Rotifers

Most rotifers are 0.1-0.5 mm long. Their body shape varies widely between groups: they can be spherical, cylindrical or elongated. The body can be soft or may have a firm covering called a lorica. Some rotifers are enclosed in a gelatinous case. Many have different types of spines and a foot. Some even have toes. The structure of the jaw (or trophi) is distinctive for each species and is used for identification (it is necessary to dissolve body tissues with a chemical, such as bleach, to observe the jaws). The cilia surrounding a rotifer's mouth form a circle, called a corona or wheel organ. The rapid movements of the cilia create water currents for swimming and feeding. A key to the orders and families of freshwater rotifers is shown in Table 7.5 (see also Figure 7.7).

Rotifer populations consist only of females under normal environmental conditions. They produce eggs that hatch into females without the need for male fertilisation (a process known as parthenogenesis). The eggs are

Table 7.5. Key to orders of freshwater rotifers (modified from Shiel 1995) (Figure 7.7).

Phylum Rotifera

Class Monogononta/Bdelloidea

1a Body with a single ovary; body often Class Monogononta 2

with a lorica or tube

1b Body with paired ovary; body without Class Bdelloidea a lorica or tube

Orders Adinetidae, Philodinidae Philodinavidae (fresh to brackish) and others 2a Mastax malleoramate Order Flosculariacea

Family Conochilidae: Conochilopsis and Conochilus Family Flosculariidae: Floscularia, Lacinularia, Sinantherina and others Family Testudinellidae: Pompholyx, Testudinella and others Family Trochosphaeridae: Filinia (Figure 7.7d) and others

2b Mastax not malleoramate ->■ 3

3a Mastax uncinate Order Collothecacea

Family Collothecidae: Collotheca and others

3b Mastax not uncinate Order Ploima

Family Asplanchnidae: Asplanchna (Figure 7.7a) and others

Family Brachionidae: Anuraeopsis, Brachionus (Figure 7.7b), Keratella (Figure 7.7e),

Notholca, Platyias and others

Family Gastropodidae: Ascomorpha and Gastropus

Family Lecanidae: Lecane

Family Lepadellidae: Colurella, Lepadella and Squatinella Family Mytilinidae: Mytilina

Family Notommatidae: Cephalodella (Figure 7.7c), Monommata and others Family Synchaetidae: Polyarthra, Synchaeta and others Family Trichocercidae: Ascomorphella, Elosa and Trichocerca (Figure 7.7f) Family Trichotriidae: Trichotria (Figure 7.7g) and others relatively large compared to the body size of females, and are normally attached to the posterior part of their bodies before being released in water. It may take less than a week for juveniles of many rotifers to become mature.

However, under certain conditions, females produce eggs that hatch into males. Fertilised female rotifers then produce special resting eggs. The resting eggs can withstand extreme temperatures, drought and other adverse conditions. The eggs can remain viable long after the female rotifers that produced them have died. The resting eggs remain dormant -buried in the sediments for many years. New populations of female rotifers can establish from resting eggs when environmental conditions become favourable.

Rotifers eat bacteria, including cyanobacteria, and phytoplankton. Some are carnivorous and eat other rotifers. Rotifers may be abundant in

Figure 7.7 Rotifers. a) Asplanchna priodonta - Foot absent. Body transparent. Specimen preserved in formalin often strongly contracts. Jaw (trophi) needs to be examined for identification of species. Scale bar 100 pm; b) Brachionus calyci-florus amphiceros - Four anterior spines (AS) on dorsal side of lorica. Long posterior spines (PS). Scale bar 50 pm; c) Cephalodella gibba - Body fusiform, with slender toes (T); d) Filinia longiseta - Body shape oval. Body with two long lateral bristles (LB) and one short posterior bristle (PB). Scale bar 100 pm; e) Keratella tropica - Three six-sided median plaques (MP) on dorsal side of lorica. Single small four-sided posterior plaque (PP). Scale bar 50 pm; f) Trichocerca chattoni-Body cylindrical, more or less squat. Single long curved spine (S) at margin of head opening. Scale bar 100 pm; g) Trichotria sp. - Head, body and foot segments distinctive and rigid. Lorica margin with small spines (S). Scale bar 50 pm.

Figure 7.7 Rotifers. a) Asplanchna priodonta - Foot absent. Body transparent. Specimen preserved in formalin often strongly contracts. Jaw (trophi) needs to be examined for identification of species. Scale bar 100 pm; b) Brachionus calyci-florus amphiceros - Four anterior spines (AS) on dorsal side of lorica. Long posterior spines (PS). Scale bar 50 pm; c) Cephalodella gibba - Body fusiform, with slender toes (T); d) Filinia longiseta - Body shape oval. Body with two long lateral bristles (LB) and one short posterior bristle (PB). Scale bar 100 pm; e) Keratella tropica - Three six-sided median plaques (MP) on dorsal side of lorica. Single small four-sided posterior plaque (PP). Scale bar 50 pm; f) Trichocerca chattoni-Body cylindrical, more or less squat. Single long curved spine (S) at margin of head opening. Scale bar 100 pm; g) Trichotria sp. - Head, body and foot segments distinctive and rigid. Lorica margin with small spines (S). Scale bar 50 pm.

both standing and running waters. A maximum of 3500 rotifers have been recorded from one litre of water in an Australian river (Kobayashi et al. 1998). It is common to find more than 20 000 rotifers per litre in some billabongs and also in some reservoirs.

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