Pleurobrachia - Part 2

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.

Thornback Rays stranding at Southend – March 26, 2009

The morning, stormy high tide at Chalkwell in Southend stranded a large ray on March 9, and the vertebra of this specimen, or more likely others, were found at Westcliff on March 11 and Southend on the 12th. The afternoon tide of March 16 stranded an undecayed specimen, curved and ventral-up on the high water mark at Thorpe Bay. It was a Thornback Ray Raja clavata lacking the yellow tag put on by Cefas for their Thames Ray Tagging and Survival 2007 - 08 study (‘’www.cefas.co.uk’’). After that the tides were generally at a lower level on the beach, but when the strandline of the afternoon tide of March 26 was studied at about the same height, the ray was found to have been converted into a curved skeleton which had moved 140 m to the east and over one groyne. It was ventral-up and showed the thin teeth of males in a head skeleton width of 125 mm, and a now partly preserved overall length of 0.5 m. Presumably it was eaten by gulls on the beach and then moved by the ides as they returned to the same elevation.