Zooming into Nudibranchs: Ercolania and the Sailor’s Eyeball
Written by Roy Kittrell 28 February 2023.
Since it’s St Patrick’s Day this month, I thought it would be fun to return to our series on ‘Zooming into Nudibranches’ with a little “St Patrick’s twist”. This month in particular, we will be looking at a very tiny species in the genus Ercolania, and perhaps it’s equally interesting counterpart, Valonia ventricosa, a species of bubble algae found commonly throughout the coral triangle, also known as the ‘Sailor’s Eyeball’. 
Despite being common and often overlooked by most divers, bubble algae are actually quite a marvel of the natural world. Valonia ventricosa is known for its unique structure, which consists of a single giant cell that can contain thousands of nuclei.
Valonia ventricosa is also of interest to researchers around the world for its potential uses in biotechnology and biomedicine. Studies have shown that the algae contains a range of compounds with potential therapeutic applications, such as antimicrobial and anticancer agents. That’s pretty amazing for something so small and easily overlooked!
The cell is surrounded by a thick, rubbery wall that helps it to resist damage. This thick wall was actually the basis for a study done in 1831 by the Scottish botanist Robert Brown*, and helped determine the chemical structure of cellulose. Later in 1957, scientists** used X-ray diffraction on the cell walls of Valonia to determine the crystal structure of cellulose – making the Valonia a very useful algae indeed! 
It is in fact this wall that makes it so useful for the Ercolania nudibranch.
Nudibranches in the genus Ercolania, a type of Sacoglossa ‘Sap Sucking Slug’. They are cousins of Costasiella kuroshimae, otherwise known as the Shaun the Sheep nudibranch that I covered in a previous article. They also have an amazing look to them, having the same green cerata and an even cooler war-paint like pattern on their face. They utilise these large algae to lay their eggs, both inside and on the surface of the cell wall, and some species have even been seen to be living inside of them. A whole slug living inside of a single-celled organism… now that is an amazing adaptation!
Join us on our next trip to Juara, Tioman and try spotting one of these algae! Be sure to take a close look to try and see if you can see any small spirals which may be the eggs of an Ercolania nudibranch, or if you’re really lucky maybe the Ercolania nudibranch itself!
* Brown, R. (1831). Observations on the Organs and Mode of Fecundation in Orchideae and Asclepiadeae. Philosophical Transactions of the Royal Society of London, 121, 449-492.
** Fraser, J. T., & Ross, J. H. (1957). A structural investigation of the cellulose of Valonia ventricosa. Biochimica et Biophysica Acta, 23(1), 47-56.