Sea spider

Sea spiders, also called Pantopoda or pycnogonids, are marine arthropods of class Pycnogonida. They are cosmopolitan, found especially in the Mediterranean and Caribbean Seas and the Arctic and Antarctic Oceans. There are approximately 1000 known species, ranging in size from 1-10 mm to over 90 cm in deep water species: most are toward the smaller end of this range.

These small animals live in many different parts of the world, from Australia, New Zealand, and the Pacific coast of the United States to the Mediterranean and the Caribbean to the north and south poles. They are found in waters as deep as 7000 meters, but more commonly in shallower waters and in both marine and estuarine habitats. Although they are relatively common, Pycnogonids are well camouflaged beneath the rocks and among the algae that are found along shorelines.

Morphology

Their reduced body size is in contrast to their large, long legs. The number of their walking legs are usually four pairs, but also species with five and six pairs exists. Because of their small size and slender body and legs, no respiratory system is needed for gas diffusion. A proboscis allows them to suck nutrients from soft bodied invertebrates, and their digestive tract has branches extending into their legs, known as diverticulae.

Pycnogonids are so small that each of their tiny muscles consists of only one single cell, surrounded by connective tissue. In terms of physical makeup, the proboscis, which has fairly limited dorso-ventral and lateral movement, and three to four appendages including the ovigers, which are used in caring for young and cleaning as well as courtship, are present in the anterior region. Yet in some species, the chelifores, palps and ovigers can be reduced or missing in the adults. In those who lacks chelifores and palps, the proboscis is well developed and more mobile and flexible, often equipped with numerous sensory bristles and strong rasping ridges around the mouth. The last segment includes the anus and tubercule, which projects dorsally. In total, Pycnogonids have four to six legs for walking as well as other appendages which often resemble legs. A cephalothorax and much smaller abdomen make up the extremely reduced body of the Pycnogonid, which has up to two pairs of dorsally located simple eyes on its non-calcareous exoskeleton. Althought sometimes the eyes can be missing, especially among species living in the deep oceans. The abdomen does not have any appendages, and in most species it is reduced and almost vestigial. The organs of this fascinating chelicerate extend throughout many appendages because its body is too small to accommodate all of them alone.

The setup of the sea spider actually creates a perfect surface-area to volume ratio for any respiration to occur through direct diffusion. The most recent research seems to indicate that waste leaves the body through the digestive tract or is lost during a molt. The pycnogonid small heart, which is long and thin like everything else in them, beats vigorously at ninety to 180 beats per minute, creating substantial blood pressure. These creatures possess an open circulatory system as well as a nervous system consisting of a brain, which is connected to two ventral nerve cords, which in turn connect to specific nerves.

Biology

Their habitats range from the shallow intertidal zones to deep water benthic environments, some 7000 metres down. Sea spiders do not swim but rather walk along the bottom with their stilt-like legs. Most are carnivorous and feed on cnidarians, sponges, polychaetes and bryozoans.

Sea spiders are generally predators or scavengers. They crawl slowly along (although some do swim), feeding. They will often insert their proboscis, a long appendage used for digestion and sucking food into its gut, into a sea anemone and suck out nourishment. The sea anemone, relatively large in comparison to its predator, almost always survives this ordeal. Studies have shown that adult taste preferences depend on what the animals were fed as young.

Subtaxa

The order Pycnogonida consists of approximately 1000 species, which are normally split into eighty-six genera. The correct taxonomy within the group is uncertain, and it appears that no agreed list of orders exists. Accordingly, families are listed in the table at the right.

Evolution and relationships

The evolutionary lineage of the sea spider group as a whole is disputed: their fossil record is sketchy at best, with the three earliest known genera originating in the Devonian.

Some believe (or once believed) sea spiders to be among the chelicerates, together with horseshoe crabs, true spiders, mites, ticks and scorpions. While sharing many morphological features with the other chelicerates, major differences (such as their unique proboscis) have caused many taxonomists to remove sea spiders from this grouping, and some consider them to form their own subphylum. Another reason for this is that it seems the appendages called chelifores are unique among present arthopods, and is not homologous to the chelicerae in real chelicerates as previously suspected. Instead of developing from the deuterocerebrum, it can be traced to the protocerebrum, the anterior part of the arthropod brain and found in the first head segment that in all other arthropods give rise to the eyes only. This is not found anywhere else among arthopods except in some fossil forms like Anomalocaris, indicating the Pycnogonida is a sister group to all other living arthopods which evolved from some ancestor who had lost the protocerebrum appendages. If this is confirmed once and for all, it would mean the sea spiders are the last surviving (and strongly modified) members of an ancient and long gone stem-group of arthropods who was once living in the old Cambrian oceans.

In regard to the evolution of Pycnogonids, the fossil record is quite lacking, although two things are fairly clear: 1. Pycnogonids possessed a coelom at one point, but it was eventually lost through evolution, and 2. Pycnogonids are relatively old, with the earliest known genera placed in the Devonian period, between 416 and 355 million years ago.

A lucid view on relationships is given by Dunlop & Arango (2005), which treat the conflicting evidence for placing of the Pycnogonida 1) outside Arachnomopha as basal Euarthropoda, supporting the concept of Cormogonida (Euarthropoda excluding pycnogonids) or 2) inside Chelicerata (which is based only in the chelifore-chelicera putative homology).

Reproduction and development

All Pycnogonid species have two separate and different genders except for one species which is hermaphroditic. Females possess a pair of ovaries, while males possess a pair of testes located dorsally in relation to the digestive tract. Scientists are aware that reproduction involves external fertilization after “a brief courtship”, but very little is known about the secret lives of most Pycnogonids. Only males care for laid eggs and young. The larvae has a blind gut and the body consist literally of a head and its three pairs of cephalic appendages only; the chelifores, palps and ovigers. The abdomen and the thorax with its thoraic appendages develops later. One theory is that this reflects how a common ancestor of all arthropods evolved; starting its life as a small animal with a pair of appendages used for feeding and two pairs used for locomotion, while new segments and segmental appedendages were gradually added as it was growing. At least four types of larvae has been described; the typical protonymphon larva, the encysted larva, the atypical protonymphon larva, and the attaching larva. The typical protonymphon larva is most common, is free living and gradually turns into an adult. The encysted larva is a parasite that hatches from the egg and finds a host in the shape of a polyp colony where it burrows into and turns into a cyste, and won't leave the host before it has turned into a young juvenile. Not much is known about the development of the atypical protonymphon larva. The adults are free living, while the larvae and the juveniles are living on or inside temporary hosts such as polychaetes and clams. When the attaching larva hatches it still looks like an embryo, and immediately attaches itself to the ovigerous legs of the father, where it will stay till it has turned into a small and young juvenile with two or three pairs of walking legs ready for a free-living existence.

Trivia

David Staples, one of the few Pycnogonid specialists in the world, notes, "An early researcher in the of Pycnogonids was T.T. Flynn, father of actor Errol Flynn."

Reference

Dunlop, J. A. & Arango, C. P. (2005). Pycnogonid affinities: a review. Journal of Zoological Systematics and Evolutionary Research, 43: 8-21.

External links

Images of Pycnogonida, and Pycnogonids in literature

Pycnogonids

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