Melanosiphon intestinalis (Twisted Sea Tubes) is a marine brown seaweed, growing as a cluster of thin, unbranched tubular fronds, up to 150 mm, from a disk-like holdfast. The tubes are often twisted. It grows in sheltered bays and estuaries, growing on overhanging rocks, in dense mats on muddy substrates, and occasionally in high-intertidal tidepools. It is native to the Pacific, where it occurs from the Aleutian Islands to California, and Japan to Russia. .In 1969, it was collected in the Bay of Fundy, Nova Scotia. In the Western Atlantic, it is now found from Newfoundland to Long Island Sound, and can be locally common. Possible vectors include fouling and ballast water.