The Solar-Powered Sea Slug: How Leaf Sheep Steal Photosynthesis

Costasiella kuroshimae, the “leaf sheep,” looks like a tiny cartoon sheep but lives more like a plant. This sacoglossan sea slug pierces algae, sucks out their contents, then keeps the chloroplasts functioning inside its own cells. Those stolen chloroplasts, called kleptoplasts, let the slug photosynthesise and survive for months on light and carbon dioxide when food is scarce. Despite lacking the algal nucleus that normally manages light damage and repairs, the slugs protect these chloroplasts through behaviour, pigmentation and clever tissue structure. Some species take this even further, deliberately severing their own bodies at the neck, then regenerating an entirely new body from the head, apparently powered by photosynthesis while they lack a gut or heart. Leaf sheep and their relatives blur the line between plant and animal, revealing how far evolution can push life’s underlying physics and chemistry.

A Solar-Powered Slug That Should Not Exist

The video opens on Costasiella kuroshimae, the “leaf sheep,” a millimetre-scale sea slug that looks strangely like a grazing cartoon sheep browsing a meadow of algae. Its cute rhinophores resemble ears, and its body is covered in leaf-like protrusions that make it look more like a tiny plant than an animal.

Sacoglossan sea slugs such as this one have evolved something extraordinary. They pierce algal cells, suck out the contents, and selectively keep the chloroplasts, the organelles that capture sunlight. Those chloroplasts remain active inside the slug’s own cells, allowing it to perform photosynthesis for days, months, or even years depending on the species.

Unlike coral, which hosts whole algal cells, sacoglossans keep only the chloroplasts. It is as if a human ate spinach once and then ran partly on sunlight for months afterwards.

Kleptoplasty: How Sea Slugs Steal Photosynthesis

The group that includes leaf sheep, lettuce sea slugs and other bright green species are sacoglossans, often called sap-sucking sea slugs. Many grow only a few millimetres long, some up to a few centimetres, and their leafy shapes make them almost indistinguishable from algae in seagrass beds and coral reefs.

Researchers first noticed in the 1960s that the green colour in these slugs was not just pigment. Under the microscope they could see intact chloroplasts inside the slug tissues. In the 1970s, experiments showed those chloroplasts were photosynthetically active.

To test whether this photosynthesis actually feeds the slugs, scientists kept individuals with and without light and food. Slugs with food and light maintained their weight. Those with light but no algae lost some weight, but survived far longer than animals deprived of both food and light. Photosynthesis does not replace feeding entirely in most species, but it clearly contributes significant energy, especially during lean periods.

The best-known example, Elysia chlorotica, can reportedly live for around nine months on light and carbon dioxide alone, effectively functioning like a slow-moving leaf.

Surviving the Dangers of Sunlight Without Plant Genes

Using sunlight is not as simple as just inserting chloroplasts into your cells. In plants and algae, excess light can produce reactive oxygen species such as superoxide and hydrogen peroxide that damage proteins, membranes and DNA. To cope, plants rely on a suite of nuclear-encoded proteins and enzymes that dissipate extra energy and neutralise these toxic molecules.

Sacoglossans do not import the algal nucleus, only the chloroplasts, so they lack much of this regulatory machinery. Nonetheless, they keep their kleptoplasts functioning for months without apparently succumbing to oxidative damage. The video outlines several strategies that seem to compensate.

Behaviourally, the slugs can crawl into shade, adjust their depth, or fold leafy extensions over their backs to limit light when it is too intense. At the cellular level they pack chloroplasts into dense clusters so outer layers take the brunt of the radiation, shielding those deeper inside. Some species also produce protective pigments that act as sunscreens, filtering harmful wavelengths.

Earlier, some biologists suspected the slugs must have stolen algal genes through horizontal gene transfer, acquiring the instructions for plant-like regulation. Genome sequencing has largely ruled this out. Instead, it appears that the inherent robustness of certain chloroplasts, combined with slug-derived protective mechanisms, is enough to keep photosynthesis running in this very alien context.

Decapitation and Regeneration Powered by Chloroplasts

The strangest sacoglossan trick is extreme autotomy: deliberate self-amputation. Many animals can drop a tail or limb when threatened and regrow it later. Some sacoglossan species go much further, cutting their bodies off at the neck and keeping only the head.

In a 2021 study, researchers observed slugs with a pale line across the neck, a “breakage plane” like a perforated edge. Using nylon thread to stimulate the area, they triggered the slug to complete the separation. The head slid free, leaving behind a body that still contained the original heart and most internal organs, which then slowly decomposed.

The detached head, however, did not die. It crawled around, grazed on algae, healed over and began to regrow a new body. Within about a week a new heart and leaf-like parapodia appeared. After roughly 17 days, the animal looked whole again. Some individuals repeated the process more than once in their lifetime.

This behaviour is unlikely to be a quick escape response to predators because it unfolds over hours and days rather than seconds. Researchers suspect it might help the slug rid itself of internal parasites or severely damaged tissue, or escape from tangles of seaweed. The ongoing photosynthesis in chloroplasts housed within the head appears crucial, providing energy while the slug has no digestive system and perhaps even no functioning heart.

Blurring the Line Between Animal and Plant

Taken together, sacoglossan sea slugs stretch our definition of what an animal is. They forage like grazers, photosynthesise like plants, cast off and regrow major body parts, and run all of these feats on a foundation of standard biophysics and chemistry.

The video closes by linking this weird biology back to fundamental ideas from physics and mathematics. Photosynthesis, chloroplast stability, oxidative stress, tissue regeneration and energy flow can all be understood as interacting systems constrained by the same principles that govern engineered machines. Evolution has spent millions of years exploring the design space that those principles allow.

By dissecting how a leaf sheep steals chloroplasts, moderates damaging light, survives decapitation and rebuilds itself, we glimpse that deeper structure. Energy capture, information storage in genomes, and mechanical resilience all follow rules that span from molecules to ecosystems. When those rules are clear, an animal that runs partly on sunlight and can regrow its entire body from a severed head stops being “impossible” and becomes a particularly dazzling example of how far life can push the physics of our planet.