Biomechanics and composition of fixed neurocranial tessellated cartilage from the neurocranium of four mobulid ray species with different depth ranges.

Mobulidae are large filter-feeding cartilaginous fishes (Chondrichthyes) that migrate in the world's oceans. However, how their skeletal tissues have adapted to varying environments remains poorly understood. In this study, skeletal adaptations were investigated in four devil rays (Mobulidae), inhabiting various depth ranges from the surface (Mobula eregoodoo 0-50 m, Mobula thurstoni 0-100 m) to deeper water (Mobula mobular 0-1112 m, Mobula tarapacana 0-1896 m). However, since these are highly protected species, often with offshore, pelagic lifestyles, samples are difficult to obtain and are often preserved by freezing or fixation. Thus, the effects of formalin fixation were assessed on neurocranial samples from a ray (M. eregoodoo), a shark (Centroselachus crepidater) and a chimaera (Hydrolagus bemisi) before the mechanical and compositional properties of fixed neurocranial tessellated cartilage (n = 2 per species) were assessed in the four mobulid ray species using uniaxial compression, micro- and nano-indentation alongside assessments of mineralization, glycosaminoglycan (GAG) content and water distribution. During the fixation experiment, mechanical properties decreased by over 30%, possibly owing to leaching or water exclusion. With this effect in mind, the mechanical behaviour in mobulid rays displayed a decreasing trend among species with increasing depth ranges. No correlation between mechanical properties and GAG content or mineralization was found; however, water displayed a negative relationship with the mechanical properties. Collectively, these results suggest a more hydrated and compliant cartilage matrix in species with increased depth range compared with shallower species. This study provides the first account of energy dissipation, toughness, strength and relaxation behaviour of tessellated cartilage in chondrichthyans, revealing new insights into skeletal adaptation in cartilaginous fishes.