This year in the marine invertebrate biology lab we received a couple of pelagic red crabs (Pleuroncodes planipes) from the Southern California dive company that supplies some of the animals we study. The pelagic red crab (Pleuroncodes planipes), pictured below (Figure 1) is a member of the crustacean family Galatheidae, a group of small, decapod “squat-lobsters” (Kato, 1974).
Figure 1. Dorsal view of the Pleuroncodes planipes, the pelagic red crab.
The pelagic phase is commonly described as the planktonic period in the life of the species, which is usually about one year. However, these crabs are awesome because they have an almost unique ecology: adults are capable of very extended pelagic existence, which may be as much as two years (Etnoyer et. al. 2004). Pelagic red crabs have abdomens and fan-shaped tails that are well adapted for swimming. Moreover, their long first pair of legs is armed with pincers, typically known as the chelipeds. They have rows of hair-like setae that are found on the front and hind margins of the walking legs (Figure 2), which aids in slowing down when descending in a water column (Kato 1974). I found this interesting because there is not a lot known about the function of setae in many crab species. Figure 2 shows a close up the hair-like setae (with a scallop attached!).
Figure 2. (left) Another view of Pleuroncodes planipes; (right) close-up of of hair-like setae, which the animal uses in part to slow it’s descent in the water column. (Note the tiny scallop that has attached.)
In nature, P. planipes are seen swimming upwards near the surface by flexing their abdomen and then descend by spreading their thoracic limbs, during which grazing by filtration is observed (Longhurst et al. 1967). They are omnivorous and versatile in their food habits. As mentioned above, they are unusual in that they can also feed directly on phytoplankton by filter-feeding (Kato 1974). Boyd briefly describes that filtration is achieved by a casting action of the fan-shaped endopodites of the third maxilliped, which beats in a simultaneous manner (1963). This feeding mechanisms can be seen in the video below.
Figure 3: Pelagic red crabs exhibiting some aspects of feeding behavior in the lab. These behaviors were observed in the absence of food, and so may serve another function. (Video Credit: Priscilla Le, Vimeo)
Though these animals are usually found off the coast of Baja California, their abundance in the coming years are going to be off-the-charts! What is even more extraordinary is that there have been mass strandings of these crabs along the coast of Southern California. The supposed reason is that water temperatures have been rising in the past couple of years, an environmental shift that benefits these crabs. So, I decided I wanted to dig a little deeper. As mentioned earlier, their general center of distribution is near the west coast of central and southern Baja California. However, when the crabs are in the pelagic phase, they are dispersed to the north by the coastal countercurrents and to the southwest by the California Current and Equatorial Current, aiding their distribution along the coast of Southern California (Boyd 1963). A 2003 survey done by Peter et. al. concluded that 70% of occurrences are in waters of 16-21oC, with their extreme habitat temperature range of 9-28OC. The average temperature range along the Southern California coast is around 14-21 oC. The region of the most abundance is over the continental shelf off southern Baja California, more specifically they correspond to the plankton-rich waters of coastal upwelling (Kato 1974). Upwelling plays a critical role in the oceans in that it supplies nutrients from the cooler to the warmer waters, stimulating new production. It is driven mainly by wind and can occur whenever large-scale offshore transport of surface water happens (Robinson et al. 2004). One of the most remarkable characteristic in pelagic crabs’ life history is the occurrence of mass strandings. These have been reported in Magdalena Bay of Baja California and along the coast of Southern California, which are pictured below (Figure 4). However, there is a significant difference between strandings in these two locations. Pelagic red crab wash-up in Magdalena Bay occur annually and is very predictable. In contrast, pelagic red crab beachings in California occur only during El Nino events when warm water is pushed much further north than usual along the Pacific coast. The species’ extreme habitat temperature range enables the populations to follow warm currents generated by the El Nino events, ultimately distributing them into the waters off Southern California (David et al. 1994).
Figure 4. Magdalena Bay and the Southern California Coast, sites included in the normal annual distribution of pelagic red crab. (Credit: Google Maps)
In these years, P. planipes play a dominant role in local ecosystems, grazing down diatom blooms in the southern part of the California Current (Longhurst et al. 1967). A 2002 study done by Robinson et al. concluded that 90% of P. Planipe’s abundance could be explained by tracking their food (as measured by chlorophyll-a concentrations, a proxy for the presence of diatioms), which was intensified in upwelled areas, showing that active upwelling sites is of considerable importance in shaping the spatial distribution of these species. Further, these results indicate that the productivity of the California Current depends on blooms of diatoms and that large populations of pelagic red crabs plays an important role in the utilization of the energy made available by primary production (Longhurst et al. 1967). The feeding habits of these species strongly suggest that the massive aggregations of the pelagic red crabs can be sustained only in certain areas where phytoplankton is highly abundant. In addition to P. planipes being one of the most significant consumers of phytoplankton and making use of available energy by primary production during coastal upwelling, some studies suggest that they are also the most common prey for marine vertebrates in the area (David et al. 1994). Due to their unique ability to feed on phytoplankton, they are an important and direct intermediate link in the food chain from primary productivity to terminal predators (Longhurst et al. 1967). Pelagic red crabs washes up along the coast in such abundance that they become prey for several species of tuna, sea birds, turtles, and whales. Alverson extensively studied the importance of P. planipes in the diet of yellowfin tunas. He found that the mean volume of crabs contained in each examined yellowfin stomach was about 0.5 liters or approximately 100 crabs (Boyd 1963). This suggests that the great quantities of crabs consumed by the yellowfin tuna shows that Pelagic red crabs do indeed play a dominant role in the biological economy of the area.
With their extensive movements into oceanic regions in response to El Nino events, it enables them to transport the accumulated organic matter far offshore while also grazing in coastal upwelling. This provides an important link in the coastal ecosystem. However, when the occurrence of El Nino events comes to a halt, the pelagic red crabs will likely go back to their original habitat range. This suggests that pelagic red crabs can be good indicators of El Nino events. They may seem small and cute, but in abundance, they dramatically contribute to the coastal ecosystem.
References:
Alverson, F. G. (1963) The food of the yellowfin and shipjack tunas in the eastern tropical Pacific Ocean. Bull. Iter-Amer. Trop. Tuna Comm. 7: 295-396.
Aurioles-Gamboa, David, and Maria Castro-Gonzalez, and Ricardo Perez-Flores (1994) Annual mass strandings of pelagic red crabs, Pleuroncodes planipes (Crustacea: Anomura: Galatheidae), in Bahia Magdalena, Baja California Sur, Mexico. Fishery Bulletin 92: 464-470.
Boyd, C. M. (1963) Distribution, trophic relationships, growth, and respiration of a marine decapod crustacean Pleurocodes planipes Stimpson (Galatheidae). University of California San Diego, PhD. Thesis. 67.
Etnoyer, Peter, and Dave Canny, and Bruce Mate, and Lance Morgan (2004) The Answer Must Be Red Crabs, Of Course. Oceanography 17: 6-7.
Kato, Susumu (1974) Development of the Pelagic Red Crab (Galatheidae, Pleuroncodes planipes) Fishery in the Eastern Pacific Ocean. Marine Fisheries Review 36: 1-9
Longhurst, Alan R., and Carl Lorenzen, and William Thomas (1967) The Role of Pelagic Crabs in the Grazing of Phytoplankton Off Baja California. Ecology 48: 190-200.
Robinson, Carlos J., and Vicente Anislado, and Antonio Lopez (2004) The pelagic red crab (Pleuroncodes planipes) related to active upwelling sites in the California Current off the west coast of Baja California. Deep-Sea Research II 51: 753-766.
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