The spined loach is Finland’s rarest fish. It is only found in the area of sea around Kotka and Pyhtää and the Hiitolanjoki river in Rautjärvi. The spined loach is nocturnal. During the day, it lies burrowed into the sand for the majority of the time, so that only its head remains visible. Under the sand, breathing is possible thanks to a small opening in the upper part of the operculum, while the lower part is covered by a membrane. When the oxygen level in the water falls, the spined loach may breathe oxygen from the air via its intestines. The fish stores oxygen in its intestines and transfers it via the surface of the intestine into its bloodstream. The spined loach thrives in clear waters with a sandy or stony bottom. Sand has been brought into the spined loach’s tank at Maretarium from its own natural environment.
The spined loach is often confused with the stone loach, but the barbels of the spined loach are shorter and its caudal fin is round unlike that of the stone loach. Good means of identifying the spined loach are the dark stripe running from its eye to the end of its snout and the small fork-tipped spine below its eye, which the fish can raise when defending itself. The spined loach grows to 10-12 centimetres in length. It feeds on planktonic crustaceans and insect larvae sifted by it from the sand. In the winter, spined loach do not eat much.