The monoaminergic system of Mytilus galloprovincialis: possible developmental functions and sensitivity to emerging contaminants

Calcifying larvae of marine invertebrates are highly sensitive to environmental stressors and contaminants of emerging concern (CEC), including endocrine disrupting chemicals (EDCs) and pharmaceutical and personal care products (PPCPs). Marine bivalves living in dynamic coastal environments have evolved sophisticated stress response mechanisms, with neuroendocrine regulation playing a critical role. Key components of this regulation include the monoamines (MAs) dopamine (DA) and serotonin (5-HT), which are pivotal neuromodulators in shell formation and larval development of bivalves and are targets of environmental stressors such as ocean acidification and CEC, notably EDCs and PPCPs. Despite the importance of monoamines in larval development, information on the ontogeny of the monoaminergic system during the early development of bivalve larvae is limited. Among PPCPs, serotonin selective reuptake inhibitors (SSRIs) are designed to bind to the serotonin reuptake transporter (SERT), yet there is currently no information regarding its presence in bivalves. To fill this knowledge gap, Mytilus galloprovincialis was used as model organism. This study aimed to characterise the presence of MA systems in M. galloprovincialis genome, alongside their expression and localisation during embryo-larval development within the neuroendocrine tissue. The involvement of MAs in development was investigated through pharmacological experiments. The effects of SSRIs were evaluated using Fluoxetine (FLX) and Citalopram (CIT), alongside an examination of their targets in the genome and transcriptome. Six MA systems were identified in M. galloprovincialis genome, with 5-HT and DA being the most expressed during development. Cells producing 5-HT and DA were localised in neuroendocrine areas, specifically in the Apical Organ/Cerebral Ganglion and Pedal Ganglia, respectively. The expression of L-Amino Acid Decarboxylase and serotonin and dopamine receptors (5-HTRs and DRs) prior to the synthesis of 5-HT and DA suggests that these components may be involved morphogenetic roles and environmental interactions. Both transcriptomic and pharmacological results indicate that 5-HT and DA play key roles in larval development and shell biogenesis. The localisation of 5-HTRs and DRs throughout development, along with their spatial relationship to the synthesis enzymes, suggests their potential involvement in synaptic and neuroendocrine functions. Moreover, the colocalisation of 5-HTR1A_inv and DR3-like implies a possible reciprocal regulation between the serotonin and dopamine systems. M. galloprovincialis possesses two types of SSRIs targets: selective (SERT) and non-selective (Nose Resistant to Fluoxetine – nrf, and 5-HTRs). Both targets were found to be differentially expressed during embryo-larval development. Exposure to high concentrations of FLX and CIT did not strongly affect larval phenotypes, suggesting some mechanisms of resistance where nrf may play a role. This work shows for the first time a comprehensive characterisation of the MA system in M. galloprovincialis; moreover, the identification of different targets of SSRIs lays the groundwork for future studies on the role of the MA system during development, and in response to different environmental stressors, including SSRIs and, potentially, other antidepressants released in coastal marine environments.