Buoyancy Control Systems

One of the great innovations in the evolution of cephalopods was the transformation of the shell into a buoyancy control device. This step frees the animal from its benthic lifestyle, allowing it to remain at any depth in the water column without expending energy swimming and enabling it to colonise pelagic habitats (Ruppert et al. 2004).

Ectocochleate cephalopods, the nautiloids (four extant species) and the ammonoids (now extinct), have chambered shells into which gas can be pumped via the siphuncle. In Endocochleate cephalopods the shell has been internalised and modified in a variety of ways, and in the case of the octopods, highly reduced leaving only a vestigial remnant. Species with greatly reduced or vestigial shells are negatively buoyant and have had to evolve other means of buoyancy control, such as extending the webbing between the arms to reduce sinking or producing dynamic lift with the fins and funnel (Ruppert et al. 2004).

In Idiosepius spp., the shell has been internalised and reduced to a gladius that runs the length of the mantle (Nixon and Young 2003). This is a chitinous structure located along the dorsal midline of the animal and is secreted from the shell sac (Young et al. 1999). It has been suggested that extracellular lipid droplets found in I. notoides may have a role in buoyancy control in this species (Eyster and van Camp 2003).