These vibrant 'star butt' urchins look like underwater bonfires

Animal common names can be confusing (just ask the bony-eared assfish), but after taking a gander at this colourful clip, it's easy to see how "star butt" urchins got their colloquial calling cards.

This seabed meet-and-greet was captured recently in Indonesia's Lembeh Strait by tourist Melissa Yeo. While the first creature you see belongs to the family Asthenosoma, the rest of the urchins in Yeo's video appear to be members of the genus Astropyga.

The genus name roughly translates to "star buttocks" – and the reason for the moniker becomes luminously clear about six seconds into the video.

That bulbous structure at the centre of the urchin's five-pointed body is an extension of the intestine called an anal sac. As for the small orbs you see cascading in line like your takings during a game of "Hungry Hungry Hippos" – that would be urchin poop.

Once more, for the people in the back:

Hence, star butts.

Aside from pentagonal posteriors, these urchins are also known for their vibrant colouration. Their flamelike hues have earned them another nickname in some parts of the world: "fire urchins". However, Echinoblog author Dr Christopher Mah explains that another group of animals – "proper" fire urchins – have claim on that name as well.

You'll notice that several species of fish appear to be taking refuge among the urchins' spines. Atsropyga spines are venomous, and the fish are likely exploiting their powers of protection (and the promise of an excrement entrée could be an added bonus).

"It's not unusual for other animals to take shelter among urchin spines," explains Mah. "But there's not really any advantage to the urchin that I'm aware of."

A crab tucked in between the spines of a fire urchin. Image: Klaus Stiefel/Flickr

In fact, a few species take the protective tactic one step further. Some speculate that a ride to new feeding grounds could be beneficial for unwitting urchin "shields" like the one in the video below, but Astropyga are relatively fast crawlers (at least as far as urchins go), so it's not clear whether being hauled around by a crab comes with any perks at all: