An animal forest that feeds in the shade
The red sea fan, Paramuricea clavata, forms red, purple or yellow colonies on dimly lit rock faces. Each polyp in this colonial cnidarian has eight branching tentacles around its mouth. These capture animal plankton and suspended organic matter. Shade can reduce competition with some algae, provided the site still receives enough food and isn’t smothered by sediment.
A current that favours capture
The current brings planktonic crustaceans, larvae, eggs and organic particles. The tentacles intercept prey and their stinging cells can help hold it. In weak flow, food is replenished slowly. In strong flow, tentacles can deform and lose their grip on prey. The speed at which capture peaks depends on the species and the shape of its polyps.
Fan orientation and exposure to flow
The flat shape puts more polyps in the path of the current when the fan faces it broadside. In a flow tank, Leptogorgia virgulata caught more prey per unit time in this position than when aligned with the flow. Where current direction varies, colonies can branch in several planes. Flexible branches can bend under drag.
