LAUSR

Neuropeptides are conserved metazoan signaling molecules, and represent useful markers for comparative investigations on the morphology and function of the nervous system. However, little is known about the variation of neuropeptide expression patterns across closely related species in invertebrate groups other than insects. In this study, we compare the immunoreactivity patterns of 14 neuropeptides in three closely related microscopic dinophilid annelids (Dinophilus gyrociliatus, D. taeniatus and Trilobodrilus axi). The brains of all three species were found to consist of around 700 somata, surrounding a central neuropil with 3–5 ventral and 2–5 dorsal commissures. Neuropeptide immunoreactivity was detected in the brain, the ventral cords, stomatogastric nervous system, and additional nerves. Different neuropeptides are expressed in specific, non‐overlapping cells in the brain in all three species. FMRFamide, MLD/pedal peptide, allatotropin, RNamide, excitatory peptide, and FVRIamide showed a broad localization within the brain, while calcitonin, SIFamide, vasotocin, RGWamide, DLamide, FLamide, FVamide, MIP, and serotonin were present in fewer cells in demarcated regions. The different markers did not reveal ganglionic subdivisions or physical compartmentalization in any of these microscopic brains. The non‐overlapping expression of different neuropeptides may indicate that the regionalization in these uniform, small brains is realized by individual cells, rather than cell clusters, representing an alternative to the lobular organization observed in several macroscopic annelids. Furthermore, despite the similar gross brain morphology, we found an unexpectedly high variation in the expression patterns of neuropeptides across species. This suggests that neuropeptide expression evolves faster than morphology, representing a possible mechanism for the evolutionary divergence of behaviors.