The sheep-leaf nudibranch

By Grant Jacobs 15/03/2015

Feeling a little sheepish? What do you think of this little creature:

Photo copyright Lynn Wu, taken in Puri Jati, Bali, Indonesia. It was an Honorable Mention in the 2014 Scuba Diving Magazine Photo Contest (image 16; lead to source details, K.J.Bishop)

I’m not a marine biologist, but when curious things come across your way on-line[1] it can be fun to track down what you can about them.

‘Sheep-leaf’ isn’t a real name, but a nickname.[2] It’s very apt, though. Apparently they’re not really nudibranches either, but I have to admit ‘sheep-leaf nudibranch’ catches the eye.

Although there is a lot to read about sea slugs in general, I can find very little about these species. They’re a sea-slug, tiny mollusks without shells that frequently have stunningly decorated bodies. The little one above is only a few millimetres long.

Like sheep, they graze – on algae. Here’s a video of two different examples, the first from the Japanese Kuro Island (Costasiella kuroshimae) and the second from the Florida’s Key Largo. (Also the name of the 1948 movie featuring Humphrey Bogart and Lauren Bacall.)

The ‘leaves’ are cerata, horned (conical) structures on the outside of the body. These provide a large surface area for respiration, in a similar way that the large surface area of our lungs do. The head-on angle of the photo is a little deceiving; the ‘leaves’ aren’t flat. Here’s another picture – this one from the French Mayotte Island, near Madagascar:


One of the things I’d like to know is if these species ingest chloroplasts. Plants use sunlight to generate energy. Within leaf cells are small organelles specialised to carry the large protein complexes used to carry out photosynthesis. These organelles are chloroplasts. Some sea slugs feed on algae, keeping the the plant’s chloroplasts for their own use.[3]

A research report (PDF file) indicates the Order these animals belong to, Sacoglossa, are present throughout the world, including in New Zealand and Australian waters.[4]

They’re widely referred to as nudibranches on-line, but apparently Costasiallidae are “sea slugs resembling nudibranchs, but are not closely related to them” – some sea slugs belong in other taxonomic groups. The name nudibranch means ‘naked gills’.

I’d love to see these wee things in their natural habitat – who wouldn’t?


1. Thanks for Tim Skellett on twitter (@Gurdur) for bringing these little creatures to my attention.

2. To make things confusing the Encyclopedia of Life names them Leaf Sheep Nudibranches.

3. There’s another slug I’d like to write about for the genetics of it’s ‘borrowed’ photosynthesis, if I can find time and have access to the research paper. (There are some reports disputing as to if all sea slugs use the chloroplasts for photosynthesis.)

4. One New Zealand member of Sacoglossa was apparently described by Hutton in 1882. Otago readers may recognise the name; the lecture theatre at the Otago Museum is named for Frederick Hutton, an early scientist in New Zealand and one time curator of Otago Museum. The map of locations in this report shows none in polar regions but other reports suggest some nudibranches are present in Antarctic waters.

Other articles on Code for life:

Monkey business, or is my uncle also my Dad?

Finding platypus venom

Aww, crap.

Thieves in gold-mining era campsites

The inheritance of face recognition (should you blame your parents if you can’t recognise faces?)

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