ZOOLOGY OF INVERTEBRATES - H. I. Shcherbak - 2008

KINGDOM METAZOA

SUBKINGDOM EUMETAZOA

DIVISION TRIPLOBLASTICA, OR BILATERIA

SUBDIVISION ECDYSOZOA

PHYLUM ARTHROPODA

SUBPHYLUM BRANCHIATA, OR CRUSTACEA

CLASS MALACOSTRACA

This Class comprises about 23,000 extant and several thousand extinct species. These crustaceans vary greatly in size (ranging from 1 mm to a leg span of 3 m in certain crabs), morphological Structure, and ecological adaptations.

Malacostracans inhabit all types of aquatic environments across various depths, and terrestrial forms are also known. Their trophic niches include predators, polyphages, phytophages, detritivores, scavengers, filter feeders, and a small number of parasites.

Morphology. The distinctive Organizational features of malacostracans include a constant number of body segments: eight thoracic and six to seventh abdominal. The male genital opening is located on the seventh or eighth thoracic segment, while the female opening is on the sixth. The Stomach is divided into gastric and filtering regions, the digestive gland (midgut gland or hepatopancreas) is well-developed, and a Heart and Blood Vessels are always present. The excretory Organs are antennal glands. They are predominantly gonochoric (dioecious); development proceeds with metamorphosis,

and occasionally (in crayfish) is direct.

The Taxonomy of Malacostraca is quite complex, divided into three subclasses and about 20 orders. Let us examine only some of them.

Order Stomatopoda (Mantis shrimps). Most species inhabit the shallow waters of tropical and subtropical seas. Several species are found in the Mediterranean Sea, with Squilla oratoria being the most common. All of them are highly specialized ambush predators. Approximately 300 species have been described.

Stomatopods are relatively large, reaching up to 60 cm in length. The body consists of a protocephalon, a gnathothorax (formed by the fusion of three jaw segments and five thoracic segments, covered dorsally by a carapace), three posterior freely articulated thoracic segments, and a long abdomen (Fig. 339). They possess a pair of highly mobile stalked compound eyes and a naupliar eye. The antennules are elongated, terminating in three filamentous branches bearing chemoreceptive organs, whereas the antennae are shortened. Stomatopods have three pairs of maxillipeds; the first pair is short and sensory, while the second pair is the longest, featuring a terminal article that resembles a sharp, serrated blade capable of folding into a special longitudinal groove on the penultimate expanded article. Using this subchelate apparatus, the shrimp captures and kills its prey. The five pairs of legs on the free thoracic segments are ambulatory. The first five pairs of abdominal appendages are biramous, covered with feathered setae, and bear branched gill processes. These abdominal limbs are used for swimming. The broadened sixth pair of abdominal appendages and the flattened telson form a caudal fin, and are also used to shovel sediment outward when burrowing.

Fig. 339. Diagram of stomatopod anatomy (after McLaughlin):

1 - antennules; 2 - stalked compound eyes; 3 - antennae; 4 - rostrum; 5 - carapace; 6 - free thoracic segments; 7 - telson;

8 - uropods; 9 - pleopods (abdominal legs); 10 - subchela; 11 - second pair of maxillipeds

Mantis shrimps are gonochoric. The female lays eggs (up to 50,000 in larger species) onto the last three pairs of maxillipeds, where an egg mass or pouch is formed. Development involves metamorphosis. The larva spends some time in the burrow, subsequently transitions to a planktonic lifestyle, molts several times, and eventually settles to the bottom, transforming into the adult form.

Most stomatopods live in burrows, with the anterior end of their body protruding outward. Upon the appearance of prey (shrimp, small fish, or other small swimming animals), the crustacea instantly lunges from its burrow and seizes the victim with its second pair of maxillipeds, much like praying mantises (hence the order's common name).

Order Mysidacea (Mysid shrimps). These are predominantly marine planktonic crustaceans inhabiting the Water Column, primarily in shallow waters; only about 50 species are found at great depths. Species from brackish and freshwaters are also known, including those in the basins of the Dnieper, Bug, and Donets rivers. Approximately 800 species have been described, 30 of which occur in the Black Sea. Their body length generally ranges from 10 to 20 mm, although certain deep-sea species can reach 40 cm in length.

In appearance, mysids somewhat resemble shrimp (Fig. 340). The body is elongated and composed of a protocephalon, a gnathothorax (unifying the jaw segments and two to three thoracic segments), and the remaining free segments of the Thorax and abdomen. The carapace of the gnathothorax overhangs the posterior free thoracic segments. Compound eyes are borne on stalks. Both pairs of antennae are very long. The appendages of the gnathothorax participate in filter-feeding, while the remaining thoracic legs are biramous and adapted for swimming. Gills on the thoracic legs are absent in many species, with gas exchange occurring through the thin integument of the carapace. Females possess specialized plates on the Base of the last thoracic legs that form a marsupium (brood pouch). Each abdominal segment bears a pair of biramous appendages, which on the final segment are greatly elongated and, together with the telson, form a caudal fan ("tail fan"). Statocysts (equilibria-sensing organs) are located at the base of the endopods of the last pair of abdominal appendages (Fig. 322).

Fig. 340. The mysid crustacean Mysis relicta (after McLaughlin)

By feeding habits, mysids are filter feeders. When plankton is scarce, they can feed on detritus from the bottom, and with the help of their mandibles, they can even chew up the remains of small animals. Mysids frequently migrate vertically and horizontally in search of food, which consists of small plankton. During these movements, they can form large aggregations.

Mysids are dioecious with internal Fertilization. Development is direct or with a slight metamorphosis (the manca larva differs from adults by lacking a portion of the thoracic and abdominal appendages).

The order Amphipoda (amphipods or scuds) is a fairly large group of marine and freshwater crustaceans. About 4.5 thousand species have been described. Of these, only 30 have been found in the freshwaters of Ukraine, and 107 species in the Black and Azov seas. Body sizes range from a few millimeters to 20 cm.

The body of amphipods is generally laterally compressed. There is a syncephalon consisting of the HEAD and one or two thoracic segments; the remaining thoracic segments are free, and the carapace is absent (Fig. 341). The eyes are faceted, sometimes fused into a single median eye. Antennules and antennae are elongated, serving as organs of Touch and chemical sense; in many species, a pair of statocysts is located under the head cuticle on the dorsal side. Mandibles and both pairs of maxillae are of the chewing type. There is a single pair of maxillipeds. All seven pairs of appendages on the free thoracic segments have different structures (hence the name Amphipoda, meaning "different feet"). Prehensile structures—claws designed for holding food—develop on the first one or two pairs. The subsequent legs bear sharp claws; in various species, these are adapted for crawling, attachment to the

substrate, and so on. All seven pairs of thoracic legs bear gills. In females, plates at the base of the second through fifth pairs of thoracic legs form a brood pouch. The first three abdominal segments bear typical biramous swimming legs. The biramous appendages of the last three abdominal segments are directed backward and provide jumping locomotion. In some amphipods (such as Skeleton shrimps and whale lice), the abdomen is shortened, and the abdominal appendages are reduced or completely absent. Amphipods swim ventral side down, but jumping movements in shallow water are performed while lying on their side (hence the Ukrainian name bokoplavy, meaning "side-swimmers").

Amphipods are dioecious. Egg fertilization takes place within the brood pouch. Development proceeds without metamorphosis. Young, fully formed crustaceans leave the chamber; sexual maturity is reached after numerous (up to 15) molts. Adult crustaceans live from one to six years, while the species *Niphargus orchus* can live up to 30 years.

Most species of amphipods are marine benthic animals, particularly abundant at shallow depths and in the intertidal (littoral) zone of seas. A significant number of them are also found along the coast of the Black and Azov seas—these are the so-called "sand hoppers," which even penetrate into the damp areas of marine beaches. Some species tolerate significant desalination well, venturing far into river mouths; There are also numerous freshwater species, the most abundant of which is *Gammarus pulex* (Fig. 341, a).

Fig. 341. Amphipods (after Birstein and Zarenkov): a — water flea Gammarus pulex; b — skeleton shrimp Caprella anatifera

Most amphipods are omnivorous, feeding on living and dead plants, small animals, carcasses, and detritus (for example, species of the genus *Gammarus*). *Pontogammarus maeoticus*, a mass species of the Azov Sea coast, belongs to the filter feeders. Subterranean species pass soil through their digestive tract and also consume the remains of various organisms. Planktonic amphipods are predominantly predators, living on and feeding on jellyfish and ctenophores. Among benthic forms, predators include skeleton shrimps (*Caprella*) (Fig. 341, b), which feed on hydroid polyps and prey on Annelids and small crustaceans.

True parasites among amphipods include whale lice (family Cyamidae), which live on the Skin of whales during all phases of development, burrowing into it.

Order Isopoda (isopods). Most species live at various depths in the seas, in freshwater bodies (including subterranean ones), and there are also terrestrial species, as well as parasites of other crustaceans and fish. About 4.5 thousand species have been described; of these, 32 are found in the Black and Azov seas, and three freshwater species have been discovered in the fauna of Ukraine. Body sizes range from a few millimeters to 40 cm.

The body is typically dorsoventrally flattened. There is a syncephalon consisting of the head and one (rarely two) thoracic segments. The eyes are faceted and sessile. The carapace is absent. The thoracic region is the longest, consisting of six to seven free segments (Fig. 342). Antennules and antennae are uniramous. Jaws are generally of the chewing type, although in some predatory and parasitic species they are modified into a piercing-sucking proboscis. There is one (rarely two) pair of maxillipeds, corresponding to the number of thoracic segments fused with the head. Seven pairs of thoracic walking legs are uniramous and structurally identical, which gives the order its name (Isopoda, meaning "equal feet"). Plates forming the brood pouch are located on the fourth to fifth anterior pairs of legs in females. The abdomen is shortened; its last segment (sometimes several) is fused with the telson in most species, forming a pleotelson. Abdominal appendages are biramous, leaf-like, overlapping one another, and directed backward beneath the pleotelson. Their primary function is respiratory. The exopodite of one of the pairs of abdominal legs is elongated, strongly sclerotized, and covers the remaining legs from below.

Fig. 342. Isopods (after Dogiel): a — woodlouse Porcellio; b — water hoglouse Asellus aquaticus

Some isopods (woodlice) have transitioned to terrestrial life. In most of them, the gills are covered with a thin layer of water and located in a special cavity on the abdominal legs. Some species breathe atmospheric oxygen: their abdominal leg exopodites contain a cavity connected by a narrow opening to a respiratory chamber, forming peculiar tracheate Lungs (Fig. 316).

Most isopods are dioecious. Hermaphrodites are known among parasitic forms. Some woodlouse species reproduce parthenogenetically. Larvae known as mancas hatch in the brood pouch, emerge into the external environment, and develop into adults.

Isopods feed mainly on plant remains and detritus; several marine species from the family Arcturidae are filter feeders. There are also quite a few predators, such as *Astacilla pusilla*. Some marine species of the genus *Limnoria* bore tunnels into underwater wooden structures, feeding on the wood.

Representatives of this order include the freshwater water hoglouse (*Asellus aquaticus*); among woodlice, the most well-known genera are *Oniscus* and *Porcellio*; the largest isopods inhabit the seas—giant isopods and sea roaches (genera *Idothea* and *Bathynomus*), growing up to 30 cm in length.

Order Euphausiacea (krill). A small group of higher crustaceans numbering about 90 species. This order includes exclusively marine planktonic animals inhabiting the entire World Ocean, except for its desalinated areas. Sizes range from 7 mm to 10 cm.

They are characterized by the presence of a protocephalon and a gnathocephalon, which includes the jaw segments and all eight thoracic segments. The carapace covers the head and thorax, but it is shortened laterally and does not cover the gills (Fig. 343). By this feature, euphausiaceans—which closely resemble shrimp—can be distinguished from them.

Fig. 343. Euphausiid crustacean Thysanopoda (LATERAL VIEW OF a male) (after McLaughlin):

1 — antennules; 2 — eyes; 3 — rostrum; 4 — carapace; 5 — telson; 6 — endopodite of uropods; 7 — exopodite of uropods;

8 — abdominal legs; 9 — gills; 10 — thoracic legs; 11 — antennae

Euphausiids are gonochoric, with spermatophore-mediated fertilization. Some species release fertilized eggs directly into the water, with fecundity scaling directly with body size—ranging from 200 to 11,000 eggs. Others brood a smaller clutch (10–60 eggs) on their abdominal appendages. Their development proceeds through four larval stages, beginning with the nauplius, with each stage undergoing multiple molts.

Euphausiids predominantly inhabit surface marine waters; however, certain deep-sea forms, such as the blind species of the genus Bentheuphausia, are found at depths of 3–4 km. In terms of feeding habits, they are primarily filter feeders. Carnivorous forms, mainly deep-sea species (such as those of the genus Thysanopoda), prey on small crustaceans and larger animals like chaetognaths and jellyfish, though some shallow-water predators also exist. A defining characteristic of euphausiids is the presence of light-emitting organs, or photophores, which facilitate conspecific recognition and schooling.

In Antarctic waters, the Antarctic krill, Euphausia superba, proliferates in immense quantities (hundreds of millions of tons), while E. pacifica is similarly abundant in Arctic waters and the Pacific Ocean. These small crustaceans are consumed by humans (commonly known as "krill") and serve as a fundamental food source for baleen whales, numerous penguin species, and various fish.

Order Decapoda (Decapods). These are the largest crustaceans, with body lengths reaching up to 80 cm and leg spans up to 3 m. Approximately 9,000 species have been described, including about 50 species inhabiting the Black and Azov Sea basins, five of which live in freshwater bodies. Among decapods, there are

marine, freshwater, and terrestrial forms; their ecological and morphological adaptations are remarkably diverse (Fig. 344).

Fig. 344. Decapods (modified from Birstein and Zarenkov):

a – spiny lobster Palinurus elephans; b – European lobster Homarus gammarus; c, d – hermit crab Pagurus bernhardus, removed from its shell,

and inside a mollusk shell with sea anemones, respectively

The body consists of a protocephalon, a gnathocephalon formed by the fusion of the head's gnathal segments with all thoracic segments, and six free abdominal segments ending in a telson. The protocephalon bears two pairs of antennae and stalked compound eyes. The basal segment of the antennules houses a balance organ, the statocyst. The gnathothorax carries three pairs of mouthparts (maxillipeds and jaws), three pairs of biramous maxillipeds, and five pairs of walking legs (hence the name Decapoda), the first of which is typically chelate (clawed). Gills are located on all thoracic limbs as well as on the body wall at the limb bases. A carapace completely covers and fuses with the head and thoracic segments, extending laterally to form branchial chambers. In terrestrial decapods, such as the coconut crab (Birgus latro), the gill cavities are modified into lung-like chambers for aerial Respiration, while the gills undergo reduction. The abdomen bears biramous swimming appendages. The final pair of abdominal appendages, the uropods, combine with the telson to form a tail fan. In certain decapods (such as true crabs and king crabs), the abdomen is more or less reduced.

Most decapods are gonochoric. Sexual Dimorphism is expressed in body size, coloration, and the shape of the abdominal appendages. Fertilization is via spermatophores. Typically, females brood their eggs on the abdominal appendages, attaching them with a special secretion from cement glands, whereas prawns of the family Penaeidae release their eggs directly into the water. Most crabs and marine lobsters (spiny lobsters) produce from several tens of thousands to 2–3 million eggs; the fecundity of freshwater decapods is considerably lower (20–600 eggs), and their eggs are large and yolk-rich.

The Development of decapods generally involves metamorphosis. The nauplius and metanauplius stages are found only in certain prawns (family Penaeidae). In most decapods, the egg hatches into a zoea larva, which leads a planktonic lifestyle and serves for dispersal. In crabs, the cephalothorax bears a long rostral spine. Through successive molts, the zoea transforms into a mysid-like larva—termed a megalopa in crabs, a glaucothoe in hermit crabs, and a phyllosoma in lobsters and spiny lobsters (Fig. 325). In freshwater crayfish and deep-sea forms, development is direct.

The taxonomy of decapods is complex and fluid, so we will focus on the most prominent groups (Figs. 344–345). Shrimps and prawns are widely known, characterized by a laterally compressed body with a long abdomen bearing swimming legs and a large tail fan. Most are marine planktonic forms that feed on small planktonic organisms.

Fig. 345. Decapods (from Dogiel):

a – northern shrimp Pandalus borealis; b – shore crab Carcinus maenas

Unlike shrimps, a large group of decapods leads a benthic, crawling existence. Their bodies are dorsoventrally flattened, and their abdominal appendages lack swimming Functions or are partially reduced. For instance, spiny lobsters (genera Palinurus, Polycheles, etc.) are bottom-dwellers; freshwater crayfish—such as the noble crayfish Astacus astacus and the narrow-clawed crayfish A. leptodactylus—inhabit burrows they excavate with their claws in steep underwater clay banks; true lobsters (genus Homarus, etc.), which are large (up to 60–70 cm long) marine crustaceans, also dwell in burrows. Most species are predatory, capturing prey with their claws, though scavengers also occur. Hermit crabs of the genus Pagurus conceal their long, soft-integumented abdomens inside gastropod shells; these are predominantly predatory forms. Some hermit crabs (species of the genus Coenobita) are terrestrial, utilizing the shells of land snails. The coconut crab Birgus latro, an inhabitant of tropical sandy beaches that feeds on the fruits of palms and other trees, returns to the sea to breed; its larvae spend a phase in gastropod shells, but adults are strictly terrestrial and do not utilize shells.

Lithodid crabs (family Lithodidae), which are closely related to hermit crabs, externally resemble true crabs because their abdomen is tucked beneath the carapace and abdominal appendages are often absent. Unlike true crabs, the final pair of thoracic legs in lithodids is small, dedicated to gill-cleaning rather than locomotion, and is largely concealed. A notable example is the red king crab (Paralithodes camtschaticus), a commercially valuable species native to the northern Pacific.

True crabs possess a broadened carapace, shortened rostrum and antennae, and a reduced abdomen folded tightly beneath the cephalothorax. The claws are invariably well developed (Fig. 345, b). Crabs are mostly predatory, though omnivorous forms exist. The edible shore crab Carcinus mediterraneus inhabits shallow waters of the Black Sea, while several freshwater crab species of the genus Potamon occur in mountain streams of Crimea, the Caucasus, and Central Asia. Semi-terrestrial crabs of the family Ocypodidae inhabit burrows dug into sandy beaches, sinking deep enough to reach the water table, and forage for prey by running across the sand. This group includes fiddler crabs (genus Uca), which grow up to 4 cm in length; males feature one dramatically enlarged, brightly colored claw used in courtship displays, and each male fiercely defends its individual burrow. Finally, land crabs of the genus Cardisoma live entirely on land, though they must return to the sea to reproduce.

In terms of feeding habits (filter feeders, detritivores, herbivores, carnivores, parasites, etc.), locomotion, and reproduction, decapods represent an exceptionally diverse and ecologically versatile group that has held a dominant position in aquatic ecosystems since their emergence at THE START OF the Paleozoic era.

Crustaceans play a vital role in the biological turnover of aquatic ecosystems, acting both as consumers at various trophic levels and as a primary food source for numerous animals, including other crustaceans, fish, and whales. Crustaceans and their larvae form the foundation of marine plankton. For instance, mysid shrimps undertake vertical Migrations forming massive aggregations—swarms of Neomysis integer can span a kilometer in length and several meters in width. Euphausiids, notably the Antarctic krill Euphausia superba, reproduce in quantities estimated in the hundreds of millions of tons. Copepods (subclass Copepoda) also contribute immense animal biomass. Furthermore, amphipods are an indispensable food source for numerous fish species, including commercially important ones.

Crustaceans make a significant contribution to the biological self-purification of water bodies, representing one of the most abundant groups of filter feeders and detritivores responsible for decomposing organic remains.

Terrestrial isopods contribute to pedogenesis and soil fertility. Desert woodlice of the genus Hemilepistus excavate extensive burrows that bring substantial amounts of soil to the surface while enriching it with their feces; their population densities in desert environments can reach up to 1 million individuals per hectare.

Crustaceans are a valuable nutritional resource for humans. Shrimps, crabs, spiny lobsters, and lobsters are major commercial targets, with global yields reaching approximately 10 million tons annually. Euphausiids are also utilized for human consumption, particularly krill, which is processed into nutrient-rich pastes, while in Southeast Asia, mysid shrimps are used to prepare savory sauces and condiments.

In artificial water bodies, ponds, and reservoirs, crustaceans substantially enhance the food web for commercial fish. Fish-farming enterprises purposefully cultivate small crustaceans such as Daphnia and Artemia.

Acorn barnacles and stalked barnacles play a detrimental role; by settling in massive numbers on underwater structures, ship hulls, and other submerged objects, they severely impair their hydrodynamic performance.

Parasitic copepods inflict considerable damage on freshwater and certain marine Fishes, posing a particular threat to juveniles in aquaculture facilities. The common carp louse Argulus foliaceus, a blood-sucking parasite of freshwater fish frequently found in our inland waters, can cause the mortality of fry during mass outbreaks.

Fossil remains of crustaceans with calcareous carapaces (such as conchostracans, ostracods, and certain barnacle species) are widely utilized in geology as index fossils.

In recent times, the populations of certain crustacean species have been declining sharply, necessitating the development of conservation measures. This is especially true for freshwater inhabitants—where anthropogenic pressure is extremely intense—as well as for heavily harvested species, notably crayfish and the Kamchatka crab.

Though less conspicuous visually, crustaceans (such as copepods, ostracods, amphipods, etc.) play a major role in the environmental biomass balance as intermediate hosts for pathogenic helminths. In their adult stage, these parasites inhabit fishes, waterfowl, carnivorous mammals, and humans (including the broad tapeworm, guinea worm, and others).