Identifying Marine Zooplankton

Fresh zooplankton - even freshly preserved and rinsed - is quite amazing to look at under the microscope, but to the naked eye the sample may seem a little disappointing after the anticipation of towing a net for 10 minutes. Remove the sticks and large jellyfish (thoroughly rinse off formalin using a fine sieve if necessary), sit down at a comfortable and well set-up microscope and enjoy the complexity, diversity and colours of these fascinating creatures. Try drawing some simple sketches of dominant types to focus your attention onto the basics of identification outlined below.

Within a sample of marine zooplankton, you may find the adults or larvae of nearly all of the Earth's living phyla, although it will usually be dominated by the crustaceans - mostly copepods (Figures 8.1-8.3). Like any arthropod (invertebrates with an exoskeleton), copepods grow by shedding their exoskeleton through a series of moults (or instars, or developmental stages), so that the diversity of shapes is potentially 10 fold greater than the number of species! You may also find drowned insects or a few rare marine insects or mites.

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Figure 8.1 a. Smaller zooplankton (~ 1 mm across) showing A-calanoid and cyclopoid copepods, B-hyperid amphipods, C-larval prawn, D-cladocerans, E-crab zoea, F-cyclopoid copepod, G-an invertebrate egg, H-larval poly-chaete worms, I-bivalve, J-pteropods, K-polychaete larvae, L-larval decapod (anomuran), M-early stage juvenile polychaete, N-ostracod, O-harpacticoid copepod, P-juvenile copepods or copepodites.

Figure 8.1 b. Smaller zooplankton caught off eastern Australia, with reference to the size of a pin (width of pin is 0.6 mm), showing A-copepods, B-ostracods, C-fish eggs, D-globigerinid shells, E-juvenile polychaete worm, F-bivalves, G-hyperid amphipods, H-juvenile krill, I-crab zoea, J-planktonic snail, a heteropod, Atlanta, K-planktonic snail, a pteropod.

Figure 8.1 b. Smaller zooplankton caught off eastern Australia, with reference to the size of a pin (width of pin is 0.6 mm), showing A-copepods, B-ostracods, C-fish eggs, D-globigerinid shells, E-juvenile polychaete worm, F-bivalves, G-hyperid amphipods, H-juvenile krill, I-crab zoea, J-planktonic snail, a heteropod, Atlanta, K-planktonic snail, a pteropod.

Figure 8.2 a. Medium-sized zooplankton caught off eastern Australia showing A-sergestid or ghost shrimp Lucifer, B-larval fish, C-planktonic snails, D-cumacean, E-larval crabs (zoeae), F-later stage crab larvae (megalopae), G-copepods, H-pteropods, I-fish egg, J-gamarid amphipod, K-ostracod, L-isopod, M-juvenile prawn or carid shrimps, N-mysid, O-brittle starfish.

Figure 8.2 a. Medium-sized zooplankton caught off eastern Australia showing A-sergestid or ghost shrimp Lucifer, B-larval fish, C-planktonic snails, D-cumacean, E-larval crabs (zoeae), F-later stage crab larvae (megalopae), G-copepods, H-pteropods, I-fish egg, J-gamarid amphipod, K-ostracod, L-isopod, M-juvenile prawn or carid shrimps, N-mysid, O-brittle starfish.

Figure 8.2 b. Medium-sized zooplankton (width of pin is 0.6 mm) showing A-calanoid copepods, B-isopods, C-gammarid amphipods, D-late stage crab larva (megalopa), E-larval crab (zoea stage), F-mysids, G-heteropods, Atlanta, H-juvenile shrimp, I-larval prawn with tail oriented upwards, J-larvaceans, K-calanoid copepods, Gladioferens, L-cladocerans Podon, M-salp or doliol-id, N-larval fish, goby, O-cnidarian, jellyfish, P-pteropods, Q-polychaete, R-mysids, note the distinctive balance organs or statocysts within the tail-fan.

Figure 8.3 a. Larger-sized zooplankton caught off eastern Australia showing A-chaetognath, B-larval lobsters (puerulus stage), C-juvenile prawns, D-ctenophore, E-larval fish including flatfish, herring, goatfish, F-stomatopod zoea, G-pteropods, H-amphipod, I-late stage crab larvae (megalopae), J-smaller chaetognaths, K-siphononophore, L-salps, M-juvenile prawns, N-three small, Glaucus (a bright blue sea slug), O-larval squid and octopus, P-polychaetes.

Figure 8.3 a. Larger-sized zooplankton caught off eastern Australia showing A-chaetognath, B-larval lobsters (puerulus stage), C-juvenile prawns, D-ctenophore, E-larval fish including flatfish, herring, goatfish, F-stomatopod zoea, G-pteropods, H-amphipod, I-late stage crab larvae (megalopae), J-smaller chaetognaths, K-siphononophore, L-salps, M-juvenile prawns, N-three small, Glaucus (a bright blue sea slug), O-larval squid and octopus, P-polychaetes.

Figure 8.3 b. Larger-sized zooplankton a with reference to the size of a pin (width of pin is 0.6 mm), A-late stage larval stomatopods, B-chaetognaths, C-late stage crab larvae (megalopae), D-tentaculate ctenophore (lobate ctenophores are too delicate to capture whole), E-polychaete, F-pelagic sea slug Glaucus, G-salp, H-siphonophore.

Figure 8.3 b. Larger-sized zooplankton a with reference to the size of a pin (width of pin is 0.6 mm), A-late stage larval stomatopods, B-chaetognaths, C-late stage crab larvae (megalopae), D-tentaculate ctenophore (lobate ctenophores are too delicate to capture whole), E-polychaete, F-pelagic sea slug Glaucus, G-salp, H-siphonophore.

A typical sample is shown that has been sorted into small (<1 mm, Figure 8.1), medium (1-3 mm, Figure 8.2) and large (>5 mm, Figure 8.3). Our minds are good at recognising characteristic shapes, so at an initial level, no dichotomous keys are necessary. Shape and body size - as indicated by the approximate scale bar - are the two essential aspects of identifying zooplankton in different orientations. The scale bar is only approximate as the actual size can vary with respect to the latitude (temperature) or rearing conditions in the laboratory.

Crustaceans are the first things we recognise - by their eyes and many limbs (Figures 8.1-8.3). The eyes are either stalked and obvious, or are sessile and compound (that is, eyes that look rather like dabs of black paint on the exoskeleton). A compound eye is made up of many elements, rather like pixels. Another useful distinction is the presence or absence of a carapace or shell that covers their main walking (thoracic) limbs and gills. For example, most prawn-like crustaceans have a carapace, but brine shrimps, copepods, amphipods and isopods do not. Some small crustaceans are enclosed by their carapace (cladocerans and ostracods). The generalised body plan of a crustacean is well illustrated by a lobster - with a head, a thorax covered by the carapace and a long abdomen. You will find the number and location of limbs on the three body sections to be a useful characteristic. Crustaceans have two pairs of antennae on the head and (like every other limb) are usually composed of an inner and outer branch joined near the base. The inner branch (endopod) often has a walking or sensory function while the outer branch (exopod) may be used for cleaning or another purpose. The mouthpart limbs (the mandibles and maxillipeds) also have this biramous structure. Similarly, adult prawns and crabs walk on the inner branch (the endopod) while the outer branch (the exopod) is reduced to a small cleaning rod or has disappeared altogether. The swimming limbs on the abdomen have very similar endopods and exopods. The uropods are the last pair of limbs on the abdomen and, together with the last segment - the telson, make up the tail-fan of the prawn or lobster. The larval development of a spade-like telson without uropods, to an adult tail fan with uropods is another useful trait for recognising larval prawns and crabs. Very basic ('primitive') crustaceans have a pair of biramous limbs associated with every segment of their bodies, from the first antenna to the uropods. Reduction from this basic form, to just a few limbs on a few segments, is one of the most fascinating aspects to the Crustacea, and one of the most useful traits for identification.

Large gelatinous zooplankton are also obvious. They comprise three groups: the jellyfish, salps and comb jellies (ctenophores). Many jellyfish

(medusae) are quite tiny (<1 mm diameter), but distinctively look like miniature versions of adults. Ctenophores are walnut- or pea-shaped balls, with eight longitudinal bands of cilia (the ctene plates, Figure 8.3b). Salps look like little gelatinous barrels, from 2 to 20 mm long (Figure 8.3a, b), while the related larvaceans or appendicularians are simply opaque blobs with a fibrous tail barely attached (Figure 8.2b). Most fish eggs are perfectly round, 0.5-1.5 mm diameter, with a clear transparent egg shell and perhaps a droplet of oil. Fish larvae should catch your attention with a large distinctive fish eye, and then you'll notice the gills and mouth (Figure 8.3). The only things with similar looking eyeballs are the baby squid and octopus (Figure 8.3a). Superficially similar to larval fish, the long and slender arrow worms - the tigers of the plankton - sometimes have large chitinous spines or jaws curving out (Figures 8.2, 8.3).

Finally, there is everything else - usually less than 1 mm and of all shapes - the larval molluscs, beach worms, starfish and sea urchins and many others. This chapter guides you to identify the distinctive shapes. For the zoologically minded, a table of taxonomic classification is provided for all the major zooplankton (Table 8.1). Refer to the recommended reading for further identification to down to genus and species - and sometimes sex.

Table 8.1. Zooplankton summary. This summary includes only dominant marine zooplankton (and excluding freshwater zooplankton). Meroplankton spend only part of the lifecycle in the plankton as larvae or medusae, while holoplankton spend their entire life in the plankton.

PHYLUM, Sub-Phylum

Class/subclass

Order

Meroplankton e.g. larvae only

Holoplankton

CHORDATA, Urochordata: Ascidiacea (sea squirts) Thalacea, Doliolida (salps) Larvacea (larvaceans)

larvae

totally, Thalia, Doliolum totally, Fritillaria, Oikopleura

CHAETOGNATHA: (arrow worms)

totally, Sagitta

ECHINODERMATA: Asteroidea (starfish) Ophiuroidea (brittle stars) Echninoidea (sea urchins) Crinoidea (sea lilies) Holothuroidea (sea cucumbers)

(pluteus larva)

bipinnaria^brachiolaria pluteus larvae pluteus larvae larvae larvae

MOLLUSCA:

Gastropoda (snails and slugs) Prosobranchia heteropods Opisthobranchia (nudibranchs and sea slugs) (shelled pteropods or sea butterflies) (naked pteropods) Bivalvia: Cephalopoda:

trochophore^veliger larvae larvae larvae larvae larvae larvae

violet shell, Janthina heteropods, Firoloida, Atlanta e.g. Glaucus totally, e.g. Creseis, Limacina totally, e.g. Clione, Desmopteris

ARTHROPODA, Crustacea: Malacostraca: Decapoda (shrimp, crabs) Stomatopoda Isopoda, Amphipoda Euphausiacea (krill) Mysidacea (mysid shrimp) Maxillopoda Ostracoda Copepoda

Cirripedia (barnacles) Phyllopoda, Branchiopoda Cladocera (clam shrimp) Anostraca (brine shrimp)

(nauplius larva)

zoea^mysis^megalopa larval stages larvae or epibenthic adults epibenthic adults

Cypris larva

- larvae

- larvae, epibenthic adult

hyperid amphipods totally few, mostly benthic most calanoids, cyclopoids only shed exuvia of adults mostly, Podon, Evadne, Penilia, Artemia in saline ponds

ANNELIDA:

Polychaeta (marine worms)

(trochophore^veliger larva)

some specialists, e.g. Tomopteris

BRYOZOA:

Cyphonautes larva

CTENOPHORA: (comb jellies)

totally, Pleurobrachia

CNIDARIA: Hydrozoa, (including siphonophores) Scyphozoa (true jellyfish) Cubomedusa (box jelly) Anthozoa (sea anemone, coral)

tiny medusa

Medusa Medusa Planula larva

Physalia, Velella

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