A Review Of Ammonia-Mediated Buoyancy In Squids (Cephalopoda: Teuthoidea)

Some deep water squids are known to achieve neutral buoyancy by storing ammonium in their body tissues. The Cranchiidae use a unique coelomic cavity to store ammoniacal fluid; in 15 other families, ammonium appears to be sequestered in either vacuoles in the active body tissues or in a gelatinous outer layer. The hypothesis that these squids form a single lineage is here reconsidered through reviews of morphological characters that could support this hypothesis and physiological mechanisms that could contribute to the repeated evolution of ammonium storage. No readily apparent character identifies all ammoniacal squids as belonging to a single lineage, although 5 families of tissue ammoniacal squids appear to be monophyletic. If the elaborate funnel locking apparatus of this group arose within the clade, it is not homologous with that in other taxa, refuting a basis on which close relationships were suggested for ammoniacal squids. Given the limited data available, we question whether some squids considered to be ammoniacal may develop these characters as a result of senescence. Squids are all thought to derive energy from amino acid catabolism, produce large quantities of ammonia and have very low blood pH's which effectively remove ammonia from cells that produce it. They also minimize H+ ion transfer between muscle cells and blood. These features may be important preadaptations for ammonium storage. Given the physiological data, and the absence of morphological evidence supporting monophyly, we argue that ammoniacal squids be best considered to be a polyphyletic group, evolved in parallel.