Ocean science. Animal function at the heart (and gut) of oceanography.

F ar more biomass is contained in marine primary producers than in marine animals at higher levels of the food chain. This relation suggests that animals—par-ticularly secondary consumers such as fishes—must play a negligible role in elemental cycling in the world's oceans (1). However, given that the midwater ecosystem is the largest on Earth, with over 99.5% of the habitable space (2), the activity and metabolism of oceanic animals across great depth ranges, especially in productive regions, should perhaps not be discounted (3, 4). Unfortunately , marine animal biomass, distribution, and function are not sufficiently well known to fully reconcile these opposing views (5). On page 359 of this issue, Wilson et al. (6) elucidate a physiological pathway , common to all marine bony fishes, that seems to contribute substantially to the marine inorganic carbon cycle. The first clue to a possible role of fish in the marine inorganic carbon cycle came from studies of toadfish, Opsanus beta. In 1991, while investigating the fate of urea-derived carbon , Walsh et al. observed pellets in the toad-fish's gut (7), which were later found to be a metastable form of calcite that contains large amounts of magnesium. The source of these " gut rocks " was not immediately obvious, but Walsh et al. reasoned that if common to all fish, they might contribute to the inorganic carbon cycle. More than a decade of detailed physiology has revealed how and why gut rocks form. As far as is known, all bony fishes regulate their internal osmolarity at a level considerably lower than that of seawater and, in seawater, must drink to remain hydrated. However, absorption of the imbibed fluid by the intestine is osmoti-cally limited by the concentrated ions in seawater. Active acid-base regulation facilitates precipitation of the divalent ions as gut rocks and promotes fluid absorption. This appears to be a universal phenomenon critical to the survival of all marine-adapted bony fishes. For most of the past century, substantial dissolution of calcium carbonate (CaCO 3) was believed to occur only in the deep waters that are undersaturated with respect to the various phases of calcium carbonate. However , more recent observations of water column alkalinity reveal that substantial dissolution of calcium carbonate must be occurring at depths well above this " chemical lysocline " (8). The most likely explanation is dissolution of more soluble forms of calcium carbonate, such as the aragonitic …