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Sea Spider

Pycnogonida

Sea spiders, or pycnogonids, are marine arthropods of the class Pycnogonida, named after the type genus, Pycnogonum. This group is also synonymously called Pantopoda, which is the only extant order within the class Pycnogonida. Although sea spiders resemble true spiders in name and appearance, they have evolved independently.  

While sea spiders are not true arachnids (arthropods of the class Arachnida) and evolved independently, molecular and genetic evidence suggests they may share a closer evolutionary relationship with arachnids than with other arthropod groups, such as crustaceans or insects. They possess ‘chelifores,’ a unique pair of limbs that are homologous to chelicerae found in other chelicerates, such as spiders and horseshoe crabs[1].

Around 1,300 sea spider species are found around the globe, from shallow intertidal zones to depths as deep as 7,000 m[2][3]

Description

Sea spiders usually range between 0.1 cm and 50 cm (0.04 to 20 in) in leg span, when fully extended, depending on their geographical distribution. However, some species found in polar regions, such as those in the Colossendeis genus from Antarctica, can grow as large as 70 cm (27 inches) in leg span.[4]

These arthropods have slender, jointed appendages that extend along their body. They usually have four pairs of legs, a feature also found in most terrestrial arachnids. However, certain families like Pycnogonidae, Colossendeidae, and Nymphonidae may have as many as five to six pairs (polymerous)[3][5]. There are nine such species; seven of them have five leg pairs and are distributed among four genera (Decolopoda, Pentacolossendeis, Pentapycnon, and Pentanymphon), whereas two species in two genera (Dodecolopoda and Sexanymphon) have six leg pairs. 

Although their bodies cannot be distinctly divided into segments, they possess the following broad divisions.

Physiology

Taxonomy

Arthropods were historically considered related to chelicerates, such as terrestrial spiders, mites, ticks, scorpions, and harvestmen[8]. Recent studies, including an analysis of the Hox gene expression, provided sufficient evidence supporting their placement within Chelicerata. These studies indicate that chelifores are homologous to the chelicerae of chelicerates and are innervated by the deuterocerebrum, which is positioned more anteriorly in sea spiders due to evolutionary modification.

According to the World Register of Marine Species, sea spiders have been further classified as follows.

Sea Spiders (Pycnogonida)

Distribution

They are cosmopolitan, found in all oceans. However, they are primarily concentrated in the Pacific waters around the United States and Japan, and are also notably present in the Caribbean and Mediterranean seas and the coastal waters of South Africa, Australia, and New Zealand. They extend to Antarctica, which harbours almost 20% of the world’s sea spider species.

Habitat

Despite harsh conditions, sea spiders exhibit remarkable diversity in polar regions, where they inhabit the seafloors beneath sea ice. There, they camouflage themselves effectively against sandy substrates and benthic algae, helping them evade predators.

They often form commensal associations with cnidarians, such as sea anemones and hydroids. By attaching to these hosts, sea spiders passively transport themselves to new locations, potentially increasing their access to food and shelter.

Diet 

These arthropods feed on soft-bodied invertebrates, including cnidarians such as jellyfish and sea anemones, hydroids, small crustaceans like copepods, and various worm-like organisms such as polychaete worms, ribbon worms, and peanut worms[9].

Behavior

Most sea spider species feed on soft-bodied invertebrates such as cnidarians, sponges, polychaete worms, and bryozoans. They use their long, tubular proboscis to pierce the prey’s tissues and extract internal fluids. In most cases, this feeding results in the death of the prey, classifying the sea spider as a predator. However, their interaction with sea anemones is an exception. Sea spiders can repeatedly extract fluids from sea anemones without killing them, which makes the sea spider a parasite, rather than a predator of sea anemones[10].  

Reproduction and Life Cycle

All sea spider species are gonochoric and have separate sexes except Ascorhynchus corderoi, a hermaphrodite having both testis and ovary in a single individual. Reproduction involves external fertilization with the male stacking on top of the female for copulation. The female then releases her eggs, which are then fertilized externally by the sperm. After fertilization, the male sea spider uses cement glands to attach the egg clusters and carries them on his ovigers until they hatch. The hatching period varies by species, typically ranging from a few days to about a month.

Each larva has a head and three pairs of cephalic appendages (chelifores, palps, and ovigers), while the thoracic and abdominal appendages develop in later stages. At least four types of larvae have been discovered in sea spiders.

Predators

Smaller sea-spider species and juveniles are consumed by benthic fish such as sculpins and flatfish.  In the intertidal zone, shorebirds such as gulls (Larus spp.) and sandpipers (Calidris spp.) have been observed picking sea spiders from exposed flats at low tide.  Rays and skates (e.g., Raja spp.) also consume sea spiders when foraging on benthic invertebrates in subtidal sandy areas[11].

Threats

In coastal continental-shelf regions (roughly 20 m to 200 m depth), bottom trawling nets indiscriminately sweep across the seafloor, from where they are caught as bycatch. Since sea spiders often shelter under small stones or within sediment interstices, the physical action of weighted trawl gear often results in their death. The biggest impact occurs on shallow and mid-depth continental shelves, where many species concentrate near prey fields such as hydroid beds and sponge communities for food. Coastal pollution, such as eutrophication and heavy metal accumulation, degrades intertidal and shallow-water sites, further decreasing their count.

Fossil Records

The fossil records of pycnogonids are scarce. Among the few specimens that are preserved, most are from the Cambrian period[12]. The earliest Cambrian fossil was of Cambropycnogon, discovered in Orsten, Sweden. Other fossil records include that of Haliestes (Silurian Age) from the Coalbrookdale Formation of England and of Devonian sea spiders like Flagellopantopus, Palaeopantopus, Palaeoisopus, Palaeothea, and Pentapantopus, discovered at the Devonian Hunsrück Slate of Germany. In 2007, Middle Jurassic fossil specimens were unearthed from the fossil beds at La Voulte-sur-Rhône, France, while the first fossil pycnogonid within an Ordovician deposit was discovered at William Lake in Manitoba.

Although only about a dozen pycnogonid body fossils and larval specimens are known, many are preserved either as tiny Orsten nodules or within deposits such as those from the Carboniferous and Jurassic Lagerstätten. For example, Cambropycnogon klausmuelleri larvae from Upper Cambrian deposits preserved in Orsten-style nodules (~497 Ma) exhibit a trunk-like posterior extension immediately behind the cephalic shield, interpreted as an ancestral opisthosomal segment that persists beyond the reduced abdomen of modern forms. Similarly, Upper Cambrian larval fossils (including Cambropycnogon and an unnamed Orsten specimen) show paired caudal processes, probably homologous to primitive telsons found in early arthropods[13]. These trunk and tail structures provide conclusive evidence of how early sea spiders retained more extensive abdominal segmentation and a terminal telson before evolving the highly reduced abdomen and losing the true tail seen in crown-group Pycnogonida.

References Article last updated on 4th June 2025
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