The Fascinating “Leaf Sheep”, Hermaphroditic Mollusks that Detach Pieces of Their Body to Escape Predators

Earlier, we talked here about Elysia chlorotica, a marine mollusk that we described as being half animal and half plant because it somehow had the ability to perform photosynthesis. Something similar happens with its cousins, the leaf sheep, which are also mollusks and were discovered in 1993.

Its scientific name is Costasiella kuroshimae, and it is a species of sea slug belonging to the order Sacoglossa, a category of marine gastropod mollusks without shells, from the family Costasiellidae. Commonly known as “leaf sheep” due to its peculiar appearance, its cerata on the back resemble the wool of a sheep. The name “kuroshimae” comes from the Japanese island of Kuroshima, where it was first observed.

Leaf sheep have a size ranging from 3 millimeters to 1 centimeter in length. Their digitiform rhinophores, located on either side of the head, are white with black tips. The eyes are very close together and are located between the rhinophores on the top of the head. A longitudinal brown pattern runs across the head, visually dividing it in two and widening and darkening after the eyes, all against a white background. This pattern is distinctive to the species, giving it a cartoonish or Pokémon-like appearance.

C. kuroshimae is one of the most colorful sacoglossans. Behind the head, the cerata that are distributed in seven rows covering almost the entire body are green due to the chlorophyll present inside them, with white tips. The green part has iridescent spots, which adds a dazzling touch to its appearance.

They can be found in tropical and subtropical waters of the Western Pacific, particularly in the Philippines, Indonesia, and the coasts of Japan. It spends most of its life among Avrainvillea algae, from which it feeds and where it lays its eggs at a depth of no more than 10 meters.

Due to its small size, it is preyed upon by many species, and for this reason, it has the ability to autotomize its cerata, a process where it voluntarily detaches parts of its body to escape predators. Additionally, it secretes mucus that helps it evade threats. Although little is known about its ecology, it is estimated that its life in the wild lasts about two years, based on data from similar species.

Costasiella kuroshimae feeds exclusively on algae, primarily Avrainvillea, but also Rhipilia and Pseudochlorodesmis. Its rhinophores are sensory organs that detect chemicals, allowing it to locate its food through specific chemoreceptors. It has the ability to extract chlorophyll from the algae it consumes, temporarily storing it in its cerata. This characteristic, known as kleptoplasty, allows it to perform photosynthesis, which would provide additional energy and enable it to survive for several days without food, although this aspect has not yet been verified.

Kleptoplasty is not exclusive to C. kuroshimae, but is also found in other species of Costasiellidae and in other groups like plakobranchioids, foraminiferans, ciliates, and dinoflagellates. This fascinating process allows these organisms to use the plastids of algae to obtain additional energy through photosynthesis.

Leaf sheep are hermaphrodites, meaning they possess both sexual organs, and their fertilization is internal. The laying of eggs is massive and is done in spirals in ribbons deposited on algae, where each structure can contain up to 500 eggs. Upon hatching, the larvae are released, and after metamorphosis, they become adult individuals.

Although it is a rare species, it is possible to keep Costasiella kuroshimae specimens in aquariums. It is preferable to keep them on Avrainvillea algae to ensure their diet. This species is very demanding regarding the species it can coexist with and should not be in contact with predators to avoid the loss of its cerata.

The recommended water parameters for the aquarium include a temperature between 22 and 27 °C, a pH of 8.1 to 8.4, a hardness of 8 to 12 dKH, and nitrate levels below 20 ppm.

The uniqueness of Costasiella kuroshimae lies in its appearance, its intriguing behavior toward predators, and kleptoplasty, a phenomenon that arouses great scientific interest since the ability to perform photosynthesis could have biotechnological implications in the future, inspiring new approaches in energy production and sustainability.

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This article was first published on our Spanish Edition on July 29, 2024: Las fascinantes «Ovejas de Mar», los moluscos hermafroditas que se desprenden de trozos de su cuerpo para escapar de los depredadores

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