Calliarthron is a genus containing two species of thalloid intertidal[1] alga.

Calliarthron reproduces by means of conceptacles; it produces tetraspores, dispores and carpospores.

The genus has lignin and contains secondary cell walls, traits which are normally associated with the vascular plants.

[2] Calliarthron is calcified, but also has uncalcified joints that allow it to flex in response to the waves to which it is subjected.

[1] After decalcifying they grow much longer, then fatten themselves up in the same way as xylem formation, resulting in secondary walls.