Rock pooling isn’t just for children!

First year Marine Biology students exploring the rock pools. Photo by Jack Davis
First year Marine Biology students exploring the rock pools. Photo by Jack Davis

Just recently, my University course took a trip down to Plymouth, during which we went down to a rocky shore covered with rock pools. Now, on a field trip at University level, you would probably expect some complex sampling, evaluation and weird but wonderful science experiments to take place. Amazingly, however, our assignment was simply to look into the pools.

As someone did joke, that is something toddlers do when they are on their summer holidays. But the lecturers did have a point with this apparently simple exercise: rock pools are not as basic as they might first seem.

The variety of life found in these isolated little lagoons was astounding. In a pool of water measuring just 1 square foot, you could find animals and organisms from so many different phyla: crustaceans, Macro algae, polychaetes, echinoderms, molluscs, bryozoans, hydrozoans. Such a diverse cross-section of life in such a relatively tiny home, and each organism with its own stories to tell and secrets to keep.

The Wordly and the Wary
Now, if there was ever an old man with a tale to tell it would be the chiton. The one we managed to find was only 2cm long and crawled around showing off its species’ distinctive body armour-plated back, which made it look almost like a cross between a woodlouse and a limpet. Incredibly, the chiton species evolved over 400 million years ago (in comparison, humans only just came along around 250,000 years ago). For me, that is an incredible thought. This species has seen so much and lived through so much, whilst it’s unlikely that the human race will survive for that long itself.

Next we come to the crabs, probably the best-known of the rock pool dwellers. Fast in their movements and partial to hiding beneath seaweed, they can evade rock-pooling beginners. However, in just one trip, we uncovered members of 3 different species: a velvet swimmer crab (Liocarcinus depurator), an edible crab (Cancer pagurus) and a common shore crab (Carcinus maenas). All display the same stereotypical crab shape, but each has its own variations. Edible crabs have blac-tipped claws and, like the name suggests, the velvet swimmer crab’s back feels velvety if you are lucky enough to get close. Of the three crabs this one is the most aggressive, so watch out for the claws!

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Balanus sp. Photo by Xanthe Ginty

The Well-Endowed Barnacle
Barnacles covered the area we explored and, whilst they usually close their shells when isolated in rock pools, we were lucky enough to find a couple actively feeding. It’s a strange sight to see – the barnacles open their shells and stick their ‘feet’ out (yes, they technically lie on their backs with their legs in the air) which look like tiny rakes that fan through the water before being pulled back inside again.

Speaking of probing protuberances, I’m very sorry, gentlemen, but prepare to feel emasculated. If a barnacle grew to the size of a human then its penis would be over 20m long! Very impressive but also a clever adaptation. Barnacles are sessile organisms (meaning they don’t move) so a male’s large penis allows him to reach females that might not be right next to him.

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Actinia sp. Photo by Xanthe Ginty

Tentacles and Terrors
Moving on, we also found in our rock pool a number of alien-like anemones, tentacles ready and waiting for something to float by to pull in and eat. The most common anemone we found was the usually red, occasionally green, beadlet anemone (Actinia equine). It’s simple to distinguish beadlet anemones from other species – just stick your finger in (gently, so as not to hurt them)! Dangle your fingers amongst their tentacles and you’ll feel them trying to pull you in. You might even feel a slight painless tingle as they try to sting you  However, once they realise you’re too big, they will close up to hide away. It’s this closing up that reveals their identity, as other species can’t draw completely into themselves.

Finally, we came across a ferocious predator – the dog whelk (Nucella lapillus). This mollusc hunts related species and drills a hole in the shell of its prey before injecting digestive enzymes. Trapped by its own shell, the prey is completely helpless as the enzymes break it down whilst it’s still alive. Once it’s reduced to a soup of body parts, the dog whelk sucks out the juices, leaving behind the coffin of its victim.

These were just a few of the different species we found on our trip to Plymouth. Our lecturers were right though, spending time just looking in rock pools can really teach you a lot. You don’t need to be an expert to enjoy rock pooling and you certainly don’t need to be an expert to identify things. A sea shore identification guide will cover the basics.

So, next time you’re heading down to a beach in England, look out for that rock pool and go exploring in a whole different world. Don’t overlook the small white barnacles clinging to the rock, keep a careful eye open for what treats you may find hidden in the cracks, and don’t forget to play with the anemones. Even 30-year-old marine biologist lecturers can’t resist that temptation!

Post by: Jennifer Rasal

References:
https://en.wikipedia.org/wiki/Human_evolution
http://www.ywt.org.uk/sites/yorkshire.live.wt.precedenthost.co.uk/files/120130%20Shoresearch%20species%20fact%20files%20CB.pdf
http://www.wemburymarinecentre.org

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