Wildland writers: Intertidal zone teems with invertebrates, barnacles and seaweeds

[UK] One might think that the rocky intertidal zone would be quite inhospitable for marine or terrestrial life.

It is alternately submerged and exposed to air, washed by fresh water during rains on falling tides and salt water on rising tides, exposed to sub-zero temperatures during winter and extreme heat in summer, and subject to intense thrashing by breaking swells.

But visit such a shore when the tide is out and we see a dense patch of mussels in one spot, barnacles in another and a dense cover of seaweeds over most of it. Few places hold more fascination for children as they discover periwinkles, starfish, crabs, mussels, sea urchins, anemones and other invertebrates among the seaweeds and in tide pools.

Indeed, the rocky intertidal zone is one of the most productive habitats on Earth. Most of the species that live there are marine species that have special adaptations and “slow down” so they can tolerate the stresses of exposure during low tide but thrive when covered by seawater. The benefits for seaweeds of living in this zone are high light, and immersion in well-mixed water carrying nutrients. For animals, the benefits are an abundance of food and oxygen, and escape from some predators. Except for a few lichens and insects, no organisms of terrestrial origin reside permanently in the rocky intertidal, although many browse there, such as mink, raccoons, bears and crows, as well as gulls and many shorebirds.

A kelp forest just below the intertidal zone at Crystal Crescent Beach.
A kelp forest just below the intertidal zone at Crystal Crescent Beach.

Perhaps the most striking aspect of the rocky intertidal zone is the way in which the inhabitants partition the space into horizontal zones within the broader intertidal zone.

Proceeding from land one first crosses the white zone, a kind of no-man’s territory between land and sea that is bereft of life, except in a few crevices. The lack of life is attributable to the intense stresses from wind, sun and salt spray. This zone appears bleached white on granite rock but doesn’t stand out as distinctively on dark rocks.

Then we move into the buff zone, the uppermost of five intertidal zones.

It is covered by seawater only on the highest of high tides but is regularly splashed at other times. Here, the rock is discoloured a light brown (seen against a lighter background as on granite, but not so distinguishable on a dark slate), with black patches of lichens and cyanobacteria. Black pigments protect these photosynthetic organisms from ultraviolet rays. Small, rough periwinkles graze this zone, gathering in cracks when the tide is down.

Next is the whitish barnacle zone, often so packed with barnacles that it assumes a honeycomb appearance. Barnacles are crustaceans (like lobsters) that live in calcified boxes with trap doors that open when the tide is up, allowing them to feed, and close when it is down to retain moisture. The top of this zone corresponds roughly to the mean high tide level — that’s useful to remember when you visit the shore when the tide is down! A sharp decline in abundance and size of barnacles marks the lower boundary, which, not by coincidence, is also the upper limit of dog whelks, a type of snail that feeds on barnacles.

Zone 3 is the fucoid zone, characterized by a dense cover of several species of brown seaweeds commonly known as wracks or rockweeds. They have flattened branches with swellings associated with reproductive functions or serving as floats. When the tide is down, these seaweeds are horizontal, or hang vertically from steeper surfaces, forming a protective canopy that harbours an abundance of invertebrates and a variety of smaller seaweeds. In tide pools (depressions that retain water as the tide falls) one can see brightly coloured seaweeds that are seen otherwise only below the intertidal zone, and here the invertebrates are most abundant. Hermit crabs in old periwinkle shells and colourful anemones with crowns of tentacles are always a hit with the kids.

The Irish moss zone is fully exposed only on the lowest tides. Irish moss is a highly branched seaweed, smaller than the fucoids, and varies in colour from green through red to brown, and sometimes the tips are iridescent blue. Other seaweeds such as the calcified coral weed, and sometimes dulse can also be abundant.

Finally, on the extreme lowest tides, we can view the top of the kelp zone. Kelps have large, flat blades, in some species dissected or with holes, attached to a tough tubular stem with a holdfast that clings firmly to the rock. The kelp zone continues into deeper water, where it forms dense stands like a forest. There are many other species of seaweeds in the shallow waters off our rocky shores that can be as colourful as any tropical reef.

Irish moss and rockweed are commercially harvested along our rocky shores, commonly with long rakes. Irish moss is a source of carrageenan, a substance used as a food thickener, especially in ice cream. Rockweed is valuable as an animal feed supplement. Dulse, that blackened seaweed Nova Scotians love to chew, occurs mostly below the intertidal zone and is red to purple when living.

I have often wondered why the common periwinkles that are superabundant in the intertidal zone and below are not more of a restaurant item in Nova Scotia. Then I think of all of shore life that benefits from them and think, that’s quite OK.


Photo: The Young Naturalist Club explores the rocky shore near Sambro on an extreme low tide. The youngsters were able to see examples of white, buff, fucoid and Irish moss zones.

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