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.

Sepia adults and Pleurobrachia at Southend-on-Sea

May 18-20 2009

During the end of a long period of relatively cool and windy weather, coincident with relatively low high tides (neap), the strandlines of dead crabs, brown and recently grown green algae still lacked jellyfish (studied Shoebury to Leigh on 19th May). They did show an only slightly bird-peck-marked cuttlebone of 179 mm length, 68.3 mm width and 20.1 thickness, deposited by the morning tide of the 19th, with the posterior end pointing 102° E. of true north on the Westcliff strandline with that trend. A hundred m or so to the west, a corpse of a female cuttlefish was aligned with the posterior end pointing towards 94° E. of N. between two strandlines spaced about 0.6 m apart and trending 104°E. of N. Both specimens were dorsal-up, and the corpse had evidently arrived earlier during the two tides of similar, slightly higher height on May 18. When removed the shell had slightly large dimensions of 191 mm by 69.6 mm by 22.2 mm thickness. Both the thickness and the length of the last septum measured in the median plane are a good guide to the maturity and races of these adults Sepia officinalis L. and the last septa were respectively 66 mm and 72.5 mm long. It is possible that the smaller one was the male of a pair that had recently mated in the Zostera beds of Canvey Island, but the female still contained numerous 6 mm diameter oocytes in a 40 mm diameter posterior-dorsal ovary. The jaws had dimensions of 25.5 mm by 24.0 mm width and 10.5 mm by 32.3 mm. The two tentacles were largely gone and the arms contracted and or similarly degraded, producing stranded body dimensions of about 210 by 88 by 60 mm, which sank with the shell insitu and the head jack-knifed after 3.0 hours in seawater

A water sample taken after the morning tide of May 20 at Chalkwell, was of interest in containing a sea gooseberry Pleurobrachia pileus (O.F. Mueller) which was still alive in the tank on the 21st and attempting to feed from the poor animal food supply in the water sample, with the mouth at the waterline and the two tentacles with spreading lassoe cells