The seaweeds dealt with in this guide are major elements of the temperate marine flora extending from southeast Alaska (60°N, just north of Homer) to central California (34°N, just south of Santa Barbara at Point Conception; Figure 1). Biogeographers refer to this stretch of coast as the Cold Temperate Region—this region is the focus of this book. The coastline west of southeast Alaska is the Arctic Region, and south of Point Conception is the Warm Temperate Region. General seawater temperatures define
Figure 1. This guide covers the Cold Temperate biogeographic region of the Pacific coast from Homer, AK, to Point Conception, CA.
Arctic Region
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About Seaweeds
each region; for the Cold Temperate Region these are 5–14°C (41–57°F) in winter and 10–18°C (50–64°F) in summer. Along the approximately 55,000 km (34,000 mi) of foreshore in the Cold Temperate Region live more than 640 species of seaweeds. We have not attempted to describe all species on our shores; instead, you will find here a curated selection of 240 species representing common seaweeds spanning the full range of lifestyles and forms in this part of the Pacific Ocean.
What Are Seaweeds?
Seaweeds are, in many ways, the plants of the sea. They have leaf-, stem- and root-like parts, and produce their own food by photosynthesis. They are sedentary and not overtly responsive to external stimuli, and are prolific reproducers. They have cell walls that give them rigidity, and are mostly multicellular and conspicuous. Yet seaweeds are fundamentally different from land plants (Figure 2). They do not have flowers, cones or other elaborate enclosed reproductive structures (like moss capsules). They do not protect themselves with bark and waxy coverings or have specialized systems for transporting nutrients internally (though some have primitive systems). Their root-like structures (holdfasts) function only as anchors and are not important in extracting nutrients and water from the soil—which would certainly be a difficult task when attached to a rock! In addition to the hundreds of seaweed species on our beaches, there are a few species of flowering plants included here as honourary seaweeds: seagrasses, commonly called surfgrass or eelgrass, which live alongside seaweeds, and shore plants that inhabit beaches and salt
Figure 2. Common seaweed structures.
Pacific Seaweeds
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marshes above the high tide line. We are excited to include shore plants as a new addition to Pacific Seaweeds.
Seaweed Morphology and Growth
The form and development of a seaweed, its morphology, begins with the basic compartmental unit of life: the cell. The cell contains those structures necessary for life: nucleus, chloroplast (responsible for photosynthesis), mitochondrion (responsible for the release of chemically bound energy) and others. Among the algae, seaweeds included, there is little cellular differentiation relative to land plants. The cells of seaweeds may be classified as parenchyma, meaning they are surrounded by thin cell walls with little or no secondary thickening and they usually retain their ability to undergo cellular division throughout their lives. Land plants have parenchyma, collenchyma and sclerenchyma. These latter cell types are characterized by secondary cell wall thickenings, resulting in woody characteristics, and by their inability to divide after reaching maturity.
Seaweed cells are differentiated by the number of nuclei they contain, their association with adjacent cells and their function. The cells may be multinucleate or uninucleate. Generally, cells visible to the naked eye are multinucleate (e.g., Urospora, p. 53). Cells are usually separated from adjacent cells by a well-defined end cell wall. During cell division, this cell wall grows as an expanding plate, separating the two daughter cells so that there is limited connection between them. In some seaweeds, the end cell wall is produced when the existing cell wall grows inward, effectively pinching the cell into two daughter cells. There is expanded connection between these cells, which are called coenocytic cells. They are usually multinucleate and may be very long (30 cm/12 in Codium, p. 68).
The surface cells of fleshy seaweeds are responsible for photosynthesis and nutrient uptake. The inner cells serve as packing and may form storage and/or primitive conducting tissues. Some cells are responsible for attaching the seaweed to its substrate (no small task). In some seaweeds, such as the green alga Ulva (p. 62), almost any cell may become reproductive; but in others, including many red seaweeds, only specific cells become reproductive. Similarly, cells that can undergo frequent division, thus being primarily responsible for growth, may be localized or occur generally throughout the plant.
Seaweeds grow in three basic ways. In the simplest way, called diffuse growth, cells divide more or less randomly all over the body. Take Ulva,
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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,