The Tidal Zone

Post written by Gar Secrist

M.S. candidate; Advisors: Mark Luckenbach and Iris Anderson

The fourth day of our adventure in Wales found us on Shell Island, a popular camping destination during the United Kingdom’s summer holidays. But we weren’t coming to pitch a tent. Instead, we were there to explore the area’s rocky shorelines, in search of flora and fauna in the intertidal zone, where the ebb and flow of ocean tides create unique habitats for marine organisms. We were greeted by a downpour of rain—a fairly common occurrence in north Wales. We sought shelter at a nearby café, and enjoyed a (second) breakfast.  The weather soon cleared up, and we set out to begin our search.

The rocky intertidal shoreline at Shell Island.

As we made our way down the beach, collecting and photographing the local wildlife, it soon became clear that this one rocky shoreline actually included several distinctive habitats for coastal critters. These environments result from different levels of exposure to the ocean due to tides. The first habitat we encountered, called the supralittoral zone, includes regions of the beach that are covered by water only at extreme high tides. This zone is also called the “splash zone,” as it is exposed only to splashes from far-reaching waves and is rarely immersed in seawater. Rocks in these regions are often covered by lichens: plant-like growths that are actually a combination of algae and fungi, working together in a symbiotic relationship.

We spent most of our time in the next zone of the rocky beach: the littoral zone. This area is the part of the beach that is submerged by the sea during high tide, but is exposed to the air during low tide. Organisms in this zone must be able to adapt to both of these very different sets of conditions. The main requirement for surviving in this zone is having the ability to hang on tightly to rocks, in order to avoid being swept out into the ocean by the force of waves. In our search, we found plenty of barnacles that were able to do just that.  While they may appear similar to coral, barnacles are actually related to crabs and lobsters, but spend their adult life as immobile filter-feeders, firmly attached to rocks in intertidal areas.

But the barnacles have more to worry about than just the tide.  We also found plenty of predators on the rocks, ready to feast on the abundance of barnacles in the intertidal zone. Among these predators were snail-like dog whelks, as well as limpets, which have a single domed shell. These mollusks have a muscular foot which they use to hang on tightly to rocks and to crawl along in search of food, before prying open and eating prey like barnacles.

Captions:  A limpet clinging to a rock in the intertidal zone; A seafood feast: dog whelks (spiral shells in top middle) and a limpet (domed shell in bottom middle) feed on barnacles covering a rock in the intertidal zone; The underside of a limpet.  The large muscular foot makes up most of the central tissue.

A limpet with barnacles and algae growing on the shell.



Another neat thing we noticed was that some barnacles were actually growing on the shells of limpets, safe from being eaten even as the limpet fed on other barnacles attached to the rocks.  Even algae would take advantage of limpets as a growing spot—we found one limpet carrying a miniature garden of algae on its shell.




Limpets feeding on barnacles while other barnacles grow attached to the limpet’s shell.

Beneath the littoral zone is the sublittoral zone, which usually remains submerged. It is often dominated by larger species of algae. Although we didn’t explore this region at Shell Island today, we may get a chance to look at similar areas later in the course during a spring tide, when low and high tides are more extreme.  Stay tuned…



Throughout our visit, we collected a surprising variety of species, from barnacles, limpets, and whelks to algae, marine worms, and more. We’ll take these back to the lab for a closer look.  Tomorrow, we plan to visit a more vertical, cliff-like intertidal zone.  It will be exciting to see what differences we find in this new environment!

About David Malmquist

David Malmquist is the Director of Communications at the Virginia Institute of Marine Science, College of William and Mary.
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