Marine hatchetfish are creatures of the deep. They have rows of light-producing organs along their bellies which shine a pale blue — matching the light from above and making the hatchetfish invisible to predators below. This fish's name comes from its body shape, which resembles the head of a hatchet.

The Japanese pygmy seahorse is the size of a grain of rice — about 1.6 cm long (0.6 in) — among the smallest seahorses in the world. It wasn't formally named until 2018, whereupon it received the specific title of "japapigu" or "Japan pig", because locals think it resembles a baby pig.

A female Pacific blackdragon is ~60 cm (2 ft) long and uses a bioluminescent lure on her chin to attract prey. Her skin is 'ultra-black', among the darkest in nature, and her teeth are anti-reflective, so as not to reflect light from her lure and alert her prey. The male is only ~8 cm (3 in) long.

A remora uses the sucking disk on top of its head to attach itself to larger marine animals like sharks, rays, and sea turtles — where it can stay for 3 months. The host provides the remora with protection, food, and a fast flow of water over its gills, which the remora needs in order to survive.

The Devils Hole pupfish is perhaps the rarest fish in the world. It lives in the desert; its entire population is found in one 33.8°C (93°F) pool — mostly on a 3.5 by 5 m (11 by 16 ft) rock shelf — inside a cave in Death Valley, Nevada.