INVERTEBRATE ZOOLOGY - H. J. Shcherbak - 2008

KINGDOM METAZOA

SUBKINGDOM EUMETAZOA

SECTION TRIPLOBLASTICA, OR BILATERIA

SUBSECTION SPIRALIA

PHYLUM MOLLUSCA

CLASS CEPHALOPODA

SUBCLASS COLEOIDEA

This subclass comprises the vast majority of extant cephalopods. Coleoids possess an internal rudimentary shell, which is completely absent in some octopuses. There are eight or ten tentacles arranged in a single ring around the Mouth, bearing suckers. In males of many species, one or two arms are modified into hectocotyli.

Coleoids have a single pair of gills, two atria, two branchial hearts, two Kidneys, and an ink sac. The Nervous system is highly concentrated; the Brain has a complex Structure and is surrounded by a cartilaginous cranium. The eyes are also complex, bearing a resemblance to vertebrate eyes.

The subclass Coleoidea includes four orders.

Order Sepiida (Cuttlefish) have a broad, flattened body; the HEAD bears ten appendages, consisting of eight arms and two tentacles. The fins appear either as a narrow fringe extending along the entire mantle or as broad oval structures on the sides of the body. The internal shell takes the form of a calcareous plate, a calcareous spiral (in Spirula), or a thin horny blade, and is sometimes entirely absent. The most typical representatives are cuttlefish of the genus Sepia, which inhabit the shallow waters of tropical and subtropical seas of the Old World. They live in the littoral zone, staying close to the bottom, though capable of active swimming. The best-known species is the common cuttlefish (Sepia officinalis) (Fig. 245, b), which inhabits the Mediterranean Sea. Fairly large species are also found, such as the broadclub cuttlefish (S. latimanus), inhabiting the western Pacific Ocean. Its mantle length reaches 60 cm, total body length up to 1.5 m, and mass up to 10 kg. One species, Spirula spirula, differs from other extant cephalopods in having a spirally coiled internal shell located in the posterior half of the mantle, which Functions as a hydrostatic apparatus (Fig. 249, b).

Order Teuthida (Squids) are distributed across all the seas of our planet—from cold polar waters to tropical coral reefs, and from the surface to the abyssal zone (the region of the greatest marine and oceanic depths). They typically possess a torpedo-shaped body with a pair of rhombic fins, and ten appendages consisting of four pairs of arms and a pair of tentacles armed with chitinous rings, occasionally bearing hooks. The shell vestige is represented by a thin cartilaginous plate known as the gladius (Fig. 249, c).

These are predominantly nektonic animals; some of them, known as neritic squids, inhabit shelf waters and rarely descend deeper than 500 m. They stay close to the sea floor but can ascend into the Water Column and surface layers, frequently undertaking long-distance Migrations. Most of them form schools. The best-known species is Loligo vulgaris (Fig. 245, c).

Another group, oceanic squids, inhabit open ocean waters. Among them are the fastest swimmers. Oceanic squids perform seasonal migrations, covering vast distances. Some reach giant sizes, such as Architeuthis dux. There are also numerous sluggish planktonic species, the so-called "ammonia squids" mentioned previously (Fig. 259). Despite their low mobility, they possess a well-developed grasping apparatus—in many species, the tentacles are robust and muscular, and the arms and tentacles bear large, sharp hooks. These animals feature well-developed photophores.

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Fig. 259. Squids (after Nessis): a — juvenile giant squid Architeuthis dux,

b — bathyscaphe squid Cranchia scabra

Order Octopoda (Octopuses) are the most well-known cephalopods. They inhabit both cold seas and tropical waters among coral reefs, occurring from shallow coastal areas to the ocean depths. They are characterized by a short, sac-like body with eight arms. The arms are frequently

connected by a membranous web, or umbrella; in some forms, it extends all the way to the tips of the arms (Fig. 245, g). Using the umbrella, octopuses can swim like jellyfish. Most octopuses lack fins.

The internal shell is absent, or represented only by small cartilaginous plates. Nearly all octopuses possess an ink sac, which in some deep-sea species contains a luminescent fluid.

The body of octopuses may be firm and muscular in coastal species, or soft, gelatinous, and even jelly-like in deep-sea forms. In males of most species, one of the arms is modified into a hectocotylus.

Most octopuses are benthic, hiding in caves, among rocks, or constructing shelters out of stones, shells, crab carapaces, and other Materials. They feed on crabs, spiny lobsters, Mollusks, and fish. Their saliva is toxic; by ejecting it, the octopus paralyzes its prey, which it then consumes by tearing off pieces. The venom of certain species is dangerous to humans as well.

A distinct ecological group is formed by planktonic octopuses, such as Chiroteuthis (Fig. 260), which reaches a length of 1.2–1.5 m. It inhabits great depths. The body of planktonic octopuses is gelatinous; the umbrella is very large and, together with the fins, serves as the primary locomotory organ.

Fig. 260. Deep-sea octopus Cirrothauma murrayi

(from Akimushkin and Filipova): 1 - umbrella; 2 - fin

Many octopuses exhibit parental care. They guard their eggs, protect them from predators and debris, and fan them with fresh water. Some species form unique brood pouches using their interconnected arms. Throughout the entire incubation period, they do not feed. However, the most peculiar adaptation for egg-brooding is found in the argonaut, or paper nautilus (Argonauta ogea), the females of which build a unique external shell: thin-walled, parchment-like, and spirally coiled over the back (Fig. 258, a). It is not divided into chambers and is a secondary formation unrelated to true shells, serving solely for brooding eggs.

The best-known species are the common octopus (Octopus vulgaris), distributed at shallow depths in temperate and tropical waters, and the giant Pacific octopus Octopus dofleini (Fig. 245, g), which inhabits the Pacific Ocean and reaches a length of 3–5 m (including the arms) and a mass of 25 kg, although significantly larger specimens are occasionally encountered.

The order Vampyromorpha includes a single species that combines the features of both octopuses and squids. This is the vampire squid (Vampyroteuthis infernalis), reaching up to 37 cm in length and possessing a velvety-black coloration (Fig. 261). It has eight arms connected by a web, or umbrella, which resembles a parasol. The arms bear a single row of suckers, flanked on either side by two rows of short cirri. A pair of paddle-shaped fins is located at the posterior end of the mantle. Like squids, it possesses a gladius. An ink sac is absent. It inhabits the open ocean at depths of 700–1500 m.

Fig. 261. Vampire squid Vampyroteuthis infernalis (from Nessis): a - female; b - larva:

1 - cirri; 2 - filament; 3 - microscopic photophores; 4 - eye; 5 - cluster of photophores on the head; 6 - fins;

7 - vestige of the posterior pair of fins; 8 - complex photophore

Cephalopods play a crucial role in marine ecosystems. As predators, they consume vast quantities of crustaceans, mollusks, Fishes, and other organisms, while in turn serving as an important food source for many marine vertebrates—fishes, birds, and mammals (pinnipeds and cetaceans).

Humans have utilized cephalopods as food since ancient times, and in recent decades, cephalopod fisheries have expanded dramatically due to the depletion of fish stocks and The Need for alternative protein sources. In nutritional value, cephalopods surpass other mollusks and even certain fishes. In Japan, they are among the most popular food products. Currently, about 30 species

of cephalopods are commercially harvested. The attractive shells of nautiluses are used to make ornaments, while the ink of cuttlefishes yields high-quality paint and ink.

Cephalopods are also utilized in medicine and perfumery. Recent studies have demonstrated that they are a source of vital and promising biologically active compounds (anti-Shock, analgesic, and antiparasitic agents, potent toxins, etc.).

Cephalopods are of great interest to researchers as model organisms: notably, squids are among the most important models for studying Nerve Impulse Conduction, while psychologists are fascinated by cephalopods as invertebrates with highly developed cognitive abilities ("primates of the sea"), among other aspects.