Rainbow Nudibranch

Image: Ed Bierman
Dendronotus iris

Is this what passes for a rainbow...

In HELL?!

Image: Ed Bierman

This incredible creature is the Rainbow Nudibranch, a large sea slug from the west coast of North America.

They can be found from Alaska all the way down to California and the mightiest giants of their kind can reach as much as 30 cm (a foot) in length. Most of them don't reach such extravagant size, though, and are more like 10 cm (4 in) long.

Image: Ed Bierman

They range in colour from demonic Hellfire...

Image: Patrick Webster

To angelic flames of righteousness, as well as all sorts of pinks, reds, greys and oranges in between.

Image: Ed Bierman

Their spectacular look is achieved by the rows of branching cerata on their back. They look like the burning roots of some inflamed, undead vegetable, granted life and seeking out the flesh of vegetarians so it might exact revenge and harvest their antioxidants.

What looks like smoke is yet more, finely branching tendrils for absorbing as much oxygen as possible. An important business when you're as large and active as the Rainbow Nudibranch.

Image: Ed Bierman

Two bundles of tendrils on their head stand out from the rest. In their midst are the two rhinophores which nudibranches use like antennae to work out what's going on around them.

The Rainbow Nudibranch also has a horde of sensitive tentacles spewing forth from just above their mouth. These help them to zero in on food, which is good because this is one sea slug who is extremely particular about what it eats.

Video: Andy Vanderheiden

Rainbow Nudibranches are not only covered in tentacles, they eat them too! They feed on a specific species of tube anemone called Pachycerianthus fimbriatus.

Tube anemones are related to sea anemones and, like them, use their tentacles to catch passing fish or crustaceans to eat. Unlike most sea anemones, however, tube anemones construct a slimy tube to escape into when predators come along. Predators like the Rainbow Nudibranch!

Video: mbdc831
I'm OK. I'm totally OK.

The Rainbow Nudibranch slowly approaches its prey. Rears up, opens its jaws and pounces. It isn't the sort of behaviour you expect from a slug! The tube anemone reacts by diving into its tube, it can do no other. The Rainbow Nudibranch stubbornly clings on. It won't release its grip until it has nipped off some of that tentacle. Sometimes they'll even be dragged halfway into the tube anemone's tube!

The nudibranch is unharmed by this impromptu visit to the tube anemone's slimy abode and soon wanders away once it has something tasty to munch on. It's not a complete disaster for the tube anemone, either, for the clipped tentacles will grow back soon enough.

Image: Peter Liu Photography

But the Rainbow Nudibranch has predators of its own to worry about, particular the likes of the unusually swift of foot/tube feet Sunflower Starfish. What a nightmarish world when a sunflower threatens to eat a rainbow who just ate an anemone. Why can't flowers and meteorological phenomena just get along?

For the Rainbow, it's time to scoot!

Video: Patrick Anders Webster

They fold their foot together along the middle and thrash their body from side to side. Before you know it, they rise from the seabed and swim away from the grasping arms of the starfish. It's like a feather boa escaping a particularly tedious cocktail party.

I don't know what I expected a demonic uprising to look like, but it certainly wasn't this!