The discovery of particularly large sea spiders, with a length up to 25 cm (10 inches), in the polar seas both Arctic and Antarctic is causing puzzlement in the scientific world because of the difficulty in finding an explanation to these animals’ size. They’re actually called pycnogonids, but are also known as Pantopoda, and are arthropods belonging to the class Pycnogonida, which means that they are not really spiders.
The precise classification of pycnogonids is still under discussion and the fact that few fossils associated with them have been found makes it even more difficult to understand the exact relation within the arthropods phylum. Generally pycnogonids are considered a class within the subphylum Chelicerata but according to many scientists they have anatomical and physiological characteristics that make them different from other arthropods so they should constitute a separate subphylum.
At best, pycnogonids are part of the same subphylum of the arachnids but all scholars agree consider them as a separate group of sea animals. Consequently, the expression sea spiders is incorrect but actually these arthropods resemble in many respects certain spiders, especially because they generally have 8 legs. Some species, however, can have 10 or even 12 legs.
The approximately 1,300 species of known pycnogonids have widely varying sizes. The oddity is due to the discovery of large specimens of species known for much smaller sizes. This discovery was made in the polar seas, in both the area close to the Arctic and the one near the Antarctica. It’s not a new phenomenon, so much so that already in 1999 an article in the journal “Nature” described what was called “polar gigantism”, although about shellfish.
One hypothesis to explain the phenomenon of polar gigantism, which also concerns other types of sea animals, is that it’s due to the concentration of oxygen present in the cold polar waters. It’s higher than in the warm waters of other sea areas and near the Antarctic coast is particularly high.
The animals that live at those temperatures have a slower metabolism and this, together with the increased presence of oxygen, could allow them to grow more than the organisms that live in warm waters. A slow metabolism results in a lower consumption of oxygen so the organisms that live in these cold seas can exploit the available amount in an optimal way.
Some experiments were conducted on the sea spiders found by placing them in different conditions of temperature and oxygen content. The result is that their metabolism was negatively affected by lower amounts of oxygen. This supports the theory of the link between oxygen and polar gigantism but those are still limited checks.
Further tests, both genetic and metabolic, are needed to better understand the phenomenon of polar gigantism in sea spiders but also in other organisms. This may look like just scientific curiosity and creatures such as sea spiders can cause repulsion but thinking about the ongoing climate changes this kind of research can help us understand what consequences there might be for organisms that live in the oceans.
