Most fish swim like you’d expect them to—facing forward. Sometimes they’ll do a headstand to probe the ocean floor for food or hide themselves in some seagrass, but this is just a temporary pose. Not so for the shrimpfish (Aeoliscus punctulatus), which does all its swimming in a vertical, head-down position.
The fish’s strange orientation is due to its center of buoyancy being farther toward their tail than their center of gravity, say biologists Frank Fish (talk about a perfect name) and Roi Holzman, who noticed the fish at an Israeli aquarium and were inspired to study their stability and maneuverability.
According to the researchers’ presentation at the recent Society for Integrative and Comparative Biology meeting, the fish’s body accommodates its posture with a wide rounded back and sides that taper to a thin belly, giving it a shape sort of like an airplane wing. While other fish normally have a fin on their back, the shrimpfish’s dorsal fin has moved to the underside of its body. There, it works together with the tail and anal fins to propel the fish forward or act like a rudder, allowing the fish to turn easily and even pirouette.
The fish aren’t stuck in this position, though. As Richard Dawkins writes in The Ancestor's Tale, “When alarmed, they are perfectly capable of flipping into more conventional, horizontal mode and they then flee with surprising speed."
You can see the fish swimming in both normal and headstand positions in these videos.