Animals That Photosynthesise

Quick Answer: No animal photosynthesises on its own. Several run partly on sunlight by borrowing the machinery: corals and giant clams host living algae inside their tissue, some sea slugs retain functioning chloroplasts stolen from the algae they eat, and the spotted salamander is the only vertebrate known to carry a photosynthetic partner inside its own cells. Biologists call the arrangement photosymbiosis, and it built the coral reefs.

Animals that photosynthesise should not exist. Photosynthesis is supposed to be a plant trick: animals eat, plants make their own food, and the line between them is one of the first things taught in biology.

The line holds until you look at a coral reef, where a very large share of the animal biomass is quietly running on light. None of those animals evolved chloroplasts. They acquired someone else's, which is arguably the more interesting solution.

How Can an Animal Photosynthesise?

There are two routes, and they are not equivalent.

The common route is symbiosis. The animal houses living photosynthetic organisms inside its tissue, provides them with shelter, carbon dioxide and nitrogenous waste, and takes a share of the sugars they produce. Both partners remain distinct organisms.

The rare route is kleptoplasty, literally chloroplast theft. The animal eats algae, digests almost everything, and retains the intact chloroplasts in its own cells where they keep working.

The two mechanisms compared:

A third category is sometimes claimed and should be treated carefully. Some animals gain incidental benefit from algae growing on them rather than in them, which is epibiosis, not photosynthesis by the animal.

Corals: The Reef Runs on Light

Reef-building corals host single-celled dinoflagellate algae, usually referred to as zooxanthellae and belonging largely to the family Symbiodiniaceae. They live inside the coral's gastrodermal cells, packed at densities of roughly one to two million per square centimetre of tissue.

The algae photosynthesise and pass a large share of their fixed carbon to the host as sugars, glycerol and amino acids. Published estimates commonly put the contribution at up to about 90 percent of the coral's energy requirement in shallow, well-lit water.

What the partnership provides each side:

• To the coral: sugars, glycerol, amino acids and oxygen
• To the algae: shelter from grazers, a stable position in the light, carbon dioxide and nitrogenous waste to recycle
• To the reef: the ability to build massive carbonate structures in nutrient-poor water

This is why reefs thrive in clear tropical water that is otherwise close to a marine desert. The nutrients are not in the water column; they are being recycled inside the animals.

It is also the reef's structural weakness. Elevated sea temperature disrupts the algae's photosynthetic machinery, producing damaging reactive oxygen species, and the coral expels its symbionts. The tissue turns transparent over the white skeleton, which is coral bleaching.

A bleached coral is not dead. It has lost its main food supply and will starve if the symbionts do not return, which makes the duration of a heat event as important as its peak.

Giant Clams: Farming Algae in the Mantle

Giant clams of the genus Tridacna host the same broad group of algae, but arrange them differently. Rather than keeping them deep in the tissue, the clam displays them in a fleshy mantle that spreads across the open shell toward the light.

The vivid blues, greens and golds of a giant clam mantle are not decoration. They come from pigment cells called iridocytes, which scatter and redistribute light into the tissue where the algae sit, and reflect away wavelengths the algae cannot use.

Notable features:

• Largest species, Tridacna gigas, can exceed 1 metre in shell length and 200 kg
• Algae are housed in a branching tubular system within the mantle
• Iridocytes act as living light management optics
• Clams still filter feed, so photosynthesis supplements rather than replaces feeding

A large clam derives a substantial share of its nutrition from its tenants. The arrangement lets it reach a size that filter feeding alone would struggle to support in nutrient-poor reef water.

Sea Slugs: Stealing the Machinery Outright

Sacoglossan sea slugs are the kleptoplasty specialists. They pierce algal cells with a specialised feeding structure, suck out the contents, and retain the chloroplasts in the cells lining their digestive diverticula.

The stolen chloroplasts keep photosynthesising. Retention time varies widely by species, from a few days in most to several months in the best-documented cases such as Elysia chlorotica.

Why this is genuinely strange:

• Chloroplasts depend on proteins encoded in the algal nucleus, which the slug does not keep
• Without resupply, chloroplasts should degrade within days
• Long retention in some species therefore requires an explanation that is still debated
• Early horizontal gene transfer hypotheses have not held up well under later genomic scrutiny

The honest position is that the mechanism behind long-term retention is unresolved. Some work suggests the chloroplasts of the specific algae targeted are unusually robust; other work points to slug-derived protective proteins. It remains an open problem.

What is not disputed is the outcome. A well-fed Elysia chlorotica is bright green, shaped like a leaf, and can survive extended periods on light.

The Spotted Salamander: The Vertebrate Exception

The spotted salamander (Ambystoma maculatum) is the only known vertebrate with a photosynthetic partner inside its own cells.

The association with green algae in salamander egg masses has been recorded since the nineteenth century. The eggs turn visibly green as algae colonise the jelly.

What changed the picture was the finding that algal cells occur inside embryonic tissue itself, not merely in the surrounding jelly. That moves the relationship from incidental to genuinely endosymbiotic.

What the partnership appears to do:

• Algae supply oxygen directly to embryos in an egg mass where oxygen is otherwise limiting
• Embryos in algae-rich eggs tend to hatch earlier and at higher rates
• Algae take up embryonic nitrogenous waste
• The association is not inherited directly and is re-established each generation

The salamander is not photosynthetic as an adult. The relationship is confined to the embryonic stage, which is precisely when oxygen supply inside a gelatinous egg mass is most constrained.

Other Photosymbiotic Animals

The four best-known cases are not the whole list. Photosymbiosis has arisen repeatedly across marine invertebrates.

• Sea anemones, including the hosts of clownfish, commonly carry zooxanthellae
• The upside-down jellyfish Cassiopea rests bell-down on the seabed specifically to expose its symbionts to light
• Some flatworms, notably Symsagittifera roscoffensis, are so dependent on algal partners that adults barely feed
• Certain foraminifera and radiolarians, single-celled but animal-like, host algal symbionts
• Some nudibranchs retain zooxanthellae acquired from the corals they eat

The pattern is consistent: photosymbiosis appears wherever light is reliable, water is nutrient-poor, and a body plan allows a partner to be held near the surface.

Why Photosymbiosis Matters

It built the reefs. Coral photosymbiosis is what allows enormous carbonate structures to grow in water with almost no dissolved nutrients, and those structures support a share of marine biodiversity far out of proportion to the area they cover.

Remove the symbiosis and the reef does not simply shrink. It stops being built, and existing structures erode faster than they accrete.

That is the practical stake in coral bleaching. The threat is not the loss of colour; it is the loss of the energy supply that makes reef construction possible at all.

Can Humans Photosynthesise?

No, and the obstacles are structural rather than a matter of degree.

Why it does not work for a mammal:

• Surface area to volume ratio is far too low; a human has nothing like the exposure a coral or leaf achieves
• Mammalian metabolic demand is enormous compared with the energy sunlight could deliver over that area
• Skin is optimised to block ultraviolet radiation, not to admit light to photosynthetic tissue
• Every known photosymbiotic animal is small, flat, sessile or aquatic, and usually more than one of those

Even a sea slug, which is thin, flat and permanently bathed in light, uses photosynthesis as a supplement and a buffer against starvation rather than a primary food source.

Related Reading on Strange Animals

See coral reefs for how the whole system fits together, brain coral for a detailed look at one reef builder and its zooxanthellae, and staghorn coral for a fast-growing branching species. The blue dragon sea slug is a different case of an animal repurposing something it ate, in that instance stolen stinging cells rather than chloroplasts.

References

This entry draws on the standard literature on cnidarian symbiosis and the Symbiodiniaceae, published work on sacoglossan kleptoplasty including studies of Elysia chlorotica, and research on algal symbiosis in Ambystoma maculatum egg masses and embryonic tissue.

Figures such as the proportion of coral energy supplied by symbionts are given as commonly published ranges. They vary considerably with depth, species, water clarity and the symbiont clade involved, and any single number should be read as an approximation rather than a constant.