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



467 



It was also noted that during handling of stranded 

 crabs an oily, orange film was left in the containers, 

 a phenomenon not previously observed in crabs col- 

 lected in more than 200 bottom trawls on the conti- 

 nental shelf (Aurioles-Gamboa, 1992). The substance 

 probably contained carotenoids, which in this species 

 have been identified as astaxanthins (Wilkie, 1972). 



Size and sex of stranded pelagic red crabs 



Pelagic red crabs collected in Bahia Magdalena were 

 12-28 mm SCL. Size distribution was similar for 



1 23456789 

 NUMBER OF STOMACHS 



2500 



1 2 3 4 5 6 7 8 

 NUMBER OF STOMACHS 



ORGANIC MATTER m INORG MATTER ^ ZOOPLANKTON □ PHYTOPLANKTON 



Figure 2 



Total number of items and gross stomach composition 

 from pelagic red crabs, Pleuroncodes planipes, collected 

 in Bahia Magdalena (A), and organisms typically found 

 on the benthos of the continental shelf off Baja Califor- 

 nia (B). POM = Particulate Organic Matter, IM = Inor- 

 ganic Matter. Numbers below the x-axis indicate num- 

 ber of stomachs. 



males and females (Fig. 3); mode and mean were 

 15.82 and 17.02 mm for females and 17.36 and 17.26 

 mm for males, respectively. Differences in mean size 

 of males and females were statistically significant 

 (two-sample *-test=2.057,P<0.05,df=l,147). In older 

 organisms, slight sexual dimorphism is evident in 

 males which are slightly larger and heavier with 

 longer and wider chelae (Serrano and Aurioles- 

 Gamboa, 1991). 



Pelagic red crabs are about 14 mm SCL at the end 

 of the first year of life (about February-March) and 

 grow approximately 1 mm per month (Boyd, 1962). 

 Because the crabs were about 17 mm SCL (mode), 

 they should have been about 14-15 months old. 

 Some individuals were about 24-28 mm SCL (Fig. 

 3), which according to Boyd (1962) were about 26- 

 27 months old. 



The number of females (605) in relation to the 

 number of males (544), deviates significantly from 

 the expected 1:1 proportion (chi-square P<0.001). 

 In contrast, the sex ratio of crabs from the conti- 

 nental shelf is slightly male biased throughout the 

 year (Serrano, 1991). 



Discussion 



Debilitated pelagic red crabs as 

 explanation of strandings 



We rejected the null hypothesis that stranded pe- 

 lagic red crabs represent a debilitated fraction of 

 the population because 1 ) the stranded crabs moved 

 vigorously during and just after stranding, 2) the 

 stomachs of crabs were full and the contents were 

 similar to those collected on the continental shelf, 

 which indicated they were feeding normally, 3) the 

 chemical composition was generally similar to those 

 collected on the continental shelf, and when differ- 

 ent, did not suggest malnutrition, and 4) timing and 

 area of strandings are better explained by physical 

 phenomena (i.e. by accidental stranding due to fun- 

 neling effect, wave action, and receding tide). Our 

 observations support the observations made by 

 Boyd (1962), that waves and receding tide play a 

 major role after the animals enter the surf zone. 

 Two available stranding reports (Boyd, 1962; Jorge 

 Llinas 3 ), indicated that beachings occurred during 

 falling tides. According to local fishermen, the two 

 higher lines of stranded crabs we found on the beach 

 of Isla Magdalena on 9 May 1991, were stranded 

 two mornings before our visit, also during receding 

 tides. 



1 LHnas, J. Centro de Investigaciones Biologicas de Baja 

 California Sur. Apdo. Postal 128. La Pax Baja California Sur. 

 Mexico, personal commun. 1993. 



