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Kodiak Laboratory

Juvenile Red and Blue King Crabs: Can They Coexist?

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Apr-May-June 2013
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Figure 1.  Photo of red king crabs in experimental setup on cobble habitat.


Red king crab (Paralithodes camtschaticus) (Fig. 1) and blue king crab (P. platypus) (Fig. 2) are two commercially important, federally managed species in Alaska, with many ecological similarities. For both species, larvae are released in the spring, are planktonic for about 2-3 months, feed on a diet of phytoplankton and zooplankton, and go through a series of four zoeal stages before molting to a non-feeding, post-larval glaucothoe stage.  Glaucothoe seek out structurally complex habitats on the ocean bottom for settling (cobble, shell-hash, tubeworms, and hydroids) and protection from predators and foraging opportunities. After settling, glaucothoe molt to the first juvenile stage and remain on the ocean bottom for the rest of their lives. As juveniles and adults, both species are opportunistic omnivores and consume a range of food items including bivalves, gastropods, echinoderms (sea stars and brittle stars), and crustaceans. 

Given their ecological similarities, it is unknown whether both species can coexist successfully. In many cases, competition between two ecologically similar species leads to the exclusion of one. The ranges of red and blue king crabs do not generally overlap much. In Alaskan waters, red king crabs occur throughout the Gulf of Alaska, in Bristol Bay, and Norton Sound, whereas the only currently healthy blue king crab population is around St. Mathew Island. Generally, blue king crabs live in cooler areas, but the relationship between the distribution of the two species and temperature is not clear-cut.  The Pribilof Islands is an area where both species co-occur and where population abundances of both species have historically fluctuated. In the 1970s blue king crab dominated the area around the Pribilofs, but in the early 1980s the population crashed.  Then in the early 1990s the populations of both species increased, but in the late 1990s the blue king crab population crashed again and has not recovered.

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Figure 2.  Photo of blue king crabs on shell hash habitat.  Photo by Scott Van Sant.


Although there is no evidence to suggest that adult red and blue king crabs compete for resources, no one has examined the potential for coexistence at the juvenile stage.  One potential mechanism that would allow coexistence is habitat partitioning, where each species utilizes different structures for food or refuge from predators.  Some field studies suggest that red king crabs prefer cobble habitat whereas blue king crabs prefer shell hash.  Our study was designed to test the hypothesis that habitat type can influence the ability of both species to coexist. To test this hypothesis, year-0 crabs were reared in the laboratory in small tubs for 13 weeks with either cobble or shell hash as the habitat.  Three species treatments were fully crossed with habitat: 10 red king crab alone, 10 blue king crab alone, and a mixture of 5 red and 5 blue king crabs.  Four replicates of each combination of habitat and species were established. Crabs were reared at 5ºC, which is well within the range of tolerance for both species, and were fed to excess three times a week.  At the end of each week, all of the habitat was removed from each tub and all of the surviving crabs were counted.  The mortality rates for each species was calculated in each tub and compared among treatments.

Habitat type and species composition affected mortality rates for red and blue king crabs.  Both species had better survival in shell hash than in cobble (Figs. 3 and 4).  However, red king crabs survived better in the presence of blue king crabs, especially in shell hash, where the mortality rate was almost cut in half (Fig. 3).  In contrast, blue king crab survival was much poorer when reared with red king crabs. Survival was especially poor in cobble where all of the blue king crab died within 9 weeks when red king crabs were present; however, the data suggest that all blue king crabs would have also died in shell hash within weeks if the experiment had not been ended (Fig. 4). Most of the mortality in the trials was probably due to cannibalism and inter-species predation as no dead crabs were removed from the tanks. 

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Figure 3.  Proportional survival of year-0 red king crabs in cobble and shell hash habitats reared alone (RKC) and with blue king crabs (RKC/BKC).  Points represent average ± 1 SE, and lines represent the best-fit trend lines assuming a constant rate of mortality.

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Figure 4.  Proportional survival of year-0 blue king crabs in cobble and shell hash habitats reared alone (BKC) and with blue king crabs (RKC/BKC).  Points represent average ± 1 SE, and lines represent the best-fit trend lines assuming a constant rate of mortality.


The data from this study suggest that habitat alone does not allow red and blue king crabs to coexist.  Although shell hash habitat did reduce blue king crab mortality compared to cobble, adding red king crabs to the trials substantially increased mortality regardless of habitat type.  Red king crab juveniles had a substantial advantage over blue king crabs in this study under all tested condition, warranting further investigations into the role of competition and predation between the species.

By William Christopher Long




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