Behavior of oyster (Crassostrea virginica) larvae in flume boundary layer flows

Larval behaviors are increasingly recognized as determining factors in the water column distribution and subsequent settlement patterns of marine invertebrates. In particular, larvae within one or two body lengths of the seafloor (= interaction zone) may be better able to control their settlement location or respond to settlement cues. Here we examine temporal changes the behavior of larval oysters (Crassostrea virginica) swimming in the bottom 1 cm of a flume boundary layer. In addition, we present a novel behavior, called "dive-bombing", that consists of an abrupt downward acceleration and subsequent contact with the bottom. Larvae were divided into two arbitrarily chosen size classes (those retained on a 153 μm screen and those retained on a 202 μm screen) to separate those that developed eyespots from those that did not develop eyespots. Behavior in a flume boundary layer was monitored from day 15 to day 21 post-fertilization. In the larger size class, the percentage of oyster larvae within close proximity (1 to 2 body lengths) of the flume bed steadily increased from 6% to 28% with larval age. In contrast, if larvae were restricted from reaching larger sizes through sieving this proportion remained constant at 12%. Dive-bombing was exhibited by a constant 4% of the larval population regardless of age or size. Kinematic analysis of swimming paths shows dive-bombing to be distinct from other swimming behaviors and from the passive sinking of dead larvae. For example, vertical acceleration at the initiation of dive-bombing is some 30–400 times that measured for other live or dead larvae. Our results indicate that larvae are capable of rapid acceleration and that they may be able to control their approach to the bottom under a much wider range of conditions than had previously been suspected.