THE GEOGRAPHICAL DISTRIBUTION OF DIGENETIC 



TREMATODES OF MARINE FISHES OF THE 



TROPICAL AMERICAN PACIFIC* 



Harold W. Manter 

 University of Nebraska 



INTRODUCTION 



Many comparisons of marine animals of the tropical Atlantic and 

 tropical Pacific strongly support other evidence (geological and paleon- 

 tological) that the two oceans were formerly connected. Panama and 

 Central America, however, have probably separated the two oceans since 

 the middle of the Miocene, between 7,000,000 and 13,000,000 years ago. 

 Specialists in almost all groups of marine animals have been impressed by 

 similarities of genera and species on the two coasts of tropical America, 

 but as a rule relatively few of the animal species are identical. The simi- 

 larity is more generally evidenced by numbers of so-called geminate or 

 twin species and by identical genera endemic to and limited to the two 

 oceans (the so-called endemic amphi-American genera). Such findings are 

 especially impressive among Crustacea and fishes. For example, according 

 to the review of this subject by Ekman (1935) of 157 genera of crabs 

 in tropical America, 33 genera are endemic amphi-American, 8 others 

 occur elsewhere only in the Eastern Atlantic, and 3 others only in tem- 

 perate South American waters. In this group of animals, however, the 

 number of identical endemic amphi-American species is small. Of 582 

 littoral species of crabs of the tropical Atlantic, only 9 are common to 

 both oceans and not found elsewhere. 



Geminate species and genera are best known among the decapods, sea 

 urchins and fishes. Rathbun (1917-1937) and Finnegan (1931) note 

 many twin-pairs of species and of genera among crabs. Jordan (1908) 

 notes "a hundred or more" twin-pairs of littoral fishes on the two sides 

 of the Isthmus of Panama. Jordan's observations of such twin-species 

 led him to formulate "the Law of Geminate Species" (Jordan, 1908), 

 which may be stated in his words as "broadly speaking . . . given any form 

 of animal or plant in any region, the nearest related form is not found 



* Studies from the Zoological Laboratories, University of Nebraska, No. 206. 



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