Copepods

Freshwater planktonic copepods comprise two major groups: calanoids and cyclopoids. The calanoid copepods have an elongated body and long first antennae (Figure 7.2a), while the cyclopoid copepods have a stout body and

Figure 7.2 Three groups of freshwater copepods. a) Calanoid (egg-carrying female, dorsal view), b) cyclopoid (egg-carrying female, dorsal view), c) harpacticoid (female, dorsal view). (I. Faulkner.)

short first antennae (Figure 7.2b). A third group, the harpacticoids, have cylindrical bodies and very short first antennae. Harpacticoids are generally benthic, being found more often in or on the bottom mud or sand (Figure 7.2c). A key to the orders of freshwater copepods is shown in Table 7.3 (see also Figure 7.3).

The bodies of calanoids are often 1-2 mm long and cyclopoids and harpacticoids are usually less than 1 mm long. The body of a copepod is clearly segmented and females are larger than males. Females and males are also distinguished by the shape of the first antennae that are attached near the anterior end of the body and by other features (see Table 7.3 for details).

Copepods have pairs of different appendages on the ventral side of the body. For calanoid copepods, the appendages under the head are used for creating water currents to collect, filter and/or capture food particles. The appendages along the mid to lower body are used for swimming. Cyclopoid copepods use their mouth parts for capturing animal prey - most species

Table 7.3. Key to orders of freshwater copepods (Figure 7.3).

Phylum Arthropoda

Subphylum Crustacea

Class Copepoda

1a First antennae long, slender body Order Calanoida

Acanthodiaptomus, Calamoecia (Figure 7.3a and 7.3b), Boeckella (Figure 7.3c), Diaptomus, Eudiaptomus, Gladioferens (Figure 7.3d), Pseudodiaptomus and others

Key to sexes

Right and left first antennae similar in shape = female

Right and left first antennae dissimilar; right antenna geniculate (with an elbow-knee-like hinge) = male

1b First antennae short; head often much wider than lower body when seen from above Order Cyclopoida

Australocyclops, Cyclops, Diacyclops, Macrocyclops, Mesocyclops (Figure 7.3e) Thermocyclops and others

Key to sexes

Right and left first antennae similar in shape = female

Right and left first antennae similar in shape, but geniculate and often strongly curved = male

1c First antennae short; cylindrical body Order Harpacticoida

Canthocamptus, Fibulacamptus, Parastenocaris (Figure 7.3f) and others Key to sexes

Right and left first antennae similar in shape = female

Right and left first antennae similar in shape, but geniculate = male

Figure 7.3 Copepods. a) Calamoecia ampulla - Body elongated, with long first antennae (FA). Small calanoid copepod. Male fifth legs need to be examined for identification of species. Scale bar 100 pm; b) Calamoecia ampulla - Male fifth legs (posterior aspect). Scale bar 50 pm; c) Boeckella fluvialis - Male fifth legs (posterior aspect). Body elongated, with long first antennae. Relatively large calanoid copepod. Scale bar 100 pm; d) Gladioferens pectinatus - Male fifth legs (anterior aspect). Body elongated, with long first antennae. Relatively large calanoid copepod in fresh and salt water. Scale bar 100 pm; e) Mesocy-clops sp. - Body relatively stout, with short first antennae (FA). Scale bar 200 pm; f) Parastenocaris sp. - Body cylindrical, with very short first antennae (FA).Bot-tom dwelling, but may also appear in plankton.

Figure 7.3 Copepods. a) Calamoecia ampulla - Body elongated, with long first antennae (FA). Small calanoid copepod. Male fifth legs need to be examined for identification of species. Scale bar 100 pm; b) Calamoecia ampulla - Male fifth legs (posterior aspect). Scale bar 50 pm; c) Boeckella fluvialis - Male fifth legs (posterior aspect). Body elongated, with long first antennae. Relatively large calanoid copepod. Scale bar 100 pm; d) Gladioferens pectinatus - Male fifth legs (anterior aspect). Body elongated, with long first antennae. Relatively large calanoid copepod in fresh and salt water. Scale bar 100 pm; e) Mesocy-clops sp. - Body relatively stout, with short first antennae (FA). Scale bar 200 pm; f) Parastenocaris sp. - Body cylindrical, with very short first antennae (FA).Bot-tom dwelling, but may also appear in plankton.

are carnivorous. The legs along the mid to posterior body of copepods are mainly used for swimming. Calanoids and cyclopoids have five pairs of swimming legs and harpacticoids have five or six pairs. The detailed structure of fifth legs in the male is useful in identifying calanoid species. Fourth and fifth legs in the female are important in identifying cyclopoid species. All swimming legs are important in identifying harpacticoid species.

Copepods moult up to 11 times before becoming adults, with body shape and size changing after each moult. There are two distinct young stages: nauplius larvae and copepodites. A nauplius larva looks very different from an adult. A copepodite has fewer body segments and appendages, but looks like a small adult.

Female copepods produce eggs that always need to be fertilised by males. Females carry the eggs in one or two sacs attached to the ventral side of the body. The egg sacs and eggs are easily observed under a microscope. Some copepods produce resting eggs that withstand drought and other adverse environmental conditions. One study reported that the resting eggs of certain calanoid copepods can live in lake sediments for as long as 400 years (Hairston et al. 1995)!

Calanoids eat a wide variety of phytoplankton species and other suspended matter such as decayed plant material and clay particles. Some eat other small zooplankton, such as rotifers and ciliated protozoans. Cyclopoids are primarily carnivorous - eating other zooplankton.

Copepods may occur in the plankton all year round, usually reaching densities of 5-20 animals per litre in ponds, lakes, reservoirs and slow-flowing rivers.

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