SEASONALITY OF POLYPIDE RECYCLING AND SEXUAL REPRODUCTION IN SOME ERECT ANTARCTIC BRYOZOANS

The seasonality of polypide cycling has been investigated for three sp ecies of erect bryozoans from Antarctica: Isoseculiflustra rubefacta ( Kluge, 1914), Nematoflustra flagellata (Waters, 1904) and Himantozoum antarcticum (Calvet, 1905). Approximately ten colonies of each species were collected monthly by SCUBA divers over a 14 mo period during 199 2/1993, and the status of each individual zooid was classified as diff erentiating/regenerating, active (feeding autozooids), degenerate (bro wn body) or sexually reproductive (ovicells present, or zooid containi ng a larva). Polypide cycling in all three species was distinctly seas onal. New zooids formed at the growth margin and typically contained a ctively feeding polypides for similar or equal to 9 mo before these po lypides degenerated into brown bodies in the austral winter (June). Ve ry few polypides were active in the period from June to August, when w ater-column food levels were at their lowest; after this period new po lypides differentiated. Individual zooids typically underwent a total of five (I. rubefacta and N. flagellata), or at least four (H. antarct icum) complete polypide cycles before becoming senescent. Polypide lif etimes generally became shorter as the age of the zooid increased. Sex ual reproduction was also distinctly seasonal in these species, with b ands of ovicells or sexually reproductive zooids being formed each yea r in late summer once a given colony had grown to a threshold size (or age). Larvae were then brooded for similar or equal to 10 mo before b eing released in January/February (N. flagellata) or February/March (H . antarcticum). The seasonal patterns of polypide cycling are related clearly to the variations in food availability, and these species appe ar to have the longest zooid lifetime (similar or equal to 5 yr) and t he slowest polypide cycling (once per year with polypide lifetimes up to 10 mo) reported for any bryozoan so far.