All About Tropical Mandarinfish: The Scaleless Stars

A female mandarinfish

Mandarinfish, Synchiropus splendidus, are tropical, marine fish living in the Western Pacific Ocean, ranging from the Philippines to the Great Barrier Reef of Australia. They are classified as dragonets in the family Callionymidae, which encompasses small, tropical fish that inhabit Indo-Pacific waters. Their habitat primarily consists of living along the ocean floor or among coral in protected lagoons and inshore reefs. They live at depths of 3 to 60 feet in warm waters ranging from 24 to 26°C (75.2 to 78.8°F). Mandarinfish can live 10 to 15 years in the wild, and roughly 4 to 5 years in captivity. They eat tiny, benthic invertebrates such as gastropods, polychaete worms, and isopods.

Mandarinfish are best characterized by their vibrant colors and gorgeous orange and green patterns. Their skin is naturally blue, caused by blue chromatophores, which are cells that contain pigment inside them. They are one of 2 vertebrate species to have blue color caused by cellular pigmentation. The name 'mandarinfish' was derived from its similarities to the coloration of Imperial Chinese garments. This brilliant coloring has led scientists to suspect that these colors could be used as an aposematic defense, where bright, unusual coloring is used to warn predators that the animals are poisonous (like poison dart frogs). These fish are also quite small, growing to 3 inches in length with a broad, depressed head and 4 dorsal spines. Mandarinfish are also sexually dimorphic, with male fish being significantly larger than females and having an elongated first dorsal spine.

A male mandarinfish, noted for its elongated first dorsal spine.

A striking feature of the mandarinfish is that, similar to catfish, they actually have no scales. This leaves them defenseless against predators, and thus they have adapted the defense of secreting toxic, odorous mucus meant to ward off predators and prevent parasites. Additionally, they are heterothermic creatures, meaning they can regulate their own body temperature or allow the environment to influence it. Mandarinfish are oviparous and pelagic reproducers, so to breed, male and female mandarinfish will gather at night and rise a meter above the reef to release sperm and eggs. These eggs are tiny, clear, and being pelagic, will float around the ocean until hatching. Interestingly, mandrinfish do not have a specific breeding season and will reproduce year-round.

Mandarinfish also help the human economy, specifically in the aquarium trade sector of Hong Kong and the Philippines. Despite being notoriously hard to care for in at-home aquariums, they are popular fish due to their stunning colors. For those who do survive in captivity, these fish are highly resistant to common aquatic diseases, a primary one being marine ich, essentially a type of parasite that inhabits the fish. Since mandarinfish do not have scales or the typical skin that many fish contracting the disease have, they can avoid it.

Overall, mandarinfish are truly bizarre yet beautiful creatures that help us appreciate the importance of conserving coral reefs to continue providing homes for animals both discovered and undiscovered.

A mandarinfish couple mating in Anilao, Philippines