Counts of crabs along each of the two transects varied with time of day- -counts 

 at night and at dusk were higher than counts during daylight or dawn. This 

 variation can be explained by the crabs' tendency to move from relatively con- 

 cealed positions during daylight to more exposed positions at night, making them 

 more available to the diver's view. 



The highest count obtained for each transect is accepted as the closest measure 

 of the total population of crabs in the transect area. Thus, the apparent popu- 

 lation size within the two 10-square-meter areas of transects A and B were 20 

 and 15, respectively. These counts were probably quite complete for all crabs 

 larger than approximately 4 mm in carapace length. Crabs less than 4 mm cara- 

 pace length were difficult to see because of their small size and tendency to 

 remain in protected locations even at night. 



The indicated population densities of 1.5 to 2.0 crabs per square meter were 

 probably maximums in the study area. As previously mentioned, the crabs usually 

 were most abundant along the marginal portions of the large reef areas such as 

 where the transects were located. Observations of crab abundance in other areas 

 of habitat similar to that of the transect areas indicated that crabs were con- 

 siderably more abundant in the transect areas than elsewhere. It appeared that 

 the proximity of the transects to the TEKTITE habitat may have had a positive 

 influence on the population density of crabs. The greater abundance of crabs 

 may have been a result of one or more of the artificial conditions created by 

 the presence of the habitat and the attendant human activity. The lights of the 

 TEKTITE habitat altered the normal night darkness conditions, especially along 

 transect A which was within the influence of the bright outside light located on 

 the west side of the structure. The high level of diver activity in the transect 

 areas with the resulting water turbulence and turbidity may have enhanced the 

 transect locations as suitable feeding areas for crabs. 



POPULATION STRUCTURE 



Samples totaling 103 arrow crabs were collected and brought into the wet room of 

 the TEKTITE habitat to obtain information on size and sex composition of the 

 population. Samples were intended to represent the general population, but 

 smaller crabs were probably not completely represented because of greater diffi- 

 culty in capturing them. 



The sex of each crab was determined by examining the shape of the abdomen. A 

 series of morphometric measurements of each crab including carapace length and 

 width, rostrum length and chela length, were made to the nearest 0.5 mm with a 

 vernier caliper. 



The ratio of female to male crabs in the sample was 55:48 indicating a slightly 

 greater abundance of females than males in the general population. Females 

 ranged from 4.5 mm to 13.0 mm and averaged 9.2 mm in carapace length. Males 

 ranged from 3,5 to 16.5 mm and averaged 9.4 mm. 



MATURITY AND REPRODUCTION 



Maturity of the arrow crabs was determined solely by examination of external 

 features. Hartnoll (1969) mentions the prominent changes in the abdomen, 

 pleopods, and sternum of most brachyuran crab females at the moult of puberty. 



VI-215 



