Molecules present in an animal's environment can indicate the presence of predators, food, or sexual partners and consequently, induce migratory, reproductive, foraging, or escape behaviors. Three sensory systems, the olfactory, gustatory, and solitary chemosensory cell (SCC) systems detect chemical stimuli in vertebrates. While a great deal of research has focused on the olfactory and gustatory system over the years, it is only recently that significant attention has been devoted to the SCC system. The SCCs are microvillous cells that were first discovered on the skin of fish, and later in amphibians, reptiles, and mammals. Lampreys also possess SCCs that are particularly numerous on cutaneous papillae. However, little is known regarding their precise distribution, innervation, and function. Here, we show that sea lampreys (Petromyzon marinus L.) have cutaneous papillae located around the oral disk, nostril, gill pores, and on the dorsal fins and that SCCs are particularly numerous on these papillae. Tract‐tracing experiments demonstrated that the oral and nasal papillae are innervated by the trigeminal nerve, the gill pore papillae are innervated by branchial nerves, and the dorsal fin papillae are innervated by spinal nerves. We also characterized the response profile of gill pore papillae to some chemicals and showed that trout‐derived chemicals, amino acids, and a bile acid produced potent responses. Together with a companion study (Suntres et al., Journal of Comparative Neurology, this issue), our results provide new insights on the function and evolution of the SCC system in vertebrates.

Solitary chemosensory cells (SCCs) and their innervating fibers are located in the respiratory system of many vertebrates, including papillae on lamprey gill pores. In order to gain stronger insight for the role of these chemosensory cells, we examined immunocytochemical and innervation characteristics, as well as abundance at the different stages of the lamprey life cycle. The SCCs were distinguished from the surrounding epithelial cells by calretinin and phospholipase C140 immunoreactivity. Nerve fibers extended into the gill pore papillae, as far as the SCCs and serotonergic fibers extended from the underlying dermis into the papillar base. Gill pore papillae were absent and SCCs were sparse during the larval stage and in newly transformed lamprey. Few SCCs were located on small nub‐like papillae during the parasitic juvenile stage, but SCCs were abundant on prominent papillae in migrating and in spawning adults. These findings show similarities between the SCCs in lampreys and other vertebrates and suggest that gill SCC function may be important during the feeding juvenile and the adult stages of the lamprey life cycle.

The post‐embryonic odor imprinting paradigm suggests Chinook salmon (Oncorhynchus tshawytscha) acquire memory to stream‐specific amino acid olfactory odors prior to emergence as fry. Because effects of olfactory experience on development can be examined by mapping olfactory sensory neurons extending into distinct territories of glomerular neuropil in the olfactory bulb, glomerular patterning from early yolk‐sac larva to fry was documented in wild salmonids, a temporal scale not yet thoroughly explored. Labeling olfactory sensory neurons with anti‐keyhole limpet hemocyanin (anti‐KLH) revealed seven spatially conserved glomerular territories visible at hatch and well established by the late yolk‐sac larva developmental stage. Because of the responsiveness of microvillous olfactory sensory neurons to amino acids, corresponding glomeruli in the lateral bulbar region were mapped using anti‐calretinin. The dorsolateral territory, distinct glomeruli of the lateral glomerular territory and the ventromedial glomeruli were immunoreactive to both KLH and calretinin. This study offers a morphological description of glomerular patterning in post‐embryonic stages in wild Chinook salmon, a temporal window previously shown to be significant for olfactory imprinting. J. Morphol. 278:464–474, 2017. © 2017 Wiley Periodicals, Inc.