The crescent gunnel was first formally described in 1873 by the American paleontologist and biologist Edward Drinker Cope with the type locality given as Sitka or Unalaska in Alaska.

[2] The specific name laeta means "joyful", "glad" or "pleasant", Cope did not explain his choice of this name but did refer to the types as "rather brilliantly colored" specimens.

[1] There are two rows of blackish crescent-shaped markings each with a yellow spot in their centers along the back, and these identify this species from other gunnels.

[1] It is an important prey item for larger vertebrate predators and these include great blue heron (Ardea herodias), pigeon guillemot (Cepphus columba), North American river otter (Lontra canadensis), American mink (Mustela vison), and larger fishes.

These fishes hide in rockpools or tiny crevices under rocks or in vegetation, they may even emerge fully from water at low tide, and are apparently able to breathe air not submerged.