Living on the sea bed in the ocean areas around Japan, Indonesia and the Philippines is a curious but somewhat cute creature called Costasiella kuroshimae, more commonly known as the Leaf Sheep. This is a species of sea slug, about 5 mm long, which is coloured a vibrant green and has a multitude of ‘horn like’ outgrowths on its back collectively called cerata. These structures can sometimes bear bright blue spots and/or orange rings or spots, as well as small dark brown dots and red or pinkish spots, the latter appearing just below the ‘tip’ of the horn. However, the easiest way to identify this creature for sure is by the two eyes that are extremely close together in the middle of its head.
The really interesting thing about the Leaf Sheep, and certain other sea slugs, is that it has the ability to steal photosynthesizing chloroplasts from the algae it eats in order to generate energy, a process called kleptoplasty. Chloroplasts are one of the features that are usually used to distinguish plants from animals. In general, animals do not have them but plants do, because plants use photosynthesis to generate energy whereas animals obtain energy by eating plants, or other animals. This means that an animal that has the ability to use chloroplasts is a fairly unique phenomenon, and one such animal is the sea slug, or certain members of the sea slug family at least.
The kleptoplasty process leaves the plastids within the algae intact, thereby allowing the animal that consumes them to generate the energy from photosynthesis. The Leaf Sheep isn’t the only species of sea slug to do this. The Eastern Emerald Elysia is another sea slug, living off the eastern coast of North America where it frequently inhabits shallow marshes, tidal pools and creeks, having the same ability. Elysia is considerably larger than the Leaf Sheep, at 30 mm in length and also differs from the Leaf Sheep in that it has the ability to absorb the algal genes into its DNA, whereas the Leaf Sheep’s engagement with algae is only temporary.
“While the next generation must take up chloroplasts anew from algae, the genes to maintain the chloroplasts are already present in the slug genome” explains Professor Sidney Pierce of the University of South Florida, who is something of a specialist on the subject.
Professor Pierce and his colleagues Julia Schwartz and Nicholas Curtis used a process called fluorescent in situ hybridization to demonstrate Elysia’s gene transfer ability, the study being published in The Biological Bulletin in December 2014.
The only animals to use kleptoplasty are these sea slugs, which belong to the family Sacloglossa. The process gives the host animals great advantages in that it means they don’t have to chase around for prey like other animals do. This is particularly advantageous in environments where other food resources are limited. Furthermore, Elysia in particular can use the process to generate energy for its entire adult life.