Rockpooling at Helford Passage

One of my modules at university this year is Invertebrate Zoology. So far I’ve been loving it of course! The first of four practical sessions for the module was at the Helford River Passage, a rocky shore estuary where we would be tasked with finding as many invertebrate groups as we could, and then classifying three organisms from domain down to species. This was a great opportunity to find (and be shown) some interesting new species, as there were about 100 of us, all searching for invertebrates!

There was a small group of sea snails that were immediately obvious, the Topshells. Unfortunately, I don’t have any photos of these species, but when I do, I’ll get them online and point out the features for telling them apart! The species that we found on the day were Purple Topshell (Gibbula umbilicalis), Grey Topshell (Gibbula cineraria) and Common Topshell (Osilinus lineatus).

Soon after the Topshells, I found both Common Oyster (Ostrea edulis) and the invasive Pacific Oyster (Crassostrea gigas). The difference between these species is relatively easy to see, as the Pacific Oyster has much larger ridges (hence also deeper troughs) in the shell than our native Common Oyster. The Pacific Oyster has escaped from hundreds of Oyster farms around British coasts, and has established itself in many locations, particularly in Cornwall. In other parts of its range, the Pacific Oyster has become so abundant, that it completely excludes other species from suitable habitats. It is a worry that our native, slower growing Oyster will begin to decline, or even disappear in areas where the Pacific Oyster is present.

I then went down to the lower shore with a very small (but free) net, to see if I could catch any fish or prawns amongst the rocks. I decided that dragging the net along the bottom, and through clumps of Serrated Wrack would probably be the most productive, as these are the places where my target species would be hiding from predators. I was proven correct, after several hauls of Common Prawn (Palaemon serratus), but unfortunately no fish. I’d managed to gain a reputation amongst my fellow rockpoolers by having a knack for catching prawns! This reputation was cemented after I managed to scoop up this gigantic prawn, earning me the title of ‘The Prawn Whisperer’. The upcurved rostrum, with serrations absent in the apical third are diagnostic of the Common Prawn, Palaemon serratus.

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I was just about getting bored with the endless supply of Prawns, and was about to head further up the shore when I spotted a very unusual looking anemone just below the surface. I bent down to pick up the stone it was attached to so I could take it up the shore and show my groupmates, but as I grabbed it, the stone started to run away! The stone was in fact a Shore Crab (Carcinus maenus), and the anemone was the Parasitic Anemone: Calliactis parasitica. The Parasitic anemone is usually found on the shells of Hermit Crab species, so finding it on a Shore Crab surprised even our lecturer! The relationship between the crab and the anemone is mutually beneficial, with the anemones stinging arms offering the crab protection, whilst the movement of the crab causes more food to pass near the anemone. This is true for a small Hermit Crab, but I’m not certain that this 8cm Shore Crab needed the anemones protection at all!

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Most people managed to find Hermit Crabs, but the only other Parasitic Anemone found that day was on another Shore Crab! It makes me wonder if this occurs more frequently than we currently know about, with all texts that I’ve read seeming to say the Hermit Crabs are the only host. It was pointed out to me that it makes the Shore Crab look like it’s wearing a beanie.

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Another highlight from that day included my first Bryozoan, the Star Ascidian (Botryllus schlosseri) which is well worth a google search, as it forms beautiful colonies, and I forgot to take a photo of it! The colony that we found was bright orange-red. Also, I saw my very first Brittle-stars. The larger individual was about 8cm across, the smaller one about 2cm.

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Finally, a little later on in the day I wondered through the woods just outside of the Penryn campus with my flatmates, when one of them found this stunning Pale Tussock moth caterpillar crawling across the path! After metamorphosis it turns into a very lovely, very furry (but very grey) moth.

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