Arachnopoda Dana, 1853 Sea spiders are marine arthropods of the class Pycnogonida,[1] hence they are also called pycnogonids (/pɪkˈnɒɡənədz/;[2] named after Pycnogonum, the type genus;[3] with the suffix -id).

[7] 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.

While some literature around the 2000s suggests they may be a sister group to all other living arthropods,[8][9] their traditional classification as a member of chelicerates alongside horseshoe crabs and arachnids has regained wide support in subsequent studies.

It consists of an anterior proboscis, a dorsal ocular tubercle with eyes, and up to four pairs of appendages (chelifores, palps, ovigers and first walking legs).

Although some literature might consider the segment carrying the first walking leg (somite 4) to be part of the trunk,[6] it is completely fused to the remaining head section to form a single cephalic tagma.

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.

[20] In a functional condition, the chelifores terminate with a pincer (chela) formed by two segments (podomeres), like the chelicerae of most other chelicerates.

[23] The chelifores are used for feeding and the palps are used for sensing and manipulating food items,[24] while the ovigers are used for cleaning themselves, with the additional function of carrying offspring in males.

[28][15] Several alternatives had been proposed for the position homology of pycnogonid appendages, such as chelifores being protocerebral/homologous to the labrum (see text)[9] or ovigers being duplicated palps.

[29] Conclusively, the classic, morphology-based one-by-one alingment to the prosomal appendages of other chelicerates was confirm by both neuroanatomic and genetic evidences.

While the fourth walking leg pair was considered aligned to the variably reduced first opisthosomal segment (somite 7, also counted as part of the prosoma based on different studies and/or taxa) of euchelicerates, the origin of the additional fifth and sixth leg pairs in the polymerous species are still enigmatic.

[17] A pair of gonads (ovaries in female, testes in male) is located dorsally in relation to the digestive tract, but the majority of these organs are branched diverticula throughout the legs because the body is too small to accommodate all of them alone.

[35] 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).

[40] 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.

[12][6] Sea spiders are mostly carnivorous predators or scavengers that feed on soft-bodied invertebrates such as cnidarians, sponges, polychaetes, and bryozoans, by inserting their proboscis into targeted prey item.

At least some species have obvious defensive methods such as amputating and regenerating body parts,[45][46] or making itself unpleasant meal via high level of ecdysteroids (ecdysis hormone).

[47] On the other hand, sea spiders are known to be infected by parasitic gastropod mollusks[48][49] or hitch‐rided by sessile animals such as goose barnacles, which may negatively affect their locomotion and respiratory efficiency.

[50] All sea spiders have separate sexes, except the only known hermaphroditic species Ascorhynchus corderoi and some extremely rare gynandromorph cases.

In this stage, the chelifores usually have attachment glands, while the palps and ovigers are subequal, three-segmented appendages known as palpal and ovigeral larval limbs.

This Cormogonida hypothesis was first indicated by early phylogenomic analysis aroud that time,[8] followed by another study suggest that the sea spider's chelifores are not positionally homologous to the chelicerae of euchelicerates (originated from the deutocerebral segment/somite 1), as was previously supposed.

Instead, the chelifore nerves were thought to be innervated by the protocerebrum, the first segment of the arthropod brain which corresponded to the ocular somite, bearing the eyes and labrum.

If that's true, it would have meant the sea spiders are the last surviving (and highly modified) members of an ancient, basal arthropods that originated in Cambrian oceans.

Under the basis of phylogenomics, this is one of the only stable topology 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).

[63] stem-groups (e.g. Palaeoisopus, Flagellopantopus, Palaeopantopus) Austrodecidae Colossendeidae Rhynchothoracidae Pycnogonidae Endeidae Phoxichilidiidae Pallenopsidae Ammotheidae Ascorhynchidae(including Nymphonella?)

[16] Phylogenomic analysis of extant sea spiders was able to establish a backbone tree for Pantopoda, revealing some consistent relationship such as the basal position of Austrodecidae, monophyly of some major branches (later redefined as superfamilies[26]) and the paraphyly of Callipallenidae in respect to Nymphonidae.

[65][33][66][26][6] While some phylogenetic analysis placing them within Pantopoda, this result is questionable as they have low support values and based on outdated interpretation of the fossil taxa.

So far only its protonymphon larvae had been described, featuring some traits unknown from other pycnogonids such as paired anterior projections, gnathobasic larval limbs and annulated terminal appendages.

Some of them are significant in that they possess something never seen in pantopods: annulated coxae, flatten swimming legs, segmented abdomen and elongated telson.

[76][77] In 2019, a new species of Colossopantopodus and a specimen possibly belong to the extant genus Eurycyde were discovered from the aforementioned Solnhofen limestone.