Nudibranchs at Similan Islands | Thailand Nudibranchs

What are Nudibranchs?

Almost all dives are familiar with the beautiful and intriguing nudibranchs. These magnificent little creatures are so widespread around the globe that it is almost impossible to choose a dive destination that does not have some resident species of nudibranchs. Most divers are familiar and have encountered these magnificent multi coloured creatures, and while most people are happy to discount them as just another wonderful marvel of the underwater world, they are in fact some of the most interesting and bizarre creatures around. The word nudibranch itself belies these wonderful little creatures, since it means “naked gill” – nudibranchs, unlike most other marine species, have their gills on the outside of their body.
These little creatures are some of the most colourful and diverse creatures on the planet, and make for some stunning photography subjects. Nudibranchs have evolved from snails and over the millennia have actually lost their shells and adopted other mechanisms for defence.
Nudibranchs live in almost every sea around the world; although they are most prevalent in warm tropical seas, they are known to exist everywhere including the Arctic and Antarctic regions of the planet.

Two types of Nudibranchs

Nudibranchs are split into two groups; Dorids and Aeolids, depending on the method and configuration of the gills they use for breathing.
Dorids are the traditional nudibranchs with a single plume of gills that protrude in a cluster near the rear of the animal. Another differentiation is that the frilly edges of Dorids do not contain any of the animal’s intestines.
Aeolids, on the other hand, are the spikiest of the nudibranch species’. These creatures have a multitude of spikes that cover their back and are used by the animal for breathing.

Defensive adaptations

In keeping with its overall amazing reputation, the humble nudibranch has developed a vast array of defensive adaptations. Some use mimicry as a form of defence by closely resembling other toxic creatures. On the other hand, most nudibranchs who feed on hydroids or sponges do not actually digest the stinging cells of their victims. Instead, the majority of these nudibranchs release the stinging cells through their skin and thus are able to create a stinging layer of cells that cover their backs, making them rather unpalatable to predators. Another well-developed defensive mechanism amongst nudibranchs is the use of acid. When some nudibranchs are irritated or touched by other creatures, they release an acid laden mucus from their skin that irritates the aggressor.

Vicious predators

When looking at nudibranchs, you would be forgiven for thinking these slow lumbering and colourfully conspicuous creatures are harmless herbivores, since they don’t seem fast enough to catch anything nor sufficiently camouflaged to ambush any other animal. Well this would be a huge mistake, since all known nudibranchs are carnivores, and actually have a reputation as voracious predators compared to their peers.
As it turns out, the tiny little nudibranch is a specialist at feeding on sponges, hydroids, bryozoans and tunicates, some even feed on barnacles and anemones. Some species will even prey on sea slugs and their eggs. At the more extreme end of the spectrum, some of these little predators are actually cannibals and will readily devour members of their own species.
One nudibranch in particular is a specialist on feeding on the highly toxic Portuguese man o’ war hydroid – the man—war dragon. These tiny little critters swallow air to keep floating near the surface and then attach themselves to the unfortunate floating hydroid and starts munching away.

Sex life

When it comes to sex, nudibranchs would make most of us blush! They are all hermaphrodites, and each nudibranch has both male and female sex organs that are fully functioning. Although they are incapable of self-fertilization, when they do mate they do note play both roles simultaneously, but instead will either choose to play the role of male or female with each mate. The process of how the animal decides is not very well understood.

Disposable penises

Recent research in Japan has discovered a truly bizarre feature of some nudibranchs; when studying pairs of a particular nudibranch they discovered that after mating, the nudibranch that used its penis to transfer sperm to the other would refrain from doing so again for 24 hours. Upon closer examination of these nudibranchs, the scientists made a stunning discovery. They found that post intercourse, the nudibranch that used its penis would wander off with its penis trailing behind it, and after about 20 minutes the creature would discard its used penis.

Upon further research, the scientists discovered that the hapless nudibranch that discarded its penis was not so hapless after all, since inside its body it actually had up to 2 spare penises ready to go. The scientist discovered that inside the nudibranch’s body there is a tube with a curled up penis, and within 24 hours the nudibranch was able to unfurl its spare penis and was prepared to mate once again.

Scientists have speculated that this behaviour has evolved in nudibranchs as a competitive mating strategy. Since nudibranch penises have tine spikes and hair at the tip, there is speculation that nudibranchs perform this bizarre practice to be able to eliminate a rival creature’s sperm from the vagina.

Do all nudibranchs have disposable penises?

This is not known yet, the Japanese study was conducted on Chromodoris reticulate, although scientists assume this practice to be fairly widespread amongst nudibranchs, since it is highly unlikely that as a mere fluke the scientists would have selected a species at random that is the only species of nudibranch that disposes of its penis.

What depths are nudibranchs found at?

The ubiquitous nudibranchs probably have one of the most varied depth ranges of any marine creatures. Although most inhabit the intertidal range and spend their time in the top 30 meters of the water column, nudibranchs regularly thrive at depths up to 700 meters. In fact, a new species of nudibranch was recently found to be happily getting on with its life at a bone crushing depth of 2500 metres.

Do all nudibranchs live on the sea bottom crawling over corals?

No, these little creatures are masters of adaption; while most will live on the sea floor and reefs, there are actually species that are pelagic and live near the surface. These species tend to be rarely encountered by divers since they spend most of their time very near the surface in the open ocean an area rarely visited by divers.

What sizes do nudibranchs reach?

Nudibranchs are some of the most varied creatures in the world by size percentage, with lengths varying from 4 mm to 600 mm. This is an incredible size range, it is the equivalent to sharks varying in length from 1 metre to 150 metres. The largest of nudibranchs is the magnificent Spanish dancer, while some of the smallest include Costasiella sp., commonly known as ‘Shaun the sheep’, which reaches a minuscule 5 mm when fully grown.