Amphipods on a deep-sea hydrothermal treadmill

Conspicuous swarms of a pardaliscid amphipod were observed at about 2520 and 2580 m depth in the East Pacific Rise vent field during dives with the submersible Alvin. Swarms occurred in association with mussels, clams and tubeworms, and were located above, and immediately downstream of cracks with emanating hydrothermal water. Numerical density sometimes exceeded 1000 individuals 1^-1, which is 3 orders of magnitude greater than any previous report on pelagic crustaceans from the deep sea. The amphipods, however, were not obligatory swarmers, and thin-layered shoals and scattered individuals were observed. Orientation of individuals was often polarized as they headed into the venting flow, swimming vigorously at 5-10 cm s^-1 to maintain their position in the current. Retention within the preferred habitat requires an average swimming speed corresponding to the average current speed, suggesting a sustained swimming of > 10 body lengths s^-1. These observations contrast with the general concept of low swimming activity in deep-sea crustaceans.