Ulva fenestrata Postels & Ruprecht
The green macroalga widely known as Ulva fenestrata is an important food source for a variety of microorganisms and herbivores, especially certain polychaetes, amphipods, and crabs, along the northeastern Pacific coast. It is often attached to shells, pebbles, rocks, or pieces of wood. Although it may always start out anchored like this, in quiet bays it may float about, except when it has been left behind by a receding tide. Floating specimens tend to grow larger than those that are attached to a substrate. The blades may reach a length of 1 meter and they are often extensively perforated, especially from Oregon northward. The specific epithet "fenestrata" refers to these "windows" in the blade. (Kozloff 1993) Recent molecular studies have suggested, however, that this fenestration and other characters used to define and distinguish this species may be unreliable. These characters may simply represent natural variation within Ulva lactuca, or possibly within a complex of cryptic Ulva species (Hayden et al. 2003; Hayden 2004). Because of taxonomic confusion within the genus, it is difficult to describe geographic distributions of many Ulva species with any confidence, but many Ulva species appear to have very wide ranges (e.g., Ulva lactuca occurring in the Atlantic and eastern Pacific: Lee 1986; Hayden and Waaland 2004).
Ulva species are isomorphic, i.e., in their lifecycle they alternate between haploid and diploid forms that look the same to the naked eye (some other algae are heteromorphic, with morphologically distinct haploid and diploid forms)."Diploid" means that each cell has two copies of each chromosome (as in the bodies of humans and other mammals); "haploid" means that each cell has just a single set of chromosomes (as in human gametes, i.e., sperm and eggs). The haploid Ulva individuals (gametophytes) produce haploid, biflagellate swimming gametes by mitosis (regular cell division). The diploid individuals (sporophytes) produce quadriflagellate swimming haploid zoospores through meiosis (the same reduction-division process by which humans and countless other sexually reproducing organisms produce haploid gametes from diploid cells). Two gametes from a male and a female gametophyte fuse to form a diploid zygote, which develops into a diploid sporophyte. The diploid sporophyte produces haploid zoospores by meiosis and these zoospores develop into the next gametophyte generation. (Druehl 2000)
Ulva species have thin distromatic (i.e., 2 cell layers thick) green blades, which are annual (Abbott and Hollenberg 1976).
Nelson et al. (2003) obtained experimental evidence indicating that extracts of "Ulva fenestrata" have allelopathic properties, inhibiting growth and/or development of other macroalgae and animal larvae. It is possible that these competitor-inhibiting compounds evolved to defend against grazers and may now impact other types of organisms as well when circumstances cause these chemicals to be released into the water (e.g., as a result of partial desiccation).
Evolution and Systematics
Systematics and Taxonomy
Recent molecular studies of algae in the family Ulvaceae have indicated the need for extensive revision of current nomenclature and taxonomy to reflect our evolving understanding of species boundaries and evolutionary relationships (Hayden and Waaland 2002, 2004 Hayden et al. 2003). The widespread Ulva lactuca was long considered to be present from Alaska to the Gulf of California, but beginning in the 1960s, the prevailing view was that U. lactuca was absent from the northeast Pacific and that only the very similar U. fenestrata occurred from Alaska to Oregon (Hayden and Waaland 2004 and references therein). Work by Hayden and Waaland (Hayden et al. 2003; Hayden 2004), however, suggests that some putative U. fenestrata in this region are in fact conspecific with U. lactuca and that U. fenestrata may be a junior synonym for the valid name U. lactuca. It is clear that more work will be necessary to resolve the systematics and taxonomy of this group.