Remarkable spatial memory in a migratory cardinalfish.

The ability to orient and navigate within a certain environment is essential for all animals, and spatial memory enables animals to remember the locations of such markers as predators, home, and food. Here we report that the migratory marine cardinalfish Apogon notatus has the potential to retain long-term spatial memory comparable to that of other animals. Female A. notatus establish a small territory on a shallow boulder bottom to pair and spawn with males. We carried out field research in two consecutive breeding seasons on territory settlement by individually marked females. Females maintained a territory at the same site throughout one breeding season. After overwintering in deep water, many of them (82.1%) returned to their breeding ground next spring and most occupied the same site as in the previous season, with only a 0.56 m shift on average. Our results suggest that female A. notatus have long-distance homing ability to pinpoint the exact location of their previous territory, and retain spatial memory for as long as 6 months.