Onboard RV G.O. Sars one day, the CTD was hauled with a 2,1 m long Pyrosoma colony wrapped around its top! "I have read about this drifter, but in 30 years of going to sea I never saw one intact and up close", writes Marsh Youngbluth from Harbour Branch Oceanographic Institution, USA.
Pyrosoma. A giant colonial salp incidentally caught on the CTD-rosette. Photo: D. Shale
Early one morning I was awakened by my French colleague Lars Stemman who burst into the stateroom we share. I did not understand what he said at first, because I had been asleep for just a couple hours and my groggy brain wasn’t filtering his urgent, accented English. “Marrshhh, wake uup, yoo must cuum and zee zometheeng marveloous that zeh CTD cawtt. Eiit’s soo beeu-tee-full.” And it was.
Initially his message didn’t make much sense. The CTD is the acronym for a framework that holds an instrument package, which continuously records conductivity, temperature and depth, and a rosette of 10-liter bottles that are used to collect water from specific depths for measurements of oxygen, nutrients and chlorophyll. When I stumbled into middle of the ship where the CTD is launched and recovered, I saw a large pelagic animal that is occasionally sampled with plankton nets, and probably rarely, if ever, has been snagged on top of a vertically-hauled CTD. But there, stretched out on the deck, was a 2.1 m long by 20 cm wide tunicate, whose name is Pyrostremma spinosum. I have read about this drifter but in 30 years of going to sea I never saw one intact and up close. The biggest pyrosome, photographed in New Zealand waters during the 1980’s as I recall, was 20 m long and large enough in diameter for a scuba diver to swim inside!
Photo: Monty Priede
One common name for pyrosomes is “corn cob jelly.” Actually, pyrosomes are not really jellyfish but rather urochordates, a faunal group that is ancestral to chordates, or animals with backbones. Structurally, the pyrosomes are hollow cylinders and composed of thousands of individuals, called zooids, joined together in a gelatinous matrix. Some forms are quite rigid and others, such as Pyrostremma, are very flaccid. They occur at the surface of the open ocean as well as in the dark, hyperbaric regions of the deep sea. These creatures can be whitish or pinkish, like the one we caught. Bits and pieces of two other large pyrosomes, probably the same species, have appeared among the mixture of fishes collected in the trawls made on Leg I and II of this expedition.
If I put my hand on her... Photo: David Shale
The tiny, pulsing zooids draw plankton-laden water into their bodies through an oral siphon. After extracting oxygen and filtering nutritive particles through the mesh of a branchial basket, the zooids expel wastes and water from a posterior siphon into the hollow cylinder of the colony. This pumping system, an activity coordinated by the zooids, enables the creature to move by jet propulsion.
Photo: Monty Priede
An amazing characteristic of the pyrosomes is their ability to bioluminesce. The waves of blue-green, chemical light they emit is dazzling. The animal literally glows brilliantly in the dark, hence it’s name “pyrosome, ” which is derived from Latin and means “fire body.” No one knows for sure why the pyrosomes bioluminesce or why this light-emitting character is so common among marine animals. A myriad of hypotheses, such as predator avoidance, prey detection, and mate recognition, have been proposed but experimental evidence to support these guesses is rare.