The unknowns: part 2

Believe it or not, I love when I get to correct myself on this blog. Being wrong is actually an important step in the scientific process. As I work through a project and gather more information, I constantly re-evaluate my assumptions. Sometimes, they turn out to be wrong.

Recently, I used DNA from my Arctic larval samples to try and identify some of the specimens to species. I shared in a blog post that I had a number of embryos in my samples, along with some larval and juvenile nemerteans (ribbon worms). I hypothesized they might be the same species. 

Well, when the DNA sequences came back, I made an interesting discovery. A sequence from one of the embryos was a reasonable match to Saccoglossus mereschkowskii, which is a hemichordate (acorn worm). I was very surprised! The common names sound similar (ribbon worm, acorn worm, what's the difference?), but nemerteans and hemichordates are completely different things. Just do a quick image search, and you'll see exactly what I mean. The two types of "worms" actually belong to different phyla - taxonomically speaking, they are as different from one another as sea stars are from fish. 

So the embryo is a hemichordate, cool. But here's the part where I really discovered I was wrong: I started looking back through my photos of what I thought were juvenile nemerteans, and I realized some of them had an indentation about a third of the way down their body. Nemerteans don't have indentations like that, so I realized maybe the indent was the beginning of the hemichordate's collar - the structure between its proboscis and its body (look here to see what I mean). I guess my "nemerteans" were hemichordates all along.

Characteristics of hemichordates (left) and nemerteans (right)
that can help me sort the specimens.

But then I remembered that some of the "nemertean" specimens actually had an obvious proboscis. Ribbon worms are characterized by what's called an eversible proboscis - basically, they keep their proboscis in a special cavity in their body and can shoot it out to attack prey. In the case of my specimens, some of them may have shot out their proboscises because of the salinity change when I preserved them in ethanol. I could even see a small barb called a stylet on the end of some of the proboscises, and stylets again indicate the specimens are nemerteans. 

So what I thought was one species is actually two. I have a ton of embryos and a handful of yellow-orange blob-like juveniles that are either nemerteans or hemichordates. It's going to be tricky to sort out who is who, but I can use characteristics like the collars, proboscises, and stylets to make my best guess. 

More importantly, my discovery means there are even more species reproducing in the polar night than I thought. So far, I'm up to 17 species from 9 different phyla, which sounds crazy! The middle of the Arctic winter is dark, cold, and low in food, but it seems more and more like it's actually a very active time of year for marine ecosystems. Very cool!