Innervation patterns of sea otter (Enhydra lutris) mystacial follicle-sinus complexes.

Christopher D Marshall,Brian Kot,Kelly Rozas,Verena A Gill

doi:10.3389/fnana.2014.00121

Abstract

Sea otters (Enhydra lutris) are the most recent group of mammals to return to the sea, and may exemplify divergent somatosensory tactile systems among mammals. Therefore, we quantified the mystacial vibrissal array of sea otters and histologically processed follicle-sinus complexes (F - SCs) to test the hypotheses that the number of myelinated axons per F - SC is greater than that found for terrestrial mammalian vibrissae and that their organization and microstructure converge with those of pinniped vibrissae. A mean of 120.5 vibrissae were arranged rostrally on a broad, blunt muzzle in 7–8 rows and 9–13 columns. The F-SCs of sea otters are tripartite in their organization and similar in microstructure to pinnipeds rather than terrestrial species. Each F-SC was innervated by a mean 1339 ± 408.3 axons. Innervation to the entire mystacial vibrissal array was estimated at 161,313 axons. Our data support the hypothesis that the disproportionate expansion of the coronal gyrus in somatosensory cortex of sea otters is related to the high innervation investment of the mystacial vibrissal array, and that quantifying innervation investment is a good proxy for tactile sensitivity. We predict that the tactile performance of sea otter mystacial vibrissae is comparable to that of harbor seals, sea lions and walruses.

Highlights

Sea otters (Enhydra lutris) are the smallest and most recent group of marine mammals to return to marine habitats
The mean number of individual mystacial vibrissae in sea otters reported here is consistent with numbers reported by Kenyon (1975) and the rostral orientation of mystacial vibrissae on the rostrum is consistent with other benthic foragers such as bearded seals (Erignathus barbatus; Marshall et al, 2006) and walruses (Fay, 1982; Kastelein and Mosterd, 1989; Marshall et al, 2006, 2008)
Sea otter F-SCs are similar to pinnipeds in that they are organized in a tripartite blood sinus system and possess common traits that pinnipeds exhibit, such as penetration of the Deep Vibrissal Nerve (DVN) at the base of the follicle

Summary

Introduction

Sea otters (Enhydra lutris) are the smallest and most recent group of marine mammals to return to marine habitats. The foraging ecology of all otters (Carnivora: Mustelidae: Lutrinae) is often categorized as either piscivorous mouth-oriented or invertebrate hand-oriented predation (Radinsky, 1968; Duplaix, 1984; Sivasothi and Nor, 1994; Jacques et al, 2009; Timm, 2013) These foraging categories manifest themselves as divergent feeding kinematics, biomechanics and performance (Timm, 2013) and were originally based on brain sulcal patterns (Radinsky, 1968) and morphological characters (Van Zyll de Jong, 1972). Sea otters use their hands, digits and vibrissae to explore their environment, likely to assist them in identifying prey items. Is sea otter mystacial vibrissae morphology similar to terrestrial taxa, pinnipeds, a hybrid of both, or does it represent a completely novel microstructure and innervation? Due to their expanded vibrissal apparatus, we hypothesized that (1) the number of myelinated axons per F - SC, is greater than terrestrial mammalian vibrissae; and that (2) the microstructure of sea otter F-SCs converge with the vibrissae of pinnipeds rather than with those of terrestrial mammals, or a de novo organization

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Conclusion