504 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



south Florida. Mugil liza (M. brasiliensis of 

 authors) penetrates to the north coast of Yucatan 

 and northwestern Cuba, but so far it has not been 

 recorded from Florida. Centropomus undecimalis 

 (snook) occurs throughout the entire perimeter of 

 the Gulf, but C. parallelus and C. pectinatus 

 penetrate only to the north coast of Yucatdn, 

 northwestern Cuba, and southern Florida. C. 

 ensiferus seems to be absent from Florida. 



SHORE FISHES 



There are certain species of coastal fishes which 

 occur along the entire perimeter of the Gulf 

 (except Cuba) but not in the West Indies proper. 

 Some of these species are enth'ely confined to the 

 Gulf, and others penetrate the Yucat&n Channel 

 and the Straits of Florida, extending along the 

 mainland of Central America and the Atlantic 

 coast of the United States, respectively. These 

 species represent a small percentage of the total 

 fish fauna of the Gulf and include tropical repre- 

 sentatives as well as forms of northern origin. 

 The well-known sheepshead {Archosargus pro- 

 batocephalus) , channel bass {Sciaenops ocellatus), 

 and the common weakfish (Cynoscion regalis) 

 among several others, are good examples illus- 

 trating this condition. These fishes are fairly 

 common in the Gulf of Mexico including extreme 

 south Florida, but they have never been reported 

 from Cuba or any other of the West Indies. 

 The ecological conditions prevailing in extreme 

 south Florida are common to northwestern Cuba, 

 and since many coastal species occur in both 

 these areas, it is difficult to explain the absence of 

 certain forms in Cuba. The species under discus- 

 sion are not tropical, and it seems reasonable to 

 assume that the Gulf Stream forms a temperature 

 barrier preventing their dispersal into the West 

 Indies. On the other hand, the Gulf Stream has 

 been a very important temperature factor favoring 

 the dispersal of most tropical West Indian fishes 

 into the Gulf of Mexico, especially its southern 

 portion. The percentage of species present in the 

 Gulf of Mexico but absent in the West Indies 

 is considerablj- higher than that of West Indian 

 species absent in the Gulf. 



As already pointed out, most of the shore fishes 

 of the Gulf of Mexico also occur in the Caribbean 

 Sea, and many of them are strictly stenobatiiic 

 forms confined to shallow water and apparently 

 unable to disperse across deep and wide water 



gaps. Despite this condition most of the species 

 occur on both sides of the Yucatdn Channel and 

 the Straits of Florida. These deep and relatively 

 wide water gaps would represent a bathic bar- 

 rier to the adult fish but not to its pelagic or 

 semipelagic larva as demonstrated for some of 

 those species. 



A third, very characteristic distributional pat- 

 tern affecting several species is well illustrated 

 by Acipenser sturio (common sturgeon) and Doro- 

 soma cepedianum (gizzard shad). These species 

 occur along the north and east shore of the Gulf of 

 Mexico as far south as central Florida and re- 

 appear along the Atlantic coast of the United 

 States, being absent around south Florida. This 

 discontinuous distribution seems to be caused by 

 the influence of the main branch of the Gulf 

 Stream acting as a thermal barrier and preventing 

 dispersal around the southern extremity of 

 peninsular Florida. Acipenser sturio and Doro- 

 soma cepedianum are temperate forms, and their 

 presence on either side of peninsular Florida may 

 be explained by the former continuity which 

 existed between the Gulf of Mexico and the 

 Atlantic Ocean across northern Florida during the 

 interglacial periods of the Pleistocene. According 

 to the paleogeographic evidence, this passage 

 existed until relatively recent times. 



Peninsular Florida and the main branch of the 

 Gulf Stream may therefore be considered as land 

 and thermal barriers, respectively, prevsnting at 

 present the exchange of temperate fishes between 

 the Atlantic Ocean and the Gulf of Mexico. The 

 isolating effect of these barriers as a factor in 

 speciation is well-illustrated by the shads, Alosa 

 sapidissima and A. alabamae, of the Atlantic and 

 Gulf coasts, respectively. These two very closely 

 related. Vicarious species seem to have evolved 

 when the original ancestral population, continuous 

 along the south coast of the United States, was 

 split by the emergence of peninsular Florida. 

 Neither Alosa sapidissima nor A. alabamae occur 

 around south Florida. (See also Ginsburg, 1952, 

 pp. 99-101.) 



The paleogeographic evidence indicates that 

 the Gulf of Mexico originated as a shallow basin 

 and according to Schuchert (1935, p. 59): 



"Previous to Middle Cretaceous time, it is 

 believed, no such deep Gulf of Mexico as the 

 present one was in existence, and the area now 

 occupied by this suboceanic interior sea was a 



