Behavioral responses to artificial skyglow vary in coastal invertebrate larvae

Light influences the biology of coastal animals by cueing behavior and biological clocks. Over the last 150 years, artificial light has increasingly illuminated the nighttime sky. Many coastal invertebrates begin life as planktonic larvae in waters exposed to artificial light at night (ALAN) along developed coastlines, but little is known about the impact of this light pollution on larval behavior. This study combined laboratory experiments and field observations. In the laboratory, we studied phototactic responses of crab, barnacle, and oyster larvae to diffuse sources of natural and artificial light representing skyglow from moonlight, a blue-filtered streetlight, and an LED streetlight. Late-stage, but not early-stage, crab larvae descended in response to increases in downwelling light, with the most dramatic responses to the LED treatment. Barnacle larvae ascended subtly in response to the moonlight and LED mimics, while oyster larvae showed no directional response to light stimuli. To assess whether photoresponses impacted larval depth in the field, we collected depth-stratified zooplankton samples at two coastal sites: a "dark" and "bright" site with low and high light pollution, respectively. During a full moon, zoeal larvae, particularly caridean shrimp, resided deeper at the bright site than at the dark site, in line with the observed photoresponses of crab larvae. Given that the most pronounced responses to ALAN were observed in relatively large crustacean larvae with complex visual systems, we predict that coastal light pollution likely disrupts depth regulation and thereby dispersal of late-stage zoeal larvae prior to settlement in adult habitat.