Look! Here is a group of "sponge sheep" that can photosynthesize

When exploring the world of marine life, most people will think of the shrimps and crabs wearing "armor", the colorful schools of fish, the coral reefs that provide refuge for other small animals, and the ferocious sharks.

However, in the vast ocean world, there is a group of small animals that are easily overlooked but very special. They are small and cute, very much like the image of little sheep in cartoons. They are the first photosynthetic animal discovered so far - the algae sea slug (Costasiella kuroshimae).

01

Cute appearance, highly recognizable

The body of the algae sea slug is soft and almost transparent. Its back looks like a layer of green leaves, and the pointed "leaves" are dyed a little pink. Its head is white, and it has a pair of small eyes like black beans and a pair of cute tentacles, which looks like a little sheep covered with leaves. It is only about 5mm long, so people call it "lamb sea slug" or "leaf sheep". Its shell has degenerated into an inner shell, and the thin white and shiny part under the green leaves on its back is its inner shell.

Although they are easy to identify as adults, they are very difficult to spot when they are young. When they just hatch from the eggs, they are still a little transparent and look completely different from their parents, but as they keep eating seaweed, their backs will slowly turn into a shiny bright green, and they will become the little green sheep we know.

Cute image of algae sea slug Image source: Wikipedia

02

A green lifestyle

Algae sea slugs live in an environment with clear water, smooth water flow and dense seaweed, and are mostly distributed in Japan, the Philippines and Singapore.

We all know that photosynthesis is the strong point of plants. Interestingly, algae sea slugs also have this ability. After swallowing seaweed, they will keep the chloroplasts contained in the seaweed in their bodies, coexist with somatic cells, and then use them to carry out photosynthesis.

Providing the energy needed for daily life through photosynthesis is a phenomenon known as kleptoplastids in biology. As early as 1870, this sea slug was discovered, but no one noticed its "special function" at the time. It was not until recent years that researchers discovered its ability.

According to investigations, algae sea slugs start eating various seaweeds from an early age. As they grow, they accumulate more and more chlorophyll, eventually turning their entire bodies bright green so that photosynthesis can proceed smoothly. When algae are plentiful, their staple food is algae.

If they don't have enough algae to fill their stomachs, they can go out to bask in the sun and gain energy through photosynthesis until they encounter abundant algae again, and then they will feast on them. Without food, they can survive for about a year by relying on the chlorophyll in their bodies. This lifestyle is really economical and green.

03

Check the "family tree" to know the family background

Having introduced so many characteristics of algae sea slugs, which family does it belong to biologically?

Algae sea slugs are one of the many types of sea slugs. They are molluscs that live in shallow waters. Sea slugs are a large family with more than 300 species distributed around the world. The shell of sea slugs has degenerated or disappeared, and they look like shell-less snails. If you really trace them back, sea slugs and snails are actually somewhat related.

Sea slugs and snails belong to the class Gastropoda under the phylum Mollusca. They both have flat abdomens and use their abdomens to attach to the surface of objects and crawl by contracting the abdominal muscles. Unlike snails, sea slugs are members of the genus Aplysia in the subclass Opisthobranchia, while snails are members of the order Stylophora in the subclass Pulmonaria, family Snailidae.

They also have different ways of breathing. Sea slugs breathe through their exposed gills on their backs, and for this reason they are classified as Opisthobranchia, while snails breathe through hollow spiracles on their bodies, just like our lungs, and are therefore classified as Pulmonaria.

Members of the Nudibranch family, to which sea slugs belong, have very different body shapes. From the outside, they are composed only of skin wrapped with muscles and internal organs. Their bodies are symmetrical on both sides, with secondary gills exposed on the outside. They breathe through their gills and rely on their abdominal muscles to wriggle on the seabed.

Molluscan lineage relationships, image source: Mobilizing molluscan models and genomes in biology

Comparison of the body structure of sea slugs and snails, Image source: "Reef creature identification: tropical Pacific", "Anatomy of a snail"

04

Defend against the enemy

In an ocean world full of predators, how do algae sea slugs protect themselves? Unlike shrimps and crabs that use hard shells to protect themselves, they have a unique "enemy defense technique".

In order to protect themselves, they will turn their "green coat" into "camouflage uniforms" to blend in with the environment, and the exposed gills on their bodies will sway constantly to simulate water plants to confuse predators. In addition, the tentacles on their heads are very sensitive to touch, taste and smell, which helps them detect enemies hiding around them, react quickly and escape.

05

They also want to pass on their lineage

Algae sea slugs are hermaphroditic animals. Although they have both male and female reproductive organs, they do not choose to reproduce themselves and need to mate with other individuals to reproduce offspring.

The mating time is mainly concentrated in the summer, and a few hours after mating, the eggs are laid in a counterclockwise spiral, with mucus on the surface, adhering to algae or gravel. They are relatively "highly productive", but the number of larvae that can be successfully hatched is very small, and the answer to this question has not yet been found.

I believe that one day in the future, zoologists will reveal more life secrets about these little cuties, allowing us to have a better understanding of these inconspicuous little animals.

References:

1. Lin Qiang. Colorful sea slugs, the elves of the ocean[J]. Civilization, 2016(05):112-125+10.

2. Hazelnut. Solar-powered sea slug[J]. Encyclopedia for Middle School Students, 2010(15):30-31.

3. Zhang Weihong, Liu Ailing, Chen Shaodan, Huang Li, Wu Songlin. The anatomy of snails[J]. Bulletin of Biology, 2010, 45(03):45-46.

4. Mobilizing molluscan models and genomes in biology. Angus Davison and Maurine

Neiman, 2021(04) 05.

5. Reef creature identification: tropical Pacific. Humann P, DeLoach N. [M]. New World Publications, 2010.

Review expert: Li Weiyang, well-known science blogger

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