Cranchiids have a unique system for providing neutral buoyancy and ventilation of the gills (Clarke, 1962), a system based on two unusual modifications.
First, the funnel retractor muscles are flattened as part of a horizontal septum (the transverse mantle septum) that divides the anterior mantle cavity into dorsal and ventral chambers (yellow color in the illustration below). Normally these muscles attach to the edge of the gladius (actually the shell sac) but in this family they attach medially to the cephalic vein and spread laterally and posteriorly onto the mantle wall. The gills lie at an opening (spiracle) in the septum. Water enters the dorsal chamber at the mantle opening, then passes through the spiracle and over the gills into the ventral chamber and out the funnel.
Second, the two major coeloms of the body have partially fused and increased in size. In squids generally, the visceropericardial (VP) coelom occupies much of the posterior region of the visceral sac, where it surrounds the stomach, caecum and gonad. The nephridial coelom consists of two portions: one surrounds the nephridial appendages and the other the digestive gland duct appendages. The latter portion fuses with the VP coelom. In cranchiids, the VP coelom has two large lobes that extend anteriorly. The coelomic wall, therefore, forms a large bilobed sac. These lobes are in broad contact medially and fuse to the mantle dorsally and the cephalic vein ventrally. The lobes, therefore, form a median septum that divides the dorsal chamber of the mantle cavity into left and right sides with one coelomic lobe on each side. The coelom is filled with ammonium chloride, whose low density provides the lift needed for neutral buoyancy (Clarke, et al., 1979). This large coelomic flotation chamber (gray color in the illustration below) is lined with muscles that can contract it anteriorly and balloon it posteriorly by peristaltic waves. This provides a mechanism that moves water through the mantle cavity. Anterior contraction of the sac draws water into the dorsal chamber over the one-way valves of the collar. The progression of the peristaltic wave moves water posteriorly and through the spiracle. As the posterior part of the sac expands if forces water out of the ventral chamber of the mantle cavity through the funnel.
Figure. Identical side-view photographs of the cranchiid, Megalocranchia fisheri. Superimposed on the upper one are illustrations of the buoyancy coelom, the horizontal septum and the path of water for ventillation.
As a result of these two modifications, the cranchiid can float passively in the water and, unlike other squid, respire without ever contracting the mantle musculature. The restriction of water flow through the spiracle greatly reduces their ability to rapidly jet by successive mantle contractions.
The mechanism that forms and maintains the ammonium chloride solution in the coelom is poorly understood. Two conditions are required: a source of ammonium chloride and a low pH to maintain the ionic state of the molecule which prevents diffusion out of the chamber. The partial fusion of the two coeloms leaves the digestive-gland-duct-appendages freely exposed in the large coelom. These appendages probably play important roles in maintaining and/or regulating the ammonium chloride content of the coelom.
