The Blue Sea Slug (Glaucus atlanticus) is a nudibranch mollusc that floats upside down at the ocean surface. It drifts around until it encounters something to eat. The Portuguese Man o’ War (Physalia physalis), often known as the ‘bluebottle’ in Australia, also floats at the ocean surface and is the usual food of Glaucus atlanticus.
Eating the Portuguese Man-of-war
Glaucus atlanticus will eat many creatures that float on the sea, including the ‘by-the-wind-sailor’ (Velella velella), the ‘blue button’ (Porpita porpita), and the ‘violet snail’ (Janthina janthina) – but according to the Sea Slug Forum it feeds: “almost exclusively on Physalia“.
Glaucus has many sharp ‘teeth’ (radular teeth), and as it consumes the Portuguese Man o’ War it takes in its stinging cells (nematocysts). These are what the Portuguese Man o’ War uses to catch its own prey, and they contain a toxin that is capable of paralysing small fish. Glaucus is not harmed by this toxin, and it uses the captured nematocysts for its own defence – they migrate to the tips of its tentacle-like ‘cerata’. According to Thompson and Bennett (reported in the Sea Slug Forum): “they are able to select the most venomous of Physalia’s stinging nematocysts for their own use”.
This makes Glaucus more dangerous than the Portuguese Man o’ War. In Australia thousands of bathers are stung by the Portuguese Man o’ War each summer, but it seems to be: “kids engaged in ‘Bluebottle’ fights – where they throw stranded Physalia at each other – who are badly stung by inadvertently playing with Glaucus” (Sea Slug Forum). The sting of the Portuguese Man o’ War is painful, but Glaucus stings are much worse!
The Blue Sea Slug Glaucus atlanticus
Glaucus has a gas-filled sac in its stomach to keep it afloat, and this results in it hanging upside down at the surface. Because it is upside down it is the underside of the animal (ventral) that is camouflaged – it is shades of blue to merge in with the ocean when seen from above. The upper surface of the animal (dorsal) is silvery, so it provides camouflage when seen from below.
Glaucus is not ‘worried’ about being seen by its prey (they have no eyes), but it could be attacked from above by birds, or from below by fish. Anything eating Glaucus would get a nasty surprise (being stung by its captured nematocysts), but all in all it’s probably better not to get eaten in the first place.
Glaucus is quite a big sea slug (nudibranch), up to 3 cm. It can be found in all warm and temperate oceans. Since it hangs passively from the surface (like its prey) it goes wherever the winds and currents take it.
Nudibranchs
Glaucus is a nudibranch mollusc, one of around 3,000 named species (although experts believe there may be as many again waiting to be found). Nudibranchs are often very beautiful, and they have a better ‘following’ than most invertebrates. There is even an iPhone App for identifying Australian nudibranchs.
Sources:
‘Glaucus atlanticus‘, Sea Slug Forum.
‘Glaucus atlanticus‘, Marine Species Identification Portal.
‘Nudibranchs’, National Geographic, 2008.
‘Nudibranchs’, World of Nudibranchs, includes a preview of a Nudibranch ID iPhone App (for Australian nudibranchs).