MARINE GAMMARIDEAN AMPHIPODA 29 



become extinct in view of the almost ubiquitous hordes of marine 

 amphipods that are potential macroherbivores. Microscopic epiphytes 

 presumably survive through rapid growth while the total amphipod 

 population may be restricted by the seasonality of epiphytes ; probably 

 a balance is thus maintained in ways similar to the diatom-copepod 

 cycle of the pelagic realm. Amphipods may benefit the macrophytes 

 by cleaning their surfaces of infesting epiphytes. 



The microherbivorous amphipod with biting-rasping mandible and 

 enlarged male gnathopod 2 may be the basic member of the am- 

 phipodan organization. To suggest that the level bottom scavenging 

 amphipod evolved first and invaded epifloras later would presume 

 that cellulase secretion was not an original part of the amphipod plan. 

 The grossly compressed bodies of the basic amphipod also attest to 

 a preadaptation for nestUng and gliding amongst anastomoses. That 

 this ungainly structure later was able to invade a host of other habitats 

 seemingly unsuitable to a flea-like morphology suggests that in the 

 course of their evolution numerous "hidden" adaptations accrued. 

 Some of these may be circumstantial: high prodigality ("success by 

 numerical pressure"), and low genetic plasticity that maintains a 

 broad adaptability to feeding conditions. A more efficient mechanism 

 than the basic mandible may be imagined for cropping micro-epiphytes 

 without radical changes in the general structure. The maintenance of 

 that basic mandible throughout so many famihes and genera of am- 

 phipods, which obviously have put it to numerous functions, suggests 

 that one key to success of amphipods is their potential omnivorous 

 habit. A measure of this manifold feeding potential is seen in one's 

 ability to trap diatom-feeding amphipods by means of a carrion-baited 

 undersea trap. 



In the dispersal to level bottoms, amphipodan morphofunction 

 almost invariably changed; the successful Ampeliscidae build tubes; 

 the Phoxocephalidae-Haustoriidae and Oedicerotidae dig burrows and 

 often become much broadened in their bodies; other genera, Hke 

 Listriella, have obtained special associations with infaunal members 

 of the level bottoms. 



Almost all of the greater Isaeidea, the tubicolous amphipods, have 

 maintained the basic mandible, palp included. Even the wood "boring" 

 Cheluridae are able to rasp wood with the basic mandible. The greater 

 Talitroidea have maintained the rasping mandible although the palp 

 has been lost. Presumably one function of the palp is the cleansing 

 of the anterior cephaHc space between the antennae. Amphipods 

 without palp often have few antennal setae projecting into that 

 space to trap particles. 



The trend to a distinct change in mandibular morphology is seen 

 in some Gammaridae and even more strongly in some of the families 



