INVERTEBRATE ZOOLOGY IN THREE VOLUMES - BOOK 2 - H.Y. Shcherbak - 1996

PHYLUM ARTHROPODA

SUBPHYLUM CHELICERATA

ADDENDA TO THE PHYLUM ARTHROPODA

CLASS SEA SPIDERS (PANTOPODA)

This Class comprises exclusively marine Arthropods which, despite their superficial resemblance to spiders, stand quite apart from all other arthropod groups.

Over 640 species are known. Most of them inhabit the sublittoral zone (the ocean and sea floor down to a depth of 200 m, permanently covered with Water) with normal water salinity, though some species have also been found at great depths (up to 5,000–7,000 m). Pycnogonids are absent from heavily desalinated seas. Seven species have been found in the Black Sea, the most widespread being *Calipallene phantoma*, and near the coasts of Crimea — *Tanystylum conirostre*.

Body length (excluding legs) ranges from 0.8 to 18 mm. The body consists of a short trunk divided into a cephalothorax and a rudimentary abdomen. Attached to the anterior end of the cephalothorax is the so-called proboscis, while laterally there are usually four, less frequently five or six pairs of walking legs (Fig. 206). The leg length is often 10–20 times the length of the trunk, giving the impression that the animal consists of legs alone (hence the Latin name, borrowed from the Greek: *pan* — all, *podos* — FOOT).

Fig. 206. Sea spiders:

a - Nymphon distensum; b - Decaiopoda australis; 1 - palps; 2 - chelifores; 3 - proboscis; 4 - walking legs; 5 - cephalothorax; 6 - abdomen

The proboscis is typically cylindrical or ovoid, directed forward, although in some species it is bent ventrally and directed downward and backward.

The cephalothorax consists of 7–9 segments. Four segments fuse to form the HEAD or ocular region, while the remaining ones may be either distinct or fused with each other and the head region.

The first pair of appendages, the so-called chelifores, arises from the anterior margin of the trunk above the Base of the proboscis; immediately behind them lies the second pair, the palps. On the ventral side of the proboscis is the third pair of appendages, the ovigerous legs. The chelifores consist of 2–3 segments and terminate in a chela; the palps are 5–10-segmented, and the ovigerous legs are typically 10-segmented. These three pairs of appendages are significantly shorter than the walking legs and do not participate in locomotion. In some species, the chelifores, palps, and ovigerous legs are more or less reduced (with a decreased number of segments) or disappear entirely (most frequently the chelifores).

Walking legs of identical Structure are attached to the posterior part of the head region and to all other segments; they terminate in a single claw or, in many species, with one or two auxiliary claws.

It is noteworthy that the size and body shape of pantopods vary depending on the habitat depths. In shallow waters, species with a short, compact body and short legs generally predominate. Deep-sea species possess long, slender legs and are capable of swimming and briefly soaring in the water Column. Furthermore, adults of certain species, such as those of the genus *Nymphon*, which inhabit the muddy bottoms of the open sea, are twice as large as those living in the coastal zone.

The body of pantopods, especially in shallow-water forms, features numerous tubercle- and bump-like outgrowths and is covered with a chitinous cuticle.

The Mouth opening is located at the tip of the proboscis, inside which lies a large, muscular Pharynx acting as a pump. Its posterior half contains a complex system of cuticular setiform and Hair-like outgrowths that grind and filter food (Fig. 207).

Fig. 207. Longitudinal section through the proboscis of the sea spider Phoxichilus vulgaris:

a - proboscis with filtering apparatus; b - region of the filtering apparatus

The remaining solid particles are subsequently expelled through the mouth opening.

The midgut is short but possesses lateral outgrowths (diverticula) that extend into almost all appendages, thereby significantly increasing the gut surface area. The hindgut opens at the tip of the reduced abdomen. Special excretory Organs are absent.

Adult pantopods feed on the soft Tissues of various marine invertebrates, including hydroid and coral polyps, jellyfish, Sponges, bryozoans, etc.

Using their claws and chelifores, sea spiders attach themselves to the body of their prey, such as a hydroid polyp colony, locate a hydrant, and, inserting their proboscis into it, suck out its contents.

The Circulatory system is open, represented by a U-shaped tubular Heart and a system of lacunae. The hemolymph is colorless.

The Nervous system consists of supra- and subesophageal ganglia connected by connectives, and a ventral nerve cord. In some pycnogonid species, the ganglia of the ventral cord are fused. The part of the nervous system located within the proboscis has a unique structure: its anterior end features a nerve ring with three ganglionic swellings connected to the Brain by three nerve roots. A nerve plexus extends from the ring along the pharynx.

Sense Organs are poorly developed. There are four primitive ocelli located on a distinct median tubercle on the DORSAL SIDE OF the anterior trunk region. Structurally, they are similar to the nauplius eyes of crustaceans. Pycnogonids can only use these eyes to distinguish the direction of light. In deep-sea forms, the eyes sometimes disappear. Numerous sensory hairs and bristles are scattered across the entire body of pycnogonids.

Pycnogonids are dioecious. Sexual Dimorphism is frequently observed, particularly in The structure of the ovigerous legs. The Gonads originate along the sides of the gut, after which the diverticula of the reproductive glands extend into the thickened segments of 4—7 pairs of legs. Gametes mature inside the legs and are expelled to the outside through pores located on the second segment of all or some pairs of legs. The eggs laid by females are gathered by males onto their ovigerous legs and cemented together by secretions from specialized adhesive glands into compact egg masses.

These masses develop into larvae possessing a short body with a proboscis, three-segmented chelifores, and two pairs of likewise three-segmented appendages.

Fig. 208. Larva of the pycnogonid Chaetonymphon

The Heart and gonads are absent in the larva; however, provisional (larval) organs are present, such as bicellular or multicellular silk glands located in the basal segment of the chelifores, with their ducts opening precisely here at the tip of a hollow spine. The glandular secretion forms a thread that hardens in water and is sometimes used by the larvae to attach themselves to the egg mass or directly to the male's ovigerous legs, where they remain until the end of metamorphosis. In most sea spiders, however, the larva leaves the male and adopts a parasitic lifestyle, attaching to a host using the secretion of these same glands. These larvae also exhibit marked adaptations designed to easily pierce the host's body wall.

The larvae parasitize hydroid polyps, sea anemones, and the mantle cavity of bivalve Mollusks. Juveniles of Decachela dogieli develop within the ambulacral grooves of starfish. Some larvae lead an endoparasitic lifestyle inside the gastrovascular cavity of hydroids (genera Phoxichilidium and Anoplodactylus). During molting, they lose their second and third pairs of appendages, allowing the larva to easily pass through the mouth of the hydroid into its gastrovascular cavity. Here the larva grows, yet its regenerated appendages remain tightly pressed against the body, giving the animal the appearance of a compact lump.