NOTE Aurioles-Gamboa et al.: Mass strandings of Pleuroncodes planipes 



465 



Figure 1 



Study area on the west coast of Baja California Sur, Mexico. 

 Dots inside Bahia Magdalena indicate the location of stations 

 sampled. The shaded area is the region where pelagic red crabs, 

 Pleuroncodes planipes, were found as indicated by positive sta- 

 tions and visual observations (from Soli's, 1991). 



The general appearance of these pelagic red crabs 

 (color and mobility) suggested that the crustaceans 

 were healthy and that the stranding could be con- 

 sidered accidental (Boyd, 1962). We questioned local 

 fishermen about crab strandings and determined 

 that mass strandings 1 ) occur annually, usually in 

 spring, from April through June, and 2 ) are common 

 on Santa Margarita and Magdalena Islands but 

 rarely seen on the peninsular coast. 



Since strandings coincide with the end of the breed- 

 ing season, we addressed the null hypothesis that 

 stranded pelagic red crabs, particularly females, were 

 in a weakened state because of energy expended in 

 reproduction, as reported for many crustaceans 

 (Hartnoll, 1985). It is known that pelagic red crabs 

 are able to breed twice in a single breeding season 



and may produce, depending on body size, 

 from 500 to 5000 eggs in each brood ( Serrano, 

 1991). Based on this fact, debilitation caused 

 by reproductive investment should be more 

 evident in females. For that reason, the 

 chemical composition of pelagic red crabs in 

 Bahia Magdalena was determined for both 

 sexes. We were aware that males were over- 

 represented in samples collected on the con- 

 tinental shelf (Boyd, 1962; Serrano, 1991); 

 thus we also wished to determine if stranded 

 animals were female-biased, as an explana- 

 tion for the unbalanced sex ratio recorded on 

 the continental shelf. 



Materials and methods 



To test whether the strandings were due to 

 weakness caused by starvation or malnutri- 

 tion, about 20 kg of live-stranded pelagic red 

 crabs were collected as soon they were 

 stranded. From this sample, a total of 1,150 

 individuals were sexed and measured for 

 SCL to the nearest 1.0 mm. Sex was deter- 

 mined by the presence or absence of modi- 

 fied pleopods, which males use to fertilize the 

 eggs (Boyd, 1962). Standard carapace length 

 was measured from the antorbital notches 

 of the rostrum to the midpoint of the poste- 

 rior border of the carapace (Kato, 1974). This 

 measurement is usually preferred over total 

 length because it does not vary with shrink- 

 age of the abdomen. 



Stomach contents were analyzed in order 

 to determine if 1 ) the animals had been feed- 

 ing before stranding, and 2) the number of 

 items and composition was similar to stom- 

 ach contents of pelagic red crabs collected on 

 the continental shelf (Perez and Aurioles-Gamboa 2 ). 

 We examined the stomach contents from a subsample 

 of nine individuals after fixing in formalin (4%), re- 

 moving the cardiac-pyloric stomach from the animal, 

 dissolving its contents in two drops of water and plac- 

 ing them on a smear slide. Perez and Aurioles- 

 Gamboa 2 determined that the average stomach 

 composition in a swarm of crabs does not vary sig- 

 nificantly after a sample of six crabs. 



Food items were identified and counted under the 

 microscope and relative abundance of major groups 



: Perez, F. R., and D. Aurioles-Gamboa. 1992. Cambios en la 

 alimentacion invierno-verano de la langostilla Pleuroncodes 

 planipes, en la costa oeste de Baja California. Paper pres. at 

 the DC International Symposium of Marine Biology, 1-5 June 

 1992. La Paz B. C. S. Mexico. 



