Family Description:
This order contains large brown seaweeds with a distinctive pattern of growth. The apex of the sporophyte is a filament with an intercalary meristem, which produces the new cells that form the main axis of the thallus. Similar meristems occur along the length of the thallus so that the entire thallus can be fringed with pigmented hairs terminating the intercalary lateral meristems. These pigmented hairs are most evident during the spring growing season. At maturity, the plant appears parenchymatous, but it is actually pseudoparenchymatous in construction. Members of the Desmarestiales probably originated in the southern hemisphere where the greatest diversity of species occur—some rivaling northern hemisphere kelps in their size and morphological complexity.

Gametophytes are microscopic. They produce eggs and sperm on the same or different thalli. A sperm-releasing and -attracting factor called desmarestene has been identified in this order of seaweeds. Other pheromones, with slight differences in chemical structure, have been found in other orders of brown algae.

Species description:
The conspicuous generation is the sporophyte, which is a dark brown, coarsely cylindrical (or wiry) alga growing only on rocks. In Europe, it is reported to be bright green when young and brown when older. The branching pattern is alternate or irregular, but some side branches are pinnate (feather-like) while others are simple. The meristems of the side branches are located near where they branch off the main axis. The main axis is up to 5 mm (0.2 in) in diameter and the whole individual can be 120-185 cm (about 4-6 feet) in length.

Individuals from southern British Columbia/northern Washington can withstand immersion for a week in water at a temperature of 23°C (74°F), but succumb if placed in warmer water.

You will need a good minus tide to see Witch's Hair where it grows, but often masses of this species wash up on the beach during storms.

Other species of Desmarestia form vast kelp forests on the shallow sea floor around Antarctica. This genus is named after A. G. Desmarest, a French naturalist.

Source:  North Pacific Seaweeds

Synonyms and Alternate Names:
Desmarestia intermedia
Fucus aculeatus