American Oceans

Sea Sheep

The Sea Sheep is a unique-looking species of sea slug. Despite its name, this creature looks similar to a wide variety of succulent plants instead of a wooly mammal.

a colorful leaf slug underwater

It has a cute little face with ear-like rhinophores and a colorful body. You could be forgiven for thinking this sea slug is a tiny sheep with a back of green succulents.   

Besides its good looks, this creature has some unique abilities, especially its talent for indirect photosynthesis.

As a result, Sea Sheep eat so much algae they can almost harness the sun’s power as plants do.

Read on to learn more about the adorable and unique Sea Sheep.

What Is a Sea Sheep?

a leaf slug underwater

A sea sheep is a species of mollusk that lives in the seas near Japan, Indonesia, the Philippines, and a wide range of the Pacific and Indian oceans.

The Sea Sheep is also known as a leaf sheep, leaf slug, salty ocean caterpillar, or its scientific name, Costasiella kuroshimae. It was first discovered in 1993 near the island of Kuroshima, Japan. 

This sea dweller may have very little in common with its land-based namesake. However, few features are similar between these two creatures.

For example, the sea sheep have rhinophores that look similar to a sheep’s ears, thanks to their black tips. Both animals get their energy from some form of grazing, with leaf slugs using indirect photosynthesis.   


2 colorful green sea sheep

As shell-less mollusks, Sea Sheep have soft bodies covered in a green certa that likely gave rise to the creature’s leaf sheep nickname.

It is those certa, which look like a sheep’s puffy wool and the leaves on a succulent plant simultaneously. However, those body parts contain digestive glands. 

They also have a face with two small black eyes and protruding rhinophores with black tips, which are similar in look to sheep ears. Those rhinophores help the slugs smell or taste chemicals from nearby food in the water. Their faces are often one of the main features that make people consider these sea creatures cute.     


Sea sheep are small, only ranging in size between five millimeters and one centimeter. Occasionally, they can reach a length of a few centimeters. On average, they are close to seven or eight millimeters in length. 


Given their tiny size, sea sheep do not weigh much at all. An adult leaf slug can weigh around .1 kilograms. Some members of the species may be bigger or smaller by a small margin, but they will not weigh too dramatically different.  


sea sheep crawling along underwater

As mentioned, Sea Sheep have a unique way of obtaining energy from their food. Costasiella kuroshimae spend a lot of time near algae called Avrainvillea.

Often, they even make their habitats on the plant. These leaf sheep sea slugs eat the algae and partially digest it.

Full digestion is unwanted because if the chloroplasts are still intact, the leaf slug can use them.

Chloroplasts are a part of a plant cell that performs photosynthesis. This process uses the pigment chlorophyll to capture energy from the sun before storing and converting it into energy.

Plants use that energy to survive, and animals gain energy when they eat plants.  

Once the Sea Sheep eats so much algae, it stores chloroplasts in its certa, which cover the creature’s body.

They use a process called kleptoplasty to absorb energy from the sun into their absorbed chloroplasts. As a result, they can use the repurposed chloroplasts to survive. 


Sea Sheep can use photosynthesis for energy, using a symbiotic process called kleptoplasty. The process starts when the leaf sheep grazes on its favorite type of algae

Many animals use digestive enzymes in their mouth and throughout the digestive tract to extract nutrients from food.

The slug is different because it only partially digests the algae, leaving chloroplasts intact. Sea sheep then store the chloroplasts in their certa where they catch the sun’s rays and photosynthesize. 

Plants usually use the process of photosynthesis to combine light, carbon dioxide, and water into a usable source of energy.

Chloroplasts, which play such an essential role in photosynthesis, may have originated from a symbiotic relationship between plant cells and photosynthetic bacteria.    

Sea Sheep are not the only sea creatures who have this ability. Mollusks, such as Elysia chlorotica and Elysia viridis can both use kleptoplasty to store chloroplasts and get energy.  

While this species is somewhat difficult to study, the information we have suggests that a leaf sheep may be able to survive for up to nine months without consuming more food as a result of photosynthesis.

For optimum success, these leaf slugs must have access to the sun during that time. 


multiple sea sheep underwater

As mentioned, Sea Sheep favor the Avrainvillea algae as a food source. They often make their home on those same algae near coral reefs.

Besides providing nutrition, the algae also provide plenty of camouflage.

Plus, areas with plenty of algae suggest good access to sunlight. So, the creature is likely to survive longer in that spot. 

While this leaf sheep may exist in other places, so far, it is only found in the Indian and Pacific oceans.

In particular, it lives near Japan, the Philippines, Papua New Guinea, and Northern Australia.

Life Cycle

a leaf slug underwater

As a relatively recently discovered species, the lifespan of the Costasiella kuroshimae is slightly mysterious.

They may live for between six months and one year. However, they may also live longer, according to some.  

Breeding Habits and Offspring

Sea Sheep are hermaphrodites, which means they have both male and female sex organs. Unlike some hermaphrodites, they can not self-fertilize.

So, two Sea Sheep are needed to reproduce. When two solar-powered sea slugs reproduce, they make a group of eggs. 

From those eggs, hatch shelled larvae that hang out in plankton for a few weeks. After that, the larvae grow out of their shells while developing into their final slug form.

The larval stage likely takes around two weeks. 

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