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212. A couple of Ocellated Sap-sucking Sea Slugs (Plakobranchus ocellatus) on a solitary Fan Green Seaweed leaf (Avrainvillea erecta)


Two Ocellated Sap-sucking Sea Slugs (Plakobranchus ocellatus) share a Solitary Fan Green Seaweed leaf (Avrainvillea erecta) on a northern Balinese sandy slope.

This species of sea slug comes in a wide variety of colours, and it is possible that these different variations are different species. In general, it is known that the Balinese variant of this gastropod species is seldom encountered, and therefor it took me a while to identify it.

This Balinese version is quite unique in colouration due to the presence of beautiful blue in its rhinophores and on the side of its body. Most specimens are cream to brown coloured and bear some spots or ocelli on their body, hence their name. They belong to the superorder Sacoglossa, commonly known as solar-powered sea slugs. The term “solar-powered” comes from the fact that they eat and ingest the cellular content, chloroplasts, from certain algae that they store in bundles on the surface of the slug’s cerata (the gill structures on both sides of the body). These chloroplasts are kept alive for long periods. This is done by giving these cells enough sunlight to survive, hence the term “solar-powered.” When food is in short supply, the slug relies on these “stolen” algae cells, which it digests in small portions. Stealing parts (cells) of other living creatures to store in one's own body is called kleptoplasty.

Due to their need for enough sunshine, they are usually found in shallow waters where the sunlight can easily reach their stolen and stored chloroplasts. Shallow water habitats like lagoons with silty and muddy bottoms are thus preferred.

It is unknown if these two Ocellated Sap-sucking Sea Slugs in this underwater videoclip are feeding on this solitary Fan Green Seaweed leaf. The careful viewer might have spotted a little sea shell on the lower left side of this seaweed leaf.

The Solitary Fan Green Seaweed leaf (Avrainvillea erecta), also known as Elephant's Ear or Elephant's Ear Seaweed, is an algae made out of a fan-like velvet blade with an average diameter of 5cm/2inch that is sitting on a 10cm/4inch stalk. This stalk is anchored into the substrate, and 80% or so of this stalk is buried in the bottom. The algae’s blade is made of a tangle of tiny filaments, which gives it its velvet-like structure.


To film small subjects that aren’t moving around, it is best to pose your camera in a very stable position. If the area is made of sand or soft substrate, you can easily position your underwater camera rig on the bottom without harming the environment.

The use of a macro lens or diopter is strongly recommended when filming any subject smaller than, for example, an apple. When using a diopter, your camera must be in a stable position; small movements of the camera will translate into jerky footage. The greater the magnification, the more your footage is subject to camera movement.

Filming location:

This short underwater videoclip has been filmed in Bali, Indonesia 🇮🇩

More about this subject:

To find out more about solar-powered sea slugs please visit our vlog post 52 or click the following link https://www.beyondscuba.com/post/melibe-sea-slug

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