Sea Spiders: Abnormally Large-Growing Arctic Sea Spiders Surprise Scientists

In the dark, cold waters of both the Antarctic and Arctic oceans, sea spiders are growing larger than usual. Researchers say this phenomenon, known as polar gigantism, may or may not be attributed to the abundance of oxygen in the seawater.

With eight exceptionally gangly legs and a long nose to match, these "spiders" are scientifically known as pycnogonids - a type of primitive marine arthropod (an invertebrate with an exoskeleton) - and can be found all over the world. While these sea creatures are generally very small, the ones found living in the Polar oceans can grow legs up to nine inches long.

Pycnogonids are not the only abnormally huge animals lurking at the bottom of the Polar oceans. In fact, many creatures of unusual size are strewn across these rather unique habitats, including certain crustaceans called copepods, echinoderms (a group of creatures with radial symmetry that includes sand dollars and sea stars), and certain mollusks that grow larger than their more equatorial relatives. However, why these creatures are growing so large remains a mystery. While several hypotheses have been proposed, scientists have not been able to prove any of them yet.

In the latest study, researchers from the National Science Foundation, the United States Antarctic Program, University of Hawaii at Manoa, and University of Montana drilled through layers of thick sea ice and dove into cold Polar waters to collect some sea spiders and get some answers. They hypothesized that the creatures' size may ultimately contribute to a higher dissolved oxygen concentration in cold water, according to Hakai Magazine. (Scroll to read more...)

Colder water can hold more dissolved oxygen than warm water, and the oxygen content in seawater is especially high off the coast of Antarctica. Colder temperatures ultimately slow the metabolisms of cold-blooded animals, meaning they consume less oxygen, thus creating supersized marine creatures.

These marine arthropods lack a respiratory system and therefore depend on simple diffusion to get oxygen into their bodies, but in order for this to work for larger creatures, there needs to be an abundant source of oxygen. For their study, researchers tested how differences in temperature and dissolved oxygen content in seawater affected the physiology of Antarctic pycnogonids. The results confirm larger sea spiders have a difficult time living on seawater with low oxygen content, supporting their hypothesis that abundant levels of oxygen influence the growth of these creatures.

As oceans continue to warm and oxygen levels fall, their findings have implications for the conservation of not only sea spiders, but for the fate of other large marine creatures, including jumbo lobsters and king crab.

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