Carmona-Suarez and Conde: Distribution and abundance of Callinectes spp^ and Arenaeus cribrarius 



13 



Figure 1 



Map of sampling sites at Ensenada de La Vela, State of Falcon, Venezuela. Stations 1 

 to 4: = foreshore sampling sites (stages 1 + 2); I = inshore sampling site, M = midshore 

 sampling site, O = offshore sampling site (stage 3). 



other 6 hours during the night. This process was carried 

 out for three to four consecutive days. No monthly or sea- 

 sonal samples were taken at these three offshore stations. 



Crab identification and characterization 



Crabs from each stage were sorted to species, their sex 

 was determined, and each crab was measured immedi- 

 ately after capture. Carapace length was measured with a 

 0..5-mm-precision Vernier caliper. This measurement was 

 selected instead of carapace width because lateral spines 

 can be abraded, making this measure of body size unre- 

 liable. This measurement method has also been used by 

 Moncada and Gomez (1980) and Williams (1984). Animals 

 were identified by using a key compiled from the informa- 

 tion and keys in Taissoun (1969, 1973b), Fischer (1978), 

 Rodriguez (1980), and Williams (1984). Juvenile females 

 are identified by a triangular pleon and adults by a semi- 

 circular pleon. Adult males have an abdomen detached 

 from the sternum; juveniles do not. 



Environmental variables and data analysis 



At each station, from January 1993 to December 1994, sur- 

 face water temperature and salinity were measured with 

 a 0.5°C precision mercury thermometer and a tempera- 

 ture-compensated hand refractometer, respectively. Dis- 

 solved oxygen was determined from November 1993 up 

 to December 1994 with a YSI® dissolved oxygen meter. 

 During the second and third sampling stages ( 1997-98 

 and 1999) only salinity values were recorded. Rainfall 

 data were retrieved from the raw records of the meteoro- 



logical station belonging to the Ministry of the Environ- 

 ment and Natural Resources (Venezuela), located at Coro 

 Airport, about 10 km west of the sampling sites. 



Population dynamics were analyzed only for the most 

 abundant species that appeared regularly during each 

 sample. Dissimilarities in body size between species were 

 compared by using a two-tailed Student's f-test of the dif- 

 ferences between two means. Sets of continuous variables 

 were checked for normality with the Kolmogorov-Smirnov 

 goodness-of-fit test. Homogeneity of variances of gi-oups of 

 data to be compared by /-tests was checked with Bartlett's 

 test (StatSoft, 1992; Sokal and Rohlf, 1995). In those cas- 

 es where normality or homoscedasticity were not met, 

 data were log-transformed. Multiple comparisons were 

 performed with the Tukey-I-Ci-amer method. The degree 

 of association between crab abundance and environmen- 

 tal variables was assessed with the product-moment cor- 

 relation coefficient. To compare abundance distributions 

 between stations and diel samplings, RxC tables were 

 analyzed with the G-test of independence. No extrinsic hy- 

 pothesis were considered. 



To describe portunid guild composition, several diver- 

 sity indices were used: Simpsons. Shannon-Weaver, and 

 Hill's numbers (Nl and N2) (Ludwig and Reynold.s, 1988). 

 Simpson's index gives the probability that two individu- 

 als drawn from a population belong to the same species. 

 The Shannon-Weaver index measures the average degree 

 of uncertainty in predicting to what species an individual 

 chosen at random would belong. Hill's numbers Nl and 

 N2, whose units are number of species, show how many 

 common and very common species appear in a guild or 

 community and lessen the weight of rare species. 



