef in Australia (October -November). In Kaneohe Bay, Zoanthus pacificus was found to be sexually active all year long but the highest percentages of fertile polyps occurred in the summer, while Palythoa vestitus was only active from May to September (Cooke, 1976). On the Great Barrier Reef, Protopalythoa were found to spawn at the same time as the mass spawning of stony and soft corals. This spawning event seemed to be connected to an increase in water temperatures. Ryland and Babcock (1991) could not find any reproductive activity from January till June while sea temperatures were either rising or falling. Peak water temperature over the year was 31°C and the minimum was 22°C.
Larval stages of zoantharia
A. Zoanthina larva of Zoanthus.
B. Late stage Zoanthina larva.
C. Zoanthella larva of Protopalythoa.
Zoanthina larvae of Zoanthus sp. collected in the waters off of Puerto Vallarta, Mexico. J. Sprung
Although the complete development of zoanthids has not yet been described, larval forms of Zoanthus and Protopalythoa have been found in plankton. Zoanthina are the larvae of Zoanthus spp., oval in shape they have a girdle of especially long cilia near the oral end. Zoanthella are the larvae of Protopalythoa spp., elongated with a ventral band of very long cilia (Hyman, 1940; see Colin and Arneson 1996 for photos). The importance of sexual
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