- Kingdom: Plantae
- Phylum: Chlorophyta
- Class: Ulvophyceae
- Order: Ulotrichales
- Family: Ulotrichaceae
- Scientific name: Acrosiphonia arcta
- Norwegian: stor grønndott
This species is also called arctic sea moss for obvious reasons. It is a green algae that forms green mats, 5-10 cm tall, made of very fine, branching filaments. The branches have rounded ends. A similar, but not so common species, A. sonderi, has thicker branches.
They thrives in exposed as well as sheltered locations. It is most abundant during spring and summer, and tends to disappear late in the summer. In the winter it exists in a reduces state, before it blooms again in the spring,
It is widely distributed on the northern hemisphere.