DECAPODA OF THE GULF OF MEXICO 



By Ellinor H. Behre, Louisiana State University, Baton Rouge, Louisiana 



GENERAL PHYSIOGRAPHIC REGIONS 



The decapod Crustacea of the Gulf comprise a 

 division roughl.v arranged in four, environmentally 

 limited, large groups. 



The first of these groups is made up of land 

 forms living more or less on the edge of the salt 

 water and spending part of their life in the sea. 

 This group consists largely of members of the 

 families Gecarcinidae, Grapsidae, and Ocypodidae. 

 Their habitat is conditioned by the nature of the 

 Gulf shores, which in the eastern part of the region 

 under discussion (southern Florida) and on the 

 southern rim are coral shores or limestone, but for 

 the rest mostly sandy or muddy. The genus 

 Cardisoma, the commonest representative, be- 

 comes in parts of this region a very conspicuous 

 inhabitant of the shores and even of the inland 

 areas. The spot distribution of this particular 

 genus, from Florida to Texas, is a matter of con- 

 siderable interest. Of its occurrence on the 

 western border of our area, nothing is known. 

 Other widely distributed forms are the family 

 Grapsidae (genus Sesarma with subgenera), and the 

 fiddler crabs, ghost crabs, and other Ocypodidae. 

 Some of the genera of this family, such as Uca 

 minax, invade brackish and even fresh water 

 shores, though they are seldom found very far 

 from water, even when not breeding. 



The second major group is the fauna of the 

 littoral, by far the best known and perhaps the 

 largest segment of the decapod population of the 

 Gulf. The northeastern portion of the northern 

 Mexican coast, especially Yucatan, shows the 

 1,000-fathom line far off shore, though the shelf 

 from the 100-fathom line is very steep. In the 

 western Gulf (400-foot contour line) the drop is 

 the steepest. The shallows extend farthest out on 

 the north (Louisiana, Mississippi) and east 

 (Florida) and on the northern shore of Yucatan. 

 The littoral therefore comprises a zone of greatly 

 varying width. The spread of the decapods over 

 this area and the ecological niches that may have 



259534 O — 54 30 



developed locally are matters of great interest 

 to our general problem. 



Roughly, the littoral here is composed of four 

 major habitats, of which two are very similar.' 

 There is first (region 1) the shore bordering the 

 mouths of the great rivers. These shore lines are 

 heavily fringed with the muddy deposits of river 

 deltas. The rivers of the southern portion and 

 also of Florida are much shorter, and there is a 

 much lesser volume of deposited material than in 

 the larger northern and western ones, chief among 

 which are the Rio Grande, the rivers of the Mis- 

 sissippi Delta, and the Alabama drainage. The 

 northern shore line appears fairly constant; but 

 some observations would seem to indicate at least 

 two faunal breaks — one at the Mississippi and 

 perhaps a second somewhere between that point 

 and the Texas-Mexico line, which seem to be other 

 than purely climatic. Along the river mouths 

 and up their muddy channels are to be found many 

 mud crabs, definitely the richest single decapod 

 element. Many species of Panopeus, Hexapanop- 

 eus and Rithro panopeus, Eurytium, and other re- 

 lated genera abound here. In general, we may 

 expect this fauna to differ little from that of the 

 muddy bays to be discussed later (region 3). 

 There is a slightly more active flow of the water 

 and more direct influence of winds, tides, and cur- 

 rents than in the back bays. Frobablj^ the single 

 greatest variant is the periodic great alterations 

 in salinity in "high water" years; and this does 

 not seem seriously to affect the decapod popula- 

 tion. A very bountiful fauna inhabits the muddy 

 river mouths and tide flats. There is also an 

 additional ecological factor in the slow run-off, 

 which has resulted in man-made artificial micro- 

 habitats in the form of jetties, where dwell the 

 rock crabs, along with many other normally rock- 

 dwelling invertebrates that have adopted this 

 territory. 



' We shall use the rather indefinite term "region" to indicate each of these 

 habitats. 



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