E D IT S (T O P U S G S (M ID D L E F R O

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 per-haps not be discounted (3, 4). Unfor-tunately, marine animal biomass, distribution, and function are not suf-ficiently well known to fully recon-cile these opposing views (5). On page 359 of this issue, Wilson et al. (6) elucidate a physiological path-way, common to all marine bony fishes, that seems to contribute sub-stantially 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 investi-gating the fate of urea-derived car-bon, 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 sea-water. 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 sur-vival of all marine-adapted bony fishes. For most of the past century, substantia