Do my eyes deceive me or is that patch of algae…moving? Well, not exactly. This sea slug (Costasiella kuroshimae) goes by many names, often affectionately referred to as the leaf sheep or even the “Shaun-the-sheep slug” by divers in the Philippines [1]. The leaf slug – like in this artistic rendition – does bear a striking resemblance to the terrestrial sheep we know and love. While not technically algae itself, this equally fascinating and adorable animal shares far more similarities with algae than you’d think.
Sea slugs are incredibly similar to the terrestrial slug you can find in your garden, with the exception of their marine habitat. Sea slugs are molluscs, known for their soft-bodies and lack of shell [2]. The leaf slug belongs to a group of sea slugs known as the Sacoglossa, a species with a special solar affinity. Sacoglossans have the incredible ability to incorporate chloroplasts from their diet into their tissues in a process known as kleptoplasty. Moreover, their chloroplasts are functional, meaning these animals actually photosynthesize just like plants [3]! The Sacoglossans are the only known multicellular organisms capable of kleptoplasty [4].
Sacoglossans achieve this truly one-of-a-kind feat by using their tiny radular teeth to access the interior of algal cells. They then slurp up the cell contents, chloroplasts and all [3]. The chloroplasts become incorporated into their bodies via their unique digestive system. Species within Sacoglossa possess a digestive gland that branches from their stomach into the rest of their bodies. This allows Saccoglossa to transport the chloroplasts directly to the surface of their skin for later photosynthetic use [6]. This is where the klepto-, the Greek root word for thief, in kleptoplasty comes in as they “steal” chloroplasts from their food source [4]. These ingested chloroplasts, however, are not permanent fixtures. Compared to other species in the Costasiellidae family, the leaf slug retains chloroplasts for a relatively short period of time. As a result, in order to maintain their ability to photosynthesize, they must have a consistent plant food source [6].
The chloroplasts ingested by the leaf slug are filled with a substance known as chlorophyll, a photosynthetic pigment. Chlorophyll gives the slug its striking green color. The bushy body of the leaf sheep is made up of protrusions called cerata, giving it that iconic sheep-like appearance [7]. These cerata increase the surface area of their skin and are heavily concentrated with chloroplasts, and thus chlorophyll, making the sea sheep green. This increased surface area maximizes their ability to absorb sunlight for photosynthesis [7]. The leaf slug also has a very characteristic set of eyes on the front of their face, lying between what appear to be horns. These “horns” are actually rhinophores – front-facing protrusions that act as a sea slug’s sense of smell and navigation. These rhinophores are capable of detecting chemical signals in the water; they also help sea slugs avoid predators! They do so through specialized nerve cells that pick up on pressure differences and movement in the water around them [8].
The leaf slug has been found in waters across the Indo-Pacific, recently also having been found in the Red Sea [7]. These waters are characteristically prime locations for algal growth. It’s no surprise that the leaf slug is largely herbivorous, solely feeding on algae in order to provide a constant source of chloroplasts for their bodies, further reminiscent of their terrestrial sheep namesake.
Many species within Costasiellidae, the family the leaf sheep belongs to, exclusively consume one species of algae: Avrainvillea. Avrainvillea, characterized by its vibrant green color, can be found adjacent to reefs in patches of fine sand [7]. The leaf slug remains on a patch of Avrainvillea for the entirety of its life. The leaf slug feeds on the algae and, when its nesting time comes around, lays four clutches of eggs in a spiral pattern nestled in the Avrainvillea [7]. At an average 5 mm long– about the width of a pencil top eraser– the sea slug camouflages seamlessly amongst its matching green Avrainvillea home, deceiving its potential predators like crabs and even other sea slugs [7, 9]. Their expert camouflage not only stumps predators, but divers as well. Little is known about the leaf slug’s social and mating behavior, but scientists have confirmed they are hermaphroditic. Hermaphrodites possess both sex organs necessary for reproduction, so any pair of leaf slugs are capable of having a clutch [1].
It’s safe to say that this sea slug is well-loved by the general public. The leaf slug has its own Build-a-Bear plush, and has even made an appearance on the Masked Singer, as former NFL player John Elway’s disguise [10]. I mean, what’s not to love about this eclectic little slug, from its adorable likeness to a beloved farm animal to its absolutely intriguing ability to photosynthesize. It’s truly amazing what this tiny sea slug can do!
Main featured image by Christian Gloor [Source].
Written by: Emily Corona is a third year undergraduate student studying animal science with a specialization in animal behavior at UC Davis. She’s currently a member of the Green Care Lab, a holistic animal neurobiology lab focused on human and animal interactions. As an animal care specialist at the lab, she has developed a keen interest in livestock and large animal welfare, and hopes to pursue it with her degree. When she’s not studying for the next midterm she loves needle felting, poetry, and cozying up with a good reality TV show.
References:
[1] Caballar, D.R. (2021, March 25). The ‘sheep’ that can photosynthesise. BBC. https://www.bbc.com/travel/article/20210324-the-odd-sea-creature-powered-by-the-sun
[2] Knutson, V. (2012, September 6). Project lab. California Academy of Sciences. https://www.calacademy.org/blogs/project-lab/sea-slugs-101
[3] Cruz, C., Calado, R., Serôdio, J., Cartaxana, P. (2013). Crawling leaves: photosynthesis in sacoglossan sea slugs, Journal of Experimental Botany, 64(13), 3999–4009, https://doi.org/10.1093/jxb/ert197
[4] University of Hawai’i at Manoa (n.d.). Weird science: kleptoplasty. https://manoa.hawaii.edu/exploringourfluidearth/biological/aquatic-plants-and-algae/structure-and-function/weird-science-kleptoplasty
[5] Hirose E. (2005). Digestive system of the sacoglossan Plakobranchus ocellatus (Gastropoda: Opisthobranchia): light- and electron-microscopic observations with remarks on chloroplast retention. Zoological science, 22(8), 905–916. https://doi.org/10.2108/zsj.22.905
[6] Christa, G., Gould, S. B., Franken, J., Vleugels, M., Karmeinski, D., Händeler, K., Martin, W., & Wägele, H. (2014). Functional kleptoplasty in a limapontioidean genus: phylogeny, food preferences and photosynthesis in Costasiella, with a focus on C.ocellifera (Gastropoda: Sacoglossa). Journal of Molluscan Studies, 80(5), 499–507. https://doi.org/10.1093/mollus/eyu026
[7] Yonow, N. (2015). Sea Slugs: Unexpected Biodiversity and Distribution. In: Rasul, N., Stewart, I. (eds) The Red Sea. Springer Earth System Sciences. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-662-45201-1_30
[8] Shipman, C. (2012, December 9). Sea slug senses part I. California Academy of Sciences. https://www.calacademy.org/blogs/project-lab/sea-slug-senses-part-i
[9] Florida Fish and Wildlife Conservation Commission (n.d.). Sea slugs. https://myfwc.com/research/saltwater/mollusc/other-molluscs/sea-slugs/
[10] Lauren, H. (2024, September 25). The Masked Singer unmasks Leaf Sheep as football legend in season 12 premiere that’s full of wild surprises. Entertainment Weekly. https://ew.com/the-masked-singer-reveals-leaf-sheep-football-legend-john-elway-8717641
[Edited by Alice Michel]