Annual mass strandings of pelagic (100-200 m), and probably invades the continental slope (Aurioles­ red crabs, Pleuroncodes planipes Gamboa, 1992). Population with­ drawal is associated with a rise in (Crustacea: Anomura: Galatheidae), bottom temperature above 16°C, and pelagic red crabs remain from in Bahia Magdalena, June through October in waters 100-200 m deep, where bottom Baja California Su~ Mexico temperature is in "the range of 12-16°C (Aurioles-Gamboa, 1992). David Aurioles-Gamboa One of the notable characteris­ Centro de Investigationes Biologicas de Baja California Sur tics in pelagic red crab life history Apartado Postal 128. LA Paz B. C. S.• Mexico are mass strandings, which have been reported for Bahia Magda­ lena and the California Coast Maria Isabel Castro-Gonzalez (Glynn, 1961; Boyd, 1962; Kato, Instituto Nacional de la Nutrici6n "Salvador Zubiran" 1974; Stewart et aI., 1984). The Vasco de Quiroga #15. Col. y Del. Tlalpan. J4000. Mexico D. F. main difference between strand­ ings in California and Baja Cali­ Ricardo Perez-Flores fornia is the frequency of ocur­ Centro de Investigaciones Biologicas de Baja california Sur Apartado Postal J28. La rence. Pelagic red crab beachings Paz B. C. S.• Mexico. in California occur during EI Niiio events, which enable the popula­ tion to move northward in warm water currents originating in the south. In contrast, pelagic red crab Pelagic red crabs (or langostilla in about 17-20 mm standard cara­ strandings in Bahia Magdalena Spanish), Pleuroncodes planipes, pace length (SCL), they become are annual, and apparently recur are very abundant galatheid crus­ benthic, making occasional move­ on the same beaches and during taceans off the west coast of Baja ments to the surface (mostly at the same season ofyear. California. Some studies suggest night) in a circadian migration InBahiaMagdalena, pelagic red that pelagic red crabs are the most (Boyd, 1967). Once the animals crabs were observed in the upper abundant species in the micro­ reach 32-34 mm SCL, they are 50 cm of water of the surf zone neck.ton, one ofthe mostimportant fully benthic as are other galat­ before stranding. Onshore winds consumers of phytoplankton, and heid species (Boyd, 1967;Aurioles­ and receding tides hasten and "the most common prey item for Gamboa, 1992). intensify stranding (Boyd, 1962>. many marine vertebrates in the Pelagic red crabs breed from Kato (1974) proposed that the area (Boyd, 1962, 1967; Black­ December throughApril; the peak presence ofpelagic red crabs near burn, 1969; LonghurstetaI., 1967; of the reproductive season is in shore is primarily due to winds, Kato, 1974; Galvan, 1988; Balart February(Boyd, 1962; Kato, 1974; waves, and currents. and Castrol ). GOmez, 1990). Females about 14­ On 9 May 1991, one ofus (D.A­ Although widely used, the com­ 15 mm SCL, have been found car­ G.) observed a mass stranding of mon name (pelagic red crab) de­ ryingeggs, but most ofthe females pelagic red crabs on a beach of scribes only the planktonic period startto breed when they are about Magdalena Island close to the (about one year) in the life of the 20 mm SCL (Boyd, 1962; Serrano, mouth ofthe Bahia Magdalena on species. Larvae, juveniles, and 1991). the Pacific coast of Baja Caliror­ young adults are planktonic. At The benthic population per­ nia (Fig. 1). About 1100 hours, a forms seasonal bathymetric move­ compact surface swarm of live ments, at least in the area from crabs was seen 1-2 m from the 1 Balart-Pliez, E., and Castro-Aguirre, lat. 24° to 26° N, in which they dis­ beach. Small groups ofcrabs were J. L. 1992. Habitos alimenticios de la merluza Bajacaliforniana Merluccius perse during winter and spring to thrown to the beach by waves, and angustimanus, en la costa occidental de occupy the benthos of the conti­ were unable to return to the sea. Baja California Sur, Mexico. Paper pres. nental shelf (0-200 m depth). M­ at the IX International Symposium of Marine Biology, 1-5 June 1992, La Paz ter the breeding season, the popu­ Manuscript accepted 4 October 1993 Baja California Sur, Mexico. lation moves to deeper waters Fishery Bulletin 92:464-470 (1994) 464 NOTE Aurioles-Gamboa et al.: Mass strandings of Pleuroncodes planipes 465 and may produce, depending on body size, liZ° from 500 to 5000eggs ineach brood (Serrano, 1991). Basedonthis fact, debilitation caused .' by reproductive investment should be more evident in females. For that reason, the chemical composition ofpelagic red crabs in Bahia Magdalena was determined for both sexes. We were aware that males were over­ represented in samples collected on the con­ b tinental shelf (Boyd, 1962; Serrano, 1991); ~ thus we also wished to determine ifstranded (') animals were female-biased, as an explana­ ... tion for the unbalanced sex ratio recorded on the continental shelf. (') Materials and methods o To test whether the strandings were due to (') weakness caused by starvation or malnutri­ III tion, about 20 kg oflive-stranded pelagic red :. crabs were collected as soon they were z stranded.. From this sample, a total of 1,150 individuals were sexed and measured for SeL to the nearest 1.0 mm. Sex was deter­ ISLA SANTA mined by the presence or absence of modi­ MAReARITA fied pleopods, which males use to fertilize the eggs (Boyd, 1962). Standard carapace length 11° was measured from the antorbital notches of the rostrum to the midpoint of the poste­ riorborderofthe carapace (Kato, 1974). This Figure 1 measurement is usually preferred over total Study area on the west coast of Baja California Sur, Mexico. length because it does not vary with shrink­ Dots inside Bahia Magdalena indicate the location of stations age ofthe abdomen. sampled. The shaded area is the region where pelagic red crabs, Stomach contents were analyzed in order Pleuroncodes planipes, were found as indicated by positive sta­ to determine if1) the animals hadbeenfeed­ tions and visual observations (from Solis, 1991). ing before stranding, and 2) the number of items and composition was similar to stom­ ach contents ofpelagic red crabs collected on The general appearance of these pelagic red crabs the continental shelf(Perez andAurioles-Gamboa2). (color and mobility) suggested that the crustaceans We examined the stomach contents from a subsample were healthy and that the stranding could be con­ of nine individuals after fixing in formalin (4%), re­ sidered accidental (Boyd, 1962). We questioned local moving the cardiac-pyloric stomach from the animal, fishermen about crab strandings and determined dissolving its contents intwo drops ofwater andplac­ that mass strandings 1) occur annually, usually in ing them on a smear slide. Perez and Aurioles­ spring, from April through June, and 2) are common Gamboa2 determined that the average stomach on Santa Margarita and Magdalena Islands but composition in a swarm of crabs does not vary sig­ rarely seen on the peninsular coast. nificantly after a sample of six crabs, Since strandings coincide withthe end ofthebreed­ Food items were identified and counted under the ing season, we addressed the null hypothesis that microscope and relative abundance ofmajor groups stranded pelagic red crabs, particularlyfemales, were in a weakened state because ofenergy expended in 2 Perez, F. R., and D. Aurioles-Gamboa. 1992. Cambios en la reproduction, as reported for many crustaceans alimentaci6n invierno-verano de la langostilla Pleuroncodes planipes, en la costa oeste de Baja California. Paper pres. at (Hartnoll, 1985). It is known that pelagic red crabs the IX International Symposium of Marine Biology, 1-5 June are able to breed twice in a single breeding season 1992. La Paz B. C. S. Mexico. 466 Fishery Bulletin 92/2). 1994 of food items was determined. Following Perez and Pelagic red crabs had been caught from the shelfin Aurioles-Gamboa2, we recorded some of the phyto all seasons, but only during mid-summer (when the and zooplankton components in four major groups: bottom-surface temperature difference is as great as 1) phytoplankton, 2) zooplankton, 3) particulate or­ 17"C) did the crabs show signs of damage as they ganic matter (POM), and 4) inorganic matter (small moved slowly and died rapidly on the deck (Aurioles­ grains of sand, clay or mud). The number of Gamboa, unpubl. data). In contrast, crabs were very diatoms, crustacean parts, foraminifers, and other active in the first minutes after stranding, andmoved components, such as small agglomerations of POM, less frequently later. The crabs were brilliant red, were counted and their numbers converted to rela­ which differentiatedthemfrom thelightercolor ofcrabs tive frequency. collected on thecontinental shelf. Basedontheirvigor­ Proximate analyses was based on 200 g 0 and ous activity, the pelagic red crabs stranded on 200 g <? of sun dried and milled pelagic red crabs Magdalena Island(Fig. 1)appearedtobeingood health. (about 4 kgs offresh crabs). The techniques usedwere those ofthe Association ofOfficial Analytical Chem­ Stomach contents of stranded ists (A.O.A.C., 1984): moisture (7.007), ash (7.009), pelagic red crabs crude fiber (7.006), crude protein (2.057), ether ex­ The total number ofitems and relative frequency of tract (7.060), carbohydrates (by difference from all the four majorgroups found in the stomachs is shown other determinations at 100%). This methodology had in Figure 2A. For comparison, the results of a typi­ been used previously to analyze pelagic red crabs cal sample taken on the continental shelfis provided sampled from the benthos of the continental shelf in Figure 2B (Perez, 1992). The minimum number of (Castro, 1993). Two-sample t-tests (Zar, 1984) were items counted was 457 and the maximum about 2,266. conducted to identify differences in food composition, This range was greaterthan thatfound in crabs from proximate composition, and mean SCL between the continental shelf (841-1,495 items).