Zooplankton dynamics

Above: The sea gooseberry Pleurobrachia pileus grazing on smaller zoooplankton using two long sticky tentacles that it trails behind it (to the right of the main body above) as it swims. The smaller circular organisms are ‘sea sparkle’ (Noctiluca scintilans), between 1 and 3mm in diameter.

I am not as up on zooplankton, but thought I should follow up my earlier post on phytoplankton dynamics in the earlier part of the year with something to indicate that the zooplankton are not (too) boring! For this article I have chosen the sea gooseberry Pleurobrachia pileus which, at about 2cm in diameter, is a member of the zooplankton which is large enough to be seen – though difficult to spot in the water as it is transparent… You can often find individuals stranded as the tide goes out, but their appearance on the beach as small lumps of jelly does not do justice to them underwater. In their element they are propelled by rows of modified cillia called ‘ctenes’ – you can see eight rows of ctenes, slightly inset into the otherwise oval body of the animal in the photo above. These ctenes can refract light to give bright and continuously changing coloured displays underwater…

The population of Pleurobrachia pileus reached a peak in the Wyre estuary in early June, when small examples of Beroe cucumis first appeared in the fortnightly samples. Initially the Beroe were only a few millimeters long – very much smaller than the Pleurobrachia. Despite this they latch on to their very much larger prey, and use modified ctenes to chew or rasp flesh from the sea gooseberry. Over time, and repeated attacks, the sea gooseberry is damaged, diminished, and will eventually die.

Above: A juvenile Beroe cucumis (bottom left) latched onto a sea gooseberry, which is using its ctenes to spin rapidly in the water, to try and dislodge its predator. Photograph from the Wyre river sample of the 14th June.

Over a couple of weeks feasting on sea gooseberries the Beroe increase in size, and can reach a total length in excess of eight centimeters. The adult Beroe are able to ‘unzip’ their mouths, opening them wide enough to ingest Pleurobrachia whole, as seen in the photograph below:

Above: By the 30th June the Beroe were large enough to consume smaller sea gooseberries whole, as you can see towards the left of the picture above.

Beroe itself is not without predators, however. In the photo below (same individual as above) you can see that the beroe is in turn being eaten by a ‘megalopa’ or juvenile crab.

Above: Beroe has bumped into the sea gooseberry to the right – it is still full of one of its bretheren, hoewever, so the sea gooseberry is likely to get lucky this time! You can see that the lower surface of the Beroe is distorted where it is being attacked by a megalopa, or crab larva.

I have often seen adult crabs eating jellyfish, and spider crabs will often climb kelp at the turn of the tide to catch moon jellies or lions manes that drift past, but this is certainly the most ambitious decapod I have ever seen! What goes around, comes around…

In the German Bight the appearance of Beroe results in a near total collapse of the Pleurobrachia plieus population in a matter of weeks(1). In the Wyre the population of Pleurobrachia falls significantly, but a continuous supply of larvae in the plankton ensures that the species is still present in our samples, and indeed it was Beroe that had disappeared from the sample on the 26th July 2022.

Reference

(1) Coastal Plankton 2nd Ed. by Otto Larink and Wilfried Westheide. Published 2011 Verlag Dr Friedrich Pfiel, Munchen (ISBN 978-3-89937-127-7). p74.

Barry Kaye, Lancashire MCS

Posted: August 2nd, 2022
Posted in Marine science update