CALCIUM PHOSPHATE RENAL CONCREMENTS IN EXTANT NAUTILUS: A FAST-ACTING, ENERGY-PRODUCING MINERAL RESERVE SYSTEM EQUIVALENT TO VERTEBRATE BONE, AND ITS POSSIBLE ROLE IN THE EARLY PALEOZOIC DIVERSIFICATION OF CEPHALOPODS

Renal uroliths (concrements) of calcium phosphate have long been known to exist in immature and mature individuals of modern Nautilus, but to date no evidence-based explanation for their existence has been available. The currently favored speculation is that they function as a calcium reserve for shell and septal calcification. Here we present new observational and experimental data that are consistent with the hypothesis that they serve as a mineral/ion reserve, allowing short-term (< 1 day) addition of ionized phosphorus to blood and other body fluids. In both in-ocean experiments and during long-term observation of captive animals, concrements disappear during two different, energy-intensive activities involving removal of anions and cations from newly secreted cameral liquid in the chamber formation cycle, and during dives to depths requiring high osmotic pressures within the canaliculi of the siphuncular epithelium to prevent flooding of previously emptied chambers due to suddenly increased ambient hydrostatic pressure. New concrements appear at other points in the chamber formation cycle and when normal living depth is restored. These observations suggest that concrements are used by Nautilus as a phosphate reserve for augmenting cellular ATP availability needed for powering energy intensive life activities in a manner similar to the mobilization of bone phosphate among vertebrates. These observations, together with the Cambrian ancestry of nautiloids, suggest that the concrement ion reserve system of Nautilus may exemplify a solution developed by the earliest nautiloids to power energy intensive life activities, not only shell construction and deep diving, but, as in vertebrates, swimming as well. It is a solution to the problem of energy supply in newly evolved swimmers of the Cambrian radiation independent of that occurring in fish. The use of renal concrements as an ion reserve may thus be a crucial adaptive breakthrough that provided a basis for the dramatic early Paleozoic diversification of buoyancy-compensated, jet-propelled cephalopods that, for a time at least, made shelled cephalopods the dominant group of swimmers in the ocean.