Fucus distichus Linnaeus

Fucus distichus Linnaeus

As per the Gabrielson et al. (2012) Key. Also known as Fucus gardneri

Common name: Rock weed

Etymology: Fucus is derived from the greek word for seaweed, phykos.

Type locality: Unspecified


Phaeophyceae: Fucales: Fucacea


Fucus distichus is one of the most common intertidal inhabitants of the San Juan Islands (Fig 1). It is readily recognized by its paired gas filled receptacles, which release a satisfying amount of mucus upon being punctured. On the surface of each receptacle are small bumps called conceptacles, which house the gametangia of the alga (Fig 2). Fucus distichus is often moss green in color, with the receptacles lighter in color than the dark, ribbon- like blades that branch profusely from a single basal stipe. Midribs are raised and run from the base of the alga and into the receptacles. It should be noted that the blades of this species lie flat, whereas those of less common Fucus spiralis, while similar in appearance, are spiraled. Swollen receptacles are not always present at the tips of the blades, as immature specimens will instead exhibit an uninflated continuation of the blade.

Figure 1. Fucus distichus in the intertidal zone of Friday Harbor Laboratories, Friday Harbor, WA

Figure 2. Small bumpy conceptacles on the surface of the receptacle of Fucus distichus









Fucus distichus is found on the Pacific coast from Point Conception, California, to the Aleutian Islands in Alaska. It can be found on virtually any shoreline on the San Juan Islands, particularly in the mid- to high intertidal zones (Druel). The Friday Harbor Laboratories herbarium for this species date as far back as 1904 and include the following collection locations:

Olga, Orcas Island, WA
Tskawahyah Island, WA
Kanaka Bay, San Juan Island, WA
Point Caution, San Juan Island, WA
Limestone Point, San Juan Island, WA
Friday Harbor Laboratories, San Juan Island, WA
Minnesota Reef, San Juan Island, WA
Botanical Beach, Port Renfrew, B.C.
East Sound, San Juan Island, WA
Blina Island, Orcas Island, WA
Mt. Dallas Beach, San Juan Island, WA
Mukkaw Bay, WA
Michel Bay, San Juan Island, WA
Golden Gardens, Seattle, WA
Eagle Point, San Juan Island, WA
Vancouver Island, B.C.
Box Island, Long Beach, Vancouver Island, B.C.
False Bay, San Juan Island, WA
Echo Bay, Sucia Island, WA
Waddah Island, Neah Bay, WA
Low Island, WA
Cattle Point, San Juan Island, WA
Mar Vista, San Juan Island, WA
False Bay, San Juan Island, WA


One voucher specimen was pressed and entered into the FHL herbarium (Fig 3). The recorded collection location and date were FHL Docks and 18 June 2013.

Voucher specimen of Fucus distichus collected 18 June 2013 from the docks in front of Friday Harbor Laboratories, Friday Harbor, WA

Figure 3. Voucher specimen of Fucus distichus collected 18 June 2013 from the docks in front of Friday Harbor Laboratories, Friday Harbor, WA

Research Notes

Life history and sexual reproduction

Unlike all other members of Phaeophyceae, species in the class Fucales exhibit a monomorphic life cycle. There is neither alternation of generations nor a free-living gametophyte stage; instead, a diploid sporophyte produces haploid gametes that go on to reproduce oogamously (Mondragon, 2003). Meiosis occurs in unilocular gametangia instead of the plurilocular gametangia seen in all other classes of Phaeophyceae. Two genera closely related to Fucus, Syringodermatales and Ascoseirales, possess tiny gametophytes within the tissues of a sporophytic body, suggesting that the exception to alteration of generations seen in all Fucales arose from the reduction and retention of the gametophyte phase by the sporophyte (Graham et al., 2009).

The aforementioned unilocular gametangia are contained within conceptacles, which themselves are embedded in the receptacles of the algae (Fig 4). Each gametangium contains oogonia and antheridia, the respective eggs and biflagellated sperm of which are released in mucilage from the conceptacles via a small apical pore. In Fucus distichus, eggs can be fertilized while still attached to the parent by mucilage or after being fully released into the water column or benthos (Schiel & Foster, 2006). The zygote then grows into a sporophyte (Fig 5). Fucus spp. sometimes release eggs still held together in the oogonium, allowing the eggs to sink faster than would a single egg (Lobban, 1985). All phaeophycean spermatozoon locate their female counterparts by the means of chemoattraction. Egg cells secrete pheromones known as octatrienes, the concentration gradient of which guides the movement of each spermatozoa (Vijayaraghavan, 1997).


Figure 4. A cross section of a Fucus distichus conceptacle.

Figure 4. A cross section of a Fucus distichus conceptacle.

Figure 5. Young Fucus distichus sporophytes in the intertidal zone.











Desiccation tolerance

On many beaches on San Juan Island, WA, one can observe a distinct intertidal zonation pattern in Fucus and Ulva (Fig 6). This is due largely to the pressures of desiccation, with Fucus distichus able to withstand greater pressure than Ulva spp. and consequently occupying a higher, more exposed spot in the intertidal. While Ulva spp. are able to maintain constant photosynthetic rates to a certain extent of water loss, Fucus distichus has been found to have a higher photosynthetic rate when exposed than when submersed (Lobban, 1985) (Dawes, 1998). A paper by Ding & Brown (1982) found that Fucus was able to tolerate and completely recover from desiccation of up to 25% of its original water content whereas Ulva was greately impaired when subjected to the same extremes (Lobban, 1985). Furthermore, the relatively thick thallus of Fucus distichus aids in water retention.

Figure 5. The distinct zonation of Fucus distichus and Ulva spp. in the intertidal zone of Cattle Point, San Juan Islands, WA.

Figure 6. The distinct zonation of Fucus distichus and Ulva spp. in the intertidal zone of Cattle Point, San Juan Islands, WA.

Response to herbivory

Fucus distichus, like other Phaeophyceae, contain phenolic compounds as a means of defense against herbivory. As described by Van Alstyne (1988), these compounds disrupt the digestive enzymes utilized by its predators and lower the nutritive value of the alga. Van Alstyne studied the effects of this defense on Littorina sitkana, a prevalent herbivorous gastropod found on Latoosh Island, an island about 100 miles west of San Juan Island. It was found that L. sitkana initially preferred newly damaged Fucus distichus specimens, but after a period of two weeks switched their preference to undamaged specimens. This switch correlated with the increase in phenolic compound concentrations measured in the damaged algae.

Literature Cited

Dawes C., 1998. Marine Botany. New York: John Wiley & Sons, Inc.

Druehl, L., 2000. Pacific Seaweeds. Madeira Park: Harbour Publishing.

Gabrielson, P., Lindstrom, S., O’Kelly, C., 2012. Keys to the Seaweeds and Seagrasses of Southeast Alaska, British Columbia, Washington, and Oregon. British Columbia: IslandBlue/Printorium Bookworks

Graham, L. E., Wilcox, L.W. & Graham, J., 2009. Algae. 2nd ed. San Francisco: Pearson Education, Inc.

Lobban, C., Harrison P., Duncan M., 1985. The physiological ecology of seaweeds. Cambridge: Cambridge University Press

Mondragon, J., Mondragon J., 2003. Seaweeds of the Pacific Coast. Monteray: Sea Challengers

Van Alstyne, K.L., Herbivore Grazing Increases Plyphenolic Defenses in the Intertidal Brown Alga Fucus Distichus.

Vijayaraghavan, M., Kaur, I., 1997. Brown Algae Structure, Ultrastructure and Reproduction. New Delhi: Aph Publishing Corporation

Schiel, D.R. & Foster, M.S., 2006. The Population Biology of Large Brown Seaweeds: Ecological Consequences of Multiphase Life Histories in Dynamic Coastal Environments. Annual Review of Ecology, Evolution, and Systematics, 37(1), pp.343–372.

Links to additional sources:

 Diagram of life cycle: http://plantphys.info/organismal/lechtml/images/fucuslc.gif

Page authors and associations:

Una Miller

University of Washington