Rock pooling is a fun activity for a day on the beach, discovering tiny ecosystems contained in pools of water across the shore. But when you peer inside at the marvellous creatures, you’re not seeing the struggles they go through day to day, hour to hour, to survive in the harsh world of tide pools.

Rock pools, or tide pools, are any pool of water among rocks along the shoreline. While these beautiful little oases of life on the shore are wonderful pools of life, the organisms within them face struggles which creatures living in the sea do not. In a matter of hours the conditions within a rock pool can change dramatically. Rock pool dwellers must be able to cope with the rapid changes in their environment in order to survive, as well as winning the fights with the other creatures around them.

Two rock pools
From tiny pools of water to expansive pools stretching over the shore, rock pools come in all sorts of shapes and sizes. Whilst larger rock pools are more resistant to changes conditions, in shallow rock pools conditions can become unfavourable very quickly. Original image sources: Jan Tilden, Lis Burke

 

What challenges are there in a rock pool?

The conditions which organisms experience in a rock pool change depending on a lot of different features. For example, rock pools further up the shore are under more stress, as they experience longer periods of time in between being replenished by the tide. Rock pools of all shapes and sizes will contain different animals, or receive different amounts of light, and so on. Every different rock pool gives its residents different challenges which they must cope with.

 

 

Chemicals within the water

As animals and plants photosynthesise and respire, the levels of oxygen and carbon dioxide change in the water. The oxygen saturation in rock pools undergoes enormous changes throughout the day, swinging between 300% saturation in the day to 3% saturation in the night. While most rock pool organisms just cope with the changes, some fish can actually climb out of rock pools to escape the low oxygen concentrations at night.

Rising level of carbon dioxide in the water due to respiration during the night makes the water in rock pools more acidic, decreasing the pH. When the pH changes, it can change the way animals act, since many rely on cues from the environment around them to dictate what they should do. For example, the hermit crab Pagurus bernhardus makes worse decisions at a lower pH, such as not choosing a better shell, or moving more slowly.

Hermit crab in a rock pool
The hermit crab Pagurus bernhardus is a common resident of rock pools, and can be negatively affected by decreasing pH levels. Image source: Nick Upton

 

Temperature

The sun can quickly heat up a rock pool on a sunny day, and rock pool organisms must be able to cope with the swings in temperature from night to day. Even within rock pools there can be different temperature conditions that organisms experience. Due to stratification within rock pools, the water is cooler at the bottom than at the surface. Temperature affects how processes in the bodies of organisms work. For example, raised temperatures decreases the ability of the prawn Palaemon elegans to tolerate changes in salinity.

When the water from a rock pool evaporates it leaves the organisms inside exposed to the air, so they must be able to survive out of the water until the tide or rain refills the rock pool, for hours or even days. Some animals have adaptions to prevent desiccation, for example the beadlet anemone Actinia equina can store water inside its body to prevent it from drying out.

As well as heat in the summer, rock pools experience extreme drops on temperature in the winter. While the sea does not often freeze, it is common for rock pools to have a layer of ice formed at the surface. The organisms underneath are usually safe from freezing if the rock pool is sufficiently deep, though the ice does prevent wind mixing oxygen into the water, and traps the creatures there.

Frozen rock pool
A rock pool with a layer of ice that has formed on the surface, trapping the organisms below. Image source: Hanneke Luijting

 

Salinity

Organisms within rock pools must be able to tolerate changing salinities in order to survive the harsh rock pool life, as rain or sun can rapidly decrease or increase the salinity of the water. For example, the larvae of the crab Armases meirsii can survive at a wide range of salinities. However, they do have their limits; extremely high or low salinities impede their development.

Other organisms

The risk of predation can be high when stuck in a small pool with no way to escape. Small fish living in the pools are in danger of being eaten by shore birds who can easily target fish trapped in a single spot. However, fish do have ways to try to avoid being eaten. For example the rock goby Gobius paganellus can change its colour to help stay camouflaged, and not be spotted by a predator.

Goby in rock pool
The goby Gobius paganellus can be hard to spot amongst the rocks with its camouflage. Image source: Robert A. Patzner

 

Rock pool animals often fight amongst themselves too since there is limited space. Anemones can use their stinging tentacles to attack other nearby anemones to clear space for themselves and their settling larvae.

 

Conclusion

Rock pool conditions don’t just change daily and seasonally, they are also changing with climate change. The temperature and acidity of the ocean is increasing. The organisms which already live at the edge of their limits are being exposed to even more extreme conditions. As well as battling external forces, these rock pool residents have to deal with fights amongst themselves for survival. To survive in a rock pool you have to be resilient, and always able to go with the flow; it’s a rough neighbourhood.

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