MARESHB GAMMARIDEAN AMPHIPODA 9 



instar); the animals are fertile yet the secondary sexual characters 

 are rudimentary; male gnathopods may be poorly developed and 

 brood plates are just beginning to develop in the female. Amphipods 

 are known to live through at least 12 instars, the females laying a 

 brood of eggs during the last five or six or in alternative instars ; how- 

 ever, terminal adult females (gerontics) may lose their brood plates, 

 apparently fail to lay eggs and develop aberrencies of an andromorphic 

 nature. Occasionally, these stages have been described as distinct 

 species. 



Amphipoda have the unfortunate habit of eating their exoskeletons 

 after ecdysis so that it is difficult to trace their molting sequence in 

 the laboratory. One must watch them continuously in order to obtain 

 ecdysial casts. Because the average instar appears to last about 15 

 days, the average maximum length of life is expected to exceed 6 

 months but some species in polar regions are estimated to Uve 5 or 

 6 years. Often the structure of the next instar may be seen within an 

 appendage. Such replication mthin the present organ can be confusing 

 to taxonomists and lead to the description of supernumerary parts. 



A few species are known to be hermaphroditic, carrying both male 

 and female gonads, and reflecting both sexes in their secondary sexual 

 characteristics. 



Paired pleopods on the first three segments of the pleon are biramous, 

 the rami multisegmented and strongly setose. Minute coupling hooks 

 on the medial edges of the peduncles are used to engage the pairs of 

 pleopods for coordinated paddling. Amphipoda usually are good 

 swimmers. Even burrowing amphipods swim well and phoxcephahd 

 males have the habit of leaving their burrows at night and swimming 

 to the seasurface from depths as great as 100 meters. They will swarm 

 around a hght suspended in the water. They may be ascending in 

 search of females even though the latter rarely swim to a night-hght. 

 Such swimming behavior may be a dispersal mechanism, especially 

 in groups having a low proportion of males. Some pelagic species 

 apparently undergo great vertical migration; several deep-sea amphi- 

 pods caught at night near the surface have been found to have ali- 

 mentary tracts full of benthic sediments. 



Variation in pleopods is rarely of sufficient extent to be used in 

 generic or famihal definitions except for Phliantidae, Talitroidea and 

 some Corophiidae. Pleopodal morphology, nevertheless, has been 

 neglected and may afford some help in taxonomic distinctions. 



There is justification in restricting the term pleon (=metasome) to 

 the first three abdominal segments bearing pleopods and utiHzing the 

 term urosome for the last three abdominal segments bearing uropods. 

 "All" Malacostraca have at- least one pair of uropods, that pair of 

 appendages on abdominal segment 6. Malacostracans generally have 



