The effects of cell concentration on colony growth and feeding behaviour were investigated in the polymorphic marine bryozoan Celleporella hyalina (Bryozoa: Cheilostomata) under controlled, laboratory conditions. We compared the relative zooidal composition of colonies cultured on Rhodomonas baltica (Karsten) for seven weeks at 18°C. We observed the pharynx of active lophophores on sudden exposure to high cell concentrations, and recorded the length of feeding episodes and ingestion rates.
Colonies grew, reproduced and produced viable larvae at all levels of food supply, but performed optimally at 50–100 cells µl-1. Cell concentration influenced somatic parameters more than sexual, the latter being more closely associated with genotype. Relative somatic investment was greatest at 10 cells µl-1 and least at 50 cells µl-1. The frequency of ovicells containing larvae remained at about 74% irrespective of food supply. Basal male investment was greatest at 10 cell µl-1, frontal male investment at 50 cells µl-1, and female investment at 100 cells µl-1.
Cell ingestion rates of previously starved zooids increased as a function of cell concentration, with an inverse relationship between length of feeding episode and concentration. Food particles were drawn from below the lophophore by lateral cilia, ciliary reversal brought them over the mouth, and ingestion then resulted from a pharyngeal current. These results suggest that, on exposure to high cell concentrations, bryozoans may exhibit specific behaviour.