Effects of currents, waves, and biofilms on motion and surface contacts by tubeworm larvae swimming above or below surfaces

Abstract

Larvae of many benthic marine invertebrates swim more slowly than the ambient water flow that carries them past surfaces where they might settle. Competent larvae of the tubeworm Hydroides elegans, abundant early colonists in warm-water fouling communities, were used to determine if active behavior by microscopic larvae carried in flowing water can affect their contacts with surfaces representing early stages of fouling community succession (clean flat surface, flat biofilm, biofilm on worm tubes). We studied larvae in waves and in the unidirectional flow used in earlier studies of larval settlement, and near surfaces above them (like those where fouling organisms recruit) or below them. We videotaped larval motion near surfaces in a small flume in which we mimicked fine-scale flow measured near surfaces in harbors. Swimming larvae within millimeters of surfaces moved up and down while being carried by horizontal flow, enhancing their contact rates with surfaces above and below them compared with dead larvae passively carried by the current. After contact, live larvae ‘bounced’ along the surface, sampling it many times per distance they were carried by the current, whereas dead larvae did not. Thus, active behaviors of larvae of H. elegans, which must touch a biofilmed surface to be stimulated to attach and metamorphose, enhance their contact with and exploration of surfaces in flowing water. These behaviors are effective in unidirectional flow and waves, for surfaces above or below larvae, and for smooth or rough surfaces, all of which are conditions that the larvae of fouling-community animals encounter in harbors.