Individual-based model of larval transport to coral reefs in turbulent, wave-driven flow: behavioral responses to dissolved settlement inducer

Abstract

Larvae of many benthic marine animals set- tle and metamorphose in response to waterborne chem- ical cues. Can the behavioral responses of microscopic larvae in the water column to dissolved chemical cues affect their transport to the substratum in the turbulent, wave-driven flow characteristic of many shallow coastal habitats? We addressed this question using an individ- ual-based model of larvae of the sea slug Phestilla sibo- gae, transported in the oscillatory flow above coral reefs. Larvae of P. sibogae stop swimming and sink in response to a dissolved inducer released by their prey, the coral Porites compressa, and resume swimming when exposed to inducer-free water. The instantaneous fine-scale spa- tial distribution of inducer in the flow above a reef is fila- mentous; hence microscopic larvae swimming or sinking through the water encounter inducer above threshold concentration in on/off temporal patterns. Model results show that using a time-averaged inducer concentration gradient to calculate larval transport rates to the reef overestimates the rates by <15% (depending on the threshold concentration of inducer required to trigger larval sinking) compared with those calculated using time-varying, fine-scale inducer distributions. Aspects of larval behavior that have large effects on rates of trans- port to the substratum are swimming speed and direc- tion, sinking speed, and sensitivity (threshold concentra- tion) and responsivity (percent of encounters eliciting a response) to inducer. In contrast, lag times to start sinking after encountering inducer or to resume swimming after re-entering inducer-free water, have negligible effect.