Despite how it may look, there is absolutely nothing wrong with this fish. Butis butis is a moderately sized species of fish (up to about 14 cm in length) widespread in warmer waters around the Indian and western Pacific Oceans. They are found in shallow bays and mangrove swamps, often entering into estuaries and lower reaches of rivers. It is known by a range of vernacular names, including crimson-tipped gudgeon or duckbill sleeper. In the aquarium trade, it often goes by the name of crazy fish, in reference to its distinctive habit of swimming hanging vertically head-down or even swimming upside-down. Its predilection for such unusual angles assists it in remaining concealed from both predators and prey; Ryan (1981) noted that the addition of anaesthetic for collection purposes to a pool resulted in the sudden appearance of several specimens of which no sign had been previously seen. Their camouflage abilities are further enhanced by the ability to change colour to a certain degree, from pale to dark. Butis butis are ambush predators of smaller fish and invertebrates that they engulf in their broad jaws with rapid lunges.
The genus Butis belongs among the gobies, a somewhat notorious group of fish from a taxonomic perspective. In the words of the South African ichthyologist J. L. B. Smith, "The Gobioid fishes are one of the major trials of ichthyologists, and when general regional collections are worked up, these fishes tend to be pushed aside, and are apparently often identified with some impatience by those not specially interested" (Smith 1958). This notoriety is mostly due to the small size of many gobies, together with a tendency to the reduction of diagnostic features. Earlier authors classified Butis within the Eleotridae, a group of gobies (commonly known as sleepers, presumably due to their benthic habits) distinguished by having the pelvic fins separate from each other (in other gobies, the pelvic fins are united into a ventral sucker or disc). This, however, is a primitive feature only, and more recent molecular phylogenies have confirmed the paraphyly of Eleotridae in the broad sense. As a result, Butis and some of its nearest and dearest have been separated out into a separate family Butidae (Thacker 2011). However, while the separation of Eleotridae and Butidae seems to be fairly widely accepted, the two groups are still not clearly defined morphologically. Characteristic features of Butis relative to other gobies include (among others) the presence of a complete covering of scales, and a bony ridge above each eye (Smith 1958). In a number of species of the genus, including B. butis, the head is low and long, and the lower jaw distinctly protruding; however, the mudsleeper B. koilomatodon has a shorter, rounder head (this latter species, though originally native to a similar range to B. butis, has become invasive in more recent years in west Africa and Brazil, presumably carried in ballast water). Butis butis differs from other species in the genus in having small secondary scales at the base of most scales on the trunk, and the bony ridges above its eyes are more or less smooth (Herre 1927).
Herre, A. W. 1927. Gobies of the Philippines and the China Sea. Philippine Bureau of Science Monographic Publications on Fishes 23: 1–352, 26 pls.
Ryan, P. A. 1981. Records of three new freshwater fishes from the Fiji Islands. Pacific Science 35 (1): 93–95.
Smith, J. L. B. 1958. The fishes of the family Eleotridae in the western Indian Ocean. Ichthyological Bulletin 11: 137–163.
Thacker, C. 2011. Systematics of Butidae and Eleotridae. In: Patzner, R. A., J. L. Van Tassell, M. Kovačić & B. G. Kapoor (ed.) The Biology of Gobies pp. 79–85. CRC Press.