The fourth entry in the Coastal Flora of the Pacific Rim series features the alga Fucus gardneri.
I find Fucus gardneri, or common rockweed, to be a gorgeous algae. Its olive-green, strap-like blades are anchored tightly to intertidal rocks via a disk-shaped holdfast. The blades transition to a yellow-green colour at the tips. They contain a jelly-like fluid, which helps the swollen tips to act as floats in order to stay well-positioned to capture the sun’s energy. I love to crouch by a tidepool and watch these tips sway back and forth with the waves. They are slightly translucent, and on a sunny day seem to almost glow against the darker parts of the thallus (the undifferentiated vegetative tissue that makes up the main body of an alga). The frond tips also perform another important function. The swollen receptacles house reproductive structures known as oogonia (female) and antheridia (male). At a low tide, the receptacles dry up and squeeze out the sperm and eggs. At the next high tide, fertilization occurs. The resulting zygote finds its own place on the rocky coast.
A visitor to the Pacific Northwest coastline is likely to encounter Fucus gardneri. In the northern part of its range, it is often the most prominent species of alga, forming a band of green along the mid to high intertidal zone. It is found on many types of coastlines, including the exposed outer coast, inland protected waters, and even on human-made structures and boats. Common rockweed inhabits an extremely demanding environment – at times submerged under rough salt waters and at other times exposed to drying winds and sun. The conditions imposed on Fucus gardneri change each day as the tides ebb and flow and each month as the seasons change. On top of this, Fucus gardneri individuals that inhabit higher elevations must contend with much longer periods of air exposure than those in lower zones. Wright, Williams and Dethier (2004) were interested in how these fluctuating conditions affect common rockweed at different life stages (from embryos to juveniles and adults). The researchers found a complex relationship between season, location within the tidal zone, and life stage. Most of the year, Fucus gardneri thalli in the lower zones survived and grew more than those that were more often exposed to air. Summer and autumn were particularly difficult on large thalli at the uppermost range inhabited by this species, which by the end of summer appeared tattered and damaged. These individuals were most often exposed to air and warm temperatures, reducing their rate of photosynthesis, and rendering their thalli brittle and susceptible to breakage. Over the winter, however, Fucus gardneri thalli in the higher intertidal zone had higher rates of survivorship and growth. Individuals in this zone also produced more reproductive tissue than those in other zones, regardless of the season.
This species also sometimes goes by the synonym Fucus distichus. For an explanation of why Fucus gardneri is not a plant, see the 2005 Botany Photo of the Day entry on Fucus gardneri.