A REVIEW OF AMMONIA-MEDIATED BUOYANCY IN SQUIDS (CEPHALOPODA, TEUTHOIDEA)

Some deep water squids are known to achieve neutral buoyancy by storin g ammonium in their body tissues. The Cranchiidae use a unique coelomi c cavity to store ammoniacal fluid; in 15 other families, ammonium app ears to be sequestered in either vacuoles in the active body tissues o r 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 mechan isms that could contribute to the repeated evolution of ammonium stora ge. No readily apparent character identifies all ammoniacal squids as belonging to a single lineage, although 5 families of tissue ammoniaca l squids appear to be monophyletic. If the elaborate funnel locking ap paratus of this group arose within the clade. it is not homologous wit h that in other taxa, refuting a basis on which close relationships we re suggested for ammoniacal squids. Given the limited data available, we question whether some squids considered to be ammoniacal may develo p these characters as a result of senescence. Squids are all thought t o derive energy from amino acid catabolism, produce large quantities o f ammonia and have very low blood pH's which effectively remove ammoni a from cells that produce it. They also minimize H+ ion transfer betwe en muscle cells and blood. These features may be important preadaptati ons for ammonium storage. Given the physiological data, and the absenc e of morphological evidence supporting monophyly, we argue that ammoni acal squids be best considered to be a polyphyletic group, evolved in parallel.