Planktonic Monsters of the Solent

The waters of the Solent and Southampton Water hold a rich planktonic flora and fauna. Plankton comprise a wide range of organisms which live at least part of their lives drifting in the water column. Their size ranges from that of a single bacterium to a huge jelly fish the size of a dustbin lid or larger. This article will focus on some of the small animals that can just be seen with the naked eye, but which under the microscope reveal themselves to be remarkably odd-looking monsters which would be quite fearsome if they were the size of a cat. Included in this article are species observed and photographed over the last year in plankton samples collected along Southampton
Water and the Solent.

Southampton Water plankton includes millions of larvae of crustaceans which, as adults, live on the sea floor as crabs and prawns. Their larvae can be remarkably odd. For example, the predatory larva of the mantid shrimp, Meiosquilla desmaresti, is like an aquatic praying mantid, with large eyes held away from the body on straight bars. This species is uncommon in British waters so the presence of larvae in Southampton Water suggests the Solent holds an important population. Another odd crustacean larva is the burrowing prawn, Axius stirhynchus. The larva has a series of defensive spines along its back. This species is also quite rare in British waters but is commonly observed in the Solent plankton. A far more abundant crab species, with a remarkable larval stage that takes defensive spines to the ultimate extreme, is Pisidia longicornis, the long-clawed porcelain crab. In early summer these larvae are remarkably abundant in the plankton.

Each year some odd shaped creatures, which normally live on the sea bed enter the plankton. Possibly this is when they are seeking a mate or when they are in search of new habitat. One of these is the skeleton shrimp, Caprella mutica, which, only recently, entered our waters from Japan probably on the hull of shipping. The head is on the left of the image supported on a highly extended neck. Another invasive species commonly observed in our plankton is the Pacific pycnogonid or sea spider Ammothea hilgendorfi. It is surprising that such a slow-moving animal is so common in the water column. It has recently become one of the more abundant invasive species in our waters.

As a final example of the odd animals which are encountered it may come as a surprise that there are planktonic worms. One of the most remarkable in the Solent plankton is Myrianida edwarsi. The males and females look quite different so images are included of both sexes. The females carry the eggs.