Cardinalfish

Your dive light glides across a dark wall, and deep within a crevice, several red silhouettes appear. The large black eyes of the cardinalfish immediately catch the light. During the day, this is often where it must be sought: sheltered, sometimes in groups, inside cavities where it waits for darkness.

But look at its mouth as well. In this species, the male takes charge of the spawn: after fertilization, he collects the mass of several thousand eggs and incubates it directly inside his oral cavity until hatching.

At a site studied in Sicily, researchers observed that courtship displays became more frequent at sunset and that directly observed spawnings took place at that time: two fish swim together, and then the spawn passes almost immediately into the male's care.

When night falls, another side of the cardinalfish appears. Monitoring conducted in fourteen caves of the western Mediterranean shows that it leaves its refuges extensively to feed outside, mostly above rocks and posidonia seagrass meadows. By morning, it returns to the cavities.

These round trips could transport organic matter there: behind the small red fish motionless in your beam hides a nocturnal traveler connecting two worlds.