Aronson, R.B., Smith, K.E., Vos, S.C., McClintock, J.B., Amsler, M.O., Moksnes, P.-O., Ellis, D.S., Kaeli, J., Singh, H., Bailey, J.W., Schiferl, J.C., van Woesik, R., Martin, M.A., Steffel, B. V., Deal, M.E., Lazarus, S.M., Havenhand, J.N., Swalethorp, R., Kjellerup, S., Thatje, S., 2015. No barrier to emergence of bathyal king crabs on the Antarctic shelf. Proc. Natl. Acad. Sci. 201513962. doi:10.1073/pnas.1513962112
Changing climate makes some habitats unsuitable for its residents while also creating opportunities for other species to comfortably move into the neighborhood. Animals at the poles run out of options, as there is no further North or South to go in search of colder temperatures. This study investigates the emergence of new predators in the Antarctic, specifically durophagous king crabs (Lithodidae). Durophagy, or bone breaking, refers to predators that consume hard-shelled prey such as crustaceans, echinoderms and molluscs. No durophagous predators have made the shallow waters of the Antarctic home in as long as tens of millions of years. Few bone breakers live in the frigid Antarctic nearshore waters, so resident species do not often face predators and so lack natural defenses. New predators could seriously mix up the food web and further threaten the ability of the native Antarctic animals to resist extinction.
Researchers conducted a photographic survey off the western Antarctic Peninsula in 2010 (Figure 1) and a trapping study in 2015 to identify the abundance and species of king crabs. A research ship pulled an underwater camera-vehicle called SeaSled along eight transects. The camera hovers on average three meters above the seafloor. The work produced 38,018 pairs of overlapping images!
Overall the king crab abundance was greater at greater depths. King crabs are known to live at the greater depths already, but this study shows the population’s movement towards shallower depths. At this location the shallow waters are colder than the deeper water. King crabs were seen in waters down to 0.43 C (their physiological limit is 0.4 C). Crabs were seen in precopulatory embraces (the male crab holding the female crab before mating begins) as well as at different age classes (juvenile and adult) suggesting a viable population exists in the area.
Prey species, including brittle stars, sea stars and snails, were observed as well. The prey species showed lower densities at depths where king crabs are present indicating the predatory crabs may be lowering their numbers (Figure 2). However, though this may be bad news for the prey species the king crabs feed on a wide variety of prey and are likely to find plenty to eat in their new Antarctic habitats wherever they go.
King crab growth could also be restricted as they encounter predators of their own such as the Antarctic toothfish, skates, octopuses, or seals. The researchers used the photos to quantify evidence of predation through counting missing legs. Missing or damaged limbs were seen in 8.9% of observed king crabs, a number much lower than what is seen in crustaceans such as the blue crab in temperate climates (one estimate at 23%). This suggests predation may not be much of a concern for king crabs, and not much of a help to keep the population in check.
Climate change brings layers of problems to the ocean- warming, sea level rise, increased acidity and changing ecosystem composition and altering the food web. Sea surface temperatures in the Antarctic summers have risen 1.5 C over the last 50 years, much faster than the global temperature change. What may seem like a small change in temperature is enough to open up a suitable habitat for new animals such as king crabs. With few predators evolved to eat king crabs and an abundance of prey we are witnessing a changing food web.
Different species are already showing different responses to changing climate and this study shows us an example of how there will be winners and some losers as things heat up. Some animals will move to newly suitable habitat but others will be pushed to extinction. Populations will die out as they are pushed to their limits and increasing predation pressure may be an added threat.