Fish of the Month: Bony-eared Assfish

This month’s fish of the month is the bony-eared assfish, Acanthonus armatus. It’s a fish living at depths of more than 1000 m to deeper than 4000 m in the world’s tropical and subtropical oceans. The fish’s claim to fame is that it has the smallest brain weight – body weight ratio of all known vertebrates! In a study of the ass-fish’s brain (better than a study of the brain-fish’s ass!) researchers found a 40 gram fish with a brain weighing less than 30 mg. That’s less than 1/1000th of the body weight!

Image of the bony-eared assfish, from Fishbase.

For comparison, a human brain weighs about 1400 grams. With a body weight of 70 kgs, that’s a ratio of about 1:50. Even though you sometimes don’t get the impression, humans are very brainy animals. But even in comparison to other fishes is the assfish very small-brained. In fact, it’s brain/body weight ratio is below such evolutionary primitive fishes as lampreys. The small assfish-brain is hence an evolutionary secondary development, it likely descended from larger-brained fishes.

What does that teach us?

– you can be perfectly successful in evolution with a very small brain. The assfish happily lives its life just like you and me do, and assfishes have existed before humans as a species came to be. Brains cost a lot of energy, and if you don’t need a big one, it’s better to reduce its size and to save that energy.

– the deep sea is a place where a small brain is OK: there are not that many enemies, and the absence of sun-light (there is some light from bioluminescence) makes vision a less of a dominating sense than on land or on shallow water. Vision is typically a sense which needs large brains for processing.

– Ichthyologists need to be more respectful when naming fishes. “Bony-eared assfish”, seriously.


Fine ML, Horn MH and Cox B (1987) “Acanthonus armatus, a Deep-Sea Teleost Fish with a Minute Brain and Large Ears” Proceedings of the Royal Society B, 230(1259)257-265.