Sea spider

'' Many sea spiders are recognised by their enormous walking legs in contrast to a reduced body region, resulting into the so-called "all legs" or "no body" appearance. The body segments (somites) are generally interpreted as three main sections (tagma): cephalon (head, aka cephalosoma), trunk (aka thorax) and abdomen. 20200205 Pycnogonida Pantopoda morphology.png|Generalized morphology of a pantopod pycnogonid 1937 Smithsonian miscellaneous collections Snodgrass 1936 p24 Fig. 07.jpg|Ventral view and leg base of Chaetonymphon spinosum The cephalon is formed by the fusion of ocular somite and four anterior segments behind it (somite 1–4). It consists of an anterior proboscis, a dorsal ocular tubercle with eyes, and up to four pairs of appendages (chelifores, palps, s and first walking legs). Although some literature might consider the segment carrying the first walking leg (somite 4) to be part of the trunk,). It usually has fairly limited dorsoventral and lateral movement. However, in those species that have reduced chelifores and palps, the proboscis is well developed and flexible, often equipped with numerous sensory bristles and strong rasping ridges around the mouth. The proboscis is unique to pycnogonids, and its exact homology with other arthropod mouthparts is enigmatic, as well as its relationship with the absence of labrum (preoral upper lip of ocular somite) in pycnogonid itself. Pseudopallene pachycheira.jpeg|Pseudopallene pachycheira, showing robust chelifores and the absence of palps. Pycnogonum littorale (YPM IZ 030249).jpeg|Pycnogonum litorale, showing the absence of both chelifores and palps. Ovigers are absent in female. Colossendeis (MNHN-IU-2013-2065).jpeg|Colossendeis sp., showing the absence of chelifores but otherwise elongated proboscis, palps and ovigers. Nymphon maculatum 2011-721 (1) (cropped).jpg|Nymphon maculatum, showing the presence of both chelifores, palps and ovigers. In adult pycnogonids, the chelifores (aka cheliphore) are variably reduced or absent, depending on taxa and sometimes sex. Nymphonidae is the only family where all of three pairs are always functional. The ovigers can be reduced or missing in females, but are present in almost all males. In a functional condition, the chelifores terminate with a pincer (chela) formed by two segments (podomeres), like the chelicerae of most other chelicerates. The scape (peduncle) behind the pincer is usually unsegmented, but could be bisegmented in some species, resulting into a total of three or four chelifore segments. The palps and ovigers have up to 9 and 10 segments respectively, but can have fewer even when in a functional condition. The palps are rather featureless and never have claws in adult Pantopoda, while the ovigers may or may not possess a terminal claw and rows of specialised spines on its curved distal segments (strigilis). while the ovigers are used for cleaning themselves, with the additional function of carrying offspring in males. The leg segmentation of Paleozoic taxa is a bit different, noticeably they have annulated coxa 1 and are further divided into two types: one with flattened distal (femur and beyond) segments and first leg pair with one less segment than the other leg pairs (e.g. Palaeoisopus, Haliestes), and another one with an immobile joint between the apparently fourth and fifth segment which altogether might represent a divided femur (e.g. Palaeopantopus, Flagellopantopus). Several alternatives had been proposed for the position homology of pycnogonid appendages, such as chelifores being protocerebral/homologous to the labrum (see text) Conclusively, the classic, morphology-based one-by-one alignment to the prosomal appendages of other chelicerates was confirmed by both neuroanatomic and genetic evidences. == Internal anatomy and physiology ==

File:Pycnogonida anatomy - tagged.png|Sagittal section of an ascorhynchid pycnogonid, showing pharynx (F), mid gut (H) and central nervous system (B). Pycnogonida anatomy tagged.png|Transverse section of a pycnogonid leg, showing gut diverticulum (C, D) and gonad (E) A striking feature of pycnogonid anatomy is the distribution of their digestive and reproductive systems. The pharynx inside the proboscis is lined by dense setae, which is possibly related to their feeding behaviour. The structure and number of the gonopores might differ between sexes (e.g. larger in female, variably absent at the anterior legs of some male). The morphology of pycnogonid creates an efficient surface-area-to-volume ratio for respiration to occur through direct diffusion. Oxygen is absorbed by the legs and is transported via the hemolymph to the rest of the body with an open circulatory system. In the case of taxa without a heart (e.g. Pycnogonidae), the whole circulatory system is presumed to be solely maintained by gut peristalsis. This third brain segment, or tritocerebrum (corresponding to the palps/somite 2), is fused to the oviger/somite 3 ganglia instead, which is followed up by the final ovigeral somata in the protonymphon larva of Pycnogonum litorale. with incorporation of the first leg ganglia into the subesophageal ganglia in certain taxa. The leg ganglia might shift anteriorly or even cluster together, but are never highly fused into the ring-like synganglion of other chelicerates. ==Distribution and ecology==

Sea spiders live in many different oceanic regions 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 , 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 are benthic in general, using their stilt-like legs to walk along the bottom, but they are also capable of swimming by using an umbrella pulsing motion, and some Paleozoic species with flatter legs might even have a nektonic lifestyle. Some species in the genus Sericosura are known to farm and consume methylotrophic bacteria on their exoskeletons. Not much is known about the primary predators of sea spiders, if any. At least some species have obvious defensive methods such as amputating and regenerating their legs, or being distasteful via high levels of ecdysteroids (ecdysis hormone). In Pycnogonum litorale, the juveniles can regrow their posterior body section after being cut off, an ability which was thought to be absent in ecdysozoans. On the other hand, some parasites of sea spiders are known, including gastropod mollusks or hitch‐rided by sessile animals such as goose barnacles, which may negatively affect their locomotion and respiratory efficiency. Reproduction and development '' All sea spiders have separate sexes, except the only known hermaphroditic species Ascorhynchus corderoi and some extremely rare gynandromorph cases. Reproduction involves external fertilisation when male and female stack together (usually male on top), exuding sperm and eggs from the gonopores of their respective leg coxae. In Pycnogonidae, the ovigers are reduced in juveniles but reappear in oviger-bearing adult males. Further details of the postembryonic developments of sea spiders vary, but their categorization might differ between literatures. As of the 2010s, there are five types identified as follows: ==Taxonomy==

Phylogenetic position }} }} Sea spiders had been interpreted as some kind of arachnids or crustaceans in historical studies. A competing hypothesis in the 2000s proposed that Pycnogonida belong to their own lineage, sister to the lineage leading to other extant arthropods (i.e. euchelicerates, myriapods, crustaceans and hexapods, collectively known as Cormogonida). This Cormogonida hypothesis was first indicated by early phylogenomic analyses around that time, in Cambrian stem-group arthropods like radiodonts (frontal appendages), which was taken as evidence that Pycnogonida may be basal to all other living arthropods, since the protocerebral appendages were thought to be reduced and fused into a labrum in the last common ancestor of crown-group arthropods, and pycnogonids did not have a labrum coexisting with the chelifores. If that is true, it would have meant the sea spiders are the last surviving (and highly modified) members of an ancient, basal arthropod group that originated in the Cambrian oceans. However, the basis of this hypothesis was immediately refuted by subsequent studies using Hox gene expression patterns, demonstrating the developmental homology between chelicerae and chelifores, with chelifore nerves innervated by a deuterocerebrum that has been rotated forwards, which was misinterpreted as a protocerebrum by the aforementioned study. Since the 2010s, the chelicerate affinity of Pycnogonida regained wide support as the sister group of Euchelicerata. Under the basis of phylogenomics, this is one of the only stable topologies of chelicerate interrelationships in contrast to the uncertain relationship of many euchelicerate taxa (e.g. poorly resolved position of arachnid orders other than tetrapulmonates and scorpions; non-monophyly of Arachnida in respect to Xiphosura). This is consistent with the chelifore-chelicera homology, as well as other morphological similarities and differences between pycnogonids and euchelicerates. Based on molecular clock and fossil records, a 2025 study suggests pycnogonids and euchelicerates diverged during the Cambrian. However, due to the pycnogonids' highly modified anatomy and lack of intermediate fossils, their evolutional origin and relationship with the basal fossil chelicerates (such as habeliids and Mollisonia) is still difficult to compare and interpret. Interrelationship }} }} The class Pycnogonida comprises over 1,300 species, which are split into over 80 genera. All extant genera are considered part of the single order Pantopoda, which was subdivided into 11 families. Historically there were only 9 families, with species of nowadays Ascorhynchidae placed under Ammotheidae and Pallenopsidae under Callipallenidae. Both were eventually separated after they were considered distinct from the once-belonged families.) and the paraphyly of Callipallenidae with respect to Nymphonidae. The topology also suggests Pantopoda is undergoing cephalic appendage reduction/reappearance and polymerous species acquisition multiple times, contrary to previous hypothesis on pantopod evolution (cephalic appendages were thought to be progressively reduced along the branches, and the polymerous condition was thought to be ancestral). Based on the stem-group interpretation and molecular clock analysis, a 2025 study suggest Pantopoda started to diversify around the early Silurian to the late Devonian, inferring a ghost lineage of Paleozoic pantopods existed long before the oldest known pantopod around the Jurassic period. (with subsequent updates on fossil taxa after Sabroux et al. (2023, • Suborder Eupantopodida Fry, 1978 • Superfamily Ammotheoidea Dohrn, 1881 • Family Ammotheidae Dohrn, 1881 • Family Pallenopsidae Fry, 1978 • Superfamily Ascorhynchoidea Pocock, 1904 • Family Ascorhynchidae Hoek, 1881 (=Eurycydidae Sars, 1891) • Superfamily Colossendeoidea Hoek, 1881 (=Pycnogonoidea Pocock, 1904; Rhynchothoracoidea Fry, 1978) • Family Colossendeidae Jarzynsky, 1870 • Family Pycnogonidae Wilson, 1878 • Family Rhynchothoracidae Thompson, 1909 • Superfamily Nymphonoidea Pocock, 1904 • Family Callipallenidae Hilton, 1942 • Family Nymphonidae Wilson, 1878 • Superfamily Phoxichilidioidea Sars, 1891 • Family Endeidae Norman, 1908 • Family Phoxichilidiidae Sars, 1891 • Suborder Stiripasterida Fry, 1978 • Family Austrodecidae Stock, 1954 • Suborder incertae sedis • Family †Palaeopycnogonididae Sabroux, Edgecombe, Pisani & Garwood, 2023 • Unknown Family • Genus Alcynous Costa, 1861 (nomen dubium) • Genus Foxichilus Costa, 1836 (nomen dubium) • Genus Oiceobathys Hesse, 1867 (nomen dubium) • Genus Oomerus Hesse, 1874 (nomen dubium) • Genus Paritoca Philippi, 1842 (nomen dubium) • Genus Pephredro Goodsir, 1842 (nomen dubium) • Genus Phanodemus Costa, 1836 (nomen dubium) • Genus Platychelus Costa, 1861 (nomen dubium) ==Fossil record==

'' '' The fossil record of pycnogonids is scant, represented only by a handful of fossil sites with exceptional preservation (Lagerstätte). While most of them are discovered from Paleozoic era, unambiguous evidence of crown-group (Pantopoda) only restricted to Mesozoic era. Due to its distinct morphology, some studies have argued that this genus is not a pycnogonid at all. However, just like Cambropycnogon, its pycnogonid affinity was questioned by some studies as well. In 2019, a new species of Colossopantopodus and a specimen possibly belong to the extant genus Eurycyde were discovered from the aforementioned Solnhofen limestone. == References ==