THOMPSON, J.T.: The evolution of mechanical function in the mantle of squids

The mechanical support for the mantle of cephalopod molluscs arises from a complex arrangement of muscle fibers, networks of connective tissue fibers, and in some cases, a rigid rod that runs the length of the mantle. Although there are over 700 species of cephalopods in 47 families, most of our knowledge of the mechanical organization and locomotor function of the mantle comes from detailed studies of a handful of species among only 3 families. Thus, our understanding of the evolution of mantle mechanical support is limited to inferences from an incomplete data set. In an attempt to describe the evolution of mantle skeletal support and to use a comparative approach to test hypotheses of the function of the connective tissue fiber networks of the mantle, I examined the muscle and connective tissue morphology of 58 species of cephalopods from 29 families. The 3 networks of intramuscular (IM) collagen fibers characteristic of a few species of loliginid squids were found in the mantles of many members of the Teuthida (the squids) and are likely to have evolved several times in parallel. In those teuthids thought to attain neutral buoyancy via the storage of low-density fluids in the mantle tissue, IM collagen fibers were replaced by a 3-D reticulated meshwork of collagen fibers. This reticulated meshwork may have evolved in parallel several times, and unlike the squids with IM fibers, it may result in portions of the mantle having nearly isotropic mechanical properties. Collagenous tunics, which were found in most of the species examined, may have evolved in parallel. Tunic morphology varied from a near crystalline crossed-fiber array in the muscular squids to a loose array of parallel collagen fibers in other species. In some of the neutrally buoyant species, a thin layer of longitudinal muscle fibers replaced the outer tunic.