E-Flora BC: Electronic Atlas of the Flora of British Columbia

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:
At first glance this does not seem to be a Desmarestia, thanks to the flattened (although highly branched) blades, but like other species in this genus, in nature Flattened Acid Kelp secretes sulfuric acid that damages other species (and even its own tissues, resulting in wide color changes and eventual disintegration of some branches).

The central axis is up to 2 cm (about 0.8 in) wide with a conspicuous midrib, but the side branches are narrower. The whole organism can reach 80 cm (2.5 feet) in length, although most of the specimens we have seen have been only half that. Many different varieties of this species are described in the scientific literature, and some workers think at least two species of ligulate Desmarestia occur in the waters of British Columbia—true D. ligulata and a much larger strictly subtidal species called D. munda.

Individuals from southern British Columbia/northern Washington can withstand immersion in water at a temperature of 20°C (68°F) for a week, but die if immersed in warmer water.

Flattened Acid Kelp is somewhat weedy; in Southern California, it can become established after Giant Perennial Kelp forests are swept away during severe winter storms.