Cladocerans

Most cladocerans are less than 1-2 mm long, but there are some notable exceptions: specimens 5-6 mm in length have been found in some water bodies. Females are usually larger than males. The body consists of a rigid, clam-like shell - called a carapace - which is transparent, but can be yellowish or brownish in colour. Pairs of appendages called thoracic limbs are inside the carapace and are important for collecting and transferring food particles to the mouth. The head of a cladoceran is usually compact, with prominent eyes and large antennae used for swimming. Some cladocerans develop conspicuous head and tail spines, helmet or 'neck-teeth' (Figure 7.4). A key to the families of freshwater cladocerans is shown in Table 7.4 (see also Figure 7.5). Clado-ceran taxonomy is constantly being reviewed and it is likely that additions of new families will occur (e.g. Santos-Flores and Dodson 2003).

Table 7.4. Key to families of freshwater cladocerans (modified from Smirnov and Timms 1983) (Figure 7.5).

Phylum Arthropoda Subphylum Crustacea Class Branchiopoda Order Diplostraca Suborder Cladocera

1a Body and swimming legs not covered with a carapace 1b Body and swimming legs covered with a carapace

-*-3

2a Body short with four pairs of swimming legs Family Polyphemidae: Polyphemus 2b Body long with six pairs of swimming legs Family Leptrodridae: Leptodora

3a Six pairs of swimming legs inside the carapace all similar 3b Five or six pairs of swimming legs inside the carapace not similar

-► 5

4 Body length much greater than body height; second antennae with large branch-like appendages

Family Sididae: Diaphanosoma (Figure 7.5e) and others

5a First antennae long and slender, like an elephant's trunk Family Bosminidae: Bosmina (Figure 7.5a) and Bosminopsis 5b First antennae usually short

-6

6a Second antennae two-branched, both with three segments; mostly small body length, hemispherical or circular in lateral view Family Chydoridae: Acroperus (Figure 7.5b), Alona, Chydorus (Figure 7.5d), Graptoleberis, Pleuroxus and others

6b Second antennae two-branched, one with three segments and the other with four segments

-7

7a First antennae not flexible and short

Family Daphniidae: Ceriodaphnia (Figure 7.5c), Daphnia

(Figures 7.4 and 7.6), Simocephalus and others

7b First antennae flexible and long relative to body length

-► 8

8a First antennae on mid-abdominal side of head; oval body Family Moinidae: Moina and Moinodaphnia 8b First antennae on frontal side of head

-► 9

9a Postabdomen with distal, terminal claw Family Macrotrichidae: Macrothrix (Figure 7.5f) and others 9b Postabdomen lacks terminal claw Family Neotrichidae: Neothrix

Female-only populations of cladocerans occur under normal environmental conditions. They produce female eggs inside a chamber on the dorsal side of the body, within which the eggs hatch. Newly hatched young - which look like small adults - remain there until they are ready to swim.

When environmental conditions deteriorate (through a lack of food or drying of the water body), the females produce eggs that hatch into males. Fertilised females then produce one or two special resting eggs encased in a thick protective covering to form an ephippium, which is released into the

Figure 7.4 A species such as Daphnia lumholtzi can produce conspicuously long head and tail spines, resulting in the extension of an overall body length. Long head and tail spines can make it more difficult for fish to eat Daphnia, thus reducing the level of predation by fish.

water (Figure 7.6). Ephippia can withstand a wide range of environmental conditions, surviving for many years in dry sediments. Cladocerans can establish new populations from ephippia when environmental conditions once again become favourable.

Cladocerans moult several times as they grow to adulthood. A new carapace is formed inside the old, which is then discarded as the body grows bigger. The discarded carapaces are called exuviae. Collections of plankton samples may contain exuviae as well as live animals. Exuviae can also be used to identify species that have occupied a habitat in the past. Those preserved in sediments can also be used to identify past occupants of habitats up to 10 000 years ago. The science of studying such remains is called palaeolimnology, and is helpful in understanding past environmental conditions and climate change.

Cladocerans, especially large Daphnia, eat a wide variety of phyto-plankton and other suspended matter, such as decayed plant material and clay particles. They may greatly reduce phytoplankton abundance. There are several genera of carnivorous cladocerans.

Cladocerans occur normally from spring to early summer, reaching densities of 10-30 animals per litre in ponds, lakes and reservoirs. In a special

Figure 7.5 Cladocerans. a) Bosmina meridionalis - Small body. First antennae (FA) relatively long, slender, not fused at their bases. Second antennae (SA) relatively small. Often a pair of spine-like elongation (S) at ventro-posterior corner of body. Scale bar 100 pm; b) Acroperus sp. - Body flattened laterally. Bottom dwelling. Normally found among water plants. Scale bar 100 pm; c) Ceriodaphnia sp. - Body shape broadly oval. Head (H) small. Short first antennae (FA). Normal eggs (E). Scale bar 200 pm; d) Chydorus sp. - Body small, spherical. Small eyes (E). Bottom dwelling, but also appears in plankton. Scale bar 100 pm; e) Diaphanosoma excisum - Body without a tail spine. Head relatively large, rectangular with large eye (EY). First antennae (FA) small. Second antennae (SA) large and well developed. Large normal egg (E). Scale bar 300 pm; f) Macrothrix spinosa - Body flattened laterally, without tail spine. First antennae (FA) situated frontal side of head. Tip of first antennae (T) wider than its base (B). Bottom dwelling. Normally found among water plants. Scale bar 100 pm.

Figure 7.5 Cladocerans. a) Bosmina meridionalis - Small body. First antennae (FA) relatively long, slender, not fused at their bases. Second antennae (SA) relatively small. Often a pair of spine-like elongation (S) at ventro-posterior corner of body. Scale bar 100 pm; b) Acroperus sp. - Body flattened laterally. Bottom dwelling. Normally found among water plants. Scale bar 100 pm; c) Ceriodaphnia sp. - Body shape broadly oval. Head (H) small. Short first antennae (FA). Normal eggs (E). Scale bar 200 pm; d) Chydorus sp. - Body small, spherical. Small eyes (E). Bottom dwelling, but also appears in plankton. Scale bar 100 pm; e) Diaphanosoma excisum - Body without a tail spine. Head relatively large, rectangular with large eye (EY). First antennae (FA) small. Second antennae (SA) large and well developed. Large normal egg (E). Scale bar 300 pm; f) Macrothrix spinosa - Body flattened laterally, without tail spine. First antennae (FA) situated frontal side of head. Tip of first antennae (T) wider than its base (B). Bottom dwelling. Normally found among water plants. Scale bar 100 pm.

Figure 7.6 Daphnia's resting eggs in an ephippium can survive in adverse environmental conditions, even after the females that produced the ephip-pium die. a) An ephippium is formed on the dorsal side of a female, b) the ephippium usually detaches after the female dies, c) young Daphnia will hatch from the resting eggs when the environmental conditions become favourable again.

Figure 7.6 Daphnia's resting eggs in an ephippium can survive in adverse environmental conditions, even after the females that produced the ephip-pium die. a) An ephippium is formed on the dorsal side of a female, b) the ephippium usually detaches after the female dies, c) young Daphnia will hatch from the resting eggs when the environmental conditions become favourable again.

case, a high density of 500 cladocerans per litre has been reported from a waste stabilisation pond (Mitchell and Williams 1982).

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