June 1, 2009
Given one change of the tank water from the sea on May 22, the Pleurobrachia continued to feed on the few available copepods until it was last seen alive at about noon on the 25th. Most of the time it lived at the surface with the two tentacles hanging below and then retracting, but when last seen it was moving down with the cillia beating and the tentacles extended below. On June 1st, the tank sediment was partly removed with the old water and appeared to preserve the corpse, which could not be seen before, as a grey triangular disc of 10 mm length extending upwards into a brown 60 mm filament in the water. It would seem that live Pleurobrachia remain at the surface by active swimming and then sink to die giving them some potential of becoming fossils despite being originally transparent and nearly all composed of water. When a Beröe cucumis Fabricus similarly arrived with the new water by accident on July 12, 2007, it was considerably more active, actively swimming up from the default position on the tank floor, where it presumably died within one day. That seawater did show flashes of light in the night, but they did not come from the moving Beröe and no light was seen at all in the Pleurobrachia water. Beröe are reported to appear later in the year because they eat Pleurobrachia, which can themselves live longer in the tank by feeding on copepods.
