The order Petromyzontiformes is one of two groups of living jawless vertebrates. Lampreys are primitive eel-like cartilaginous fishes that inhabit cool-temperate marine and fresh waters. Two families live in Australian waters: the Geotriidae or Pouched Lampreys, and the Mordaciidae or Short-headed Lampreys.
Lampreys have a sucker-like oral disc or mouth, armed with rows of horny teeth, and a toothy rasping tongue. They have one or two dorsal fins, lack paired fins and their gill openings are a series of enlarged pores along the sides. They lack scales and paired fins.
Many lampreys are anadromous. The adults breed in freshwater rivers and streams and the larvae hatch and grow in freshwater. Juveniles migrate downstream to the sea to mature, and return as adults to spawn. Most lay their eggs in clear freshwater streams where the blind larvae (known as ammocoetes) remain buried in fine silt for several years, filter-feeding on plankton, detritus and microscopic algae. They usually metamorphose after about three years, developing eyes and teeth, and some species migrate to sea to mature.
Lampreys are specialised predators and the adults of some species are parasitic on other fishes. They use their sucking disc to attach to prey and rasp flesh with their toothy tongue to feed on tissue and body fluids. After several years, these lampreys cease feeding and return to freshwater rivers and streams to spawn, dying shortly after. Some land-locked populations are entirely freshwater.
The order comprises 40 species in three families, two of which are found in Australia. These ancient fishes have a fossil record dating back to the Devonian Period, 360 million years ago.
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