Hydrodynamic morphology and behavior of a free-living sediment-dwelling bryozoan, Alcyonidium disciforme (Smitt)

Abstract

The free-living bryozoan Alcyonidium disciforme (Smitt) is a pliable slightly convex disc with a central opening and lophophores on the upper surface. Field and flow-tank experiments on live and preserved specimens from the Bering and Chukchi Seas as well as model colonies indicate that this morphology 1. (1) maintains a correct convex-side up position on the sea floor 2. (2) resists dislodgement and transport by flow 3. (3) when lifted into the water column, falls to the substratum basal-side up 4. (4) when basal-side up on substratum, is easily reorientated at low flow (≤ l0cm· s −1. Strong negative lift (> - 12 mN) was produced by flow over upright colonies, enabling them to resist suspension and maintain their orientation through a wide range of current velocities (≤ 60 cm · s −1). Experimental closure of the colonies' central opening reversed the direction of lift and resulted in colonies being swept away at much lower current velocities ( ≤ 30 cm · s −1 ). The self-righting ability of A. disciforme is the result of significantly higher drag and lift experienced by inverted vs. upright colonies at all flow speeds. Observations of live A. disciforme suggest the existence of a colony-wide escape response. When disturbed, split and coiled colonies tightened their coil and increased their height. Plastic models used to mimic this response showed “extended” colonies were transported at significantly lower flow velocities (mean = 46 cm · s −1) than colonies of normal aspect (mean = 57 cm · s −1). The shape of A. disciforme aided in directing experimental current flow between the overlapped ends and thus across the covered lophophore regions of split and coiled fragments. The high proportion of colony fragments found in the Bering Sea, many of which had grown into overlapping coils exceeding 360°, suggests that asexual reproduction is important. These findings show that the morphology and behavior of A. disciforme, unique among Bryozoans, enable it to live in high energy, wave swept, sedimentary environments, which are equally unusual for the phylum.