Man O’War-eating nudibranch

The variety of organisms living on this planet constantly astounds me. Each one has its little role in the ecosystem and the simplicity or complexity of it is equally important. A group of these nudibranchs was found a couple weeks ago offshore just floating along the surface of the Gulf. 

What is a nudibranch you might ask? They are a shell-less gastropod (sea slug).  Gastropods are Molluscs, animals that live in shells – like oysters or snails. Most gastropods have passive defense mechanisms, which means they don’t actively fight off predators but use things like shells for safety. These guys don’t have shells, but utilize a different passive defense mechanism.

This nudibranch is Glaucus atlanticus. Some call them the ‘butterflies of the sea’ and rightly so – they are beautiful and delicate although this one is particularly deceiving!  It feeds on a highly venomous organism of which you are aware – the Portuguese man o’ war. Glaucus’ little legs are called cerata – dorsal structures that are used to draw attention away from more important parts of the body and are quickly replaced if damaged. These cerata have ‘cnidosacs’ that can hold nematocysts, essentially an ammo bag of stinging cells that get released on predators! They acquire the stinging cells from the man o’war they eat! One of those predators are humans, so if you ever find one, I wouldn’t recommend picking them up with your hands. 

As you can see from the video (below), they are brightly colored on the ‘top’ and pale underneath. Actually, the colored part is their ventral side (front/stomach) and the pale side is their back. They float upside down. Many aquatic animals have coloration this way – the pale side faces the water below so when animals look up towards the surface, they blend in with the sun-lit water. Darker colors on top for the same reasons – they blend in with the dark water below.

They interact with each other if they happen to touch, but they don’t actively swim. They float around in the currents, which makes sense that man o’war are their prey because they also float in the currents as well.

I hope you enjoyed this post! The Gulf of Mexico is not just Red Snapper and Amberjack, there is way more than meets the eye. 


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