Trimmatom nanus, adult body length about 10 mm (Photo by Yi-Hsiung Chen)	Actual specimen size of Trimmatom nanus (Photo by Yi-Hsiung Chen)

Species Profile:

　　Trimmatom nanus Winterbottom & Emery, 1981 is an omnivorous small coral reef benthic fish. Taxonomically, it belongs to the class Actinopterygii, order Perciformes, and family Gobiidae. It is a rare but not endangered species. It is primarily distributed in the western Indian Ocean and the central to western Pacific Ocean, inhabiting crevices along reef slopes and submarine trenches with coral substrates. The species was first discovered in the Indian Ocean. Current literature records its presence in Australia, the Chagos Islands, Indonesia, the Maldives, New Caledonia, Papua New Guinea, and Tonga. There have been no records from Taiwan’s main island; the specimen collected from the outer Dongsha Atoll represents the first recorded occurrence in Taiwan.

　　Trimmatom nanus typically inhabits coral reef areas at depths of 5–30 meters (optimal depth 20–30 m). It tolerates water temperatures ranging from approximately 17.99–33.14°C (optimal 25.22–28.94°C) and salinity between 33.04–36.52 psu. The body is semi-transparent with an orange hue, featuring 12 dark orange vertical bands along its sides. Dark brown pigment spots are scattered on the anterior trunk before the anus, as well as on the membranes of the dorsal and anal fins. It has two dorsal fins, and the caudal fin is nearly straight with only a very slight notch. Body depth is approximately 16.4% of total length (TL), and the eyes are large, with eye diameter about 42.4% of head length (HL).

　　Although Trimmatom nanus is recognized as the “world’s smallest coral reef benthic fish,” it ranks second when considering the “world’s smallest fish” overall. The current record holder is Paedocypris progenetica, a freshwater cyprinid discovered in swamp areas of Sumatra, Indonesia, with an adult body length of only 7.9 mm. Nevertheless, the discovery of this tiny species in the waters of Dongsha Atoll is truly remarkable.

The trench topography of the outer Dongsha Atoll provides excellent shelter for marine life (Provided by the Marine National Park Headquarters)