Asexual Reproduction by Transverse Fission and Some Anomalies in the Sea Anemone Nematostella vectensis

Abstract

Nematostella vectensis is one of only 5 sea anemones known to reproduce asexually by transverse fission. Sibling individuals of this species divide at highy variable rates with some individuals dividing rarely or not at all, while others may divide many times a year. Field populations are frequently unisexual, and such populations may be clones derived from a single founder. As individual anemones grow, the asexual fragments produced do not necessarily become larger, nor is the time taken to regenerate a functional oral crown on a fragment related to the size of the fragment. The inclusion of a bolus of undigested food in an aboral fragment may delay completion of regeneration as compared to fragments without a bolus. Increased food intake increases the frequency of fission and results in smaller fragments but does not significantly influence the time fragments take to regenerate. Starvation suppresses fission in individuals but does not totally eliminate it. Multi-crowned anomalies are common in natural and in laboratory populations. Subsequent fission of multi-crowned individuals produces normal, single-crowned anemones and meets the definition of budding, a truly rare phenomenon in sea anemones. Additional key words: Cnidaria, Actiniaria, regeneration, clones The most common means of vegetative asexual reproduction among sea anemones are longitudinal fission and pedal laceration (Stephenson 1929; Shick 1991). Both have been studied in a number of species and no common regulator for these methods has been found (Sebens 1980; Shick 1991). Two less common modes of vegetative reproduction are transverse fission and budding, the latter being restricted to boloceroidarian sea anemones which regenerate new individuals from their tentacles (Shick 1991). Transverse fission occurs in more diverse sea anemones, but is known for only 5 species: Anthopleura stellula (Ehrenberg 1834) (see Schmidt 1970), Aiptasia mutabilis (Gravenhorst 1831) (formerly Aiptasia couchii Cocks 1850: see Stephenson 1935), Gonactinia prolifera (M. Sars 1835) (see Chia et al. 1989), Fagesia lineata (Verrill 1873) (see Crowell & Oates 1980), and Nematostella vectensis Stephenson 1935 (see Lindsay 1975; Williams 1976; Frank & Bleakney 1978; Hand & Uhlinger 1992). Nothing is known about the control of budding in sea anemones, and to date only the reports of Schmidt (1970) on Anthopleura stellula and Hand and Uhlinger (1992) on Nematostella vectensis provide any data on a To whom correspondence should be addressed. the control of transverse fission. Both these studies showed that fission rates were enhanced at lower sa-