Sea spider is the common name for all arthropods that belong to the class pantapoda, which is also commonly known as pycnogonida. Sea spiders are most common in polar regions but are found in benthic habitats all over the world; they have been found in all seas except the inner Baltic and the Caspian. They exist at many depths as well; sea spiders may inhabit the shallows of an intertidal region or abyssal depths as far as 6500 m below sea level and anywhere in between. Pycnogonids come in a wide range of sizes, varying from the smallest species measured at 2 mm to the largest at 50 cm. Lifestyles vary within the class pantapoda as well: sea spiders can be found clinging to their prey, walking about on the sea floor or even embedded in a shellfish.
The class name Pantapoda means "all legs" and accurately describes these animals; pycnogonids have a reduced body attached to long lateral appendages that are relatively enormous. The pycnogonid body is metameric, meaning the body is organized into segments called metameres. The sea-spider body is divided into two parts. The anterior metamere is known as the prosoma and can be compared to the cephalothoracic segment that is observed in many crustaceans. The posterior metamere is commonly referred to as the opisthosoma and can be compared to the abdomen segment that is observed in many insects. In pycnogonids the body is also organized into smaller segments called somites; each somite supports a pair of lateral appendages.
Located on the prosoma of the typical sea spider are seven pairs of lateral appendages, a tubercle containing four simple eyes and a proboscis that may be longer than the body itself. The first pair of appendages (the most anterior) is known as the cheliceras. The cheliceras are used for feeding; on the end of each chelicera is pincers that are used to grasp prey of to tear off bits of food and transport them to the mouth, which is located on the anterior end of the proboscis. The next pair of appendages is palpi. The palpi are used as sensory organs; many of the larger species sweep their palpi back and forth across the substrate to detect possible food sources. Sea spiders usually posses a pair of ovigers, which are located behind the palpi. The ovigers are used by the males and the females of many of the larger species to clean their appendages. The cleansing function of the ovigers is advantageous because the slow moving sea spiders often become encrusted with various organisms; this encrustation slows the spiders even more and makes them susceptible to predation. For the male sea spiders the ovigers also function as a means to carry egg masses. The cheliceras, palpi and ovigers may be reduced or absent in various species. In species that lack chelicera and palpi the proboscis is more developed and more mobile; the proboscis of these species often include numerous sensory bristles and strong rasping ridges around the mouth. The remaining four pairs of lateral appendages that are present on the prosoma of the typical sea spider are the walking legs. The function that the walking legs serve for the animal differs among species of sea-spiders: the walking legs of sea-spiders that live in intertidal regions are not adapted for walking at all, but for clinging to their prey and the sea-spiders of the deep ocean are larger and have longer legs, this makes them more mobile. Sea spiders that inhabit deep ocean habitats can move forward, backward, sideways and at any angle without turning their body. Most sea spiders possess four pairs of walking legs, but there are some species that have developed one or two additional pairs of walking legs.
The opisthosoma consists of a greatly reduced abdomen and is nearly nonexistent.
The typical sea spider feeds upon soft-bodied invertebrates including cnidarians, bryozoans, sponges, mollusks, echinoderms, tunicates and various worms. Some species have been known to feed upon detritus and algal material as well. The pycnogonid mouth is located at the anterior end of the proboscis, this is where food is sucked in. The interior walls of the proboscis are lined with teeth and spines. The teeth grind food material into fine particulate and the spines filter the particulate material, allowing only fluids and small, digestable food particles to pass into the midgut. Once the food enters the midgut, digestion occurs intracellularly and nutrients reach various parts of the body through branching cavities that extend into all of the walking legs, the cheliceras and the proboscis.
No specialized excretory glands exist in the sea spider. It is believed that waste is excreted through the body surface.
Pycnogonids posses no specialized respiratory glands. Respiration is believed to take place by diffusion through the body surface.
Sea-spiders posses an open circulatory system, it consists of a single dorsal tubular heart with two or three pairs of lateral ostia.
The nervous system of the typical sea spider is characteristic of the arthropod nervous system and includes a dorsal ganglion with connectives to the esophagus and a chain of paired ventral ganglion. For species that posses extra pairs of walking legs, extra pairs of ventral ganglion exist for each extra pair of walking legs.
Reproduction in sea spiders is sexual. The male and female gametes are released through pores in the legs and undergo external fertilization. The male then gathers the egg cluster into his ovigers where he will carry them until they are nearly ready to hatch. In some species the adult male will not drop the eggs before they hatch and it will transport young sea spiders in its ovigers for a short time. Male sea spiders have been known to mate repeatedly and have been observed with as many as 14 egg clusters, each one from a different female. In some species the female also mates repeatedly, secreting only a small portion of her eggs for each mate. When the eggs are close to hatching the male may deposit them on a colony of organisms that will serve as suitable hosts for the developing larvae (for example a hydroid colony). In most species the eggs will hatch into an ectoparasitic larval form; the parasitic larvae attach to host organisms where they feed and develop into adult sea spiders.
The relationships that pycnogonids share with other arthropod classes have been a source of debate among scientists. It is generally agreed that sea-spiders are related to the chelicerates, especially the arachnids and the horseshoe-crabs, due to similarities in the use of the first pair of appendages (for feeding), segmentation of the body, possession of 8 walking legs, the lack of mandibles and antennae and certain aspects of the digestive and nervous systems that the pycnogonids and chelicerates share. However, pycnogonids exhibit various aspects that are unique among arthropods; the pycnogonid oviger is a unique characteristic, as well as the existence of 10 and 12 legged species, other unique characteristics include: a greatly reduced abdomen, multiple genital pores on the legs and certain aspects of the developmental cycle of the pycnogonid larvae. These distinctive characteristics have lead some scientists to believe that pycnogonids should be placed in a subphylum of their own. The question still remains: do pycnogonids and other chelicerates share a common ancestor or have they simply followed a similar evolutionary pattern?
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