Today we pulled purple grape juice up from the ocean – gallons and gallons of it.
Upon closer inspection, it was actually seawater full of thousands of blue-and-red doliolids. They had apparently been growing like crazy in the surface waters surrounding our boat. Not many other creatures were in our nets, which keeps us wondering where did they come from, and how long will they last? Only more sampling over the next few days will tell us the development and fate of this mysterious mass of creatures.
Perhaps you’re wondering what a doliolod is. Picture this: a small, clear, gelatinous barrel shape, slightly larger than a pencil eraser, with eight to nine purple-blue muscle bands circling its girth like hoops. The gut and gonads form a pair of red dots inside, producing the blend of colors that give our net tows their intriguing purple goo. Because of their watery gelatinous composition, doliolids are often mistaken for jellyfish. Surprisingly, however, doliolids are not very closely related to jellyfish, and are actually some of humanity’s closer relatives. Within the kingdom Animalia, humans are in the phylum Chordata, meaning that we have certain developmental characteristics such as a notochord and gill slits as embryos. It turns out that doliolids have several of these same characteristics, making them more similar to us than to things like sea stars and jellyfish.
Doliolids are rather mysterious ocean creatures. They are usually present in very low abundances within the zooplankton community, but every couple of years, without warning, they bloom and completely dominate surface waters. When we were sampling in offshore waters for Cycle 2 this past weekend, we saw a fair number of larval doliolids and pyrosomes, and wondered if a bloom would materialize. As we moved closer inshore and began sampling in a higher-productivity nearshore upwelling zone for Cycle 3, our nets became absolutely clogged with doliolids. Every net we’ve brought up yesterday and today, day and at night, has been dripping with doliolids. Our post-processing time has also increased dramatically. Usually, the three filtering processes that we put a Bongo net sample through take an hour to complete, combined. This morning’s Bongo net took four hours to complete, with everyone in our lab (four graduate students plus two volunteers) filtering nonstop (in addition to deploying our daytime MOCNESS net, running through a weekly ship fire drill, and taking 10-minute shifts for lunch). Other experiments happening onboard have also been swamped with doliolids. One of my labmates, Cat Nickels, looks at egg production in female copepods (a type of small crustacean generally present in high numbers). Her copepod sorting over the past two days has slowed down significantly, as she shuffles through hundreds of doliolids to find a few stray copepods that she can use for her experiments. Another member of our cruise, Mike Stukel, deploys sediment traps at several depths in the water column at the beginning of each of our three-day sampling cycles. These traps collect particulate matter and dead plankton that rain down through the water column. When we haul the sediment traps up at the end of the cycle tomorrow night, they may be completely full of dead doliolids.
Normally each bongo sample takes 5 dishes. This one took 28!
We had to split the top MOCNESS sample in half to get it to a manageable size to preserve.
Despite the nuisance of doliolids to our sampling efforts, they are an incredible study opportunity. Little is known about what causes doliolids and salps to rapidly reproduce into water column-dominating ‘blooms’, and they can be very hard to find and study because of our inability to predict when and where they will occur. The current high numbers of doliolids that we are finding may give us clues as to what physical ocean conditions favor these blooms. If we can better understand what constitutes good conditions for these organisms, we may also be able to use future blooms in turn as indicators of variations in ocean conditions.
What will tomorrow’s sampling bring? That’s the question that keeps us going as we throw nets into the ocean several times a day (including middle-of-the-night MOCNESS deployments). Processing takes a lot of extra time and effort, but sailing into the middle of an event like this reminds us that we have to take advantage of this rare glimpse into the secrets of the sea.
This post written by Laura Lilly, our filterer extraordinaire.