Monoamines (MA) are neurotransmitters or endocrine modulators that contain an amino group connected to an aromatic ring by a two-carbon chain, on whose basis they are normally classified into three groups: 1) those containing the indolamine or tryptamine group, which include serotonin (5-hydroxytryptamine [5-HT]) and melatonin, characterized by an indole ring structure; 2) the histamine group, including histamine, derived from histidine decarboxylation, and 3) the catecholamine (CA) group, which includes dopamine (DA), norepinephrine (NE), and epinephrine (EPI) and also trace amines octopamine (OA) and tyramine (TA), mainly occurring in invertebrates (Rosikon et al., 2023).

In mammals/humans MA play a role in complex behaviors, cognitive functions, such as learning and memory, as well as in fundamental homeostatic processes such as sleep and feeding (Goulty et al., 2023). MA are present in all vertebrates studied so far, where evolution of the MA systems has always served to provide the adaptation to challenging environments (Savaliya and Georrge, 2020). A recent phylogenomic analysis showed that most of the genes involved in MA production, modulation and reception, originated in bilaterians, indicating that the monoaminergic system is conserved in invertebrates and vertebrates (Goulty et al., 2023). A neurosecretory/neuroendocrine system appeared very early in metazoans, so that all invertebrates possess neuroendocrine cells releasing signaling molecules in their body fluids. Moreover, in most invertebrates, except for arthropods, the lack of a canonical endocrine system implies that the modulation of physiological functions is essentially neuroendocrine (Malagoli et al., 2017; Seralini and Jungers, 2021). However, due to the large variety of invertebrate groups and species, information on their neuroendocrine systems is still fragmentary.

In this work, we review available information on MA systems in invertebrates, with focus on bivalve molluscs. Bivalves are sessile, filter feeding organisms widespread in different aquatic environments (freshwater, brackish, marine), where they are subjected to a variety of environmental stressors, including exposure to pollutants. It has long been recognized that different MA are present in high amounts in bivalve tissues, where they can play multiple roles in modulation of physiological processes when facing variable environments.