Biota of Lakes and Reservoirs - Hydro-Environmental Analysis: Freshwater Environments - page 431

Euglena
gracilis

Peridinium

FIGURE 15.18 Examples of protistan zooplankton. (Images courtesy of Ohio University Algae Home Page.)

forms are planktonic. They are generally smaller than crustaceans and often have long spines to
reduce settling. Rotifers have cilia, which function in locomotion but also aid in moving food toward
the mouth of the zooplankton (see Figure 15.19). They exhibit a wide range of morphological varia-
tions and adaptations.

15.3.1.2.1.3 Cladocerans Cladocerans (called waterleas) are generally small (0.2-3.0 mm;
Wetzel 2001), with a distinct head, a body covered with bivalve carapace, and a light-sensitive eye.
They typically feed by iltering water to remove particulate organic matter, and the size of the par-
ticles eaten is a function of the setae. They are typically herbivorous, but there are some raptorial
predators that grasp protozoans, rotifers, and small zooplankton with their forelegs. Cladocerans
have hard exoskeletons and molt many times as they grow. Many species show changes in shape
as they grow—characteristic of age, season, or environmental conditions (Balcer et al. 1984). They
have large swimming appendages (second antennae; Wetzel 2001). Perhaps the best known of the
cladocerans is the Daphnia (Figure 15.20).

15.3.1.2.1.4 Copepods The copepods (class Cristacea) comprise of three groups: the suborders
Calanoida, Cyclopoida, and Harpacticoida (Wetzel 2001). Of these, the Harpacticoids are primar-
ily benthic with mouthparts for seizing and scraping, while the other two groups can be suspension
feeders or raptorial predators. The body of a copepod consists of the anterior cephalothorax bearing

Ascomorpha ovalis

Keratella cochlearis

Keratella hiemalis

Conochilus unicornis

Hexarthra mira

FIGURE 15.19 Examples of rotifers. (Courtesy of the NOAA Great Lakes Water Life Photo Gallery.)