Neuroendocrine regulation of the metamorphic transition of giant clams

For many benthic marine organisms, metamorphosis of larvae into juveniles is accompanied by a shift from the plankton into the benthos. This pelagobenthic transition must be precisely regulated and integrated with external environmental cues to allow the larva to settle at the right place, time, and conditions. Though the molecular mechanisms of the metamorphic processes in marine invertebrates remain relatively understudied, increasing evidence highlights the significant role played by neuroendocrine signaling in the modulation of this process. In this study, we examined the effect of the neurotransmitters gamma-aminobutyric acid, serotonin, epinephrine, and L-DOPA on the settlement and metamorphosis of the giant clam, Hippopus hippopus. We then determined the expression of genes encoding neurotransmitter-related enzymes and receptors during early development using transcriptomes of H. hippopus and a related species, Tridacna gigas. Epinephrine emerged as a promising candidate for inducing settlement and metamorphosis, while serotonin and L-DOPA may be involved in other settlement-related functions such as larval swimming and seeking behavior. This is supported by the upregulation of transcripts encoding enzymes involved in neurotransmitter biosynthesis during the competent pediveliger stage. The likely involvement of neurotransmitters in the regulation of the metamorphic process suggests their potential to boost spat yield in giant clam aquaculture. Beyond its applications in mariculture, these results provide insights into the complexity of the giant clam larval neuroendocrine system.