Incredible plants: Costaria costata

Small sporophytes of Costaria costata in an intertidal
pool at Carmel Pt., Monterey Co., CA, June 2014.

Kelps are undoubtedly one of my favorite groups of plants – “plants” in the broadest sense of the word since they belong to an order of brown seaweeds (Laminariales) that are quite distinct evolutionarily from land plants. Among the dozens of kelp species along the world’s coasts, Costaria costata is one of my favorite species. It tends to be just rare enough that it is a pleasant surprise to find it during a visit to the rocky intertidal, and it also has such a remarkable and intriguing shape.

Like many of the smaller kelps that don’t form tall canopies in kelp forests, the macroscopic stage of Costaria consists of a single large blade. However, the blade is very distinct, making the species easy to identify in the field. It has 5 raised ribs that run longitudinally along the blade. In between these ribs, the surface of the blade is raised and lowered in textured undulations. Sometimes the blades have holes in them and oftentimes the end of the blade is tattered and torn from thrashing among the rocks and surf. The blade of the plant is held to the rocks by a short stipe and a holdfast of branching haptera that resemble roots, but the holdfast function is largely for anchoring the kelp to the rocks. The blades can reach up to 2-3 m in length. The stipes have a corrugated rather than smooth surface, a feature that I think is unique among all the kelp species along the western US coast.

Specimens of Costaria costata from Pacific Grove, Monterey Peninsula (left) and Iwate Prefecture, northern Japan (right) collected in 1897 and 1986 respectively (UC Berkeley herbarium specimens UC96712 and UC1829920). The five midribs and bullation on the surface of the blade are obvious on the specimen from California. the blade in the plant from Japan has numerous round perforations which are only seen on some individuals.

Costaria costata (right) and a related kelp, Dictyoneurum
californicum (left), from Mendocino Co., CA, July 2008.
Thecorrugated surface of the stipe is easily observed in
this photograph.

Costaria is an annual like its cousin the sea palm that I highlighted in a previous post (Druehl 2000). As with other kelps, it has a microscopic gametophyte stage that grows cryptically on the rocks. Only the large sporophyte is visible to the casual observer, and being an annual it will most likely be easiest to find during the summer. The sporophyte produces spores in the blade from summer to fall that eventually make their way to the substrate to germinate into male and female gametophytes.

Broadly speaking, Costaria is reportedly distributed from southern California through Alaska to northern Japan in the northwestern Pacific. However, the actual site-by-site occurrence along the coast is much more spotty. Unlike very common kelps such as Egregia menziesii or Laminaria setchellii, one won’t find it at most stretches of rocky intertidal coastline. I have personally observed Costaria at Carmel Point (just south of Monterey, California); Glass Beach (in Mendocino County, CA); on San Juan Island, WA; at Botanical Beach in southern British Columbia; and at a few other west coast locations. From herbarium records I’ve compiled at regional museums (UC Berkeley, Humboldt State Univ., etc), other locations where the species has historically been found include: the Monterey Peninsula; Shelter Cove; Humboldt County; Sunset Bay and Newport, Oregon; Whidbey Island, Washington; southeast Alaska; and Hokkaido, Japan. Skimming through my herbarium notes, I haven’t seen any specimen records farther south on the US Pacific coast than Big Sur, California.

Costaria costata and Cymathere triplicata in the low intertidal at Botanical Beach,
Vancouver Island, British Columbia, summer 2000. These two kelps co-occurred on
rocky substrate with numerous urchins nearby. Urchins are typically voracious
consumers of kelp, but these plants had so far escaped herbivory. Cymathere, a kelp
from the Pacific Northwest, is distinguished from Costaria by having a smooth blade
and only 3 longitudinal ribs.

Links to other web resources on Costaria costata:

-          The late Tom DeCew’s Guide at the University Herbarium, UC Berkeley

-          British Columbia coastal biodiversity page by the Starzomski lab

-          Seaweeds of Alaska on-line flora

References

Abbott IA, Hollenberg GJ. 1976. Marine Algae of California. Stanford University Press.

Druehl L. 2000 Pacific Seaweeds. Harbour Publishing.