Tuesday, 29 January 2019

Alien Invaders from the Deep?

Not quite. However, our rocky shores are under threat from small creatures that are arriving in our waters from foreign seas. Last week, with the help of the Natural England Marine Team, we were searching the rockpools for marine invasive non-native species; plants and animals that shouldn’t be found on our coastline and are consuming resources that should be reserved for our native species. Many marine species reproduce by releasing embryos into the water column, which float around as plankton until they are ready to permanently settle on a hard surface. Therefore, a suitable place to settle and grow is the most sought-after resource for rockpool flora and fauna, as many species are sessile and do not move from the surface they land on. Therefore, competition between both animals and plants for space is fierce.

Reserve staff and Natural England's Marine Team surveying the rocky shore

The invasive non-native we were focusing on was the Orange Tipped Sea Squirt (Corella eumyota), which have arrived on the UK coastline from the southern hemisphere. They are small invertebrates, whose appearance has been described as that of a Werther’s Original. They belong to the class Ascidiacea, and actually have a distant relationship to humans; the tadpoles they release have a very simplified version of a backbone and nerve chord characteristic of fish, birds and us! The Orange Tipped Sea Squirt is thought to have arrived in our waters attached to ships, and a combination of sea temperature rise and the ability to out-compete native species have led to it spreading around the rocky shores of the UK in only 10 years. 

Orange Tipped Sea Squirt (Corella eumyota)

They are found as individuals and are not a colony species. However, they can form large clusters on the underside of rocks, in crevices and even on large seaweeds, as they hold their embryos inside them until they are ready to permanently settle and begin feeding for themselves. This process often results in the embryos settling on or next to their parent, or anything else already growing on that surface, forming dense clusters, smothering other sessile native species and occupying valuable space.

Multiple Orange Tipped Sea Squirts forming a large clump on the underside of the rock competing with Breadcrumb Sponge (Halichondria panicea) for space. 

Species most at risk from the Orange Tipped Sea Squirt are other sessile species including Star Ascidian (Botryllus schlosseri) and various species of Bryozoans. They are all filter feeders, which means they take in sea water and sift out tiny pieces of food for consumption, and so not only compete with the Sea Squirt for space but also food. 
Star Ascidian (Botryllus schlosseri)

Unfortunately, we did find a large number of Orange Tipped Sea Squirt last week, and we will continue to monitor the rockpools to get a baseline understanding of the distribution of this species on the rocky shores around the Reserve. Once this is known we will carefully consider how we manage this sensitive issue and will post updates on our progress soon.

During our search, we also came across a vast array of weird and wonderful creatures, from beautifully elegant Nudibranchs to bold and daring Squat Lobsters and Porcelain Crabs. Native sessile species of Breadcrumb Sponge and Star Ascidian are still thriving and are as beautiful as ever.

Long-Clawed Porcelain Crab (Pisidia longicornis)

Dahlia Anemone (Urticina felina)

The rocky shore is an amazing place and is one of the most diverse habitats on earth. If you are visiting the rockpools, please remember to put rocks back exactly where they were found, to ensure the species living on the underside of the rocks remain in the water!

Sea Hare (Aplysia punctata)

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