Filmed on a shallow sandy bottom on Bali's northern coast (Indonesia), this Green Melibe sea slug (Melibe viridis, scientifically also known as Melibe fimbriata) is on the hunt for tiny crustaceans, which are its main food source. It does so by scraping the substrate with its expandable oral hood, much like a fisherman who throws a round cast net to catch little fish, or like the Venus Flytrap, a carnivorous plant that slams together its two sides to catch a fly.
This disk-like veil or oral hood is equipped with rows of sensory papillae around its inner edge. When these papillae detect prey, the oral hood rapidly closes, trapping the prey, usually tiny crustaceans, which are then ingested. The Green Melibe crawls over the sandy bottom while constantly scraping the substrate for food. Other nudibranch species are equipped with a radula, a raspy tongue that is used to scrape and cut food before swallowing.
The Green Melibe lacks this radula and is obliged to swallow its prey whole. This Green Melibe has an opportunistic feeding style and engulfs all life material that is caught in its veil. Although their primary food source of the Green Melibe is little shrimp-like crustaceans, they often unwillingly consume fish larvae.
The Green Melibe nudibranch is also capable of swimming. It does so by vigorously bending its body from side to side, which propels it forward in the water column. Furthermore, Melibe nudibranch are capable of autotomy, the behaviour whereby an animal discards a body part in an attempt to escape a predator. Gill structures located on the cerata are often shed when attacked by a predator, but these shed cerata will completely regenerate over time.
Some Melibe species live in a symbiotic relationship with single-celled algae known as zooxanthellae, which are stored in bundles on the surface of the slug's cerata. When food is scarce, it relies on the algae cells, which it digests in small portions. However, the Green Melibe, which is the nudibranch featured in this underwater video, is a species that does not store zooxanthellae in its cerata.
Although the Green Melibe nudibranch is an Indo-Pacific species with its original habitat ranging from South Africa over the Indonesian and Philippine archipelago to Japan, it is now a common and regularly seen species in Turkey, Greece and southern Italy, where it as probably been introduced there accidentally. How this happened still remains a mystery, as it is unlikely that the introduction to these waters originated through the Suez Canal. Because the Green Melibe does not occur in the waters of the Red Sea, the introduction into Mediterranean waters must have had another origin.
Filming a pale semi-transparent animal on a pale background requires a boost in colours and some extra contrast in post-production. The green of the underwater vegetation was also enhanced, and sounds were added during the editing of this underwater videographer to increase the veracity of the scene.
The Green Melibe nudibranch's unique feeding behaviour and characteristics make it a fascinating subject for underwater videography. As an underwater videographer, capturing the movement and feeding habits of nudibranchs like the Green Melibe can provide valuable insights into the behaviour of these fascinating creatures.
For a more in-depth description about autotomy please go to our vlog post 15 or click on this link: https://www.beyondscuba.com/post/decoy-defence
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