Comparative morphology of cephalic cartilage and statocysts of Mediterranean cephalopods using magnetic resonance imaging (MRI)

Organs responsible for equilibrium of cephalopods, the statocysts, possess numerous similarities with the vestibular systems of vertebrates. The statocysts are embebbed in the cephalic cartilage protecting the brain. Although statocysts have been largely described, few studies focused on the cephalic cartilage and its morphology. The aim of this study is to describe the morphology of both statocysts and cartilage, to compare them between different species and to determine which are the parameters influencing their morphology. For species of Decapodiformes (Sepia officinalis, Sepiola rondeletii, Loligo vulgaris and Illex coindetii) and two species of Octopodiformes (Octopus vulgaris and Eledone cirrhosa) have been studied. Cephalic cartilages from these species have been scanned by magnetic resonance imaging (MRI) and volumes and linear measurements have been taken on the 3 dimension reconstructed models of the cartilages and statocysts. Results show that Octopodiformes possess a globular cartilage which surrounds almost totally the brain. On the other hand Decapodiformes posses a cartilage with bigger lateral and anterior foramens and then surrounds a smaller part of the brain. The morphology of the statocysts also varies between the two superorders. The statocysts of Octopodiformes are divided into two parts: endolymph and perilymph while those of Decapodiformes aren’t. These morphological variations between the two groups might be related to their ecology. Eledone cirrhosa and Octopus vulgaris are typically benthic species and then might receive shocks more often than Decapodiformes that possess a more pelagic lifestyle. Octopodiformes might then need a better protection for their brain than Decapodiformes.