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FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



records from the Gulf proper are starred in the 

 check list. The Texas records are of exceptional 

 interest, however, as only three hydroids had 

 previously been recorded from those waters; 

 they were included in the writer's account of the 

 hydroids of Louisiana and Texas (Deevey 1950), 

 a zoogeographic discussion that was founded 

 primarily on a small collection made by J. W. 

 Hedgpeth. 



ZOOGEOGRAPHY 



The list of 183 species looks impressive, but it 

 would be idle to pretend that the hydroids of 

 the Gulf are well-known. Tropical regions gen- 

 erally have a wealth of species, but hydroid 

 habitats are probably no more extensive in the 

 tropics than elsewhere. The rarity of the rarest 

 species is correspondingly greater, and it is un- 

 likely that more than half the hydroids living in 

 the Gulf have yet been found there. Partly 

 because of inadequate collecting and partly be- 

 cause shallow water hydroids are always under 

 suspicion as fouling organisms, little can be said 

 about the meaning of their geographic distribution. 



Of the total of 183 hydroid species known from 

 the Gulf of Mexico, 95 are also found in the 

 Caribbean and another 18, though not yet re- 

 corded from the Caribbean, are known from the 

 eastern tropical Pacific. The remaining 70 species 

 include some of the most interesting. 



Some of the 70, of course, are known only from 

 the Gulf, but while a few true endemics are to be 

 expected it is too early to say which ones they are; 

 at any rate, no common species is known to be 

 endemic. The interesting members of the group 

 of 70 are those whose main range is otherwise 

 boreal. The boreal species among the hydroids 

 of Texas and Louisiana have already been dis- 

 cussed (Deevey 1950). They include several, 

 such as Podocoryne carnea, that are unknown in the 

 warmer parts of the Gulf, and at least one, 

 Tubularia crocea, whose ecology is well enough 

 understood to indicate that it could not flourish 

 in southern Florida. To the list of supposed 

 relicts of a glacial age of the Pleistocene given in 

 the earlier paper may be added the name of 

 Cladocarpus jiexilis, a moderately deep water 

 species taken at three stations off Mobile but not 

 otherwise known south of Cape May. In several 

 other cases we have the familiar phenomenon of a 



shallow water boreal species occurring at consid- 

 erable depths in the Gulf (and Caribbean): 

 Eudendrium tenellum, Lafoea dumosa, L. gracillima. 

 Another way of looking at the facts is this: of 

 156 species known from the Caribbean, 61 are not 

 known from the Gulf; 29 of them, at least, are 

 common enough in the Caribbean to have been 

 taken at more than one station. Hydroid sta- 

 tistics are scarcely necessary to prove the point, 

 but it is obvious that the Gulf is not a strictly 

 tropical body of water. Low winter temperatures 

 and low and variable salinity, particularly along 

 the northwestern shore, are only some of the fac- 

 tors that must be responsible for maintaining a 

 different "fauna" in the Gulf of Mexico. 



The 95 species that are common to the Gulf 

 and the Caribbean present different problems. 

 Most of them are definitely warm water types, 

 although a high proportion belong to the tropical 

 flotsam (especially sargassum) fauna and so may 

 not be true residents of the Gulf. Seventy of the 

 Gulf hydroids and 59 of the 156 Caribbean species 

 are represented in the much larger list of 312 

 species recorded from the eastern tropical Pacific. 

 The richness of the Pacific fauna, which is known 

 almost exclusively from the Allan Hancock collec- 

 tion (Fraser 1948), is another indication that the 

 Caribbean hydroids have been inadequately col- 

 lected; it is surely correct to suppose that the 18 

 species common to the Gulf and the eastern trop- 

 ical Pacific will appear in the Caribbean, along 

 with many others. Wliat is surprising is that 

 nearly half of the tropical Atlantic hydroids cross 

 the Isthmus of Panama without undergoing 

 specific differentiation. If one cares for statis- 

 tical statements, the "strength of the relation- 

 ship" between the Gulf and Pacific faunas is 

 nearly as great (38 percent) as is that between 

 the Gulf and Caribbean faunas (52 percent). 



The Isthmus of Panama is not very old, and 

 many biogeographers have supposed that it is 

 younger than most of the species of marine organ- 

 isms (Schuchert 1935). The problem is related to 

 that of Tethyan distributions; pan-tropical species 

 (more usually genera) are supposed to have had 

 their ranges established in the Tethys Sea, of 

 Cretaceous and early Tertiary age, only to have 

 them sundered by the rise of central Asia (see 

 Ekman 1934, 1935, for review). If disjunct 

 distributions in the Mediterranean and the 



