Seasonal and annual growth in erect species of Antarctic bryozoans
Seasonal and annual growth was monitored in situ at Signy Island in two species of bryozoan of different growth morphologies, using photographic and tagging techniques. Cellarinella watersi, a heavily calcified species, exhibited marked seasonality of zooidal production, leading to the formation of annual skeletal growth check lines. These growth checks allowed colony age to be determined, and individual colony growth history to be studied. Alloeflustra tenuis appeared to grow without obvious seasonality, despite seasonal variation in many environmental factors, to a maximum age of about 26 yr old. The amount of organic growth (ash-free dry mass) per year, was very similar in these two species, although there were large differences in dry mass because of differences in skeletal investment. Cellarinella watersi produced comparable annual dry mass to an erect temperate bryozoan species, whereas A. tenuis produced much less. Nematoflustra flagellata, similar in morphology to A. tenuis, was monitored for a single summer only. It grew much faster than A. tenuis, reaching a much larger size at similar maximum age; its summer growth rate was comparable with the temperate species Flustra foliacea. The maximum size obtained by the rigidly erect Cellarinella watersi in shallow waters appeared to be limited by current damage. It is possible that in all these bryozoans, maximum age (and size) is limited in the long term by frequency of iceberg impact. These bryozoans provide a first insight into the growth of Antarctic colonial suspension feeders, which dominate many of the dense benthic faunal assemblages known around Antarctica.