Monday, 14 April 2014

Week 47: Sea Slugs ('Nudibranch')

Sea Slugs were @SpeciesofUK from 20th to 26th January, 2014.
Sea slug is a common name given to thousands of marine gastropod molluscs that lack external shells.[1] They are related to our familiar land-dwelling slugs and snails.

Polycera quadrilineata
[Flickr Creative Commons © Frank Gloystein]

The main grouping of sea slugs is the so-called ‘nudibranchs’.[2] It is these that form the subject of this post. 

Nudibranchia is a clade (subgrouping) of gastropods.

Nudibranchs are found right across the world, from Antarctica to the UK. However they are most abundant in tropical waters. Most (but not all) spend their lives crawling on the sea floor.[3]

There are more than 3,000 known species of nudibranch. And new ones are being identified all the time.[4]

They are usually oblong-shaped, sometimes quite thick, other times rather flattened. They vary from 0.5-31 cm long.[5]

Ancula gibbosa
[Flickr Creative Commons © Ken-ichi]

Some nudibranchs live for only a couple of weeks. But others can live to the ripe old age of one year.[6]

The word ‘nudibranch’ comes from the Latin for naked and the Greek for gills.[7]

There are two main types of nudibranch – dorids and eolids.

Dorid nudibranchs have gills shaped into branchial plumes in a rosette on their posterior end.[8]

By contrast, eolid nudibranchs have obvious cerata (finger-like appendages) on their back which functions as gills by enabling gaseous exchange directly into the body.[9]

All nudibranchs are molluscs so they have a shell, but it's shed in the larval stage, leaving them with just a fleshy covering, or 'mantle'.[10]

The White Fleshy Mantle of Polycera quadrilineata
[Flickr Creative Commons © Eric Burgers]

The mantle can extend out in the form of growths, for example the elegant cerata of eolid nudibranchs or as ‘tubercules’ (spotty growths).[11]

Coloured Cerata on Cuthona caerulea
[Flickr Creative Commons © Christophe Quintin]

Tubercules on Archidoris pseudoargus
[Wikimedia Commons © Vulpecula23]

On a nudibranch’s head there are two ‘rhinopores’. There are sensory tentacles, rhino being Ancient Greek for nose.[12]

Nudibranch Rhinopores
[Wikimedia Commons © Scott F Cummins et al]

The rhinophores work by detecting chemicals dissolved in the water and sometimes currents. Most species can withdraw them into a protective pocket.[13] They are similar in principle to the sensory tentacles of terrestrial snails.[14]

In many nudibranchs, the gills form feather-like structures around the anus.

Gills of a Nudibranch (rear of this image)
[Flickr Creative Commons © Asbjørn Hansen]

The eyes of a nudibranch are simple and able to detect little more than light or dark.[15]

Sea slugs can be extremely colourful!

There is a huge amount of variation in the colours and patterns of sea slugs. Many are downright drab, but others have become renowned for their spectacular, bright colours.[16]

Janolus cristatus
[Flickr Creative Commons © Antoni]
Flabellina pedata
[Flickr Creative Commons © Manuel Sánchez-Mateos Paniagua]

Sea slugs derive their colour from the food they eat. This explains some of the brightness but also why many are actually very well camouflaged.[17]

They are carnivores.

Nudibranchs eat sponges, anemones, corals, barnacles and even other nudibranchs.[18]

They feed by scraping off bits of their prey using their radula (a chitinous ‘tongue’).[19]

Some eat poisonous species and cleverly retain the poisons themselves and secrete these as a defence against their own predators.[20]

Nudibranchs have both male and female reproductive organs.

This is extremely beneficial to these solitary, slow-moving animals. When a nudibranch meets another nudibranch, it knows it’s definitely a potential mate, as it has all the right parts![21]

Nudibranchs mate by lining up side by side with their right sides together. They both pass a sperm packet to each other, and they both lay eggs.[22]

Mating Nudibranchs
[Flickr Creative Commons © Lenny Cliffbanger]
Mating Nudibranchs
[Flickr Creative Commons © Dan Hershman]

After about a week nudibranch eggs develop into free-swimming larvae, and later settle on the ocean bottom to grow into an adult.[23]

Strange but true...

Some sea slugs are solar-powered. They store algae in their outer tissues and live off sugars produced by the photosynthesis.[24]

Pteraeolidia ianthina, a Solar-powered Sea Slug
[Flickr Creative Commons © John Turnbull]

Sea slug species.

Here’s a quick run through of some of the sea slug species you could find around the UK.

Facelina annulicornis has a translucent body with scattered spots of white pigment.  It’s uncommon, and found in the southern parts of the British Isles.[25]

Facelina annulicornis
[Flickr Creative Commons © Géry Parent]

Diaphorodoris luteocincta has a white mantle with a yellow rim and central red blotch on the back. It’s found all around the UK.[26]

Diaphorodoris luteocincta
[Flickr Creative Commons © James Lynott]

Onchidoris bilamellata has a brown pigment pattern and many club-shaped papillae on the mantle. It has numerous retractable gills in a horseshoe.[27]

Onchidoris bilamellata
[Flickr Creative Commons © Ken Bondy]

Acanthodoris pilosa has a unique ‘fluffy’ appearance. The colour is variable from white to brown, purple and black.[28]

Acanthodoris pilosa
[Wikimedia Commons © Minette Layne]

Polycera quadrilineata is translucent white with patches of yellow or orange pigment forming five longitudinal lines. Some individuals have fine black spots over the body.[29]

Polycera quadrilineata
[Flickr Creative Commons © Frank Gloystein]

Tritonia hombergii is the largest UK nudibranch, growing up to 20cm in length. The colour varies from white to pinkish-brown. It has many irregular sized gills arising from the mantle edge.[30]

Tritonia hombergii
[Flickr Creative Commons © James Lynott]

Tritonia nilsodhneri is pale pink or white. It is extremely well camouflaged on its prey, the gorgonian coral Eunicella verrucosa.[31]

Tritonia nilsodhneri
[Flickr Creative Commons © Christophe Quintin]

Aeolidia papillosa is the largest eolid nudibranch in the UK. It is known as the ‘shaggy mouse’.[32]

Aeolidia papillosa
[Flickr Creative Commons © Ken-ichi]

Cuthona caerulea is transparent, with yellow yips to the oral tentacles and rhinopores. The cerata have bright blue pigment in the mid region, with an orange or red band above.[33]
Cumanotus beaumonti is translucent with speckles of gold pigment, and numerous tapering long cerata. It’s scarce in the UK.[34]

Cumanotus beaumonti
[Flickr Creative Commons © Certo Xornal]

Flabellina pedata is pink-purple in colour. The cerata occur in bunches and are joined at their bases.[35]

Flabellina pedata
[Flickr Creative Commons © Géry Parent]

Dendronotus frondosus is very variable in colour. It has large arborescent gills, oral processes and rhinophoral sheaths.[36]

Dendronotus frondosus
[Flickr Creative Commons © Geir Friestad]

Janolus cristatus has numerous swollen cerata with iridescent bluish-white pigment in the tips.[37]

Janolus cristatus
[Flickr Creative Commons © Antoni]

Cadlina laevis has a flat oval body with small gills that retract into a pocket. It has characteristic lemon-yellow glands towards the mantle margin.[38]
Greilada elegans is orange in colour with brilliant blue iridescent spots.[39]

Greilada elegans
[Wikimedia Commons © Bernard Picton]

Thecacera pennigera is translucent white with tiny orange, yellow and black spots. It occurs in the south and west of the UK and feeds on the bryozoan Bugula plumosa.[40]

Thecacera pennigera
[Flickr Creative Commons © crawl_ray]

Archidoris pseudoargus is one of the most common nudibranchs on UK shores. It is known as the sea lemon’. It has a mottled variable colour on the mantle, which bears many short blunt tubercles.[41]

Archidoris pseudoargus
[Flickr Creative Commons © kqedquest]



  1. Just found your blog when I was looking for information on Springtails, will be popping back to look through your older posts..thanks Amanda..

    1. Thanks for your comment. Hope you enjoyed the rest of the blog :)