Elysia Viridis, commonly known as the Solar-powered Sea Slug or Sap-sucking Sea Slug, was first described by Montagu in 1804. They are common in the UK and Ireland but can be found from Norway across France, Spain, Portugal, Italy and even the Azores. On iNaturalist there are currently nearly 1,500 research-grade observations and over 4000 observations on the Global Biodiversity Information Facility (GBIF).

Elysia viridis are large ‘Sacoglossan’ sea slugs. Sacogloassans are a superorder of around 300 herbivorous slug species characterized by having specialised teeth in their ‘radula’ (mouthparts), that are adapted to pierce algal cell walls to suck out the contents. Hence the sacoglossans are often referred to as ‘sap-sucking sea slugs’. This group of sea slugs are in a different evolutionary branch to nudibranchs, which are all carnivorous.

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Solar-powered sea slugs normally reach 3-5cm in length in the UK, though a specimen of 70mm was found in the Netherlands! They have a large head with two tightly enrolled rhinophores that usually have white tips, but no oral tentacles.

They have two large wing-like extensions of the mantle called parapodia running from behind their head right to their tail. These have lobed or frilled edges which you can see when the slug unfurls them, to a shape that looks almost like a leaf from above, to increase their surface area.

In digestion, they can separate the chloroplasts sucked out of algae and retain them in their own cells close to the surface of their body, to provide energy for them from photosynthesis. This is called ‘kleptoplasty’, and you can see the ‘stolen’ chloroplast with a magnifying glass, they can be seen as tiny green spots, mainly in the parapodia of the slug. Solar-powered sea slugs can keep the chloroplasts alive in its body for weeks to months!

The parapodia, head and body are normally covered in tiny glistening red, blue and green specks which have been known to move location on individuals. There is some information to suggest that the iridescent pigments are stored in round cells in the haemolymph (slug blood). They may help in camouflage through breaking up the slugs silhouette against the algae it consumes, though their true origin or purpose is not known.

Parts of the digestive gland and albumen gland (a gland that secretes substances to form and nourish egg capsules in many molluscs), are sometimes visible through the skin.

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The body colour typically varies with diet from green to brown and almost red, dependent on which algae the slugs have been consuming. Pale green to yellow and almost white specimens are an indication the slug has been starved for enough time to deplete its chloroplasts and colouring.

Slugs may also have white patches on them, especially toward the edges of their parapodia, and black markings on the head or body.

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Solar-powered sea slugs each have both male and female reproductive organs. A pair lines up next to each other to fertilise both individuals' eggs at the same time (simultaneous hermaphrodism). They then lay a small egg mass in a coil of hundreds of eggs encased in a gelatinous membrane. These masses colouration depends on the seaweed the adult slug has been eating. As slugs in different countries tend to consume different species of seaweed this egg colouration effectively varies by country, with specimens in Denmark typically laying reddish-yellow eggs, and UK specimens' eggs ranging from lemon-yellow to bluish white. These hatch in 5 to 12 days (mainly dependent on temperature), to microscopic larvae. These tiny planktonic larvae drift for 30 to 45 days before settling on their preferred food algae and metamorphosing into slugs. These slugs have a 12 to 15 month lifespan.

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Elysia viridis is the only Elysia species so far known in the UK, making adults quite distinct. Very young and tiny adults can be harder to identify as they only start to grow parapodia at a length of 3mm. However, younger individuals also tend to be quite translucent, with some bright green colouring as they have not yet built up many chloroplasts. They will also still have obvious, if a little stubby rhinophores. This distinguishes them quite easily from any potentially similar looking species like Limapontia capitata. However, in any reasonably size individual, the identification should be very easy!

Diet

This species is found on a variety of shallow-water algaeacross Europe but populations in each area tend to be specific feeders ofcertain species of macroalgae and inidviduals struggle to adapt quickly toother species when their environment is changed. In the UK, Elysia viridisfeeds most commonly on Codiums or ‘sponge-finger seaweed’. The favoured algaeis thought to be Codium fragile as it has a thinner ‘utricle walls’ (xx) whichis easier to pierce for feeding. Solar-powered sea slugs’ growth rate and finalbody size is dependent on the type and volume of algae consumed, and the amountof energy received from retained chloroplasts.

noted favorability towards C. fragile, C. sericea, and C.rupestris Less . viridis has been found to be less selective of algalhosts than other members of Elysia.

In contrast, of 886 thalli examined of C.fragile subsp.atlanticum from eleven sites all around Scotland, not one had E.viridis on it (Trowbridge & Todd, 2001).Historically, E.viridis may have frequently fed on Cladophora (figs. 1 & 17) but it is now only rarely used at sites where the preferred alien C.fragile subsp.fragile is still absent (pers. obs. I.F.S. and Trowbridge & Todd, 2001). Experiments showed that those born from adults feeding on the alien lacked the ability to feed or grow on Cladophora. There may have been a historic host shift from Cladophora to Codium fragile subsp.fragile (Trowbridge & Todd, 2001). Smaller oncladophora.

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