Gorekia! A worm that lives INSIDE a Sea Urchin!

(From Fig. 1a of Schiaparelli et al., 2011)

We ALL love stuff that comes INSIDE other stuff! The Internet tells us that this is so!! For example, this website tells us that we LOVE cherry pie INSIDE a chocolate cupcake!!!
and here we have a Philly Cheese steak wrapped INSIDE a pizza!!
And so today I thought we could talk a little bit about WORMS that live INSIDE the GUTS of Sea Urchins!!

Data for this comes from a new paper published this year (2011) in Antarctic Science by Stefano Schiaparelli, Maria Alvaro and Ruth Barnich.

First, the players...

So.
They live in Antarctica.

There's this worm. Gorekia crassicirris (family Polynoidae)

(Here's a close up of its head... The worms are small-about 2 to 4 mm long and 6 to 13 mm wide.)

(Figure 2a of Schiaprelli et al., 2011)

and as it turns out, it lives in the INTESTINE of this sea urchin, Abatus nimrodi
The authors collected the urchins from bottoms of moderate depth (90-148 meters).

Upon observing the animals alive, they began noticing this worm crawling in and out of this sea urchin's mouth!

Previously, Gorekia was reported as free-living. That is, they just live out in the open on their own. (They like sandy bottoms)

But Gorekia living in this urchin? Sometimes behavior among species varies? Or perhaps there are advantages to living for worms that have access to these sea urchin species??
Here's a diagram!

(courtesy of Echinoblog Art Department)

This presents us with a good example of a commensal relationship. That is, one in which there is essentially a benign relationship between the host and the "guest". In other words, the host isn't harmed or negatively affected.
In fact, this is considered a SPECIAL case of commensalism called inquilinism in which one organism lives WITHIN the other, usually in some part of the alimentary tract or respiratory chamber, without being parasitic or causing the host any arm.

The case of Gorekia and the two urchins shown here? Only the SECOND ever recorded and the FIRST from the Antarctic!

The authors recorded the sequence showing the behavior of the worm on its host! Read from the top left to right (a to b to c..etc.)

These show the worm basically hanging out around the mouth of the urchin, and ultimately just retreating back into the gut. The bottom part of the figure shows where they cut open the urchin to look...and to verify that indeed the worm lives INSIDE the intestine.

(Fig. 3 from Schiaparelli et al.)

Out of 14 specimens of sea urchins examined, eight of them had a worm in its gut!

Interestingly, the authors checked OTHER Antarctic sea urchins to check for infestations.

And lo and behold..it turns out ANOTHER species harbors the worms in its gut! May I present, Brachysternaster chesheri...
And what's interesting? Brachysternaster "picks up" in the range where the urchin Abatus nimrodi (above) "leaves off"!!

So, the worms find the hosts important enough that they switch out different species along their range!

Okay. So. It lives in sea urchins. So What? Why do it? Why one of these urchins?

1. Protection. So, if ya' look at a close up of one of these sea urchins, you see that the surface is covered with spines and other bumpy bosses. The protective structures not only protect the animal but apparently also "guests" living inside...
2. Food?
The authors didn't extract any gut contents from the worms, but believe it or not, OTHER species of worms have this SAME kind of relationship with other sea urchins.
And in those instances, the worms can presumably just take any of the food from the intestine that they want! (hence the moochie guest taking your food out of a well-stocked refrigerator analogy)

3. Why choose an "irregular" versus "regular" urchin to inhabit?
Location, location, location.

So, if you're a "regular" urchin...that is one of the sea urchins that you typically see with long spines and such.. there are two major anatomical differences.

1. The Aristotle's Lantern or Jaw mechanism sits over the mouth opening (and blocks access to the intestine).

2. the mouth opens on the bottom and the anus sits on the 'top'

(from the U. Michigan Animal Diversity web)

The "irregular" urchins such as the ones above? These are related to sand dollars and have special adaptations to digging through sediments!!

1. They LACK an Aristotle's Lantern. Thus, there is easy, convenient access to the intestine!

2. Instead of the intestine tracking from the mouth to anus going vertically (i.e., UP), the mouth sits at the front and the anus sits at the rear. Thus the intestine is laid out horizontally (i.e., ACROSS). So basically...you have a flat house without having to expend the energy to walk "upstairs."

Which kind of urchin will the worm like?

The "irregular" urchin! Its perfect polychaete worm housing!
Another reason why things inside other things is NEATO!