Vertical distribution of decapod crustacean larvae: field and experimental studies

Joanne Kathryn Breckenridge

We investigated the factors affecting vertical distributions of decapod crustacean larvae via both a field and a laboratory study. In our field study, we addressed vertical distribution in relation to predator avoidance and transport. We predicted that if behaviours aiming to maximize predator avoidance and seaward transport were present in decapod larvae, they would be evidenced by shallower vertical distributions and/or higher abundances on nighttime ebb tides. We collected larvae from discrete depths in Willapa Bay, Washington and related their abundance and vertical distribution to a suite of environmental variables. While vertical distributions were variable, Neotrypaea californiensis zoea were shallower at night, supporting our prediction for predator avoidance, and during one sampling, pinnotherid zoea were shallower on ebb tides, supporting our prediction for seaward transport. Abundances varied more predictably with tidal and diel phases than did vertical distributions, with both taxa most abundant on ebb tides, suggesting the presence of behaviours to enhance seaward transport. In a mixed estuary, hydrostatic pressure (water height) would be a reliable indicator of tidal phase. The abundances of both taxa increased with water height and N. californiensis were shallower when the water column was deeper. In our laboratory study, we investigated the impact of haloclines and a light/dark cycle on the vertical distribution of N. californiensis. Resource patches are often associated with physical discontinuities in the water column and the ability to cue to these discontinuities to locate prey could increase foraging success. We recorded the depth distributions of larvae in salinity stratified and vertically homogenous tanks using video cameras that panned the vertical extent of each tank hourly over periods of approximately 24 hours. Our results indicate that N. californiensis will aggregate in the presence of a physical thin layer and exhibit diel vertical migration, but that this response is plastic and possibly dependent on pre-conditioning. Our results add to the understanding of larval migration and will increase our ability to predict dispersal and recruitment dynamics. In addition, the aggregation of decapod larvae around thin layers has implications for the formation and/or erasure of thin layers. KEYWORDS: Selective tidal stream transport; biological thin layers; decapod behaviour; vertical distribution; diel vertical migration; zooplankton behaviour

Vertical distribution of decapod crustacean larvae: field and experimental studies

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