28 U.S. NATIONAL MUSEUM BULLETIN 271 



often return to the stenopodous condition even though gnathopod 2 

 has obviously not returned to a walking function because of its pe- 

 culiar morphology. 



The seemingly primitive members of the isaeid stock also maintain 

 the enlarged gnathopod 2 but the advanced members show either an 

 axial reversal, a shift of domination to gnathopod 1, or an enfeeble- 

 ment. Potentiality for enlarged gnathopod 2 is fully maintained in the 

 Ampithoidae and Ischyroceridae and the ultimate peculiarity is 

 reached in the Cheluridae, one species of which has gnathopod l! ex- 

 panded into a fully prehensile appendage like that of Maera. Although 

 some of the most diverse and, thus, presumably successful shallow- 

 water (and primarily tropical) genera have the fully enlarged and 

 prehensile gnathopod 2, the trend in gammaridean evolution has been 

 a secondary return to stenopody. Retention of the primitively en- 

 larged gnathopod 2 in the Podoceridae and the Caprellidea, even the 

 Cyamidea, is one more mark of their relationship. Those very success- 

 ful tropical gammaridean genera with enlarged gnathopod 2 (in males) 

 occur in several distinct evolutionary lines: Elasmopus, Maera, and 

 Ceradocus in the Gammaridae ; Hyale in the Talitroidea ; Gammaropsis 

 in the Isaeidae; the axially reversed condition expressed in Lembos 

 of the Aoridae ; Podocerus in the Podoceridaej the inquUinous Stenothoe 

 of the Stenothoidae and Leucothoe of the Leucothoidae. These genera 

 clearly have their highest diversity in the tropics and subtropics 

 whether they had their origin there or not. The ecologist's attention 

 should be drawn to this curious matter. 



Nontropical gammarideans with enlarged gnathopod 2 are partic- 

 ularly conspicuous in the demersal eusirids, the lUjeborgiids, various 

 stenothoids and the Gammaridae. 



Correlation of mandibular functions with morphology are poorly 

 understood. Biting, chewing, grinding, piercing, and rasping functions 

 are obvious, but mandibular variations are far more numerous than 

 just those five categories. Gammaridean amphipods have been thought 

 of primarily as scavengers, feeding on debris and detritus, carrion and 

 dead plant fragments. The basic mandible seems to be adapted to 

 biting off chunks with the incisors and grinding those chunks with the 

 molarial rasp. The lack of emphasis on herbivorous habits of Gam- 

 maridea in the literature is surprising in view of the properly adapted 

 mandibles and the strong infestation of marine plants by amphipods. 

 Macroscopic algae and marine grasses infested with amphipods rarely 

 show gross cropping or evidence of bites having been removed. 

 Stomach contents of a few phycophilous amphipods demonstrate that 

 they probably feed on microscopic epiphytes. Undoubtedly the larger 

 and slower growing algae have evolved mechanisms to limit the success 

 of marine herbivores; poorly adapted macroscopic algae must surely 



