ways. In the simplest way, called diffuse growth, cells divide more or less randomly all over the body. Take Ulva,
15
About Seaweeds
the sea lettuce; if you were to draw a circle on the Ulva blade (can be done with India ink) and follow the change in the circle as the plant grew, you would see the circle expand but retain its original proportions. In the second way seaweeds grow, called intercalary growth, cells divide only in certain locations, not all over the whole body and not at the base or tip of the seaweed. In kelp, for example, cell division occurs between the stipe and blade. If you drew a grid across the upper portion of the stipe and base of the blade, the grid would expand unevenly over time. The grid points nearest the blade base would be farther apart than the points more distant from the blade base, because most of the active cell division takes place within a few centimetres of the base. In the third mode of growth, termed apical growth, most of the growth is associated with one (e.g., Fucus, p. 181) or many (most red algae) actively dividing apical cells located at the outer extremes of the seaweed. A grid marked on a seaweed with apical growth would expand along the margins of the seaweed and remain relatively unchanged near the seaweed centre.
Seaweeds take on a bewildering range of forms. In their extreme, they range from a single-celled green sphere about 1 cm (0.4 in) in diameter (Derbesia, p. 57) to what appears to be a large (up to 36 m/118 ft long) brown onion (Nereocystis, p. 226). Seaweeds may be filamentous, branched or not. They may be fleshy crusts, blades, tubes or bushes, flattened or radially branched. All of these forms are found in the red, green and brown seaweed groups. This repetition of form has led many researchers, like Diane and Mark Littler (Smithsonian Institution), Patrick Martone (University of British Columbia) and various colleagues, to consider the functional role of form. Does a particular form convey a competitive advantage? Are some forms better able to withstand high-energy waves? Do different forms dominate during different stages of succession (the development of
Figure 3. Common seaweed branching patterns.
Pacific Seaweeds
16
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
