Chronic exposure to environmentally relevant levels of citalopram affects crawl-out behavior and morphological antipredator plasticity in a freshwater gastropod.

Citalopram is a commonly prescribed antidepressant and selective serotonin reuptake inhibitor (SSRI). Low wastewater treatment plant removal rates facilitate its ubiquity in natural surface waters. In non-target organisms, SSRIs interact with evolutionarily conserved stress-hormone pathways alongside other neurological receptors to impact behavior, physiology and morphology. However, we still know little about the consequences of citalopram for aquatic invertebrates, including whether it impacts their ability to plastically respond to other environmental challenges. Thus, here we investigate the effects of citalopram on antipredator plasticity in Physella acuta, a freshwater gastropod. Following a split-clutch design, we raised siblings in a fully-factorial 2 × 2 antidepressant/perceived predation risk treatment combination. They consistently received either 18 ng l-1 citalopram (i.e., the median worldwide level of citalopram in natural water bodies) in hydrochloride form or 2 ng l-1 HCl as a vehicle control. This was combined with either conspecific alarm cues (high-risk) or a low-risk water control. In adult snails, we investigated crawl-out behavior, body mass, shell size alongside morphological antipredator defenses such as shell thickness, shell shape and mantle pigmentation. Citalopram exposure reduced crawl-out behavior and induced larger apertures and short globose shells, comparable to the shell shape of high-risk snails. Both high-risk and citalopram non-additively reduced body mass and shell thickness. Furthermore, high-risk snails consistently developed smaller shells, but this effect weakens under citalopram presence. Mantle pigmentation increased under high risk but was unaffected by citalopram. Overall, these fitness-reducing consequences of environmentally relevant levels of citalopram on aquatic gastropods likely have far-reaching ecological consequences.