Imagine trying to use a net to haul lumps of jelly unharmed through water for hundreds, or even thousands, of meters. Well, it’s slow going, and in the end your globs are likely to be in less than pristine condition no matter how careful you were. This explains to a large extent why gelatinous organisms have traditionally been somewhat neglected in favor of studying things that are easier to get hold of in good condition, such a fish and crustacean plankton.
This preference given to hardier organisms means that lots of interesting questions about jellies remain unanswered. What kinds are there and where? How do they behave? What do they eat and how much energy do they consume? What’s their role in the ecosystem? On the RV G.O. Sars –cruise last summer, we used traditional nets as well as modern remote operated vehicles (ROVs) in hope to increase our knowledge a bit.
Gelatinous zooplankton. Photo: Tone Falkenhaug
Numerous net samples of plankton were taken during the G.O. Sars -cruise. The jellies from many of these samples were fixed in formalin, and are waiting for less hectic times when I’ll have the chance to sit down by the dissecting microscope and identify them. Because of their gelatinous structure the jellies don’t stand fixation quite as well as for example crustaceans do, and lose some of their beauty and –unfortunately– features. Identifying the animals already mauled by the net can therefore sometimes be quite a challenge – but it’s one that I am looking forward to. Hopefully the samples will offer interesting insight into the gelatinous fauna of the MAR.
The real treat, however, was to get to see the jellies in their natural environment during the cruise. With the help of the two ROVs Aglantha and Bathysaurus we had a rare opportunity to observe these beautiful animals live. Some of them would just hang motionlessly in the water, spreading their tentacles and waiting for prey to swim by. Others were more active: jellyfish would pulsate by and siphonophores glide soundlessly through the darkness.
A ctenophore, member of a common group of gelatinous zooplankton. Photo: D. Shale
Besides being great fun, using the ROVs has scientific benefits compared to the more traditional nets. The vertical transects from the surface to the bottom allowed us to record exact depth distributions for the animals - something that is nearly impossible to do with the nets that haul up everything in their path into one big bucket of soup. We were also able to observe the natural behavior of the animals way below the scuba diving depths where this is usually feasible.
The ROVs also enabled us to capture jellies one by one and bring them to the surface unharmed for further studies normally impossible with the moribund animals caught with a net. Granted, this proved to be a bit tricky - catching the animals with remote controls and live video feed on a small screen was easier said than done. Maybe more practice with a Playstation might have helped… However, the pilots did manage to bring up a number of small red medusae of the species Aeginura grimaldii from several hundred meters for respiration measurements. These fellows seemed to be unharmed by their long ascent and were happily swimming around in the oxygen measurement chambers, oblivious to us continuously monitoring their metabolic rate.
Exciting as the cruise was with all the live animals and ROV dives, some of the novelty has worn off by the time you watch the same 3-hour dive tape for the nth time, in slow motion, trying to decide whether a particular faraway lump is a siphonophore or a doliolid. Sitting in the lab glued to the microscope, counting hundreds of tiny medusae, can also at times get a bit dull – at least until you find something you haven’t seen before. But hopefully nice results will in the end pay off for all the hard work.