Subject:
An Aeolid nudibranch is crawling on the sandy bottom of a Balinese reef in Indonesia. This Aeolid nudibranch belonging to the genus Trinchesia (it is very likely a Trinchesia yamasui) carries its defence mechanism on its back. The colourful cerata (derived from the Greek word “ceras” meaning “horn”.
They are dorsal and lateral horn-shaped anatomical structures that grown on the nudibranch body and aid in the nudibranch’s respiration) are packed with nematocysts. Nematocysts are stinging cells of cnidarians (stinging celled animals such as anemones, jellyfish, hydroids, etc. …) that this nudibranchs has eaten. These stinging cells or nematocysts are passed through to the digestive system of the nudibranch and stored in cnidosacs at the tip of the cerata. These horn-shaped cerata are now loaded with stinging cells collected over time by the nudibranch and are used as a powerful biological weapon to deter predators.
The with stinging cells loaded cerata of this nudibranch are also beautifully and brightly coloured. This is called aposematism. Aposematism is bright colouration used as a warning to potential predators that it is not worth attacking or eating. The most efficient colours to prevent an attack are red, orange and yellow in combination with white and/or black, and high-contrast patterns such as stripes and dots. In general the brighter and more noticeable the warning, the more toxic the animal usually is. The method of defence relies on the memory of the predator, having experienced the foul-tasting or stinging sensation before, the predator might not want to take its chances again. Marine critters with aposematic characteristics are able to wander more in the open and exposed areas of the reef as they are less or not targeted by predators, resulting in more foraging time to find more and better quality food.
In some nudibranchs like the ones of the genus Ceratophyllidia, cerata are used as decoy tactics. Normally, these in the case of Ceratophillidia nudibranchs, round shaped cerata, often called tubercules are not armed with nematocysts, but when attacked, the nudibranch will shed its them. This process is called autotomy. The rejected cerata sometimes produce a sticky secretion and wriggle or squirm for some time after being cast off, causing enough distraction to the predator and allowing the nudibranch to escape.
Technique:
To highlight the little critter, which in this videoclip is only 2,5cm/1inch in length, a vignette mask was added to the final clip. The little yellow tips of the animal’s cerata were saturated to emphasise the aposematic characteristics of the nudibranch. The crawling sound was also added to make the scene more convincing.
Filming location:
This short underwater videoclip has been filmed in Bali, Indonesia 🇮🇩
For a more in-depth description about the decoy and autotomy defence strategy of the Ceratophyllidia nudibranch genus please go to our vlog post 15 or click on this link: https://www.beyondscuba.com/post/decoy-defence
For a more in-depth description about aposematism please go to our vlog post 111 or click on this link: https://www.beyondscuba.com/post/inimicus-lll-aposematism
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