Annual mass strandings of pelagic 

 red crabs, Pleuroncodes planipes 

 (Crustacea: Anomura: Galatheidae), 

 in Bahia Magdalena, 

 Baja California Sur, Mexico 



David Aurioles-Gamboa 



Centra de Investigationes Biologicas de Baja California Sur 

 Apartado Postal 128, LA Paz B. C. S , Mexico 



Maria Isabel Castro-Gonzalez 



Instituto Nacional de la Nutricibn "Salvador Zubir^n" 



Vasco de Quiroga #15. Col y Del Tlalpan 14000. Mexico D. F 



Ricardo Perez-Flores 



Centra de Investigaciones Biologicas de Baja California Sur Apartado Postal 1 28, La 

 Paz B. C S . Mexico. 



Pelagic red crabs (or langostilla in 

 Spanish), Pleuroncodes planipes, 

 are very abundant galatheid crus- 

 taceans off the west coast of Baja 

 California. Some studies suggest 

 that pelagic red crabs are the most 

 abundant species in the micro- 

 neckton, one of the most important 

 consumers of phytoplankton, and 

 the most common prey item for 

 many marine vertebrates in the 

 area (Boyd, 1962, 1967; Black- 

 burn, 1969;Longhurstetal., 1967; 

 Kato, 1974; Galvan, 1988; Balart 

 and Castro 1 ). 



Although widely used, the com- 

 mon name (pelagic red crab) de- 

 scribes only the planktonic period 

 (about one year) in the life of the 

 species. Larvae, juveniles, and 

 young adults are planktonic. At 



1 Balart-Paez, E., and Castro-Aguirre, 

 J. L. 1992. Habitos alimenticios de la 

 merluza Bajacaliforniana Merluceius 

 angustimanus, en la costa occidental de 

 Baja California Sur, Mexico. Paper pres. 

 at the IX International Symposium of 

 Marine Biology, 1-5 June 1992, La Paz 

 Baja California Sur, Mexico. 



about 17-20 mm standard cara- 

 pace length (SCL), they become 

 benthic, making occasional move- 

 ments to the surface (mostly at 

 night) in a circadian migration 

 (Boyd, 1967). Once the animals 

 reach 32-34 mm SCL, they are 

 fully benthic as are other galat- 

 heid species (Boyd, 1967; Aurioles- 

 Gamboa, 1992). 



Pelagic red crabs breed from 

 December through April; the peak 

 of the reproductive season is in 

 February (Boyd, 1962; Kato, 1974; 

 Gomez, 1990). Females about 14- 

 15 mm SCL, have been found car- 

 rying eggs, but most of the females 

 start to breed when they are about 

 20 mm SCL (Boyd, 1962; Serrano, 

 1991). 



The benthic population per- 

 forms seasonal bathymetric move- 

 ments, at least in the area from 

 lat. 24° to 26° N, in which they dis- 

 perse during winter and spring to 

 occupy the benthos of the conti- 

 nental shelf (0-200 m depth). Af- 

 ter the breeding season, the popu- 

 lation moves to deeper waters 



( 100-200 m), and probably invades 

 the continental slope (Aurioles- 

 Gamboa, 1992). Population with- 

 drawal is associated with a rise in 

 bottom temperature above 16°C, 

 and pelagic red crabs remain from 

 June through October in waters 

 100-200 m deep, where bottom 

 temperature is in the range of 

 12-16°C (Aurioles-Gamboa, 1992). 



One of the notable characteris- 

 tics in pelagic red crab life history 

 are mass strandings, which have 

 been reported for Bahia Magda- 

 lena and the California Coast 

 (Glynn, 1961; Boyd, 1962; Kato, 

 1974; Stewart et al., 1984). The 

 main difference between strand- 

 ings in California and Baja Cali- 

 fornia is the frequency of ocur- 

 rence. Pelagic red crab beachings 

 in California occur during El Nino 

 events, which enable the popula- 

 tion to move northward in warm 

 water currents originating in the 

 south. In contrast, pelagic red crab 

 strandings in Bahia Magdalena 

 are annual, and apparently recur 

 on the same beaches and during 

 the same season of year. 



In Bahia Magdalena, pelagic red 

 crabs were observed in the upper 

 50 cm of water of the surf zone 

 before stranding. Onshore winds 

 and receding tides hasten and 

 intensify stranding (Boyd, 1962). 

 Kato (1974) proposed that the 

 presence of pelagic red crabs near 

 shore is primarily due to winds, 

 waves, and currents. 



On 9 May 1991, one of us (D.A- 

 G.) observed a mass stranding of 

 pelagic red crabs on a beach of 

 Magdalena Island close to the 

 mouth of the Bahia Magdalena on 

 the Pacific coast of Baja Califor- 

 nia (Fig. 1). About 1100 hours, a 

 compact surface swarm of live 

 crabs was seen 1-2 m from the 

 beach. Small groups of crabs were 

 thrown to the beach by waves, and 

 were unable to return to the sea. 



Manuscnpt accepted 4 October 1993 

 Fishery Bulletin 92:464-470 ( 1994) 



464 



