OLYMPUS DIGITAL CAMERAWhen I was young, seaweed was the gross stuff piled on the beach that I had to tiptoe through at my grandparents’ cottage to get to the heavenly sand flats at low tide.  A missed step may have landed my foot on a partially buried jellyfish.

When I started teaching biology, one of the labs that the biology teachers carried out in previous years was an observational lab of native seaweed.   Since I live only hundreds of feet from the beach, I volunteered to collect the seaweed specimens.

seaweed anatomy

I was struck by my complete obliviousness to any scientific name for seaweed. Even the common names I was unsure about.  All these seaweed types had been right under my nose (or toes) on so many hot summer days of my childhood; this apparently didn’t stir my interest enough to appropriately identify them.

I have now taken a much keener interest to what washes up on the beach.  I no longer tiptoe over the seaweed, but instead search underneath the rockweed (etc.) that clings to the treacherously slippery rocks of the intertidal zone. As I walk along the water’s edge, I now can appreciate a nice wrack consisting of numerous species discarded by the sea.

The main body of seaweed is called the thallus.  When describing seaweed, the holdfast is what you would think of as the root, the stipe is what you might consider the stem,, and the blade or frond is what you would call the leaf.  The plant terms are replaced because although they may resemble their plant counterparts in appearance, they don’t share all the same functions, such as having internal vascular tissue for conducting water and nutrients.   Seaweed instead obtains what it needs via diffusion.

Seaweed is considered to be multicellular algae from the Kingdom Protista, although many field guides call them plants (1).  The three main phyla of seaweeds and their characteristics are shown in the table below:

Table 1: Characteristics of Seaweed Phyla (1)

Phylum Color Pigments Light
Chlorophyta Green Chlorophyll require substantial   sunlight, common in intertidal zone and in shallow water
Phaeophyta Brown Chlorophyll, additional   brown pigments intertidal zone and   subtidal, common on rocky shores
Rhodophyta Red Chlorophyll, additional   brown and red pigments require less light than   the other two phyla and are found further from shore in the intertidal zone   (1)

Many types of seaweed also have other accessory pigments, such as xanophylls (yellow) and carotenes (1) that contribute to their color.  Although color is useful for identification, specimens may lose their dominant color due to too much sunlight or decay and other pigments may show through making identification problematic.



  1. Gosner, Keneth L. (1978), Peterson Field Guides: Atlantic Seashore – A field guide to sponges, jellyfish, sea urchins, and more

2. Miller, K.R., Levine, J. (2004). Biology. Prentice Hall

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