Comparative Brain Morphology of the Greenland and Pacific Sleeper Sharks and its Functional Implications

Kara E Yopak,Aaron T Fisk,Christian Lydersen,Kirk W Feindel,Shaun P Collin,Bailey C Mcmeans,Christopher G Mull,Kit M Kovacs

doi:10.1038/s41598-019-46225-5

Abstract

In cartilaginous fishes, variability in the size of the brain and its major regions is often associated with primary habitat and/or specific behavior patterns, which may allow for predictions on the relative importance of different sensory modalities. The Greenland (Somniosus microcephalus) and Pacific sleeper (S. pacificus) sharks are the only non-lamnid shark species found in the Arctic and are among the longest living vertebrates ever described. Despite a presumed visual impairment caused by the regular presence of parasitic ocular lesions, coupled with the fact that locomotory muscle power is often depressed at cold temperatures, these sharks remain capable of capturing active prey, including pinnipeds. Using magnetic resonance imaging (MRI), brain organization of S. microcephalus and S. pacificus was assessed in the context of up to 117 other cartilaginous fish species, using phylogenetic comparative techniques. Notably, the region of the brain responsible for motor control (cerebellum) is small and lacking foliation, a characteristic not yet described for any other large-bodied (>3 m) shark. Further, the development of the optic tectum is relatively reduced, while olfactory brain regions are among the largest of any shark species described to date, suggestive of an olfactory-mediated rather than a visually-mediated lifestyle.

Highlights

In recent years, a comparative approach to understanding brain evolution has provided great insights into correlations between complex brains and behavior
Despite what is presumed to be a diminished visual capacity and that locomotory muscle power is often depressed at cold temperatures[57], such that the sleeper sharks are extremely slow swimmers[58,59], S. microcephalus and S. pacificus remain capable of capturing active prey, feeding on a wide range of mid-water and benthic invertebrates and fishes[52,60]
In both Somniosus microcephalus and S. pacificus, the brain (Table 1) occupies only a small proportion (

Summary

Introduction

A comparative approach to understanding brain evolution has provided great insights into correlations between complex brains and behavior. Significant differences in the size, surface area, and density of the peripheral sense organs of cartilaginous fishes have been correlated with a range of ecological parameters[31,32,33,34,35] This variability suggests there may be differences in sensitivity, acuity, and/or detection thresholds[36,37], the links between various peripheral characteristics and improved function remains uncertaine.g.38,39. Despite what is presumed to be a diminished visual capacity and that locomotory muscle power is often depressed at cold temperatures[57], such that the sleeper sharks are extremely slow swimmers[58,59], S. microcephalus and S. pacificus remain capable of capturing active prey, feeding on a wide range of mid-water and benthic invertebrates and fishes[52,60]. No study to date has examined the relative organization of all major brain regions in these sharks and contextualized it within a comparative framework

Objectives

Methods

Results

Discussion

Conclusion