Pyrosoma, a large pelagic colonial sea squirt
Two Pyrosoma weighing >10 kg each
Phronima, inspiration to the movie "Alien"?
Author: Uwe Piatkowski (IFM-GEOMAR, University of Kiel) and Mikko Heino (Institute of Marine Research, Bergen)
We are still operating on our major southern transect about 200 nautical miles north of the Azores. The day brought a variety of different catches and kept all people on board quite busy. In the early morning hours we had another catch with the big pelagic trawl that yielded a good number of large fishes and invertebrates. A big surprise and definitely the event of the day was the occurrence of two big jelly-like pyrosomes in the catch (Fig. 1). Pyrosomes are colonial pelagic sea squirts that have all the characteristics of a primitive vertebrate but lack a backbone, so they are invertebrates. They can consist of thousands of small barrel shaped animals. Although the individual animals are small the communal animal can become quite large, with some up to 4 meters in length. The genus name Pyrosoma is derived from the Greek, pyros (fire) and soma (body), referring to the bright bioluminescence characteristic of this sea squirt group. The ones we caught weighed more than ten kilograms each and were several meters long, but so delicate that they broke into pieces when we carefully sorted them from the catch.
Later the day we successfully completed our fourteenth haul with medium sized pelagic trawl. Like the day before, the catches were characterized by high species diversity, notably in lanternfishes, squids, and midwater shrimps. The scientists enjoyed new records of species that have not occur in our catches before, and which definitely shows that we are now working in the northern extension of sub-tropical water masses.
During the early night we then completed another haul with the krill trawl which efficiently samples the macroplankton and micronekton fraction of the biota inhabiting the water column. These animal groups are largely composed of numerous species of crustaceans, but also small fishes, squids, pelagic worms and snails as well as a variety of gelatinous creatures such as sea nettles and moon jellies. Many gelatinous animals serve as convenient travelling homes or resting places for numerous other creatures. Certain types of larval fish and crustaceans are the primary users of this resource, defining them as hitchhikers of the open sea realm. A conspicuous representative of this group – and a common component of our recent catches – is Phronima, a pelagic amphipod belonging to a crustacean group also called beachfleas (Fig. 2). Females of Phronima actually take over the tests of certain pelagic sea squirts and swim while covered in their modified protective “barrel”. Phronima broods eggs within the barrel, and the hatchlings then consume their home before searching for new gelatinous victims.
Working in the fish lab, deep in the berth of the vessel, was mentally more demanding than before because the sun was shining all day long - and it did so for the first time during our cruise. So we hope that weather conditions will keep on friendly during the few remaining days of our expedition to give us a chance to change our pale faces into a nice suntan.
Tomorrows expected highlights
The biological diversity in the area we are working in is high, and every trawl haul is an adventure. We thus anticipate several species that are new to our expedition. At the same time, analyses of the materials collected during the survey continue, and step by step, we are gaining a better overall picture of the structure of the ecosystem along the mid-Atlantic ridge.