When I found out that I would have the opportunity to examine a handful of nudibranchs under a microscope, I nearly fainted in excitement. No … really.

After letting out an overwhelmingly loud 'Ermagerd-nudies!', I promptly fell out of my chair and remained on the floor for a much-needed moment of relaxation before grabbing the first specimen.

Nudibranchs posses a wide variety of strange abilities from tasting like lemons, to smelling like watermelon candy ... but this one, the opalescent nudibranch (Hermissenda crassicornis) won me over with a far less extraordinary, but equally spectacular behaviour: pumping its tiny, transparent heart.

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The complex closed 'track' of the vertebrate circulatory system Image: Eric Villalba/Flickr
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In an open circulatory system, the blood (or hemolymph) is pumped directly into the body cavity, where it bathes the tissues in oxygen and nutrients. Image: Earth Touch

At first, I didn't know what I was looking at. I found myself wondering why I could see the heart pumping, but could see no evidence of blood inside of it. As it turns out, in nudibranchs and (most) other mollusks, the makeup of blood, and the path that blood takes is very different from that in our human scenario. 

When parts of your body need oxygen, your blood is pumped through a vast system of arteries, veins and small capillaries to the tissues that need it, before returning to the heart and lungs via the same system. Your blood functions like a toy car on a closed track. The circulatory system of a nudibranch, however, is open (it lacks a track for blood to flow around).

Each atrium of the two-chambered heart feeds into one side of the gills, where the blood (called hemolymph) is oxygenated ... pretty normal so far. But here is where it gets bizarre: rather than pumping blood through veins, the oxygenated blood is pumped out into the body cavity (called the hemocoel).

"Basically, you have the organs and tissues being directly swathed in oxygen and nutrients," deep-sea biologist Dr. Craig McClain explains. 

From there, a set of uptake vessels (large open suction tubes) pump the blood back through the heart and into the gills to be re-oxygenated ... but why can't you see it?

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The transparent hemolymph is nearly invisible as it moves through the nudibranch heart Image: Sarah Keartes

Nudibranch blood is called hemolymph. Because it lacks hemoglobin, the protein that gives our blood its colour, it's hard to spot through the nudibranch's transparent skin. 

Hemolymph acts as more than just blood. It contains a cocktail of various nutrients (hormones) and fats, which allow it to take on many of the tasks carried out by the lymph in vertebrates (like carrying waste away from tissues and protecting the body from infection).

Mollusks aren't the only animals to use hemolymph – arthropods (like insects and crustaceans) are also filled with the stuff, which is why they don't leave a red splatter if you squish them (not that we encourage that type of behaviour).