Conchas negras - black clams- are one of the most prized ingredients for cebiche in Peru. Native to the warmer waters nearer the equator, where they grow amidst the mangrove roots, black clams are harvested and shipped daily to Lima, which makes them a bit more expensive than other ingredients - at the market one pays for them by the unit. However, as their flavor is dominant, other blander fishes can be mixed in without significantly reducing the clam flavor in the dish.
However, the best way to have them is alone in a cebiche. They have an almost effervescent quality to them which seems to increase alertness -no wonder they are reputed to have aphrodisiac qualities. They also are the only shellfish in Peru that is used entire, with no part other than the shell being discarded.
Aside from the clams themselves, the cebiche included lime juice, chopped aji limo, salt, aji-no-moto (monosodium glutamate), and chopped onion, with some beans, boiled corn, and toasted corn (cancha) added on top.