By Matt Walker
Editor, Earth News
Sea slug: a light harvester
Certain species of sea slug live by the light, possessing a remarkable ability to perform photosynthesis, the only animals known to do so.
Now scientists have uncovered details of how they capture the sun's rays.
The sea creatures steal chloroplasts from algae, using them to harness the sun's energy just as plants do.
But they also swim in a way that boosts the amount of energy available, with some species being capable of living for months by light alone.
The study on solar-powered sea slugs is published in the Journal of Experimental Marine Biology and Ecology.
There are thought to be more than 280 species of sea slug, a type of ocean-going mollusc.
The animals have their shells on the inside, freeing their bodies to swim in beautiful undulating motions through the water.
The majority of "solar-powered" sea slugs are coloured green.
Their colour holds the clue as to how they are able to harness the power of the sun.
Sea slugs feed on green algae, which like plants possess green structures called chloroplasts that capture light and convert it into useful energy.
Many sea slugs are capable of incorporating the chloroplasts they ingest into their own tissues, some being able to retain the chloroplasts for more than six months.
"Sea slugs are born without chloroplasts and have to acquire them by 'stealing' from their food source," says Dr Bruno Jesus of the Oceanography Centre at the University of Lisbon, Portugal.
A sea slug sits on the source of its power: a green alga
"So far, [sea slugs] are the only animal group we know that is able to do this.
"The animal acquires a new metabolic feature and can photosynthesise as a plant."
That much, scientists already knew.
"However, we know very little about the extent of this functionality. Is the chloroplast fully functional within the animal cell or just partially functional?" says Dr Jesus.
He and colleagues Dr Gonçalo Calado and Dr Patricia Ventura, from Portugal's Institute of Malacology in Albufeira, studied sea slugs (Elysia timida) collected from the Mediterranean Sea and held in aquaria.
They made some startling discoveries.
Some species of sea slug can harness light's power better than even algae, the organisms they stole their chloroplasts from.
The ocean-going molluscs do this in two ways.
First, they boost the amount of energy the chloroplasts produce by harnessing a physiological mechanism called the xanthophyll cycle, which involves using pigments to transfer electrons.
Plants usually use this chemical process to prevent damage caused by excessive light.
The sea slugs also use this mechanism to stop their tissues being damaged by intense light, but they can also use it to absorb light for much longer periods than algae can.
Second, the sea slugs fall back on their skills as an animal to move and behave in certain ways that vary the amount of light they are exposed to.
If the light is too intense, they will curl up like a ball, to limit how many chloroplasts are exposed.
In low light they flatten their bodies into a leaf-shape, maximising how much of their body can harness the sun's rays.
"Solar powered sea slugs take a little bit of the best of both worlds," says Dr Jesus, using both animal and plant abilities.
That may have allowed the species to colonise very shallow waters where light is plentiful.
"Some species can survive solely on the photosynthetic products, essentially living from light," says Dr Jesus.