Endoscopic Observations Of Invertebrate Larval Substratum Exploration And Settlement

In the marine environment, competent larvae of sessile invertebrates are influenced by water flow and a variety of biological, chemical and physical cues. Most research has focused on how these biotic and abiotic factors influence where individual larvae ultimately settle. Much less is known about post-contact exploration prior to metamorphosis. This is, in part, due to limitations associated with directly observing small larvae (100 to 500 μm) in flowing seawater. A study was conducted in Beaufort, North Carolina, USA to understand how larvae of the barnacle Balanus amphitrite and the bryozoan Bugula neritina respond to a variety of flow rates (0, 1.3, 6.1 and 8.3 cm s-1) and surface types (clean, biofilmed, 1 and 2 wk fouled). Larval behavior was studied by means of endoscopy in a running-seawater chamber. Larval movements were observed at 30 frames s-1 for individuals that remained in contact with surfaces from \u3c1.0 s to 44.5 min. Both flow rate and surface type significantly influenced the behavior of the species examined, although larvae of B. amphitrite and B. neritina often responded very differently to the same treatment conditions. Larvae of B. amphitrite explored more surface area (fractal dimensions) in moving water than in still water, but flow did not influence the direction of travel. Mean exploration rate of B. amphitrite did not vary among treatments and ranged from 0.16 to 0.21 mm s-1. More cyprid larvae explored surfaces with macrofouling and spent significantly longer times on these surfaces than on clean ones. In still water, larvae of B. neritina repeatedly contacted, explored and swam away from the test surfaces. In contrast, in flow, larvae of B. neritina never swam away from any surface after contact was made. Individuals of B. neritina crawled directly upstream on clean and biofilmed surfaces at all flow rates unless individuals encountered filamentous structures (biofilmed surfaces only). When this happened, larvae of B. neritina frequently remained attached to filaments as the filaments moved with the flow. These larvae were then either dislodged or immediately resumed crawling upstream upon contact with the plate surface. A limited number of larvae of both species settled during our observations (15% B. amphitrite, 18% B. neritina). Settlement of B. amphitrite was not correlated with flow rate or surface type; larvae of B. neritina settled only on 2 wk fouled surfaces