The a-like octopamine receptors (OctR) are believed to be the evolutionary precursor to the vertebrate a2-adrenergic receptors (a2-ARs) based upon sequence similarity and the ability to interact with norepinephrine and a number of compounds that bind with high affinity to a2-ARs. Barnacles and fruit flies are two prominent model marine and terrestrial representatives of the Arthropoda phylum, and although a-like OctRs have been cloned from Balanus improvisus (BiOctR) and Drosophila melanogaster (DmOctR), little is known about the structure–activity space for these important species. A diverse panel of 22 probes spanning different structural classes were employed to interrogate the structure–activity of the BiOctR and DmOctR. While BiOctR and DmOctR exhibited similar functional profiles for mammalian biogenic amine G protein–coupled receptor agonists and antagonists, some ligands had dramatically different mechanisms of action. For instance, significant differences in the efficacy for some agonists were observed, including that vertebrate biogenic amines structurally related to octopamine acted as superagonists at the DmOctR but partial agonists at the BiOctR, and the two species diverged in their sensitivities to the a2-AR antagonist [3H]rauwolscine. Furthermore, sodium enhanced [3H]rauwolscine’s interactions with the BiOctR, but not at a vertebrate a2-AR. Molecular mechanistic studies indicate that rauwolscine interacts with the BiOctR, DmOctR, and a2C-adrenergic receptor at an allosteric site. In addition, compounds that acted as agonists at a cloned a-like BiOctR also induced a hyperactivity response in Balanus cyprids mediated by the a-like OctR, suggesting that the receptor may serve as a higher throughput proxy for discovering compounds with potential cyprid deterrent properties.