They may be distant relatives, but sea squirts are more closely related to us that many other creatures you can find on the shore. Their larval form has something called a notochord, which helps to organise the nervous system and which is present in all Chordates. All animals with backbones (that’s us) are grouped within the larger animal grouping (Phylum) Chordates, together with Tunicates (the squirts) and Cephalochordates (small fish-like invertebrates).
You might not see too much of a family resemblance with your clan, though. UK squirts are small, sometimes completely transparent and stuck permanently to surfaces, from lower down in the intertidal zone and further out into shallow seas to vast depths. They have an outer wall (tunic) called a “test”, which holds in a filtration system (branchial sac), so the seawater which is drawn in through the inhalant, “oral” syphon can be cleaned of nutrients before being ejected through the exhalant, “atrial” syphon. It is through the exhalent syphon some of them may squirt out this water, when disturbed, which gave them their name. Squirts can live as individual animals or as a colony of zooids which share the same outer test. Colonial squirts will often share the atrial syphon with a small group of others within the colony. Colonial squirts can also reproduce asexually as well as sexually which is the only reproductive option available to the unitary form.
For those in the (plant-life) know, you’ll be thrilled to learn that tunicates are the only group of animals able to make their own cellulose-type substance. They use this in their outer test, creating a tough defence. Together with many molluscs and sponges, tunicates provide a filtration service which is vital for our seas. As our closest invertebrate relative, they are also helping to inform scientists about our own health – for instance they have a certain physiology which is helping researchers learn more about our innate gut immunity and since they filter out plastics (of which there is an abundance in our seas) and store them in their test, they can be investigated to see what effect microplastic pollution might be having on other Chordates like us.
Identifying squirts can be tricky, but there are a few which can be fairly easily identifiable so here’s a very quick introduction to those:
Star Ascidian/chwistrell serennog (Botryllus schlosseri) – is a colonial sea squirt, whose zooids sit together in small groups, which create shapes likes stars or flowers. Their shared pore is placed in the centre of the colony and the zooids lie attached to the surface, flattened splayed away from the centre longer than wide. They can be a wide variety of colours, often blue-ish, but also green, orange, purple, red and brown. They live attached to rocks and algae, but also sometimes to other ascidians.
Baked bean sea squirt/chwistrell fôr gôch (Dendrodoa grossularia) – although these are classed as unitary squirts they still often form large clusters. They’re unitary in terms of how they grow within that cluster. They can actually exhibit different growth forms, so they don’t always looks bean-like which gives them their English common name. They can grow on rocks, shells or algae and tend to be reds or browns.
Lightbulb sea squirt/chwistrell wydr (Clavelina lepadiformis) – a delightful find, this sea squirt lives in loose colonies under rocks. They’re clear with a strip of white running along the length and rims of the siphon openings, which makes each zooid look like a glass lightbulb with the filament lit up. The zooids are only attached at the base through a stolon-type system attached to the rock.
Yellow-ringed sea squirt/chwistrell fôr (Ciona intestinalis) – tall with a colourful edge to both siphons, this squirt likes to be sheltered and can often be found attached to jettys, pier and buoys. In areas which are more exposed it is more likely to live on its own, whereas when less exposed to wave action it forms clusters. These animals contract when disturbed.
Fascinating fact: The squirt heart (basically a tube that contracts) is able to reverse its pumping direction every few seconds and has been found to be very similar to the mammalian heart functioning.
Keep an eye out for our Shoresearch surveys to get a better understanding of our intertidal life including our squirts.