There is a bag of hydrochloric acid inside each of us. To digest our food, our stomachs secrete stomach acid, which is powerful enough to dissolve bone and erode teeth. But not all animals are walking bags of caustic acid. Crustaceans’ stomachs are literally a thousand times less acidic than ours and are about the same pH as fresh rainwater. Crabs rely far less on stomach acid than we do, so Alex Quijada-Rodriguez, Dirk Weihrauch and colleagues at the University of Manitoba, Canada, studied crabs to find the consequences of using less stomach acid. When our stomach lining pumps in acid, it leaves our blood more alkaline, but if crustaceans use less stomach acid, does eating make them alkaline too?
To investigate, Quijada-Rodriguez and colleagues fed green shore crabs (Carcinus maenas) a hearty meal of scallops and sampled the pH within and outside their stomachs. Yet, even while digesting their meal, the acidity of the crabs’ stomachs never changed. However, when the team tested samples of the crabs’ haemolymph (the crustacean equivalent of blood) after dining, they found something unexpected. Instead of becoming more alkaline, the haemolymph became more acidic.
Being acidic is dangerous, whether you're a human or a crab. Acid impairs our hearts and brains, so animals keep a tight rein on their pH. Normally, crustaceans get rid of extra acid in the same way we do: they breathe it out. Our blood converts excess acid into carbon dioxide, which we exhale to reset our pH, and crabs do too. So Quijada-Rodriguez and colleagues sampled the crabs’ haemolymph after a meal to measure carbon dioxide, but things looked dire. Even 6 h after eating, carbon dioxide was ∼50% above normal. This means the crabs couldn't breathe out the carbon dioxide fast enough to reset their pH. Left unchecked, this build-up of acid could prove fatal. But, of course, crabs don't die after every meal. Surely, something else was expelling the acid.
Acid is not the only by-product of digestion. Protein in the crabs’ diet produces ammonia, a toxic chemical used in household cleaners. And when the team checked ammonia concentrations in the crabs’ bodies after dining, the toxin doubled, and peaked at the same time as acid in their haemolymph. Yet, instead of compounding each other's toxicity, acid and ammonia may cancel each other out. The University of Manitoba team recorded how quickly acid and ammonia left the animals. Although the crabs secreted only a small amount of acid, they cleared ammonia at a much higher rate. However, ammonia appeared to carry acid away with it. Weihrauch's lab had previously shown that crustaceans secrete ammonia as a weak acid called ammonium. In essence, ammonia soaks up the crabs’ acidity before it's quickly thrown out. Just like us, after a messy meal, crabs use ammonia to clean house.
We humans go through all the trouble of producing stomach acid – an acid so corrosive that laboratories like Quijada-Rodriguez's label it with ‘Danger!’ and lock it away in specially made safety cabinets. Perhaps in time, the team will clarify how crustaceans can digest without such extreme digestive systems, given that crabs’ stomachs are less acidic than the coffee I'm drinking now.
