GULF OF MEXICO 



211 



Duriii<}: tho pcM-iods of lower mean soa level in 

 January aiul February, the lowermost zone, below 

 the brown algae, is exposed. This zone consists 

 of hydroids, Bryozoa, and encrusting sponges. 

 Inshore, near laud and on concrete pilings at Port 

 Aransas, the middle zone is also occupied by 

 oysters. Mussels do not occur at Port Aransas 

 but are found at Frecport and Galveston on the 

 jetties. 



Although not occurring in the Gulf of Mexico 

 proper, the zonation in the Florida Keys and at 

 Beaufort described bj' the Stephensons (1949, 

 1950, 1952) have aspects in common with that at 

 Port Aransas. The most conspicuous difference 

 is, the generally lower arrangement of the entire 

 zonal pattern at Port Aransas in relation to tide 

 zero, a phenomenon apparently associated with 

 the pronounced seasonal differences in sea level 

 on the Texas coast (fig. 54) and the higher level of 

 the tide zero in relation to the tidal cycle. 



There is a tendency toward the formation of 

 sub-zones in Texas and Florida which may be 

 induced by irregular tidal cycles; this complex 

 pattern seems much less developed at Beaufort, 

 where the tidal cycle is more regular (Hedgpetli 

 1953, pp. 188-194). 



SAND BEACH COMMUNITIES 



The communities of the sand beaches are evi- 

 dently similar to those of the Beaufort area which 

 were studied by Pearse, Humm, and Wliarton 

 (1942) since many of the same species, or closely 

 related species, occur on the sandy beaches of 

 Texas and Louisiana. LaFleur (1940) briefly 

 described the biota of sand beaches of Grand 

 Isle. Neither of these are studies of communities, 

 in the strict sense of the term. The most notice- 

 able bottom community of the sandy beach is that 

 of Donax which occurs in large beds, moving up 

 and down with the tides. Immediately offshore 

 there are evidently large communities composed 

 of such bivalves as Dinocardium robustum, Area 

 and Anadara, Dosinia and Teliina, predaceous 

 gastropods, and such echinoderms as Mellita and 

 Astrojjecten. This assemblage appears to be a 

 counterpart of the sandy-bottom Teliina commu- 

 nity of European waters. 



The characteristic inhabitant of the sand 

 beach is the ghost crab, Ocypode albicans, which 

 seeks refuge during daylight hours in burrows well 



25953-1 0—54 15 



above high tide lines. Beyond this region, at 

 Port Isabel and in southern Florida, there occurs 

 the larger land crab, Cardisoma guanhumi. Occa- 

 sional individuals are found at Port Aransas, but 

 established colonies of them are uidinown north 

 of Port Isabel except at Grand Isle (Behre 1950). 

 Lower down on the beach, associated with the 

 windrows of algae (sargassum in spring and sum- 

 mer and various reds in winter) are the amphipods, 

 Orchestia grillus, 0. platensis, and Talorchestia 

 longicornis. 



Intensive study of the animal life of this most 

 characteristic of Gulf coast environments has 

 hardly begun. Caspers (1951), in a study of the 

 arthropods of the Bulgarian coast, characterized 

 the community of the sandy beaches as the 

 "Orchestia variation of the Pachygrapsus bio- 

 coenosis." From the vantage point of the 

 Texas coast where Pachygrapsus seems most 

 abundant on the jetties and the sand constitutes 

 the major part of the environment, we might say 

 that Pachygrapsus is a "variation" of the "Or- 

 chestia (or Ocypode) biocoenosis." 



THE SHRIMP GROUND COMMUNITY 



Offshore in the muddy bottoms between the 

 foot of the sandy beach and the 10- to 15-fathom 

 line there occirrs a large community which we 

 recognize principally as that from which white 

 shrimp, Penaeus setijerus, are taken in commercial 

 quantities. Several sedentary invertebrates are 

 characteristic of these bottoms. The most con- 

 spicuous of these is the sea pansy, Renilla miilleri, 

 which must pave the bottom in some localities. A 

 gorgonian, Leptogorgia setacea, also flourishes in 

 this region. Other characteristic members of this 

 shrimp ground community include tube building 

 worms of the family Onuphidae, crabs of the genera 

 Hepatus, Calappa, and Persephone, the anemone, 

 Paranthus rapiformis, and certain gastropods, e.g.. 

 Busy con, Murex, Dolium, and Fasciolaria. In the 

 larger abandoned shells of these snails there occurs 

 the large red hermit crab, Petrochirus bahamensis. 

 UsuaUy the shells bear one or more anemones, 

 Calliactis tricolor, and inside, living commensally 

 with the hermit crab, is the porcelain crab, Por- 

 cellana sayana. Also common, but perhaps oc- 

 curring in irregular colonies, is the stomatopod, 

 Squilla empusa. 



