Flexion of hydroid colonies in a current

Suspension feeding is a common feeding strategy among small sessile invertebrates. While active suspension feeders use metabolic energy to draw water through their feeding apparatus, passive suspension feeders depend on local currents to feed (Sponaugle and LaBarbera 1991). Erect colonies of passive suspension feeders therefore face a potential contradiction: how to increase the feeding surface such that sufficient flow will occur, while keeping drag forces on the feeding modules and colony low? Flexibility is a feature of many sessile organisms, and a strategy for dealing with drag: bending or stretching with the current allows for a morphological reconfiguration that alters the drag footprint of the animal. Vogel (1984) argues that this shape deformation can be adaptively useful, and is not necessarily merely a consequence of drag force on the organism. Several studies have investigated the effects of drag on Cnidarian colonies. Harvell and LaBarbera (1985) find that the natural flexibility of Abietenaria rigida (Cnidaria: Hydrozoa) colonies reduced drag on and velocity around polyps, in contrast to artificially stiffened colonies, which experienced a much greater drag force. Sponaugle and LaBarbera (1991) find that flexion in Gorgonian colonies in flow appears to function at least in part to reduce local flow velocity around polyps. Other studies (e.g. Hunter 1989) highlight the importance of flow velocity, directionality, and colony architecture in different flow regimes. This study aims to address questions of flexion in the hydroid Obelia geniculata Linnaeus 1758 (Cnidaria: Hydrozoa)