Influence of food concentration on polypide regression in the marine bryozoan Electra pilosa (L.) (Bryozoa: Cheilostomata)

The effects of different food concentrations on polypide regression and colony growth rate and form in the marine bryozoan Electra pilosa (L.) were investigated under laboratory conditions. Colonies were reared on the cryptophyte Rhodomonus sp. at four different food concentrations (1 × 10²-1 × 10⁵ cells·ml^-1). Three replicated genotypes were used to test for genotypic effects on the chosen parameters. Polypide regression and regeneration were followed through three or more successive cycles and showed the same pattern as described for other cheilostome species. The life spans of the feeding functional polypides decreased with increasing food concentration, and differed between genotypes. There was significant food concentration × genotype, and food concentration × polypide generation interaction. Colony growth rates were influenced significantly by food concentration and genotype and there was genotype × food concentration interaction. There were significant effects of food concentration on colony form, the different genotypes had different forms and there was significant food concentration × genotype interaction. The results show that, in the absence of other external stimuli, polypide regression is controlled both by the amount of food ingested by the polypides and by genotype. Selection for polypide turnover rates could act variously according to the control of zooidal longevity: assuming no rejuvenatory advantage, if zooidal longevity is predominantly genetically controlled, or if there is a maximum possible number of polypide generations per zooid, low rates of polypide turnover might be favoured. If, however, zooids are beneficially rejuvenated by the process of polypide regression, an elevated turnover rate could maximize zooidal lifespans, with concomitant increases in whole-colony energetic throughput and reproductive output.