Some sea slugs run on solar power, others cast off their body parts, and still others steal firearms from other animals, reusing them. They can even produce sulphuric acid! Marine scientists are discovering that some sea slugs use solar power, breaking all the rules of the animal world as they’re switching from being an animal to a plant, soaking up sunlight to produce a sugary snack.



This remarkable group of plant-like animals, the sacoglossan sea slugs, looks more like terrestrial butterflies than slugs, displaying lovely green wings as they slide gracefully across the seafloor. Their graceful appearance masks their darker nature: they’re thieves, stealing chloroplasts and plastids from their seaweed lunches.
   
These sea slugs use the stolen chloroplasts as a back-up energy supply when food is running short, like using solar energy panels to supplement your hot water system at home.
   
There’s even one species of sea slug that has been established as an anti-cancer agent and is under clinical trials in the U.S. These amazing shell-less mollusks, some living on the plant/animal frontier, may even hold a cure for cancer.
   
Sea slugs are a specialized group of snails and are classified as animals. Some are herbivores, while others are carnivores, often eating only a very specialized group of animals.
   
“There are two fascinating groups of sea slugs, called ‘solar powered,’ as they have become very plant-like in their behavior. Sea slugs can only behave like plants by either farming small plants in their bodies – as with nudibranchs harboring zooxanthellae, or by keeping the plastids, which are the photosynthesising factories in plant cells alive – as with the sacoglassans. These two groups of solar-powered sea slugs have evolved ways of using the ability of plants to convert the sun’s energy into sugars and other nutrients,” explains Dr. Bill Rudman, of the Australian Museum.
   
“The herbivorous sacoglossan sea slugs suck the cell contents from the seaweeds they feed on. From this cell sap, they keep alive and functioning the plastids – those parts of the plant cell that convert the sun’s energy into sugars. This conversion of the sun’s light energy into food for the plant is called photosynthesis.”
   
In green plants the plastids are green and are called chloroplasts. Most sacoglossans are coloured by the plant pigments they keep in their bodies.
   
Among the nudibranchs, which are all carnivorous, a number of different families have evolved ways of keeping microscopic single-celled plants alive in their bodies. These single-celled plants are called zooxanthellae. Although they have free living relatives in the ocean’s plankton, they have adapted to living in the tissues of the sea slugs.
   
Some nudibranch species have evolved branches in their gut and contain plastids, the photosynthesising “factories” from the algae, which are alive and operating. In many cases the plastids are chloroplasts but “Sacoglossans, which feed on red and brown algae, are also known to keep the plastids from these algae alive,” says Bill.
   
“In the nudibranchs, many have evolved similar ways of keeping whole single-celled plants – zooxanthellae – alive in their bodies. Mostly zooxanthellae are obtained from their food, often cnidarians that already have symbiotic zooxanthellae in their bodies,” he added.
   
Using stolen chloroplasts, sea slugs have found a way to bridge the gap between plants and animals by using the chloroplasts to produce up to a quarter of their food from photosynthesis. Sea slugs have found a way to break the rules of biology and use stolen chloroplasts to live as part-time plants.
   
The pilfered chloroplasts are called kleptoplasts and sea slugs are able to maintain the kleptoplasts in full working condition in their gut cells. There’s no need for the sea slug to go searching for food; it can simply lay in the sun, letting the enslaved kleptoplasts do all the hard work.
   
So far, sea slugs are the only multicellular animals known to photosynthesis using stolen chloroplasts. This process, called kleptoplasty, differs from the symbiosis that takes place in other marine invertebrates; as kleptoplasty is no partnership, the sea slugs have found a way to enslave the seaweed’s chloroplasts after destroying and digesting the rest of the plant.
   
Some nudibranchs are brightly colored as a warning to predators that they taste bad, called “aposematic,” and are believed to sequester toxins from the foods they eat, to use in turn for defense. These defense chemicals are called allomones.
   
There still hasn’t been much experimental work done on aposematism since the pioneering work of a researcher named “Crossland,” who 90 years ago was throwing bits of chromodorids, a kind of nudibranch, off his houseboat in the Sudanese Red Sea to hungry fishes.
   
A spectacular example of allomone use is found in the nudibranch family chromodorididae – where all species concentrate chemicals obtained from the sponges they feed on in special glands around the mantle edge. “In South-Eastern Australia many species have evolved a red-spotted color pattern in a great geographical display of defensive mimicry,” says Dr. Bill Rudman.
   
Sea slugs are essentially snails that have lost their shells and have had to evolve ways to protect their soft flesh from fish, crustaceans, and other hungry marine animals. Some have evolved glands in their skin that secrete noxious, distasteful, and sometimes poisonous chemicals.
   
Even the most primitive sea slugs, such as the bubble shells, produce a milky white acidic mucous secretion from glands around the edge of their mantles. In other opisthobranchs, i.e., nudibranchs, clusters of white glands called repugnatorial glands can be seen as part of the color pattern.