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You said Nudibranchs?

Why are so many divers mad about these minuscule creatures? Why are they so often models for photographers with a passion for macro pictures? Certainly, because amongst the small underwater life, they are some of the most attractive, coloured and beautiful animals.

Nudibranchs are molluscs.  There are more than 3000 species worldwide of Opisthobranchs (subclass including the nudibranchs) living in salt water only, the Indo-Pacific having the greatest distribution. In the Indian Ocean, the areas of Philippines, Malaysia and Indonesia are famous places where you may find beautiful specimens.

NUDIBRANCH BODIES

Literally “naked gills”, nudibranchs are evolved gastropods without a shell. The absence of the shell is compensated by their ability to secrete chemical acids from specialized cells on their mantle. Others extract sponge toxins or coral defensive cells from their food and store them. Some nudibranchs have the ability to neutralise the effects of these cells by digestion and to store them in colourful sacs on the upper surface far away from vital areas of the body. Upon contact with these sacs, predators (fishes and crustaceans) escape or die.

As a snail, they move using a muscular foot. The locomotion is helped by the form of the shell, muscular contractions and movement of cilia on the bottom of the foot. They are also good swimmers too.

The bright colourful mantle might be a way to warn predators of their toxicity or might be used as camouflage on some substrate where they feed. Then, being not very edible, their only predators are their carnivorous congeners.
      
On their mantle are various appendices for breathing. The more archaic species have a hole in the cavity of the mantle. The evolved nudibranchs have external gills on the posterior part of the body of which some nudibranchs are able to retract into a branchial pocket in case of danger.

Others do not possess gills but breathe through pustules on the back of their body. The head is well defined, wearing a pair of sensitive antennae. Eyes can be reduced or non-existent replaced by light captors. All sea slugs have a rasp-like tongue. Depending on species, they are herbivorous or carnivorous (sponges, cnidarians).
Their lifespan depends of the food they eat. Nudibranchs that feed on sponges, anemones and gorgons live around one year whereas those feeding hydroids and bryozoans live only a few weeks. The size varies from a few millimetres to 30 centimetres.

They dwell in all habitats except those where their soft bodies could be hurt. They can be found from shallow water to 180 metres but they tend to avoid estuaries and inertial pools.
NUDIBRANCH BABIES

Survival depends on food, camouflage and warning but also on reproduction.

All nudibranchs are hermaphrodite; meaning that there is a simultaneous presence of male and female sexual organs in each individual. These organs are situated inside the body on the right side and the gonophore, where the exchange of sperm occurs, is at the right side of the head.

Two nudibranchs of the same species meet and take a head to tail position that puts their penis-like organs in contact. First one introduces it into the partner and fertilises the internal ovules, and in turn the other partner does the same.

Then, they separate and crawl away to lay the spawn on the suitable substrate. Shape, colours and arrangement of the eggs are specific to the species.

The eggs hatch giving birth to planktonic larvae wearing a shell. After a variable duration of time, they settle on the bottom to metamorphose into juveniles who will lose the shell and become adult sea slugs.

LOVE STORY PARTY

I had the chance to witness a sexual party like an orgy. It occurred in July in the Maldives. The waves on the shore did not help for taking photos.

In a lagoon, in very shallow water, Gymnodoris ceylonica (GC) were meeting their congeners for the big day in a grass area where other nudibranchs, Stylocheilus longicauda (SL), were doing the same. Was the presence of these two species in the same place an accident or not?

Hundreds of them were working on their future. My attention was focused on observing the GC behaviour of reproduction. They seemed insatiable, passing non stop from one partner to another, going to lay a yellow spawn on the grass. On the way, they were crossing SL specimens. At this moment, a crazy voracity began.
They started without restraint to eat the small SL, squashing the poor nudibranchs who finished in their stomach. Some blue volutes emanating from the purple pigment of SL were visible during the action; this was proof of their deaths.

Amazing! Hundreds of Stylocheilus longicauda disappeared to give more power to the Gymnodoris ceylonica for reproduction.

Did Gymnodoris ceylonica choose this place on purpose? Do they do this all the time? Why did Stylocheilus longicauda not escape? How many deaths for a few new births?

There are so many questions without answers…
 

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