plates on the female’s thoracic legs called oostegites. Amphipods are the dominant
benthic crustaceans in fresh-water, salt-marsh, shelf, and upper-slope sediments.
Typically their bodies are flattened laterally, taking an arched shape (Fig. 13.8a).
Downslope they are progressively replaced by isopods and tanaids (Fig. 13.9). The
defining differences between amphipods and isopods are technical, but these groups
do sort out along the depth gradient. Isopods in shallow habitats most typically are
dorso-ventrally flattened and of simple shapes (like the common terrestrial forms
known as woodlice or pill bugs), but in the deep sea they take a variety of forms (Fig.
13.8b). For animals living buried in mud, they have amazingly sculptured and
decorative shapes. The significance of this level of morphological complexity is not
known; it definitely is a trend opposite to that described by Hartman and Fauchald for
polychaetes. Tanaids (Fig. 13.8c) are elongate, almost worm-like, but they carry
prominent chelae on the first thoracic limbs. Their strong sexual dimorphism often
results in separate species names for males and females. Tanaids compose about 60%
of deep-sea crustacea, the remainder being isopods and a few amphipods.
Fig. 13.8 Principal groups of benthic crustacea, all pericaridans: (a) a 6.0 mm
amphipod, Ampelisca of the Gammaridea
(after Myers 1985),
(b) a dendrotionid deep-sea isopod
(^) (after Hessler et al. 1979),
and (c) a tanaid, Neotanais.
(After Gardiner 1975.)