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Nuts About Nudibranchs

Welcome to guest blogger, marine biologist Sam Craven, from Mad As A Marine Biologist.

As a marine biologist and a diver I feel incredibly privileged to have seen many of the delightful examples of life that the most biodiverse ecosystem on our planet, the coral reefs, have to offer, but nothing has kept by attention and enthusiasm as much as the group of shell-less molluscs, the nudibranchs.

Nudibranchs (prounounced Nude-dee-branks) are gastropods of the order Opisthobranchia that have evolved to throw out the fashion of shells after their juvenile stage. As Adults, their soft bodies are exposed and vulnerable. Unlike other snails, their breathing organs, their gills, stick out of the body, and without a shell, they are exposed too. This is reflected in the name ‘Nudibranch’, a combination of the Latin ‘nudus’ for ‘naked’ and the Greek ‘brankhia’ meaning gills.

© Hypselodoris infucata, Tioman 2
Without a shell, Nudibranchs have evolved other defence mechanisms. Some are poisonous, their bodies containing distasteful and toxic chemicals. Animals with this trait often have bright colour patterns, which advertise their toxic nature and warn away potential predators. This is called aposematic colouration. Others may eat the stinging cells of animals like Hydroids, and use those stinging cells in their gills or other body parts to sting predators.

Most nudibranchs are carnivorous, and prey on other animals using their rhinophores to touch and smell, and primitive eyes which act as light sensors. Some Nudibranchs even eat other individuals of their own species! And a select few species actually harness their energy from the sunlight, as they provide a home to special bacteria that photosynthesise  within their tissues.
When it comes to mating, Nudibranchs are double trouble. They are hermaphroditic, so when two individuals find each other, they mate, and both individuals will lay eggs in the form of egg ribbons!
Nudibranchs are one of the most fashionable groups of animals. Their bright colours and wide variety of body shapes make them a well-loved group by many scuba divers. There is a skill to spotting a Nudibranch, because despite their extraordinary body patterns, they are still living on the rainbow of colours that is the Coral Reef. On top of that, they can range in size from just 20 millimetres to 60 centimetres and come in an incredible array of shapes!
Of all the marine species I’ve learned about, Nudibranchs have helped me with my taxonomic skills the most. After seeing some on a dive, it’s straight to the identification books for me and after a few dives, the patterns between different orders, families and genus’ start to jump out at me! They are an incredible branch of life, and it isn’t hard to see why so many divers and marine biologists alike are nuts about them!