Seaweed Basics

The study of seaweeds and other algae is a branch of botany called phycology, the term stemming from the Greek words phykos or phykion meaning seaweed and logos meaning discourse. Algae can be both freshwater and marine. Marine phycology is the study of marine algae, which include the familiar kelps we see washed up on beaches (the macroalgae) and microscopic forms such as diatoms (the microalgae).

Seaweed is a catchall term for plants growing in the sea, both in the shallow ocean and along the shore. But it is especially reserved for the macroalgae, to which we will limit ourselves in this guide. All seaweeds contain chlorophyll and produce their food by photosynthesis, but they contain other pigments as well. Seaweeds are divided into three main groups (Phyla) according to their color which is governed by dominating pigments:

   Green Algae (Chlorophyta) - chlorophyll, both a and b, dominates giving them a green color.

   Brown Algae (Phaeophyta) – fucoxanthin, which reflects yellow light, gives brown algae their brownish color.

   Red Algae (Rhodophyta) – phycoerythrin gives a red color; phycocyanin reflects blue light.

Green algae usually look green, brown algae usually appear brown or yellow-brown, but red algae, because of combinations of pigments, can appear anywhere from bright red to black, and some may even look brownish.

In addition to algae, seagrasses are often covered in seaweed guides, and two genera are covered here, Zostera and Phyllospadix. These are true flowering plants (but not true grasses) that happen to be marine. 

The study of seaweeds, like most scientific disciplines, is replete with terms and jargon, and though I try to avoid as much of both as possible, some unfamiliar words are necessary, and I usually define them at least once in the text of the guide. To help, though, I have included a glossary.

While writing this guide to seaweeds and presenting information on how they live, it was almost impossible, no matter how hard I tried, to avoid a discussion of algal life cycles. So, I have included it here. Several “plant” groups, such as ferns, fungi, and algae, have complex life cycles consisting of separate and alternating phases of free-living individuals with different ploidy (some addition to or multiple of the basic number of chromosomes) and modes of reproduction. Many seaweeds have a biphasic or what is called a haploid-diploid life cycle, and these species alternate between haploid (having a single set of chromosomes in the nucleus, designated by the letter ‘n’) and diploid (having two sets of chromosomes in the nucleus, designated by ‘2n’). Diploids produce haploid spores by meiosis, a set of two special divisions of the cell nucleus. These spores develop into male and female haploid adults called gametophyseaweed-basics-1tes. The adult gametophytes produce eggs and sperm which can unite to grow into diploid adults, the sporophytes, completing the life cycle.

This is the generalized life cycle of seaweeds. There are some complexities. Usually, male and female gametophytes are dioecious, that is the sexes are in separate individuals, and there are male ‘plants’ and female ‘plants’. Some species, though, are monoecious, both sexes being housed in the same individual ‘plant’. Normally, gametes (eggs and sperm) are released into the water where the egg is fertilized and forms a diploid zygote which settles to the bottom, attaches to the substrate, and grows into a mature alga. However, red algae have a modified biphasic cycle – actually a triphasic cycle - with the addition of a short-lived carposporophyte, a diploid stage formed on the surface of the female gametophyte thallus by the union of haploid gametes (Thornber, 2006). This carposporophyte lives on the female thallus from which it acquires nutrients. It produces spores, the diploid carpospores, which are released into the water, settle, and grow into mature, free-living tetrasporophytes. The tetrasporophytes, by meiosis, produce tetraspores (each spore is formed in a group of four) that are released into the water column where they settle to become male and female gametophytes.

Another complication is that mature free-living phases of some species have morphologies that are heteromorphic with independent stages that are distinctly different from each other, and other species are isomorphic with phases that look almost identical to each other. Those algae with isomorphic phases sometimes have different demographics; they are differently distributed as gametophyte or sporophyte populations, depending on seasons, the amount of shelter or wave exposure, and other environmental factors.

For more information on seaweed biology, life cycles and identification, I recommend the following books:

Abbot, Isabella A. and George J. Hollenberg. 1976. Marine Algae of California. Stanford University Press. Stanford, California.

Dawson, E. Yale. 1966. Marine Botany – An Introduction. Holt, Rinehart and Winston, Inc., New York.

Druehl, Louis. 2000. Pacific Seaweeds.  Harbour Publishing, Madeira Park, British Columbia.

Gabrielson, Paul W., Thomas B. Widdowson, Sandra C. Lindstrom, Michael W. Hawkes, and Robert F. Scagel. 2000. Keys to the Benthic Marine Algae and Seagrasses of British Columbia, Southeast Alaska, Washington and Oregon. University of British Columbia, Physiological Contribution Number 5.

Kozloff, Eugene N. 1983. Seashore Life of the Northern Pacific Coast – An Illustrated Guide to Northern California, Oregon, Washington, and British Columbia. University of Washington Press, Seattle, Washington.

Lamb, Andy and Bernard P. Hanby. 2005. Marine Life of the Pacific Northwest – A Photographic Encyclopedia of Invertebrates, Seaweeds and Selected Fishes. Harbour Publishing, Madeira Park, British Columbia.

Mondragon, Jennifer and Jeff Mondragon. 2003. Seaweeds of the Pacific Coast – Common Marine Algae from Alaska to Baja California. Sea Challengers, Monterey, California.

O’Clair, Rita M. and Sandra M. Lindstrom. 2000. North Pacific Seaweeds. Plant Press, Auke Bay, Alaska.

Snively, Gloria. 1978. Exploring the Seashore in British Columbia, Washington and Oregon – A Guide to Shorebirds and Intertidal Plants and Animals. Gordon Soules Book Publishers, West Vancouver, British Columbia.


Thornber, Carol S. 2006. Functional properties of the isomorphic biphasic life-cycle. Intg. Comp. Biol. 46:605-614.

Jim Young
Oceanside, Oregon

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