Sea spiders are often called multi-legged animals. They belong to the class of Helicerovs, the type of these creatures is Arthropods. Also acceptable is the classification by which the term "Chelicerae" is defined as a subtype from which sea spiders are allocated in a separate class. There are several other options for scientific names for this class - Pantopods, Pycnogonids, and others.
Some general information
The concept of "sea spider" includes more than 1300 different species from a dozen families. They live in the seas around the world. Meet marine arthropods at different depths. Some species prefer the lower littoral (tidal section of the coast), while others descend to the abyssal (deep zone). In salty and slightly saline waters, multi-legged are much more common than in desalinated inland seas. In coastal areas, spiders settle in the thickets of algae and on the ground.
Deep-water and littoral species of spiders have differences in body structure and size. In the deep layers of water, the sea spider will be larger, it has much longer and thinner legs, on which long hairs can be. These devices reduce the speed of immersion. The spider does not just swim, but as if floating in water. To sink to the bottom, it is enough to compactly fold the long limbs under the body.
Coastal forms are more compact. Their legs are thicker and shorter, but they have developed tubercles and spikes that are necessary for hunting and protection.
Structural features
Any sea spider, both deep-sea and coastal, has a typical structure. The body is divided into two tagmas (departments). Their name is segmented millet and non-segmented desome. A millet is characterized by a cylindrical or discoid shape.
The body of sea spiders is smaller than the limbs and is covered with a chitinous cuticle. There is a division into the cephalothorax and abdomen (is vestigial). In the cephalothorax from 7 to 9 segments, 4 of them fused together. The fused part of the cephalothorax is called the cephalic segment. The remaining segments can be either intergrown or dissected. In front of the head segment is the trunk of a cylindrical or ovoid shape. On the lateral parts of the trunk 2 pairs of limbs are fixed: heliphores and palps. A third pair of limbs (ten-segmented egg-bearing legs) is fixed on the ventral side of the head. One of the structural features of sea spiders is that 3 front pairs of legs do not reach the ground and do not participate in walking.
Walking legs of the sea spider are fixed on the lateral processes of the head segment of the trunk. Most often there are 4 pairs, but some representatives have 5-6 pairs.
Digestive system
The sea spider has a digestive system in the form of a poorly differentiated through tube with diverticula. The diverticulum in this case is the process of the intestine that goes into each leg. Digestion in these arthropods is combined. Both the cavity and intracellular form are shared.
Diet
It is easy to guess what sea spiders eat. Most of them are predators. In their diet, sessile and inactive invertebrates. It can be polychaetes, bryozoans, ciliates, sea anemones, gastrointestinal and cephalopod mollusks, small echinoderm starfish. Extraction is held by claws on heliphores. They tear off pieces of food and go into the mouth.
Gigantomania
Not so long ago, a giant sea spider was found in the waters of the Antarctic. Studying the individual, scientists drew attention to a mysterious phenomenon, which was called polar gigantism. For some reason that is not yet known, the icy waters of Antarctica turn ordinary species of sea spiders into giants. Perhaps for increased growth is responsible for the amount of oxygen, which in cold water contains more than in warm water.
It has been established that gigantomania in Arctic waters affects not only spiders, but also some mollusks, crustaceans and echinoderms. Research is ongoing.
