OBSERVATIONS ON THE BIOLOGY AND BEHAVIOR OF THE ARROW CRAB, 

 STENORYNCHUS SETICORNIX (HERBST) , IN LAMESHUR BAY, ST. JOHN, VIRGIN ISLANDS 



by 



Louis Barr 



National Marine Fisheries Service 



Auke Bay, Alaska 



ABSTRACT 



A study of the arrow crab population near the TEKTITE II habitat showed 

 that these crabs are present in densities of up to 2 per square meter of 

 reef area. These crabs mature at about 8.5 mm in carapace length and 

 appear to have an extended, or year-round, reproductive season. They 

 perform a diel migration, leaving the crevices in the stony corals at 

 dusk and spending the nights on the soft corals and gorgonians . They 

 serve as scavengers on the reef and appear to be subjected to little 

 predation. 



INTRODUCTION 



The arrow crab, Stenorynchus seticornis (Herbst) , family Majidae, is widely 

 distributed in the tropical and subtropical Atlantic. Its known range is from 

 North Carolina and Bermuda to Rio de Janeiro in the western Atlantic and from 

 Madeira and Canary Islands to Angola in the eastern Atlantic (Williams, 1965). 

 Arrow crabs have been collected at depths ranging from very shallow to 1,A88 

 meters (814 fathoms), on rock, coral, pebble, sand, and sand-shell substrates 

 and on wharf pilings and rock jetties (Williams, 1965). 



During my participation from July 23 to August 12, 1970, in the TEKTITE II 

 saturation diving experiment in Lameshur Bay, St. John, Virgin Islands, I con- 

 ducted an intensive in situ study of the biology and behavior of the arrow 

 crabs near the TEKTITE habitat. Approximately 40 hours were spent making 

 in situ observations of the crabs during routine excursions from the TEKTITE 

 habitat on standard scuba and closed-circuit, oxygen rebreather equipment. In 

 addition to the in situ observations, approximately 100 crabs were collected 

 and brought into the wet room of the TEKTITE habitat for further behavioral 

 observations and for determination of sex and size composition of the popula- 

 tion in the study area. 



HABITAT 



The study area ranged in depth from 8 to 22 meters and included several types 

 of substrate. Much of the bottom was extensive live coral reefs. Areas of sand 

 substrate occurred among the various large reefs and in an unbroken expanse west 

 of the reefs. On the sand areas, generally near the interface between coral 

 reef and sand substrate, were numerous patch-reefs of living coral, ranging from 

 a fraction of a meter to several meters in diameter. 



Arrow crabs were more abundant in areas of coral substrate than in sand substrate 

 areas. Although crabs were found throughout the coral areas, they were generally 

 most abundant at or near the edges of the reefs; that is, near the coral-sand 



VI-213 



