Any particular reason? Something about their cell structure or something?
One reason which applies to seafood in general:
The temperature of the ocean, almost anywhere, is remarkably stable, compared tothe temperature of the air almost anywhere.
For example, within 100 mile of the South pole, temperatures have been recorded of almost +100F and -40F (that I know of, I’m sure the winter windchill is far lower). In temperate zones, the air temperature varies by 10-20F during the course of almost every day, By comparison, shallow regions of the “deep ocean” (well away from the continental shelf) rarely vary by as much as 10F during th course of an entire year, and the deeper strata rarely vary by 5F.
Colder waters dissolve more oxygen, and are generally more biologically productive. Carribean waters are so clear because few microorganisms grow in them, and therefore they support relatively little larger sea life, as well, aside from coral-associated species. The polar and near-polar zones, however, are the ocean’s ‘breadbasket’, producing vast quantities of plankton, krill, etc.
Commercial shellfish almost invariably come from the continental shelf, where temperature variation is slightly greater, but water temperature may remain below, say 60-65F for decades or centuries. These relatively narrow regions are, however, often quite productive because of nutirent runoff from the land/rivers. Commercial shellfish have guts full of a complex mix of microbes fram many biomes, and “garbage” from the overlying water strata , and land sources. When the shellfish dies, these can run amok and hasten decomposition. This is why many shellfish are sometimes stored or transported live in tanks and allowed to “detox” on a ‘clean diet’ like cornmeal before being sold.
Since many sea organisms may have lived for millions of years without seeing a temperature of 68F-70F, their proteins are optimized for thermal stability or balanced action at "room temperature’ or even “refrigerator temperatures”. (Parts of a shelffish that you buy at a grocery story, bring home, and refrigerate may easily reach 50F or more for an appreciable length of time)
Arrhenius’ Law (more a rule of thumb, when discussing organic or bio- chemistry) states that for every 10C (22F) degree rise in temperature, the rate of a reaction (or enzymatic catalysis) typically doubles) therefore small changes in temperature can cause fairly large increases in reaction rates, and since different reactions or catalyses occur at different rates, the physiological conditions inside the seafood can change quite a bit.
Cold water/ deep sea fish are much less stable than shellfish, but they are usually ice-packed or flash frozen on the boat, as soon as the nets are brought in.
Wow, that’s fascinating! I had no idea it had to do with temperature stability!
But shellfish above all else seem more prone to spoilage than fish. (no idea on the shelf life of dolphin…ahem!). Is there something about their particular biochemistry that makes them rather appetizing to whatever bacteria happen to be around? Spoiled shellfish can make people violently ill; is this because of the time of microorganisms they harbor? (much like how dangerous uncooked pork can be)