Crustacea

These are the most conspicuous element in the permanent zooplankton, commonly amounting numerically to at least 70 per cent of the total. The predominant class is the Copepoda. These are present in incredible numbers and are represented by many species. There are about 1200 species in the British sea area, of which Calanus (Marshall and Orr, 1955), Acartia, Centropages, Temora, Oithona, Pseudocalanus and Paracalanus are especially common (Figure 2.10).

Another abundant group of planktonic Crustacea is the Euphausiacea. The best know of these is a large species, Euphausia superba (Figure 2.11a), which occurs in enormous numbers in the Southern Ocean south of the Antarctic Convergence. It constitutes the 'krill' upon which the giant baleen whales of the Antarctic feed, including the blue whale (Balaena musculus). In the north-east Atlantic, species of Nyctiphanes, Meganyctiphanes (Figure 2.11b) and Thysanoessa are common.

Other crustacean groups that are sometimes numerous in marine plankton are the Cladocera (water fleas), for example Podon and Evadne (Figure 2.12); the Ostracoda (seed shrimps), for example Conchoecia, Philomedes; and the Amphipoda, for example Parathemisto (Figure 2.13). Mysids or opossum shrimps (Mysidacea) mostly live close to the bottom but are sometimes found in coastal plankton, especially in estuarine regions. In addition to these holoplanktonic forms, great numbers of larvae are contributed to the temporary plankton by benthic crustacea. Larvae of crustaceans such as crabs, barnacles and lobsters are very different in form from the adults and often have striking adaptations of shape that prevent them from sinking.

Figure 2.10 Planktonic copepods common around the British Isles. (a) Calanus finmarchicus, (b) Temora longicornis, (c) Acartia clausi, (d) Centropages typicus, (e) Pseudocalanus elongatus.

Chaetognatha

This phylum, the arrow worms, is widespread and well represented in most plankton samples. Around the British Isles there are several species of Sagitta (Figure 2.14d), and Eukrohnia hamata occasionally enters the North Sea from the Arctic. Chaetognaths are of special interest as 'indicator species' (see Section 4.7.1). They are an important group of planktonic predators.

Protozoa

The zooplankton includes a variety of Protozoa. We have mentioned earlier that some flagellates are holozoic. Foraminifera and Radiolaria are sufficiently

Figure 2.H (a) Euphausia superba^ the 'kriW of Antarctic seas. (b) Meganyctiphanes norvegica^ a common euphausid of the North Atlantic.
Figure 2.12 Two species of Cladocera common in spring and summer around the British Isles. (a) Podon polyphemoides, (b) Evadne nordmanni.

Figure 2.13 Parathemisto gaudichaudi^ a hyperiid amphipod which is epipelagic in colder parts of the north and south Atlantic.

numerous for their skeletons to form a conspicuous part of some deep-water sediments (see Section 6.1.2). Ciliates are found mainly in coastal plankton.

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