Ribbon eels: when your junk is in the trunk

The Central Florida Aquarium Society recently posted this mesmerizing footage of a ribbon eel undulating across the seafloor.  While the eel looks like something out of a Gymnastics ribbon routine, for this fish, it is simply swimming. It just looks fancy. It’s the striking colors and unusually elongate and narrow body that creates the effect. And as noted, it’s part of why the ribbon eel is considered unique. But there’s more to this fish’s novelty than meets the eye.

For one thing, the ribbon eel is a sex-changer. The gorgeous blue and yellow fish in the post is a male, likely a mature one. If it lives long enough, it will morph into a female, blushing golden all over. Soon after, it will spawn and then die. But that’s not actually all that unique. Lots of marine species transition from male to female, including most of the shrimp in your shrimp cocktail. What is unique is that ribbon eels are one of the only vertebrates that has its gonads located BEHIND its anus…

Yes. That’s right. It stores the family jewels in its tail, which is an aberrant (if not totally unique) arrangement for any vertebrate.  Nature’s always got a first.

According to Fishelson 1990, there is an interesting consequence of this anatomical situation:

“This means that in Rhinumuraena spp. during reproduction, the sex cells within the gonad will move anteriorally towards the respective openings, and not posteriorally as in other fish.”

Translation: unlike every other known vertebrate, with sperm and eggs flowing down and out, the ribbon eel’s gametes swim upstream, towards it’s head. Which makes one (ok, me) wonder for a moment if that’s why they are so unusually long: otherwise they’d be spawning themselves in the face all the time.  Then I remember they are fish, swimming in ocean, which has a rather strong diffusive effect. It would take quite a bit of explosive force to shoot gametes with any directionality. Still, for the ribbon eel, a little extra length certainly doesn’t hurt, given their unique organ configuration.

Reference: Fishelson,L. “Rhinomuraena spp. (Pisces: Muraenidae): the first vertebrate genus with post-anally situated urogemtal organs.” Marine Biology 105: 253-257.

More weird and wild sex lives of marine life to come as Sex in the Sea continues its journey towards publication February 9, 2016.