INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

KINGDOM MULTICELLULAR ANIMALS (METAZOA)

SUBKINGDOM TRUE MULTICELLULAR ANIMALS (EUMETAZOA)

DIVISION TRIPLOBLASTIC (TRIPLOBLASTICA) OR BILATERAL (BILATERIA) ANIMALS

SUBDIVISION MOLTING ANIMALS (ECDYSOZOA)

PHYLUM ARTHROPODS (ARTHROPODA)

SUBPHYLUM CHELICERATES (CHELICERATA)

SUPERCLASS MEROSTOMANS (MEROSTOMATA)

CLASS HORSESHOE CRABS (XIPHOSURA)

Only five extant species are known, which are considered "living fossils," whereas in the Paleozoic, horseshoe crabs were abundant and diverse. Typically, they inhabit shallow marine waters, although some species can migrate across the continental shelf to depths of up to 250 m or, conversely, into river estuaries. They are found along the Atlantic coast of North America (Limulus polyphemus) and Southeast Asia (three species of the genus Tachypleus and one Carcinoscorpius). Being among the largest modern Arthropods, some individuals reach a length of 50-90 cm.

Anatomy. The body of horseshoe crabs is divided into two movably connected tagmata: the prosoma (cephalothorax) and the opisthosoma (abdomen), ending in a long, strong, movable sword-like appendage known as the telson (hence the Class name). The prosoma, convex dorsally and somewhat concave ventrally, is covered on the back by a thick carapace bearing two pairs of eyes: simple eyes in the middle and compound eyes on the sides of the shield (Fig. 289). The latter consist of numerous ommatidia covered by a common transparent cuticle, the cornea. On the ventral side, in the center of the prosoma, lies a longitudinal Mouth opening surrounded by six pairs of appendages. The chelicerae are shorter, while the pedipalps are longer and structurally similar to the walking legs. All of them (except the last pair) terminate in chelae (claws), and their expanded basal segments (except for the chelicerae) bear gnathobases (masticatory processes). All appendages are multifunctional: they are used for walking, capturing and grinding food, and burrowing into sand or mud. Posterior to the prosomal appendages lie the appendages of the last rudimentary segment, the so-called chilaria—small, unsegmented appendages bearing spines, the function of which remains unknown.

Fig. 289. Anatomy of a horseshoe crab (after Dogel): a - dorsal view; b - branchial leg; c - ventral view:

1 - prosomal shield; 2 — facet eye; 3 - abdominal carapace; 4 - movable spine; 5 - telson (tail spine);

6 - branchial appendage; 7 - segmented leg; 8 - mouth; 9 - chelicera; 10 - pedipalp; 11 - walking legs;

12 - branchial operculum; 13 - gill-bearing legs; 14 - genital opercula; 15 - chilaria

The DORSAL SIDE OF the opisthosoma is also covered by a tough carapace with movable spines along its lateral margins. The ventral side features six pairs of lamellar, leaf-like branchiferous (gill-bearing) appendages. The first pair is enlarged to form the opercula, on the lower lateral sides of which the genital openings are situated, hence their designation as genital opercula. They cover the subsequent pairs of abdominal appendages, which bear numerous gill lamellae on their inner surfaces. The abdominal appendages perform respiratory and swimming Functions. Horseshoe crabs swim upside down (ventral side up). The telson helps the animal right itself if overturned by a wave, or AIDS in burrowing into sand or mud. During burrowing, the animal bends at the junction of the prosoma and opisthosoma, braces the telson against the substrate, and digs in using the sharp anterior margin of the shield.

The integument is characterized by a powerfully developed carapace, which is thickest on the dorsal side of the prosoma. The cuticle includes a well-developed epicuticle, which enables horseshoe crabs to venture onto land during the breeding season.

In addition to the exoskeleton, horseshoe crabs possess a true endoskeleton, the main component of which is the endosternite—a robust plate with several processes located in the prosomal region. Well-developed bundles of specialized Muscles that ensure the movement of all body parts attach to both the external and internal skeletons.

Digestive System. The foregut consists of the Pharynx, Esophagus, and a large muscular (gastric) mill lined with a thick layer of cuticle bearing three Teeth, which provide additional grinding of food. The ducts of the large digestive gland, the hepatopancreas (Liver), which occupies almost the entire volume of the prosoma, open into the midgut. It is here that the final stages of food Digestion, absorption, and intracellular digestion take place. The hindgut opens via the anus located near the Base of the telson.

Horseshoe crabs are predominantly nocturnal predators, feeding on Annelids, Mollusks, and other benthic organisms. In aquarium conditions, they also consume meat and can ingest Algae.

Excretory Organs are represented by four pairs of modified coelomoducts—coxal glands located in the prosoma. Two common excretory ducts extend from them and open at the base of the fifth pair of prosomal appendages (Fig. 290, a).

Fig. 290. Internal anatomy of a horseshoe crab (after Dogel): a - excretory (coxal) gland; b - Nervous system; c - FEMALE Reproductive System:

1 - lobes of the glandular body; 2 - opening of the excretory duct; 3 - excretory duct; 4 - Brain; 5 - circumoesophageal connectives;

6 - nerves innervating the appendages; 7 - ventral nerve cord; 8 - lateral longitudinal nerve trunks; 9 — Ovary

The Circulatory system is well developed (Fig. 291). The elongated tubular Heart features eight pairs of ostia that connect it to the pericardial cavity. Anteriorly, The Heart extends into the aorta, while posteriorly it ends blindly. Lateral Arteries also branch off the heart, merging into two major longitudinal arteries that subdivide into numerous smaller vessels. This forms an extensive arterial network ensuring the efficient supply of oxygen- and nutrient-rich hemolymph to organs and Tissues. In the terminal capillary beds, the hemolymph pours into a system of lacunae within the mixocoel, through which it reaches the gills to be oxygenated before returning via the pericardial cavity and ostia to the heart. Horseshoe crab hemolymph is blue because it contains the respiratory pigment hemocyanin. The hemolymph also contains amebocytes, which, among other functions, play a protective role.

Nervous system. The supraesophageal ganglion is undivided and innervates the eyes. It also gives rise to vestigial antennal ganglia, which are retained only in horseshoe crabs (recall that chelicerates lack antennae). Nerves extending from the circumesophageal connectives supply the chelicerae and all prosomal appendages. The ganglia of the ventral nerve cord innervate the opisthosomal appendages (Fig. 290, b).

Sense Organs. In addition to the visual organs already mentioned, Touch and taste receptors are present in the form of simple or feathered sensilla.

Reproductive system. Horseshoe crabs are dioecious. Females possess paired Ovaries (Fig. 290, c), whereas males feature numerous small Testes. The paired genital ducts open on the first abdominal segment beneath the genital opercula.

Reproduction. During the breeding season, horseshoe crabs migrate to the shore, where males seek out females. The female digs a shallow pit in the sand (up to 15 cm deep) in which she deposits her eggs, while the male fertilizes them with seminal fluid. Fecundity varies among species: a female Tachypleus gigas can lay up to 8,000 eggs per year, T. indentatus up to 20,000, and Limulus polyphemus up to 88,000.

Development is direct: a fully formed Organism hatches from the egg, with only the caudal spine remaining underdeveloped. Growth is accompanied by periodic molting.

Fig. 291. CIRCULATORY SYSTEM OF a horseshoe crab in dorsal view (after Mikkelsen):

1 - anterior aorta; 2 - severed dorsoventral muscles; 3 - lateral prosomal artery; 4 - longitudinal artery; 5 - heart; 6 — ostia

In North America, horseshoe crabs were long harvested for use as fertilizer, whereas in Southeast Asia they served as a food source. In the latter half of the 20th century, it was discovered that the hemolymph of these animals clots in the presence of certain groups of Bacteria,

primarily Gram-negative ones, and that the amebocyte lysate derived from horseshoe crab hemolymph serves as a sensitive reagent for the rapid detection of bacterial endotoxins contaminating Pharmaceuticals, injectable solutions, and medical instruments—toxins that are not always inactivated by sterilization. Until then, the only detection method involved multi-day rabbit pyrogen tests. Horseshoe crab hemolysates are also utilized in diagnosing certain medical conditions. Today, protocols have been developed to extract hemolymph from these animals with minimal harm. Research has shown that horseshoe crabs can fully recover within three to seven days after having 30% of their hemolymph extracted.