Seaweeds are a fascinating and diverse group of organisms living in the earth's oceans. You can find them attached to rocks in the intertidal zone, washed up on the beach, in giant underwater forests, and floating on the ocean's surface. They can be very tiny, or quite large, growing up to 30 metres long!

Although they have many plant-like features seaweeds are not true vascular plants; they are algae. Algae are part of the Kingdom Protista, which means that they are neither plants nor animals. Seaweeds are not grouped with the true plants because they lack a specialized vascular system (an internal conducting system for fluids and nutrients), roots, stems, leaves, and enclosed reproductive structures like flowers and cones. Because all the parts of a seaweed are in contact with the water, they are able to take up fluids, nutrients, and gases directly from the water, and do not need an internal conducting system. Like true plants, seaweeds are photosynthetic; they convert energy from sunlight into the materials needed for growth. Within their cells seaweeds have the green pigment chlorophyll, which absorbs the sunlight they need for photosynthesis. Chlorophyll is also responsible for the green colouration of many seaweeds. In addition to chlorophyll some seaweeds contain other light absorbing pigments. These pigments can be red, blue, brown, or golden, and are responsible for the beautiful colouration of red and brown algae.

Seaweed structure

Instead of roots seaweeds have holdfasts, which attach them to the sea floor. A holdfast is not necessary for water and nutrient uptake, but is needed as an anchor. Holdfasts are made up of many fingerlike projections called haptera.

The stalk or stem of a seaweed is called a stipe. The function of the stipe is to support the rest of the plant. The structure of the stipe varies among seaweeds; they can be flexible, stiff, solid, gas-filled, very long (20 metres), short, or completely absent.

The leaves of seaweeds are called blades. The main function of the blades is to provide a large surface for the absorption of sunlight. In some species the blades also support the reproductive structures of the seaweed. Some seaweeds have only one blade, which may be divided, while other species have numerous blades.

Many seaweeds have hollow, gas-filled structures called floats or pneumatocysts. These help to keep the photosynthetic structures of the seaweed buoyant so they are able to absorb energy from the sun.

The term thallus refers to the entire plant body of a seaweed.

  Structure of seaweed


Seaweeds play very important ecological roles in many marine communities. They are a food source for marine animals such as sea urchins and fishes, and are the nutritional base of some food webs. They also provide shelter and a home for numerous fishes, invertebrates, birds, and mammals.

Large seaweeds can form dense underwater forests, called kelp forests. These forests provide a physical structure that supports marine communities by providing animals with food and shelter. Kelp forests act as underwater nurseries for many marine animals, such as fish and snails. The lush blades form a dense forest canopy where invertebrates, fishes, birds, otters, and whales can find lots of tasty food and a good home. Beautiful sea slugs and kelp crabs can be seen on the blades and stipes of the seaweeds, while other small marine animals like worms find their homes in the the holdfasts. Kelp forests are a huge food source for sea urchins and other grazing invertebrates.

Seaweeds are affected by the physical characteristics of their environment. Because seaweeds absorb gases and nutrients from the surrounding water, they rely on the continual movement of water past them to avoid nutrient depletion. The constant motion of ocean water also subjects seaweeds to mechanical stress. Ocean waves and currents are sometimes strong enough to rip seaweeds right off the rocks! Seaweeds cope with mechanical stress by having a strong holdfast, a flexible stipe and blades, and bending towards the substrate as waves move over them.

Many seaweeds live in rocky intertidal communities. Because they cannot get up and follow the water when the tide goes out, intertidal seaweeds are subjected to the stresses associated with exposure to air and weather conditions. To survive in the intertidal, seaweeds must be able to tolerate or minimize the effects of evaporative water loss and temperature and salinity changes. When exposed to air seaweeds lose water through evaporation. Some seaweeds can dry out almost completely when the tide is out, then take up water and fully recover when the tide brings water back to them. Seaweeds living in tidepools are exposed to changes in water temperature and salinity caused by weather conditions. On hot, sunny days the water in tidepools warms up and evaporates, which increases the salinity of the water. When it rains the opposite happens, the salinity of tidepool water decreases. On cold days, seaweeds can be damaged by freezing.

When the tide is out mobile intertidal animals must also try to minimize water loss. One way they do this is by seeking out a moist hiding place under some seaweed. As well as providing shelter for invertebrates, intertidal seaweeds are also a food source for grazing animals.


Seaweed life and reproductive cycles can be quite complicated. Some seaweeds are perennial, living for many years, while others are called annuals because they live for only one year. Annual seaweeds generally begin to grow in the spring, and continue throughout the summer. During powerful fall and winter storms the stipes and blades of seaweeds are often ripped off. If the holdfast manages to survive through the winter, new blades will begin to grow from it in the spring. Perennial species may also lose many of their blades during the winter and their growth is reduced.

Seaweeds can reproduce sexually, by the joining of specialized male and female reproductive cells, called gametes. Like the cells that make up our bodies, the cells of adult seaweed plants are diploid, which means that they contain two sets of chromosomes. Diploid plants are called sporophytes because they produce and release spores. Spores are produced by meiosis, a cell division process that halves the number of chromosomes and forms new cells containing only one set of chromosomes (haploid cells). After they are released from the sporophyte, the haploid spores settle and grow into male and female plants called gametophytes. The gametophytes are also haploid, and they produce gametes (sperm or eggs). The sperm and eggs are either retained within the gametophyte plant body, or released into the water. Eggs are fertilized when the sperm and egg fuse together, and a diploid zygote is formed. Zygotes develop and grow into diploid sporophytes, and the life cycle continues.

Seaweeds display a variety of different reproductive and life cycles and the description above is only a general example of one type, called alternation of generations. If the gametophyte and sporophyte forms look the same, it is called isomorphic (same form) alternation of generations, and if they vary in appearance, it is called heteromorphic (different form).

Seaweeds can also reproduce asexually (a reproductive process that does not involve gametes) through fragmentation or division. This occurs when parts of a plant break off and develop directly into new individuals. All offspring resulting from asexual reproduction are clones; they are genetically identical to each other and the parent seaweed.


Seaweeds are classified into three major groups; the green algae (Chlorophyta), the brown algae (Phaeophyta), and the red algae (Rhodophyta). Seaweeds are placed into one of these groups based on their pigments and colouration. Other features used to classify algae are; cell wall composition, reproductive characteristics, and the chemical nature of their photosynthetic products (oil and starch). Within each of the three major groups of algae, further classification is based on characteristics such as plant structure, form, and shape.  

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Druehl, L. 2000. Pacific Seaweeds. Harbour Publishing, Madeira Park, B.C., Canada.

Harbo, R.M. 1999. Whelks to Whales. Harbour Publishing, Madeira Park, B.C., Canada.

Waaland, J.R. 1977. Commom Seaweeds of the Pacific Coast. J.J. Douglas Ltd., Vancouver, B.C., Canada.

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