Small And Irregular Zooplankton

The small, irregular zooplankton remaining in a 200 ^m mesh plankton sample are part of a very diverse group, and are of immense importance for the food web. Some comparatively large phytoplankton (less than 0.1 mm) will often be caught up in your sample, but fortunately they are quite distinctive (diatoms, and the dinoflagellate Ceratium, Figure 8.8A). Other single-celled animals include various star-shaped radiolarians and beautifully shaped foraminifera (the forams, Figure 8.8B). Radiolarians produce an internal silica test, or shell, with part of the cell extending out through tiny perforations and along spines for feeding (Figure 8.8B, Acantharia). Forams produce a calcium carbonate test, which is often altered by temperature or stress. The deposits of these distinctive tests often provide clues to past oceanographic environments, as well as modern day integrators of water quality. While forams are typically benthic, some well known planktonic forms are Globigerina, Globigerinoides, Neogloboquadrina, Orbulina and Turborotalia (Figure 8.8B2). A third group of protozoans are the cone or vase-shaped tintinnids (Figure 8.8C, Tintinnopsis, Codonellopsis, Favella and Rhabdonella). When undisturbed, they extend a crown of cilia around the top of the cone, which is able to capture diatoms. The unar-moured (naked) dinoflagellate Noctiluca also captures diatoms and other plankton (see Box 1, Chapter 1). It is large (around 1 mm diameter) and entirely carnivorous, and contains no photosynthetic pigments. Noctiluca look like translucent, reddish balls (like peaches, with single tentacles,

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Figure 8.8 Small irregular plankton showing: (A1) large dinoflagellate Ceratium, (A2) chain forming diatoms Chaetoceros, (B1) radiolarian, (B2) foram or globi-gerinid, (C1-C5) tintinnids, (D) larval bryozoan (cyphonautes larva), (E) Noctiluca, one with diatom prey inside, (F1-F4) larval echinoderms (pluteus larvae), (G1) larval nermetean worm, (G2-G3) trocophore larvae (larval polychaete or mollusc), (H1-H3) jellyfish (cnidarians), (I1-I2) siphonophores, (I3) egg mass (Sources: Dakin and Colefax 1940; Wickstead 1965).

Figure 8.8 Small irregular plankton showing: (A1) large dinoflagellate Ceratium, (A2) chain forming diatoms Chaetoceros, (B1) radiolarian, (B2) foram or globi-gerinid, (C1-C5) tintinnids, (D) larval bryozoan (cyphonautes larva), (E) Noctiluca, one with diatom prey inside, (F1-F4) larval echinoderms (pluteus larvae), (G1) larval nermetean worm, (G2-G3) trocophore larvae (larval polychaete or mollusc), (H1-H3) jellyfish (cnidarians), (I1-I2) siphonophores, (I3) egg mass (Sources: Dakin and Colefax 1940; Wickstead 1965).

Figure 8.8E). They can bloom in the estuary or coastal ocean in response to their preferred prey - diatoms - which, in turn, have bloomed in response to nutrient upwellings or sewage. Noctiluca tend to bloom within a critical temperature range around 20°C and may numerically dominate the zooplankton (Figure 8.8E).

Adult bryozoans form an encrusting sheet of colonial, filter-feeding animals, found on rocks, kelp and any other firm surface. Their larvae (known as cyphonautes larvae) are distinctive little triangular bivalved animals, with a row of cilia along their longer convex side (for example, Bugula, Figure 8.8D). Larval bryozoans provide a useful bioassay of heavy metals and other environmental impact assessments.

Echinoderms may be apparent in your plankton sample as distinctive larvae (Figure 8.8F). The larva of a sea star (Asteroidea) is known as a bipinnaria, which is characterised by the growth and folding of the ciliated band to form two loops. This larva settles to become a brachiolaria, with arms and a sucker; it then metamorphoses into the young sea star and frees itself from the remains of its attached larval form. The characteristic larval stage in the brittle stars (Ophiuroidea) and sea urchins (Echinoidea) is the pluteus which has an external apical plate with a tuft of cilia and a single, curved ciliary band. Pluteus larvae are most apparent in 200 ^m mesh plankton samples. Larvae of these two groups of echinoderms differ, but, in both, metamorphosis is dramatic and often rapid.

Larval snails and beach worms hatch into a tiny free-swimming, ciliated, trochophore larva around 0.2 mm across (Figure 8.8G). The trochophore stage is followed in the gastropods and bivalves by a veliger larva, with a foot and shell. The veliger then settles to the bottom as a young adult.

There are other phyla not illustrated here, whose larvae occasionally appear in the plankton, including the tornaria larva of the peanut worms (Sipunculida). The remaining zooplankton are the jellyfish and their relatives. There are many small jellyfish among the zooplankton (Figure 8.8H), including the related hydrozoan-siphonophores (Figure 8.8I).

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