Most people tend to think that sea spiders are merely some kind of spiders that live in the sea, however this isn’t the case at all. In actual fact sea spiders are completely unrelated to land spiders, and are only called spiders at all because of their superficial resemblances.
The class name for sea spiders is Pycnogonida, and there are eighty-six separate genera within this. Unlike land spiders, sea spiders are not arachnids, and in fact belong to the family of arthropods which includes insects and crustaceans as well as arachnids.
There are currently over 1300 known species of sea spider, which range in size from under one millimeter, to nearly 1 meter long in some species. Although most have four pairs of legs, similar to land spiders, there are also some species with five and six pairs. Compared to land spiders, they also tend to have small, slender bodies and much longer legs, which enables them to move around easier in the water.
Sea spiders generally spend their time traveling slowly along the ocean floor, usually looking for food. Although they are rarely seen in the wild by most people, sea spiders can in fact be found in nearly every sea in the world. Also most species can be found in fairly shallow water, although the larger species tend to live deep in the Antarctic.
The fact that sea spiders can be found in virtually any water temperature means that they are clearly a very successful species. Also despite the lack of any fossil record, they are thought to be an extremely old species, perhaps being older then the dinosaurs. The lack of any fossil record is largely due to the fact that they have soft bodies, and also tend to be eaten on the seldom occasions that they die naturally.
Sea spiders are thought to have few natural predators, largely because they are often superbly camouflaged against their potential predators. Also the way that they move means that they are often mistaken for seaweed. They are sometimes eaten, although due to their size and lack of much of a body, they are usually only prayed upon opportunistically.
Despite the fact that sea spiders are slow moving and incapable of being noticeable aggressive, they are in fact nearly all predatory. They have a mouth part called a proboscis, which is like a sharp straw, which they stab into prey in order to drink the nutrients within. However, despite this sounding violent, their prey is primarily stationary species such as anemones, polychaetes and sponges.
Furthermore, usually the sea spider’s prey species survive being fed on, which means that while feeding, they don’t deplete the numbers of any other species. This is particularly useful in Antarctica, where the numbers of marine species are much less dense than in warmer waters. It is also thought that this is the reason that species tend to get a lot bigger, and live a lot longer in the extreme cold, so that they have enough time in their lifetime to find a mate in the vast empty space that is the ocean floor.
Although sea spiders may appear to be quite large and intimidating when into he water, they are in fact harmless to humans. Even the largest species are extremely slow moving, and lack the strength required to lift their limbs when removed from the water. Even if encountered while diving, sea spiders would be unable to pierce human skin, much less a diving suit, and are in no way aggressive or reactive, even if handled.