From that very moment, both scientists and divers were able to admire and discover the wonders of the sea, unachievable for them until then. Among the fascinating creatures that live under the sea, nudibranchs are the most stunning of all of them due to their incredible colours, their beauty and their exotic forms. Also are known as sea slugs and butterflies of the sea due to their undulating way of swimming. They are considered the most beautiful creatures of the sea and as they move very slowly, they are very popular among submarine photographers.
(unranked): clade Heterobranchia
Nudibranchs are little marine animals between 3 mm to 15 cm long, though some species can reach 30 cm. They can be very different in shape, but generally are elongated and symmetrical. They are benthonic species, living in nearly every marine bottom and can be found all over the world, from Antarctic to tropics. Their small eyes are located below the tissue of the animal on the head near the brain, and they cannot see actual images but only take measures of light intensity.
In most molluscs the shell is the primary defence in which the animal can retreat from danger. But shells are heavy and make difficult to move and slow down growth. So the loss of the shell gave them freedom to evolve spectacularly shaped bodies, they could growth more rapidly and move faster. But sea slugs had to evolve a series of strategies to protect themselves from predation. Those amazing, effective and sophisticated defence systems not only reduce death in direct predators’ attacks but also have dissuasive effect, which means that enemies that had attack a nudibranch before will not do it again because they remember that bad experience. These defence systems, real chemical and biological weapons, could have been either a post-adaptation to shell loss, either a pre-adaptation that made the shell loss possible.
One of the easiest defence strategies is escape. Some nudibranchs have evolved muscular foot that can secrete mucus to be able to escape, sometimes even swimming, though never very quickly. But they also present some other defences like tubercles that vary in size, shape and number and calcareous spicules in the mantle tissue that can heart the enemy. There are a number of species that have evolved the ability to drop parts of their body, which can be rapidly regenerated, to confuse predators and escape. This behaviour is called autotomy.Other species have developed colour patterns entirely for defence against predators, mainly fishes. The first line of defence is to try to become invisible. In some cases both the texture and the colour of the body of the nudibranch match the colour and the texture of the prey or the substrate they live on. It is called cryptic coloration. Nudibranchs can obtain the pigments from their diets, though sometimes they can produce pigments themselves. Their translucent body permit the digestive system to be seen and also nutrients that travel through it. In many cases, slugs can vary their colour as they change the food source.
Some other nudibranchs have a bright and spectacular colour patterns. These colours are used to warm potential predators that these animals are full of distasteful chemicals and not worth to attempting to eat. This is known as aposematic colouration. Even have the ability to change their colours when danger. Normally these slugs contain distasteful or even toxics compounds and one a fish has attacked one nudibranch it is not likely to forget this odd experience, which will correlate with this colour pattern, and will not attack it again. Nudibranchs can obtain the pigments from their diets, sponge or cnidarians, or even they can produce de novo themselves. Sometimes different species, both toxics, have evolved similar colour patterns to “help” predators to remember their danger. This is known as Müllerian mimicry. But in other cases, a non-toxic specie evolved the toxic specie’s colour pattern to confuse and to avoid been attacked by predator. This is known as Batesian mimicry.
But usually nudibranchs have a chemistry or biological active defence. Sometimes slugs possess skin glands that secret toxic products, like sulphuric acid. In some cases these compounds are known to be toxics and can kill animals near them, but in other cases they are not so toxics but still enough repellents to confuse predators. But the sophisticatest defence system is known cleptodefence. Slugs are evolved to be immune against other invertebrate defences, swallow them and transport them along tracts to the cerata, where they are used for defence, being replaced every few days. Some species are able to use really toxic compounds from sponges and accumulate them in some glands ready to be used if is in danger. Some others, like aeolids, fed on cnidarians (hydrozoans, corals, anemones, jellyfish, etc.) that content cnidocytes, sting and toxic cellules with nematocysts used to defence themselves from predators. To avoid them, some aeolids are able to secrete mucus, which change with cnidarian species, and that inhibits nematocyst discharge. Other species acquire the mucus from the cnidarian and trick their prey. Once the aeolids eat them, cnidocystes are transport and stored in cnidosacs in the tip of their dorsal cerata ready to be used.Defence compounds of nudibranchs present biological activity, and are being investigated to be used against some really dangers disease like cancer.
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