Purple ochre stars clinging to barnacle-studded boulders, harbour seals beach-bound with transient orcas in hot pursuit, and Dungeness crabs scuttling between waving blades of eelgrass: on Mayne Island, we don’t have to venture far or wide to see an incredible amount of biodiversity! Look no further than the intertidal zone.
What is the intertidal zone, you may ask? It’s the area of shore between the lowest and highest tides. From sea stars to tidepool sculpin, an amazing array of different creatures call this harsh environment home. There are different types of intertidal environments. Mud flats, sandy beaches, and rocky shorelines all provide unique habitat for local creatures. This article focuses on rocky intertidal areas on Mayne, showcasing three of our favourite locations: Piggott Bay, Miners Bay, and Georgina Point. During any low tide you can head out to experience these ecosystems for yourself.
A Hard Place to Live
What makes the rocky intertidal zone such a difficult place to survive? Most organisms are adapted to live within a narrow range of environmental conditions and require specific temperature, salinity, moisture and oxygen availability. Take humans, for example: we need continuous access to air and wouldn’t last long if totally submerged in saltwater. On the other hand, intertidal lifeforms have evolved to be incredibly resilient to huge environmental fluctuations. They’re constantly exposed to air and then covered by water with the changing tides. Along with desiccation, flooding, wave action, etc., there are also two sets of predators to contend with: those hunting at low tide such as shorebirds, and those hunting when the tide comes in such as fish. Intertidal creatures must defend themselves against both.
If life in the intertidal area is so demanding, why do so many things live there? This is an extremely productive zone, with great access to sunlight and nutrients, and loads of organisms taking advantage of those factors. In fact, there are so many animals trying to squeeze into the narrow intertidal zone that space is often limited: Picture a shoreline covered in mussels, barnacles and whelks (a type of sea snail).
To contend with the stress of constantly changing tides, intertidal animals have developed incredible ways of competing for space, keeping themselves from drying out, holding fast to the rocks they live on, and protecting themselves from predators. We’ll talk about some of these adaptations below.
An interesting interface between a mud flat, eelgrass meadow, and rocky intertidal zone, Piggott Bay is teeming with life and habitat diversity.
What kind of sea creature could produce a robin’s egg blue? Chitons use eight plates of armour as a defense against predators and desiccation. Like limpets and whelks, chitons clamp down tight to rocks when the water goes down. When the tide returns, they release their grip and move around in search of food, using specialized teeth called radula to scrape algae off rocks.
Rockweed covers many areas of the rocky intertidal zone and is extremely slippery. Be sure not to step on it as you explore! This incredible species of brown algae can dry to a crisp when the tide is out and will rehydrate when underwater again.
Nudi-what? Nudibranchs are sea slugs that breathe by absorbing oxygen from seawater through external gills on their backs. This rainbow nudibranch was spotted floating adjacent to Piggott Bay’s intertidal zone during one of the Conservancy’s eelgrass surveys. Growing up to 30 cm long, rainbow nudibranchs are among the biggest sea slugs we have on the west coast.
Speaking of sea slugs, there are at least eight barnacle nudibranchs in this photo – can you spot them all? Barnacle nudibranchs grow to be only 2 cm long. Their goopy white egg ribbons cling to the rocks above these tiny sea slugs.
With strong currents sweeping through Active Pass daily, Miners Bay receives an ever-shifting stream of nutrient-rich water. River otters, gulls, seals, and eagles are common visitors to the productive intertidal area here, scouring the rocky surface when the tide is low for their next meal.
While predators like eagles and raccoons visit the intertidal zone during low tide, sea stars are a voracious predator in this zone when the tide comes in. This blood star was found belly up at the time it was photographed. Never pull a sea star off a rock, as you’ll rip off the tube feet they use to cling to hard surfaces. They can regrow these tube feet, but it takes precious energy and time to do so.
Japanese wireweed is an invasive species that takes up the already limited space of the intertidal zone, making it difficult for native species to establish and compete. It was likely accidentally introduced to British Columbian waters and is now present throughout the coast from Mexico to Alaska.
Rows of tube feet can be seen on these orange sea cucumbers. As suspension feeders, they eat when the tide is up by catching plankton out of the water with their tentacles. They can grow to be 20 cm long and spend their days anchored to rocks. Their cousins, California sea cucumbers, are bigger, walk along the sea floor instead of attaching to rocks, and eat sediments (eg. mud and sand) instead of filter feeding.
Just like other sea stars, leather stars control their tube feet through a specialized circulatory system that distributes water throughout their bodies. They control this system by bringing in sea water through their madreporite. The madreporite can be seen in this photo: look for a light, small circle on the sea star’s back.
At the shore below the lighthouse at Georgina Point, you can see distinct bands of largely sedentary creatures vying for space in the intertidal zone. A kelp bed sits adjacent to the intertidal zone, providing additional habitat for more mobile intertidal creatures like crabs and sculpin.
Bands of different creatures demonstrate the fierce competition for space in intertidal areas. In general, biological factors like predation and competition within and between species determines the lower extent that these animals live. Environmental factors that result from changing tides, like moisture content and temperature, determine how high creatures can live in the intertidal zone.
Crevices in the rocks are great places to find creatures, like this ochre sea star. As extremely effective predators, these sea stars have a huge impact on the community structure of the intertidal zone. They roam around at high tide, inverting their stomach to digest mussels, barnacles, and other invertebrates externally. By preying on sedentary creatures, sea stars create space for new animals to take hold and help shape species distributions in the intertidal zone.
Creatures need different ways of holding onto rocks to deal with wave action, especially in exposed areas like Georgina Point where ferry wake and winds combine with currents. Mussels have risen to the challenge by developing strong fibres called byssal threads that connect them securely to rocks. They live in dense mats, and having neighbours pushed up beside one another provides additional stability.
Ever wonder what it feels like to be a barnacle? Try standing on your head and eating with your legs! Similar to many intertidal creatures, barnacles start their lives as free-floating larvae. When they find a place to settle, they cement their forehead to a hard surface and begin building their shell for protection. When the tide goes down, they close the plates of their shell to keep the heat and predators out. When the water comes back up, they stick out their legs and grab tiny pieces of food going by.
Guidelines for Intertidal Exploring
- Look at local tide charts before you go. Any tide lower than 1 m is sure to provide viewing opportunities of interesting creatures exposed to the air.
- Watch where you step: Intertidal animals are experts at camouflage and can be hard to see.
- Many creatures that live on the underside of rocks will dry out if left to face the sun. Flipping rocks can also disturb fish spawn and interrupt their lifecycles. Avoid moving rocks and look instead for deep crevices where you can observe intertidal animals without disturbing or harming them.
- Unless it’s human-made garbage, leave what you find on the beach. Hermit crab make homes from discarded snail shells, and rocks and shells form important surfaces for other animals to live on.
- Give marine mammals, like seals and their pups, a wide berth.
- Bring a guidebook! There’s nothing more satisfying than identifying a mystery creature on the spot.