Pestzilla

Phestilla lugubris. Photo by Matthew-James Bennett.

We were leaning over the tank together. Matthew illuminated the coral nubbins with a red filter on his phone's flashlight. 

"Do you know what this is?" he asked. 

My first guess was a polynoid polychaete - a scale worm. If not that, probably a mollusk of some kind, maybe a nudibranch. Whatever it was, it was surprisingly large, and it was definitely eating our corals. 

"How do we get it off?" Matthew queried. 

I shrugged. "Scrape it off with a scalpel," I offered, digging in the back of my mind to remember if I had packed forceps. "Make sure you get underneath it, because otherwise, it might break." 

Matthew wanted to keep the animal alive, so he ended up using a zip tie as a very gentle tool to pry it away from the coral nubbin. Once we had it in a petri dish, we stuck it under the microscope. It had a muscular foot, so it had to be a mollusk. Its shape and number of dorsal tentacles suggested it was a nudibranch. After scanning the ID book, talking to another researcher, and looking around online, we settled on an identification: Phestilla lugubris. 

Phestilla lugubris eggs. Photo by Matthew-James Bennett.

By the end of the night, we had found three individuals on our corals, and we were pretty curious how they got into the tanks. PICRC's seawater system pulls unfiltered water from the ocean nearby (they can do that because the water is so clear in the oligotrophic tropics, the pipes never get clogged), which means that maybe the nudibranchs got sucked in from the coral reef just below the dock. It's possible that the nudibranchs had been hiding out for a few days - we did have some mysterious, unexplained coral deaths. 

Finding the nudibranchs is both good and bad. Obviously, we don't want them eating all the corals in our experiments, so we'll have to keep diligent watch in case there are more. On the other hand, Matthew had recently begun planning an experiment to evaluate susceptibility to predation in baby corals, so he can actually use the nudibranchs as his predator in that experiment! 

The closer we looked at the corals that had nudibranchs on them, we started seeing small white ribbons attached to the skeleton. I recognized immediately what they must be: nudibranch eggs. Our little Phestilla lugubris adults were laying eggs on the corals in our tanks! We're going to keep the eggs, see if we can get them to hatch, and rear the larvae. Since Matthew's experiment uses baby corals, newly-settled baby nudibranchs would actually be the best fit for his predation trials. Hopefully the arrival of a coral predator turns into an ideal scenario for his experiment!