Sea spiders are also known with the names Pantopoda or pycnogonids. Sea spiders are the marine arthropods, which belong to class Pycnogonida. Sea spiders can be found world wide. They are found especially in Arctic, Antarctic Oceans and Mediterranean and Caribbean Seas. There are around 1300 species of sea spiders. There sizes varies from 1mm to 90 cm. The large spiders are found in the deep waters, while the smaller ones are found in the shallow depths. The large sea spiders are found in the Antarctic waters.
Sea spiders have small body and long legs. They generally have eight walking legs (four pairs), while some species also have five and six pairs. Sea spiders are very small in size that each of their muscle contains only one cell surrounded by connective tissue. The front part of the spider contains proboscis, by which they suck the nutrients from soft-bodied invertebrates. Their proboscis has a limited lateral and dorsoventral movement.Their digestive track has diverticula extending into the legs.
They have three to four appendages including the ovigers, which they use to carry their young and also for cleaning and courtship. The appendages often seem like legs. The ovigers, palps and chelifores are missing in some species as well as in adults. The proboscis is well developed in those species which lacks the chelifores and palps. Also they are more flexible and have several sensory bristles and strong rasping ridges around the mouth. The last part of their body is anus and tubercle, which are located dorsally.
A cephalothorax and small abdomen makes a reduced body of the sea spider (Pycnogonid). They can have up to two pairs of eyes located dorsally on its non-calcareous exoskeleton. Sometimes the eyes are missing, especially in the species from deep sea. The abdomen does not contain any appendages. It is reduced in many species or not fully developed. The organs of chelicerate are generally extended to its appendages, as it can not hold all of them alone.
Pycnogonids leave the waste from body through its digestive tract or during moulting. The heart of Pycnogonids beats at the rate of 90 to 180 beats per minute, which creates the blood pressure. Pycnogonids have open circulatory system and nervous system. The brain is connected to the ventral nerve cords, which are again connected to specific nerves.
Pycnogonids are cosmopolitan. They can be found in Australia, New Zealand, and the Pacific coast of the United States, to Caribbean and Mediterranean. They are found in large numbers in shallow waters. However, they can also found in the deep sea of about 7,000 metres depth. They live in both marine as well as estuarine locations. They are generally found below the rocks and between the algae that are found along shorelines.
Pycnogonids live on eating sea sponges, bryozoans, cnidarians, and polychaetes. They insert their proboscis into their food and suck the nutrients.
Except hermaphroditic, all the species of pycnogonid have male and female sexes. The female spiders have two ovaries and males have a pair of testes located dorsally relative to the digestive tract. Very little is known about the secret lives about pycnogonids that their reproduction involves external fertilisation.
The male pycnogonid takes care of the egg and the young. The body of larva contains only the head and three pairs of cephalic appendages i.e. the ovigers, chelifores, and palps. The other parts of their body: abdomen and thorax develops later. This is how the common ancestor of arthropods is evolved. Its life begins as a small animal with fewer appendages, some use for feeding and some for locomotion. However the other parts and appendages grow gradually.
There are four kind of larvae found in pycnogonids: encysted larva, the typical protonymphon larva, atypical protonymphon larva, and the attaching larva. Most common among the four is protonymphon larva, which is free living, and gradually grows as adult. The encysted larva is a parasite. It hatches from the egg and shelters in a polyp colony, where it turns into cyst. It does not leave this shelter before turning into a young juvenile.
Very less is known about the atypical protonymphon larva. Its adults are free living, whereas the larvae and juveniles lives inside the temporary shelters like polychaetes and clams. The attaching larva looks like an embryo still after hatched from the egg. After hatching, it instantly attaches itself to the ovigerous legs of its father. It stays there until turn into a young juvenile having two or three pairs of walking legs.