Alcyonidium disciforme: an exceptional Arctic bryozoan

Abstract

The ctenostome bryozoan Alcyonidium disciforme is the only known free-living Arctic bryozoan. Alcyonidium disciforme has a strictly Arctic circumpolar distribution. The material studied was collected during 1995–2002 in West Spitsbergen, Svalbard. Densities of the bryozoan in Van Veen grab and SCUBA diving samples are presented together with environmental data. Alcyonidium disciforme is restricted to the inner part of Kongsfjorden—main area of study. The activity of the tidal glacier situated in the inner part of the fjord results in strong environmental gradients, meltwater outflows causing salinity and temperature fluctuations together with high concentrations and rates of mineral material sedimentation. The wide bathymetric range of the species (8 to 240 m) suggests that depth is not its limiting factor within the fjord. High densities of A. disciforme recorded in shallow glacial bay waters indicate that it is highly tolerant of fluctuations in salinity. It also tolerates high mineral sedimentation rates; this is unusual for a suspension-feeding animal. The species has a very regular disc-like shape. Colonies develop a central hole when they reach approximately 3 mm in diameter. There are significant positive correlations between wet mass vs colony diameter, wet mass vs central hole diameter, and diameter of the whole colony vs central hole diameter. Alcyonidium disciforme is adapted, through its colony shape and zooids filled with sand particles acting as a ballast to survive in a dynamic sedimentary environment.