Sea spiders, scientifically known as pycnogonids, are marine arthropods that belong to the class Pycnogonida and order Pantopoda. Despite their name and appearance, they are not true spiders (arachnids) and have evolved independently of their terrestrial lookalike. However, the traditional classification system indicates they are more closely related to arachnids than other arthropod groups like crustaceans and insects.
They possess ‘chelifores’, a unique pair of limbs that are not found in any other living arthropod.
Around 1,300 sea spider species are found around the globe, from shallow intertidal zones to depths as low as 7,000 m.
Sea spiders usually range between 0.1 cm and 50 cm (0.04 inches to 20 inches) in leg span, depending on their geographical distribution; however, certain species living in the polar regions can reach as long as 70 cm (27 inches).
These spiders have slender, jointed appendages arranged around their significantly reduced bodies. Like their terrestrial counterparts, they usually have four pairs of legs, though individuals of certain families like Pycnogonidae, Colossendeidae, and Nymphonidae may have as many as five to six pairs. There are nine such extra-legged (polymerous) 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 discreetly divided into segments, they possess the following broad divisions.
Although many sea spider species have been discovered, their correct taxonomy within the group remains unclear. These arthropods were long considered chelicerates, like terrestrial spiders, mites, ticks, scorpions, and harvestmen.
However, some scientists proposed the ‘Cormogonida hypothesis,’ a competing idea suggesting that sea spiders were not closely related to other arthropod groups like Chelicerata, Crustacea, Myriapoda, or Insecta. They argued that the chelifores in sea spiders were controlled by the ‘protocerebrum,’ a part of the brain, which in other arthropods usually controls the eyes and the labrum, rather than the ‘deuterocerebrum’ that controls similar limbs in chelicerates. In contrast to the hypothesis, Hox gene expression patterns revealed that chelifores are identical to the limbs of chelicerates and are controlled by the deuterocerebrum, which has rotated forward in sea spiders. Thus, the scientific community finally decided on Chelicerata as the preferred taxon.
According to the World Register of Marine Species, sea spiders have been further classified as follows.
Although cosmopolitan, sea spiders are primarily concentrated in the Pacific waters around the United States and Japan. However, they are equally abundant in the Caribbean and Mediterranean seas and countries like South Africa, Australia, and New Zealand. They extend southwards to Antarctica, the continent harboring 20% of the world’s sea spider species.
They inhabit a wide range of habitats, from shallow, intertidal zones to the abyssal depths of oceans. They are surprisingly diverse in the polar regions, primarily residing under the sea ice by camouflaging against the sandy ocean bed and benthic algae.
They often associate symbiotically with cnidarians, such as sea anemones and hydroids. By attaching themselves to these creatures, sea spiders get transported to new areas where they might find food or suitable habitats without expending much energy.
These arthropods primarily feed on soft-bodied invertebrates, such as cnidarians like jellyfish and sea anemones, hydroids, small crustaceans like copepods, and worm-like organisms.
They use their long, tubular proboscis to puncture the prey’s skin and suck out fluids from their bodies and are often branded as ‘suctorial predators.’ However, this extraction of body contents does not necessarily kill the prey but simply weakens it; hence, sea spiders are sometimes considered parasitic.
All sea spider species are gonochoric and have separate sexes except Ascorhynchus corderoi, a hermaphrodite having both testis and ovary in a single individual. After a brief period of courtship, the male mounts the female and adjusts his body till the genital pores (gonopores) on the second segment of their appendages perfectly align. The female then releases her eggs, and external fertilization with the sperm occurs outside her body. The males attach the fertilized eggs to their ovigers with a glue-like material and carry them until they are ready to hatch.
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.
They are generally preyed upon by starfish, crabs, rays, fishes, and shore birds.
Since sea spiders are bottom-dwelling creatures, they are highly vulnerable to fish trawling nets that sweep through the ocean beds to collect fish.
The earliest sea spider fossils were of Cambropycnogon from the Cambrian period, 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, 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 fossil records of pycnogonids are insufficient, some specimens possess an extended ‘trunk’ behind the abdomen, and two fossils contain prominent tails, thus shedding light on the evolutionary journey of sea spiders.
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