Animals - Freshwater Ecology: Concepts and Environmental Applications - page 165

Phylum Tardigrada
Water bears (tardigrades) are microscopic animals with a cosmopoli-
tan distribution (Nelson, 1991); about 350 species are reported worldwide.
They have a bilaterally symmetrical body with four body segments, each
with a pair of legs (Fig. 9.5N). Most adults are from 250 to 500 m long.

Hermaphoroditism, parthenogenesis, and sexual reproduction all occur in
the Tardigrada.
Most species are able to withstand drying, a characteristic related to
their unusual habitat requirements. The tardigrades are rarely found in the
plankton; aquatic species mainly inhabit benthic habitats in which they can
reach very high densities, notably in the capillary water in wet sand of
beaches. Most species are associated with droplets of water on terrestrial
mosses and liverworts and are only semiaquatic. They are some of the only
animals found in extreme habitats, such as Arctic and Antarctic lakes,
streams, and ice sheets (McInnes and Pugh, 1998; Pugh and McInnes,
1998).

Phylum Arthropoda
Arthropods are found in all continental surface waters. They are im-
portant components of aquatic biodiversity and central to ecosystem func-
tion. They are characterized by a chitinous exoskeleton and stiff jointed
appendages (including legs, mouthparts, and antennae). There are three
subphyla common in freshwaters: the Chelicerata (class Arachnida—water
mites and aquatic spiders), the Uniramia (insects and Collembola), and the
Crustacea (e.g., crayfish, shrimp, amphipods, isopods, mysids, fairy shrimp,
Cladocera, and Copepoda).

Class Arachnida
Water mites belong to several families. More than 5000 species of wa-
ter mites have been described worldwide; about 1500 are estimated to oc-
cur in North America, but only half of them are named. They can be ex-
traordinarily diverse, with as many as 75 species from 25 genera/m 2 in a
eutrophic aquatic weed bed or 50 species from 30 genera in a single stream
riffle (Smith and Cook, 1991).
Water mites have a mouth region and a body that has a fused
cephalothorax and abdomen (Figs. 9.5O and 9.5P). Six pairs of appendages
are present; the last four pairs, the legs, are the most conspicuous. The ap-
pendages can have setae, or spines, which are used as characteristics for
identification.
Water mites inhabit a variety of benthic habitats, including springs, rif-
fles, interstitial habitats, lakes (with mostly benthic but a few planktonic
forms), and temporary pools. The majority of water mites are carnivorous
or parasitic (primarily on aquatic insects). More sedentary species may feed
on carrion or possibly detritus.
Among the true spiders, no North American species are completely
aquatic. Several species live near water and are able to run on the water sur-
face and even dive beneath it. Many spiders build their webs on emergent