Guest written by Brenton Prigge and Jake Baldry
Many divers dream of finding treasure under water. Plymouth Sound abounds in treasure just waiting to be found. But one person’s treasure is another person’s… sea slug? Ok, beauty is in the eye of the beholder, but we would argue one of the most spectacular and often overlooked treasures of the underwater domain is the diverse world of Sea Slugs.
Most British divers are aware that there are Nudibranch in the UK, but it may come as a surprise to many that this particular treasure bounty is, well … bountiful…
Nudibranch are present in UK waters in abundance… if you know where to look.
Jake Baldry and Brenton Prigge, authors of this blog post (and self-proclaimed “nudi-nerds”) have spent the last couple of years on a quest to identify and record as many Nudibranch species as they can and are currently up to 76 species in Devon – 72 of those in the Plymouth Sound alone. They are in no way scarce, they are just not easy to spot – unless you know where (and how) to look. Want to know the secret? Read on!
But first, what are they?
What are Nudibranch?
Nudibranch (new-dee-brank) comes from the Latin for “naked gills”. Technically the order of Nudibranchia is restricted to those sea slugs which breathe through a “feathery” looking structure of gills protruding from their body (as visible in the picture of an Okenia elegans below) but the term has come to be used more widely of sea slugs in general. Because let’s face it, the word “slug” conjures up something a lot less beautiful than these jewels of the ocean!
He / She / They ?
Nudibranchs are hermaphrodites, meaning they carry both male and female sexual organs. If you closely at this photo of a mating pair of Polycera kernowensis, you will see that both have their sexual organs inserted into the other Nudibranch, so they can both inseminate the other individual and have their own eggs fertilised. A perfect give-and-take relationship!
How do Nudibranch breathe?
Nudibranchs “proper” - the Dorid group of sea slugs - breathe through the tuft of “naked gills” on their back. But there are other kinds of sea slug with other cool ways of breathing underwater.
Take the Aeolids, which are known for their vibrant colours and unique shapes. Perhaps the most striking features of aeolid Nudibranchs is their cerata – the finger-like projections on their backs (as visible in the photo of a Coryphella lineata). They don’t just look cool, they also serve important functions.
Each of these cerata contains a branch of the digestive tract, and Aeolid sea slugs will consume prey like anemones and hydroids and will store the stinging cells from their food source in the tips of the cerata. Their digestive system is also their defence mechanism… and they also use the cerata to breathe!
It gets better. Some Nudibranch in the UK are “side-gilled” slugs, like this Berthella plumula. They have their gills hidden under their “skirt” on one side of their body (as can be seen in this photo).
Side-mount enthusiasts will no doubt be suitably impressed.
How to find Nudibranch in the UK
Many sharp-eyed divers will no doubt already be familiar with some of the larger, more colourful species of Nudibranch in the UK like the Okenia elegans above, and the Edmundsella pedate pictured here. However, many Nudibranch are missed because of their tiny size. They can often be just a couple of millimetres long fully grown.
Tergipes tergipes for example, seen here on Kelp Fir (Obelia geniculata), average just 3-5mm.
Others are well camouflaged on their food source, as can be seen in the next two pictures.
There is a Jorunna tomentosa on Mermaid’s glove sponge (Haliclona oculata) in both pics, but the one on the right is in super stealth mode!
So the secret to finding Nudibranch in the UK is really quite simple:
Learn to spot the food source, and look there.
Find Mermaid’s Glove… and look for Jorunna tomentosa.
See Kelp, look for Kelp Fir… and… if you have sharp eyes maybe you will spot Tergipes!
But look very closely and carefully. You could be looking right at a Nudibranch and still not see it.
But we do encourage you to learn to spot these precious jewels of the marine world. Nudibranchs in their natural habitat are a showcase for the incredible diversity and complexity of ocean life found right here on our doorstep.
Care to join us for a treasure hunt?