As a child, I remember spending hours staring into rock pools amazed at all the small critters moving around and going about their lives in these tiny pockets of water. From afar these rockpools may look like a calm and relaxing environment in comparison to the choppy waves of the ocean, but don’t be fooled - many factors make these microhabitats an extreme place to live. This blog post will teach you what determines the species that live in rock pools, how their placement up the shore affects these microhabitats and what common species you can see when you go rock pooling for yourself!
What are rock pools
As the name suggests, rock pools are pools of water found in the rock crevices along shorelines. They are also known as tidepools as they are created when seawater becomes trapped within these crevices as the tide goes out. Rock pools are found across coastal shores, which are areas on the shore that can be split into three main categories of tidal zones - high, mid and low. High tidal zones are flooded only during a few hours in the day when the tide is at its highest; Mid tidal zones are submerged for the majority of the day except for a short period when the tide is low; and lastly, low tidal zones are almost always submerged unless there is a very low tide - rock pools in this area usually have more biodiversity and more marine vegetation like seaweed.
What factors affect the species found in rock pools?
Like many habitats, the composition, abundance and distribution of species in rock pools are affected by different natural processes. These natural processes could be physical (e.g. topography), chemical (e.g. changes in oxygen, salinity and temperature) and biological processes (e.g. predation and competition for food and space). Rock pools are often isolated and distributed in patches along shores, which creates lots of microhabitats. These microhabitats are highly varied depending on where they are found along the shore, resulting in a range of different species that have developed different adaptations to allow them to tolerate sudden fluctuations in temperature, salinity and oxygen levels.
Rock pool structural complexity plays a role in determining which species are found in them. Structural complexity can be things like crevices in rocks, rock size, shells and sand at the bottom of the rock pool. It directly affects fish abundance and diversity by supplying shelter for predators, prey and creates different areas to harbour a larger number of species that can coexist. These structural features also provide shelter from environmental factors like waves, temperature and salinity changes. The depth of the rock pools plays a similar role to structural complexity. A deeper rock pool can mean more space for different species, and a greater volume of water is less likely to have large fluctuations in salinity, temperature and oxygen levels, creating a more stable environment.
The length of time seawater is trapped in rock pools before being replaced by new seawater when the tide comes in is one of the biggest factors in controlling the types and abundance of species that can occupy rock pools. Within a few hours, the environment in a rock pool can change drastically, temperatures can increase by 15°C, salinity can increase by 3ppt (parts per trillion) and oxygen can reach hypoxic conditions. Rock Pools in high tidal zones are only flooded when the tide is at its highest point, which can occur 1-3 times a day, meaning that rock pools in these zones go a long time before being submerged with new seawater. This causes high tidal rock pools to go through extreme changes in oxygen, temperature and salinity, meaning only species who are highly adapted to these fluctuations can inhabit them.
Why are rock pools important to coastal ecosystems?
You may be surprised to find out that almost all coastal fish species are thought to utilize rock pools at some stage in their lifetime, either by searching for food, using them as a refuge or to develop and reproduce. The species that use rock pools can be split into three categories: (1) Residents: these are species that spend the majority of their lives in the rock pools and therefore are adapted to the sometimes extreme conditions; (2) Secondary residents: these are species that utilize rock pools when they are juveniles and still developing, then move off-shore once they reach adult life (when a habitat is used in this way it’s called a nursery); and (3) Transients: these are species that are only found in rock pools occasionally or by accident - this usually happens when they enter the pools during high tide and then become trapped at low tide. The use of rock pools by species for food, hiding from predation and reproducing and developing makes them an integral part of coastal ecosystems and a key habitat in coastal fish species diversity and abundance.
Key species you can see in rock pools
So as you’ve learnt by now rock pools are pretty impressive habitats that are teeming with wildlife, meaning there are lots of amazing coastal creatures for you to see! Some of the most common species found in UK rock pools are:
Common limpets (Patella vulgata) are a greyish-white colour and sometimes have a slight yellow tint. They use suction to attach to the sides of rocks and are near impossible to remove!
Common shore crabs (Carcinus maenas) can grow up to 10cm in width and adults are mostly green, but colour varies between different life stages.
Bladderwrack seaweed (Fucus vesiculosus) is found higher up the shore and can be identified by short forks at the end of the branch and round bumps on their fronds which are air bladders.
"File:Common limpets1.jpg" by Tango22 is licensed under CC BY-SA 3.0 (far left), "Common shore crab (Carcinus maenas)" by blackartz is licensed under CC BY-NC 2.0 (centre), "Bladder Wrack" by Akuppa is licensed under CC BY 2.0 (far right)
Common starfish (Asterias rubens) are mostly orange and usually have five arms, but can sometimes have six! They don’t like being out of the water and the tips of their arms will start to curl if they become stressed.
Beadlet anemones (Actinia equina) are bright red and have lots of tentacles that sway in the water, when there is low tide their tentacles recede and they look like small red blobs.
Common sea urchins (Echinus esculentus) are covered in small spines. They feed on algae which help to regulate the algae’s population.
"Asterias rubens (Common starfish / Gewone zeester)" by Bas Kers (NL) is licensed under CC BY-NC-SA 2.0 (far left), "Beadlet Anemone" by Will_wildlife is licensed under CC BY-NC 2.0 (centre), "Common Sea Urchin / Rød kråkebolle" by asbjorn.hansen is licensed under CC BY-NC-ND 2.0 (far right)
How to be safe and respectful while exploring rock pools
Before you go running off to explore this newfound world of rock pools there are a few things you should consider first:
Rock pools are sharp and can be very slippy due to seawater and algae so you want to make sure you are wearing footwear with grip and take care when climbing across the rocks.
When standing next to a rock pool you want to try and not cast a shadow over it, your shadow will cause the marine life inside to hide as they’ll think you’re a predator. So to get the best look at the creatures inside the pool check to see where the sun is before approaching it.
Taking a see-through container like an old Tupperware means you can collect species in the rock pools to have a closer inspection - make sure anything that is collected is returned to the place you got it from and you treat it with respect.
If you do collect species from the rock pools make sure to only take one at a time - you don’t want to accidentally trap prey with its predator in a small space!
Be mindful of what species you are collecting, there are species in rock pools that could cause you harm, so it’s always a good idea to wear gloves and have an ID book/sheet to help you steer clear of any accidents.
Take pictures! You won’t always be able to tell what something is even when you have an ID book so it’s best to take pictures in case you want to better ID something at home or just to remember what you saw!
Rock pools are one of Britain's amazing coastal habitats and are a great way to get both the young and old interested in marine life! So next time you visit the coast have a look and see if you can spot any of the species mentioned in this post and if you do show us a photo on Instagram or Twitter!
If you want to learn even more about rock pools and the amazing species that reside in them you can do so with our friends over at The Rock Pool Project. They run rock pool safaris if you live near the coast and some amazing online content that allows you to see rock pools from the comfort of your own home!
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