This is a marine species of green algae that is found in the North Pacific. It "occurs on boulders and bedrock in the mid to low intertidal of protected to semi-exposed habitats in spring and summer" (Lindeberg and Lindstrom 2010).

Family Description:
Members of this family have a heteromorphic life cycle. The gametophyte is filamentous and can be branched or unbranched. Cells are multinucleate in most species. The single chloroplast per cell is reticulate and perforate (full of holes) and contains numerous, conspicuous pyrenoids. Biflagellate gametes are produced in ordinary vegetative cells, which can become slightly to markedly swollen when fertile. The gametes are released through an opening with a lid, called an operculum. The sporophyte that develops from the zygote is a microscopic, ovoid to elliptical unicell that is also multinucleate. It can be stalked or sessile and is often endophytic in fleshy red algae. The entire contents of the unicell are presumed to undergo meiosis and form the quadriflagellate zoospores that germinate and grow into the filamentous phase.

Species description:
The gametophyte of Arctic Sea Moss forms bright green clumps or tufts of sometimes coarse, repeatedly branched filaments up to about 6 cm (about 2.25 in) in length. If you hold a tuft of this species in your hand and pull some of the filaments, as the filaments separate they pull their branches and the tuft becomes messy (compare with Ulothrix below). With a hand lens you will notice that the branchlets are not hooklike, as some are in the closely related Green Rope.

You are unlikely to encounter the unusual sporophyte that alternates with most gametophytes in this genus because it is unicellular and lives protected within the tissues of species of red algae such as Northern Red Blade (Neodilsea borealis) or Red Sea-cabbage (Turnerella mertensiana) where it looks like tiny green dots. This unicellular sporophyte was formerly called Chlorochytrium inclusum before its true identity was determined.

Source:  North Pacific Seaweeds

Synonyms and Alternate Names:
Acrosiphonia spinescens
Chlorochytrium inclusum
Cladophora acta (Dillwyn) Kützing
Conferva arcta
Conferva cohaerens
Spongomorpha arcta
Spongomorpha spinescens

Read more about the life history of this species.