MARINE GAMMARIDEAN AMPHIPODA 7 



1 and 2 belonging with gnathopods 1-2 and numbers 3-7 with 

 pereopods 1-5. 



Gills are thoracic and generally are attached to the medial surfaces 

 of coxae 2-7, occasionally only on 2-6, or ?3-6. In females the medial 

 surfaces of coxae 2-5 (?or 2-6) carry brood lamellae. They are simply 

 buds in young females, but as body growth proceeds they become 

 longer and more heavily setose and are interlocked by their setae to 

 form a cradle enclosing the eggs. As the female increases in age and 

 size, the number of eggs laid after each molt becomes larger. Some 

 amphipods lay such large yolky eggs that a young female can carry 

 only one of them; at maturity she may be able to carry three or four. 

 Not all of the extruded eggs hatch; mortahty of about 25-50 percent 

 may occur even before hatching ; stunted degenerating eggs often may 

 be seen in the broods (as well as commensals such as spherical copepods 

 and reniform ostracods). Some large Amphipoda lay and carry more 

 than 200 eggs at a time. 



Commencing with leg 3, the thoracic appendages are termed pereo- 

 pods (walking legs), so that five pairs of thoracic legs represent 

 pereopods (many specialists commence numbering pereopods mth 

 gnathopod 1). There is justification in distinguishing gnathopods by 

 name because of their specialization in analogy to the pereopods of 

 other Crustacea (e.g., posterior maxillipeds of decapods). The first 

 two pairs of pereopods are useful in cleaning the gnathopods and other 

 anterior appendages and as a balance when alighting from a swim. 

 The last three pairs appear rather immobile and less adapted for 

 walking than in isopods. Amphipods are poorly balanced for walking, 

 hence their mobility usually depends on swimming. Their body plan, 

 however, permits a motility through dense masses of hydroids and 

 algae. Perhaps this is one of the reasons why amphipods are one of 

 the most abundant macroscopic crustacean groups in algae and other 

 anastomoses. 



Some Amphipoda living on the sea-bottom have immensely elon- 

 gated pereopods, which are spread out in the fashion of a spider and 

 prevent the amphipod from sinking into the mud. To lower the center 

 of gravity the body of the amphipod hangs upside down in its cradle 

 of legs. 



A few families of Gammaridea have the habit of burrowing into 

 benthic sediments and their pereopods are armed densely with strong 

 spines which aid in burrowing. Elongate flexible setae, as well as stiff 

 spine-like setae may occur on pereopods 3-5 of fossorial Amphipoda. 

 Even though a few other nonfossorial Amphipoda, such as Ampelis- 

 cidae, have these elongate setae, the term "fossorial pereopods" is 

 usefully applied to the condition. The glossary contains a precise 

 definition of fossorial pereopods. 



