Tosmopolitan in temperate waters. Epilithic and on other hard substrates. Forming tangle masses of green, yellowish-green or brownish-green fronds, favouring habitats with brackish water, but also in full strength seawater. Often above the high tide mark in areas of freshwater run-off. The morphology and development of the tubular thallus of Capsosiphon superficially resembles that of Enteromorpha and Blidingia. However, Capsosiphon differs from these genera in the arrangement of cells, and in its life history. The presence of a cyst-like sporophyte in C. groenlandicum and in at least some populations of C. fulvescens suggests affinities with Monostroma and other algae in the Ulothricales.