Water flow and coral colony size: Interhabitat comparisons of the octocoral Alcyonium siderium.

Colonies of the octocoral Alcyonium siderium Verrill form aggregations on subtidal vertical rock walls in the Gulf of Maine. Maximum and mean colony sizes increase dramatically from protected embayments to sites exposed to ocean swells and from the bottom to the top of each vertical wall studied. Water movement along the substratum, generated by surface waves and tides, also increases along both gradients. Although plankton concentrations were similar at all sites during a 2-yr period, increased encounter rates of zooplankton with coral tentacles will result in greater energy intake and, thus, faster growth rates at the more exposed sites with greater flow. Other hypotheses for the proximate cause of the size gradient, such as differential mortality of large colonies or generally higher mortality rates at the protected site, are not consistent with available data.