Monday, 9 May 2016

Hermit crabs, sea slugs, and mystery eggs!

One of my favourite habitats is the shoreline, and in particular rockpools. I think that they are a very special place, not only because of the animals that live there, but also because it is where children can connect with nature. Since most children nowadays very rarely get the opportunity to do this, a trip to the beach is about as connected to nature that they get, as they paddle about in the rockpools looking for crabs.

 A previous rockpooling trip at Bridlington.

As a lover of rockpools and rockpooling, whenever I visit the beach, I don't make sand castles and throw balls about - instead, I'm knee-deep in the water, usually trying desperately to catch some kind of aquatic animal. This is exactly what I was doing yesterday (8 May 2016), and I'm happy to say that I had one of the most succesful and rewarding rockpooling trips that I have ever had.

My Mum and I went to Filey, a seaside town in North Yorkshire. The Filey Brigg, a thin strip of land riddled with rockpools, is one of my favourite places to go on a day-out, and it has plenty of wildlife to be found there.

Our first finds in the upper to middle shore were the usual common molluscs and crustaceans - common periwinkles (Littorina littorea), common muscles (Mytilus edulis), and acorn barnacles (Semibalanus balanoides), amongst others. After that, I found several beadlet anemones (Actinia equina) - one of which was attached to the shell of a periwinkle! I have never seen this sort of behaviour in beadlet anemones before, and on further research on the Internet, I could find no other account of such a thing. Some anemones are known to hitch a lift on hermit crabs, but beadlet anemones, as far as I am aware, do not practice similar behaviour. After placing the periwinkle and its hitch-hiker in a plastic tub, the anemone later seemed to be peeling off of the shell, with only a small portion of it still clinging on.

 As we further explored the middle shore, my Mum had picked up a handful of dog whelks, and was bringing them over for me to look at, when she suddenly let out a rather amusing little 'ooh!' of surprise. On closer inspection, it turned out that the shells were no longer inhabited by dog whelks. They had new tenants: common hermit crabs (Pagurus bernhardus). This discovery came as a bit of a surprise to us, for we have searched long and hard for hermit crabs on Filey Brigg, and we have never been succesful. I now think that we simply had never looked far enough down the shore, as when you start to enter the middle shore, the finds start to become more rewarding.
When my Mum and I returned to the rockpool where she had found the hermit crabs, we soon saw that the empty shells litttered around on the bottom were not quite as empty as we had supposed. As we watched, antennae and claws poked out, and the shells started to crawl about. After years of looking for hermit crabs at Filey, within an hour of beginning rockpooling, we had found 20 or so individuals - and by the end of the day, we had seen well over 50.

We also found 3 chitons, probably grey chitons (Lepidochitona cinerea). These fascinating molluscs have segmented shells which protect their soft body, and they can be found clinging to rocks where they scrape away algae using their radula. Interestingly, they also have organs called 'ocelli' in their shell. These are their equivalent of eyes, and they have a retina and lens, capable of telling the difference between light and dark. These ocelli are not visible as they are small and embedded in the shell.

While searching for more hermit crabs, we discovered a grey sea slug (Aeolidia papillosa). These common sea slugs are covered in what look like small, stubby tentacles, called cerata. This particular species feeds almost entirely on sea anemones, and it is capable of storing the stinging cells of its prey in its cerata. This re-used defence mechanism is used to keep it safe from predators.

Our next find was a small edible sea urchin (Echinus esculentus), which can be pink, purple, or green. The individual that we found was green, and not fully grown, as it was just a few centimetres wide. This species is so-named because it is edible - its gonads are eaten in some parts of the world.

Other rockpoolers had passed on some of their finds for us to look at before they were released. One man had quite a collection of fish, including a large butterfish, Pholis gunnelllus, ("Baby conger eel." he incorrectly told us) and a pair of long-spined sea scorpions, Tauralis bubalis (these he believed were monkfish, while his 2 young daughters were stroking the fish, unaware that these were not monkfish, but sea scorpions, which carry 4 spines on their back, capable of causing a minor but painful injury . . .). Sea scorpions are unusual fish, with bulbous eyes, a spiny, bony head, and extendable mouths capable of swallowing prey much larger than themselves. 
Another man had a small 5-bearded rockling (Ciliata mustela), a fish that can grow up to 25 cm long, and has 5 whiskers on its face, which allow it to sense prey. He didn't seem to care how many sensory whiskers it had, nor exactly how it found its prey, despite my best efforts to explain how fascinating it was. His only interest was, "Can I cook it and eat it?".

Another creature that a person gave us to look at was a sea hare (Aplysia punctata). These unusual sea slugs are named after their rhinophores (small tentacles on their head area), which resemble the ears of a hare. The sides of their body curve up, meeting above their back, which I always think makes them look like the wings of a swan. Interestingly, when faced with danger, they can release a bright purple cloud of sulphuric acid in an attempt to deter a would-be predator.

We also found some unusual eggs in the rockpools. Bright green jelly-like blobs that my Mum found turned out to be the eggs of the green leaf worm (Eulalia viridis). However, the other eggs that we have found proved more difficult to identify. They were in spiralling, tube-shaped sacks made of a jelly-like substance, containing many tiny peachy-white orbs, which we believe were eggs. On further research, I think that these were the egg sacks of the grey sea slug (Aeolidia papillosa), an adult specimen of which we had found earlier, and I have already mentioned previously in this post.

Overall, it was a great day out - not only fun, but educational too. I hope to return to Filey Brigg soon to continue our exploration . . .

Until next time, keep on the wild side!

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