K's Kreature Feature is an educational presentation at our monthly general staff meetings here at the Two Oceans Aquarium, but it’s only right that we extend the knowledge and fun to our readers here on our website. Every month PA to Head of Education Katja Rockstroh presents a different animal that we have here at the Aquarium and tells our colleagues more about it. Usually she uncovers some rather interesting facts about the various animals she researches. She also makes sure to entertain her co-workers …

This is one of my favourite sea animals, ever. They are also one of the tiniest creatures in the sea, but are by far one of the most impressive when it comes to the things they can do. But we will get to that in a moment.

I am cheating slightly, as this animal is not always in the Two Oceans Aquarium building. In fact, when it is here, it usually arrives by accident, on board some sea plants that our collections team collected for our exhibits.

The lettuce sea slug (Elysia crispata) is the most common Sacoglossan in the Caribbean. Photo courtesy LASZLO ILYES at Wikipedia (licensed under CC BY 2.0)

I like genealogy and classification, possibly because I also like order, and genealogy and classification order things into logical groups and sections. Boy, was I not prepared for the classification of this little animal. In the grand scheme of things, where we belong to the class Mammalia, sea slugs and other snail-like creatures belong to the class Gastropoda. And that is where the complications begin. There are clades, super-families, sub-orders and more. What is clear, however, is that the plant-sucking sea slugs, called the Sacoglossans, are not nudibranchs. They might look the same, but physiologically they are sufficiently different to belong to separate sub-orders/clades. Nudibranchs belong to the sub-order Nudibranchia.

Elysia viridis. Photo courtsy Genet at German Wikipedia (licensed under CC BY-SA 3.0)

The specific species that is sometimes found at the Aquarium’s microscope exhibit is called Elysia viridis. The word Elysia comes from the ancient Greek word “elysium”. Elysium means blissful, but was also used for the ancient Greek equivalent of heaven. Viridis, the species name, means “green”. And by looking at the colouration of the plant-sucking sea slug, it is very clear why they were named that.

George Montagu. (Licensed under Public Domain)

An Englishman by the name of George Montagu first described Elysia viridis. He lived from 1753 to 1815 and was well known as an ornithologist and naturalist. He published a book on British birds in 1802 and then in 1804 published another book on molluscs, describing over 470 different species, the plant-sucking sea slug being one of them.

Several species are, commonly, named after Mr Montagu, such as Montagu’s harrier, Montagu’s snapper and Montagu’s blenny. Quite anti-climatically, Mr Montagu died of tetanus after stepping on a rusty nail. Never too late to get those shots!

Slug food: codium fragile, AKA upright codium. Photo courtesy Flyingdream (Licensed under Public Domain

The distribution of this little slug, which only grows to about 3 to 5cm, is quite astonishing. Stretching from the Norwegian Atlantic coast all the way to the Mediterranean, it also occurs along most of the South African coast. It feeds on codium fragile, or upright codium.

Elysia viridis. Photo by Jim Anderson

Now this is where the true magic of this animal reveals itself. The plant-sucking sea slug, and in fact all of the Sacoglossans, are able to do something called “kleptoplasty”. Klepto means “thief” and plasty refers to “cells”. Plant-sucking sea slugs have evolved into being able to "steal” the chloroplasts of plants by eating the plant itself.

Thanks to a very specialised mouth, the membranes of the chloroplasts – cells that contain chlorophyll and in which photosynthesis takes place – are not broken and are then absorbed into the tissue of the sea slug. This results in photosynthesis within the tissues of the plant-sucking sea slug, producing nutrients for the animal. Behold the solar-powered sea slug!

The fact that this creature can do this at all is the most amazing thing, but on top of that it is the only multi-cellular group of animals that is able to do this. The only other organisms that do kleptoplasty are types of dinoflagellates (classified as Protista and known as phytoplankton, some of which cause red tides) and foraminiferans (ameboid Protista), which are both single-celled. Plant-sucking sea slugs are truly astonishing creatures.

On your next visit to the Aquarium, stop by at the microscope and ask our friendly staff member whether there is a plant-sucking sea slug in one of the dishes. It is definitely not an animal to be missed.

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