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Pycnogonida

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The Pycnogonida (sea spider) are of the Anthropoda phylum with various families and species including the Pseudopallene circularis. Sea spiders are usually found in the Mediterranean Sea but are also found in the Atlantic and Artic Oceans, and the Caribbean Sea. They look similar to normal spiders but are generally larger. An adult male can be as big as 2.8 feet (76 centimeters) and as small as 5 centimeters, and can weigh up to .7kg (1 pound) or as little as .01kg (.1 pounds). The largest region of the body contains four small eyes and a mouth at the end of a long tube called the proboscis. Sea Spiders appear to have come from a crab and an early spider. They mainly feed on sponges. Their single defense is a poison in their legs.
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Pycnogonida, The Sea Spiders

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The Pycnogonida, also known as the Sea Spider, is a class of unidentified aquatic Chelicerata’s in the Pantopoda order and has many genera including the Pseudopallene circularis.

The Sea Spiders are mainly found in the Mediterranean Sea but can be seen in the Atlantic Ocean, Artic Ocean, and Caribbean Sea. They have a similar look to a common spider but can be larger; their size varies on their environment and has longer legs than most Arachnids. These aquatic animals are best known for its long legs and similar look to Arachnids though they are in the Chelicerata subphylum.

An adult male can be as large as 76 centimeters (Around 2.8 feet) in length from leg to leg and as small as 5 centimeters. It is a very light weight class as heavy as .7kg (1 pound) and as light as .01kg (.1 pounds). Sea Spiders have a proboscis out of its mouth that sucks out the nutrients of prey.They have a small center and 8 legs but can have 10 or 12 legs. They craw or swim with its legs pushing down the water. The color greatly varies from yellow to even blue.

They have no found origins yet because only two complete fossils have been found. It seems they are most often found in the Atlantic Ocean near Europe and in the Mediterranean Sea. The fossils were found in these areas. These animals have many connections and seem to have originated from crabs and a form of early spider. They have only one common ancestor that we know as and that is to the prehistoric monkey-spider hybrid.

The Pycnogonida are predators of many soft tissued invertebrates. They mainly feast on sponges. They keep to themselves and stay hidden most of the time. They have no one predator that lives off of them. They live in small communities and are rarely found. Their only defense is their legs with contain a poison of sorts that disables the attacker or prey.

There are no recorded discoverers of the Pycnogonida. Many biologists are trying to discover the connections of the Sea Spiders and discover more genus and species. There are many genus recorded but no intense studying of just one.

The Pycnogonida Sea Spiders are a very elusive and new class. They are magnificent in their ways or live and should have more in-depth studies in my view.

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Brief Summary

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The Pycnogonida are a group of marine arthropods known as "sea spiders" for their resemblance to true, terrestrial spiders. Although there has been much debate during the past century regarding which arthropod groups are their closest relatives, growing consensus places the pycnogonids as an early branching lineage that is the sister group to the chelicerates (arachnids plus horseshoe crabs) (Giribet and Edgecombe 2012).

There are around a thousand described species of pycnogonids (and probably hundreds more not yet described). They are found worldwide from the intertidal zone to depths of nearly 7,000 m. Most pycnogonids are small, with leg spans of less than a centimeter (in some species, just a few millimeters), but some deep-sea species have leg spans of up to 60 cm. Many species are errant (i.e., they actively move about), but others live on seaweeds or on other invertebrates, such as sea anemones, hydroids, ectoprocts, and tunicates (at least one or two species live on the bells of pelagic medusae and pycnogonids have also been observed on the huge vestimentiferan worms living in the hydothermal vent community of the Galapagos Rift).

The walking legs of pycnogonids are 9-segmented. Most of the commonly encountered intertidal species have short, thick legs and are quite sedentary, moving very slowly. Deep-water bottom-dwelling species tend to have longer, thinner legs and to be more active, walking on the tips of their legs. Many pycnogonids are also known to swim periodically.

Most species are generalist predators (and, in many cases, scavengers), although a few feed on algae. Food is obtained by sucking it through the proboscis (often just body fluids and tissue fragments from their prey.).

Pycnogonids are dioecious (i.e., with separate males and females) and embryos are brooded by the male. In many species, distinctive 6-legged larvae (immature forms) are released by the brooding male that live in some sort of symbiotic relationship with cnidarians, molluscs, or echinoderms.

(Brusca and Brusca 2003)

Bamber and El Nagar provide an online database on pycnogonid names, taxonomy, museum specimens, and literature.

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Sexual Dimorphism

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Negligible Sexual Dimorphism in size (males about 1% larger); males have specialized appendages (ovigera) for brooding of eggs
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Fairbairn DJ (2013) Odd couples: extraordinary differences between the sexes in the animal kingdom. Princeton: Princeton University Press. http://www.worldcat.org/oclc/820118780
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Sea spider

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Sea spiders, also called Pantopoda or pycnogonids, ('pycno-' closely packed, 'gonid' gonidia) are marine arthropods of class Pycnogonida.[1] They are cosmopolitan, found in oceans around the world. There are over 1300 known species, with a leg span ranging from 1 mm (0.04 in) to over 70 cm (2.3 ft).[2] Most are toward the smaller end of this range in relatively shallow depths; however, they can grow to be quite large in Antarctic and deep waters.

Although "sea spiders" are not true spiders, or even arachnids, their traditional classification as chelicerates would place them closer to true spiders than to other well-known arthropod groups, such as insects or crustaceans. This is in dispute, however, as genetic evidence suggests they may be the sister group to all other living arthropods.[3][4]

Description

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Anatomy of a pycnogonid: A: head; B: thorax; C: abdomen 1: proboscis; 2: chelifores; 3: palps; 4: ovigers; 5: egg sacs; 6a–6d: four pairs of legs

Sea spiders have long legs in contrast to a small body size. The number of walking legs is usually eight (four pairs), but species with five and six pairs exist. Because of their small size and thin body and legs, no respiratory system is necessary, with gases moving by diffusion. A proboscis allows them to suck nutrients from soft-bodied invertebrates, and their digestive tract has diverticula extending into the legs.

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Pycnogonid grazing on a hydroid

Certain Pycnogonids are so small that each of their tiny muscles consists of only one single cell, surrounded by connective tissue. The anterior region consists of the proboscis, which has fairly limited dorsoventral and lateral movement, and three to four appendages including the ovigers, which are used in caring for young and cleaning as well as courtship. In some species, the chelifores, palps and ovigers can be reduced or missing in adults. In those species that lack 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 tubercle, which projects dorsally.

In total, pycnogonids have four to six pairs of 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, though 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 chelicerate extend throughout many appendages because its body is too small to accommodate all of them alone.

The morphology of the sea spider creates an extremely well suited surface-area to volume ratio for any respiration to occur through direct diffusion. Oxygen is absorbed by the legs and is transported via the hemolymph to the rest of the body.[5] The most recent research seems to indicate that waste leaves the body through the digestive tract or is lost during a moult. The small, long, thin pycnogonid heart beats vigorously at 90 to 180 beats per minute, creating substantial blood pressure. The beating of the sea spider heart drives circulation in the trunk and in the part of the legs closest to the trunk, but is not important for the circulation in the rest of the legs.[5] Hemolymph circulation in the legs is mostly driven by the peristaltic movement in the part of the gut that extends into every leg.[5] 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.

Reproduction and development

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All pycnogonid species have separate sexes, except for one species that is hermaphroditic. Females possess a pair of ovaries, while males possess a pair of testes located dorsally in relation to the digestive tract. Reproduction involves external fertilisation after "a brief courtship". Only males care for laid eggs and young.

The larva has a blind gut and the body consists of a head and its three pairs of cephalic appendages only: the chelifores, palps and ovigers. The abdomen and the thorax with its thoracic appendages develop 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 appendages were gradually added as it was growing.

At least four types of larvae have 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 cyst, and will not 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 until it has turned into a small and young juvenile with two or three pairs of walking legs ready for a free-living existence.

Distribution and ecology

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A pycnogonid in its natural habitat

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 Sea and the Caribbean Sea, to the north and south poles. They are most common in shallow waters, but can be found as deep as 7,000 metres (23,000 ft), and live in both marine and estuarine habitats. Pycnogonids are well camouflaged beneath the rocks and among the algae that are found along shorelines.

Sea spiders either walk along the bottom with their stilt-like legs or swim just above it using an umbrella pulsing motion.[6] Most are carnivorous and feed on cnidarians, sponges, polychaetes, and bryozoans. Sea spiders are generally predators or scavengers. 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, large in comparison to its predator, almost always survives this ordeal (hence the sea spider is more properly a parasite of the anemone, rather than a predator).

Classification

The class Pycnogonida comprises over approximately 1,300 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 taxobox, all considered part of the single order Pantopoda.

Sea spiders have long been considered to belong to the Chelicerata, together with horseshoe crabs, and the Arachnida, which includes spiders, mites, ticks, scorpions, and harvestmen, among other, lesser known orders.[7]

A competing theory proposes that pycnogonida belong to their own lineage, distinct from chelicerates, crustaceans, myriapods, or insects. This theory contends that the sea spider's chelifores, which are unique among extant arthropods, are not in any way homologous to the chelicerae in real chelicerates, as was previously supposed. Instead of developing from the deutocerebrum, they 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 arthropods, except in some fossil forms like Anomalocaris, indicating that the Pycnogonida may be a sister group to all other living arthropods, the latter having evolved from some ancestor that had lost the protocerebral appendages. If this is confirmed, it would mean the sea spiders are the last surviving (and highly modified) members of an ancient stem group of arthropods that lived in Cambrian oceans.[8] However, a subsequent study using Hox gene expression patterns consistent with a developmental homology between chelicerates and chelifores, with chelifores innervated from a deuterocerebrum that has been rotated forwards; thus, the protocerebral Great Appendage clade does not include the Pycnogonida.[9][10]

Recent work places the Pycnogonida outside the Arachnomorpha as basal Euarthropoda, or inside Chelicerata (based on the chelifore-chelicera putative homology).[11]

Group taxonomy

According to the World Register of Marine Species, the order Pantopoda is subdivided as follows:[12]

including the following genera:

This taxonomic classification replaces the older version in which Pantopoda is subdivided into families as follows:

Fossil record

Although the fossil record of pycnogonids is scant, it is clear that they once possessed a coelom, but it was later lost, and that the group is very old.[citation needed]

The earliest fossils are known from the Cambrian 'Orsten' of Sweden, the Silurian Wenlock Series of England and the Devonian Hunsrück Slate of Germany. Some of these specimens are significant in that they possess a longer 'trunk' behind the abdomen and in two fossils the body ends in a tail; something never seen in living sea spiders.

In 2013, the first fossil pycnogonid found within an Ordovician deposit was reported from William Lake in northern Manitoba.[16]

In 2007, remarkably well preserved fossils were exposed in fossil beds at La Voulte-sur-Rhône, south of Lyon in south-eastern France. Researchers from the University of Lyon discovered about 70 fossils from three distinct species in the 160-million-year-old Jurassic La Voulte Lagerstätte. The find will help fill in an enormous gap in the history of these creatures.[17]

References

  1. ^ "WoRMS taxon details – Pycnogonida". World Register of Marine Species. Retrieved 27 March 2015..mw-parser-output cite.citation{font-style:inherit}.mw-parser-output q{quotes:"""""'"'"}.mw-parser-output code.cs1-code{color:inherit;background:inherit;border:inherit;padding:inherit}.mw-parser-output .cs1-lock-free a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/6/65/Lock-green.svg/9px-Lock-green.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-lock-limited a,.mw-parser-output .cs1-lock-registration a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/d/d6/Lock-gray-alt-2.svg/9px-Lock-gray-alt-2.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-lock-subscription a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/a/aa/Lock-red-alt-2.svg/9px-Lock-red-alt-2.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration{color:#555}.mw-parser-output .cs1-subscription span,.mw-parser-output .cs1-registration span{border-bottom:1px dotted;cursor:help}.mw-parser-output .cs1-hidden-error{display:none;font-size:100%}.mw-parser-output .cs1-visible-error{font-size:100%}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration,.mw-parser-output .cs1-format{font-size:95%}.mw-parser-output .cs1-kern-left,.mw-parser-output .cs1-kern-wl-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right,.mw-parser-output .cs1-kern-wl-right{padding-right:0.2em}
  2. ^ "Sea spiders provide insights into Antarctic evolution". Department of the Environment and Energy, Australian Antarctic Division. 22 July 2010. Retrieved 27 December 2017.
  3. ^ Regier, Jerome C.; Shultz, Jeffrey W.; Zwick, Andreas; Hussey, April; Ball, Bernard; Wetzer, Regina; Martin, Joel W.; Cunningham, Clifford W. (2010). "Arthropod relationships revealed by phylogenomic analysis of nuclear protein-coding sequences". Nature. 463 (7284): 1079–83. Bibcode:2010Natur.463.1079R. doi:10.1038/nature08742. PMID 20147900.
  4. ^ Sharma, P. P.; Kaluziak, S. T.; Perez-Porro, A. R.; Gonzalez, V. L.; Hormiga, G.; Wheeler, W. C.; Giribet, G. (2014). "Phylogenomic Interrogation of Arachnida Reveals Systemic Conflicts in Phylogenetic Signal". Molecular Biology and Evolution. 31 (11): 2963–84. doi:10.1093/molbev/msu235. PMID 25107551.
  5. ^ a b c Woods, H. Arthur; Lane, Steven J.; Shishido, Caitlin; Tobalske, Bret W.; Arango, Claudia P.; Moran, Amy L. (2017-07-10). "Respiratory gut peristalsis by sea spiders". Current Biology. 27 (13): R638–R639. doi:10.1016/j.cub.2017.05.062. ISSN 0960-9822. PMID 28238655.
  6. ^ Deep-Sea News: Sea Spiders[self-published source?]
  7. ^ Margulis, Lynn; Schwartz, Karlene (1998). Five Kingdoms, An Illustrated Guide to the Phyla of Life on Earth (third ed.). W.H. Freeman and Company. ISBN 0-7167-3027-8.[page needed]
  8. ^ Maxmen, Amy; Browne, William E.; Martindale, Mark Q.; Giribet, Gonzalo (2005). "Neuroanatomy of sea spiders implies an appendicular origin of the protocerebral segment". Nature. 437 (7062): 1144–8. Bibcode:2005Natur.437.1144M. doi:10.1038/nature03984. PMID 16237442.
  9. ^ Jager, Muriel; Murienne, Jérôme; Clabaut, Céline; Deutsch, Jean; Guyader, Hervé Le; Manuel, Michaël (2006). "Homology of arthropod anterior appendages revealed by Hox gene expression in a sea spider". Nature. 441 (7092): 506–8. Bibcode:2006Natur.441..506J. doi:10.1038/nature04591. PMID 16724066.
  10. ^ Pharyngula[self-published source?]
  11. ^ Dunlop, J. A.; Arango, C. P. (2005). "Pycnogonid affinities: A review". Journal of Zoological Systematics and Evolutionary Research. 43: 8–21. doi:10.1111/j.1439-0469.2004.00284.x.
  12. ^ http://marinespecies.org/aphia.php?p=taxdetails&id=1358 accessed 18 July 2016
  13. ^ http://marinespecies.org/aphia.php?p=taxdetails&id=379601 accessed 18 July 2016
  14. ^ http://marinespecies.org/aphia.php?p=taxdetails&id=379600 accessed 18 July 2016
  15. ^ http://marinespecies.org/aphia.php?p=taxdetails&id=379602 accessed 18 July 2016
  16. ^ Rudkin, Dave; Cuggy, Michael B.; Young, Graham A.; Thompson, Deborah P. (2013). "An Ordovician Pycnogonid (Sea Spider) with Serially Subdivided 'Head' Region". Journal of Paleontology. 87. Retrieved 23 September 2017. Here we report the first known occurrence of fossil pycnogonids from rocks of Ordovician age, bridging a 65 Myr gap between controversial late Cambrian larval forms and a single documented Silurian specimen. The new taxon, Palaeomarachne granulata n. gen. n. sp., from the Upper Ordovician (ca. 450 Ma) William Lake Konservat-Lagerstätte deposit in Manitoba, Canada, is also the first reported from Laurentia. It is the only record thus far of a fossil sea spider in rocks of demonstrably shallow marine origin.
  17. ^ "Fossil sea spiders thrill experts". BBC News. August 16, 2007.

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Sea spider: Brief Summary

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Sea spiders, also called Pantopoda or pycnogonids, ('pycno-' closely packed, 'gonid' gonidia) are marine arthropods of class Pycnogonida. They are cosmopolitan, found in oceans around the world. There are over 1300 known species, with a leg span ranging from 1 mm (0.04 in) to over 70 cm (2.3 ft). Most are toward the smaller end of this range in relatively shallow depths; however, they can grow to be quite large in Antarctic and deep waters.

Although "sea spiders" are not true spiders, or even arachnids, their traditional classification as chelicerates would place them closer to true spiders than to other well-known arthropod groups, such as insects or crustaceans. This is in dispute, however, as genetic evidence suggests they may be the sister group to all other living arthropods.

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