This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Neuroscience. Please check back later for the full article.
Within the phylum Mollusca, cephalopods encompass a small and complex group of exclusively marine animals that live in all the oceans of the world with the exception of the Black Sea. They are distributed from shallow waters down into the deep sea, occupying a wide range of ecological niches. They are dominant predators, and are themselves prey, with high visual capability and well developed vestibular, auditory, and tactile systems. Nevertheless, their world is chemically driven so that water-soluble and volatile odorants are key mediators of many physiological and behavioral events.
Conventionally, olfaction, referring to the sense of smell, perceives chemical stimuli emanating from a distant source. The other chemosensory organ generally requires physical contact with the source for detection. This sensory modality is called gustation.
For cephalopods and for other aquatic animals, chemical cues convey a remarkable amount of information critical to social interaction, habitat selection, defense, prey localization, and courtship and mating, affecting not only individual behavior and population level processes, but also community organization and ecosystem functions.
Cephalopods possess sensory systems that have anatomical similarities to the olfactory systems of land-based animals, but the molecules perceived from a distance are different because, in water, solubility matters. Many insoluble molecules that are detected from a distance on land must be perceived by contact in aquatic systems. Most studies regarding olfaction in cephalopods have been performed considering only water-born molecules that can be detected by the olfactory organs. However, cephalopods are equipped with gustatory systems consisting of receptors distributed on the arm suckers of octopods, the buccal lips on decapods, and the tentacles on nautiluses.
To date, what is known about the olfactory organ in cephalopods comes from studies of nautiluses and coleoids (decapods and octopods). In nautiluses, the olfactory system consists of a pair of rhinophores located below each eye and open to the environment via a tiny pore, whereas coleoids have a small pit of ciliated cells, present on either side of the head, below the eyes and close to the mantle edge.
Several behavioral and functional studies have been conducted on nautiluses and decapods, demonstrating the role of olfactory organs in mate choice, predation improvement, defense strategy, and spatial orientation. Recent functional and morphological studies of octopods have revealed new perspectives about the role played by olfaction in the complex behavioral patterns shown by these fascinating animals.