Figure 3. Common seaweed branching patterns.
Pacific Seaweeds
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a mature community from virgin substrate)? These and related questions are discussed in Identifying Pacific Seaweeds (p. 33) and Seaweed Ecology (p. 237).
Along with form comes an equally bewildering array of traits. Kyle Demes (Hakai Institute), whose PhD thesis explored seaweed material properties, likens seaweeds to the superpowers of comicbook superheroes. Some species can “shape-shift” (individuals look drastically different when grown in different environments; e.g., Callophyllis, p. 132), some can turn to stone (coralline algae and other calcifying species, p. 76, 81), others can clone themselves (reproduce asexually), some have extreme extensibility (e.g., Nereocystis, p. 226) and many can “fly” in the water using gas-filled floats (e.g., Macrocystis, p. 232).
Seaweeds and the Tree of Life
Seaweeds have a fascinating relationship to each other and to other groups of life. First, seaweeds around the world are divided among three groups: the green, red and brown seaweeds. Each group has distinctive storage products, cell wall components and, most noticeably, pigmentation; hence their common and formal names: Chlorophyta (Greek=green), Rhodophyta (Greek=red) and Phaeophyceae (Greek=brown). Second, you’d be forgiven for assuming that all seaweeds are relatively closely related given that many green, red and brown species overlap in form and live side by side; however, the three groups are actually separated by many millions of years of evolution. On the family tree of all life, the ancestral lines leading to modern-day green seaweeds and land plants separated roughly 1.2 billion years ago, with more than 1.5 billion years since red and green seaweeds shared a common ancestor! Brown seaweeds are a much younger lineage and so different that they are not considered related to either red or green seaweeds, much as we consider a jellyfish (now called a “jelly”) and a finfish not to be related. Thus, and third, the term “seaweed” encompasses an artificial assemblage of organisms. This artificial assemblage becomes obvious if you look at a family tree of the major groups of life (excluding bacteria and archaea, Figure 4). Since red and green seaweeds share a common ancestry with plants and their relatives, and brown seaweeds do not, we will only refer to red and green seaweeds as “plants” throughout this book.
The features shared by seaweeds mostly define a larger artificial assemblage, the algae (singular: alga, adjective: algal). Seaweeds share the
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About Seaweeds
beach with other algae, the blue-green algae and diatoms. Most of these are inconspicuous and microscopic, which is not to say inconsequential. The blue-green algae are photosynthetic bacteria whose presence is indicated by dark, approaching black, variously shaped little slimy colonies. The diatoms, which are normally encountered floating in the sea (phytoplankton), exist as microscopic epiphytes living on seaweeds and as dark brown macroscopic strands attached to rock or seaweeds. These diatom strands may be distinguished from brown seaweed filaments by grinding them between your fingers: the diatom strands will disintegrate, the brown algal filaments will not. Interestingly, brown seaweeds are more closely related to the microscopic phytoplankton diatoms than to other seaweeds (Figure 4).
Figure 4. A family tree of the major groups of life (excluding bacteria and archaea). Coloured ovals highlight relevant lineages within select major groups (major group names in bold); the black circle at centre represents the common ancestor.
Adapted from P. Keeling (2004), American Journal of Botany 91(10), 2004: 1481–1493.
Slime Moulds
Choanoflagellates
Dinoflagellates
Brown Algae
Diatoms
Animals
Fungi
Red Algae
Green Algae
Charophyte Algae
Land Plants
Plantae
Rhizaria
Excavates
Unikonts
Chromalveolates
Pacific Seaweeds
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Seaweed Life Histories
The life cycle of humans, like that of almost all animals, is simple and straightforward. Your cells are diploid (having two sets of chromosomes). Some of these cells will undergo meiosis, a type of cell division that reduces the chromosomes to one set, producing haploid (one set of chromosomes) eggs or sperm. The fusion of the egg and sperm introduces two chromosome sets into a zygote. The zygote increases in cell number by mitosis, a type of cell division that does not change the number of chromosome sets, and develops into a mature diploid individual.
The life cycle of plants, particularly the seaweeds, is much more elaborate and varied than that found in animals. In plants, the products of meiosis are rarely eggs and sperm but rather a haploid plant body. Thus a plant can have two bodies, a haploid one and a diploid one. The diploid body is called the sporophyte because it produces spores by meiosis. These spores give rise to the haploid body, which is called the gametophyte because it gives rise to gametes (eggs, sperm or other sexually active cells).
The gametophytes and sporophytes may appear very similar, a condition referred to as isomorphic (same form), or they may be dissimilar, or heteromorphic (different form). The green seaweed Ulva (sea lettuce, p. 62) has a life cycle that alternates between morphologically similar gametophytes and sporophytes (isomorphic; Figure 5). Some seaweeds have gametophyte and sporophyte generations that are markedly different in appearance (heteromorphic). Green seaweeds such as the green filamentous Urospora (p. 53) have a single-celled sporophyte and a multicellular filamentous gametophyte. At the other extreme is the brown seaweed Fucus (p. 181), where the sporophyte is multicellular and the gametophyte is reduced to eggs and sperm as in the typical animal life cycle (Figure 6). In between are many species whose gametophytes and sporophytes are both multicellular but of different sizes and forms. In the kelp Nereocystis (p. 226), the sporophyte is a very large plant—often longer than 30 m (98 ft)—and