INVERTEBRATE ZOOLOGY - H. J. Shcherbak - 2008

KINGDOM METAZOA

SUBKINGDOM EUMETAZOA

DIVISION TRIPLOBLASTICA, OR BILATERIA

SUBDIVISION ECDYSOZOA

PHYLUM ARTHROPODA

SUBPHYLUM BRANCHIATA, OR CRUSTACEA

This subphylum comprises Arthropods that have most successfully conquered the aquatic environment: they inhabit temporary puddles, freshwater basins, and marine environments, populating the entire Water Column from deep-sea trenches to the hyponeuston (the subsurface water layer). Most of them are free-living, motile animals, though some are sessile, attached to a substrate, or parasitic on other aquatic animals. A small group of crustaceans has transitioned to a terrestrial lifestyle. Over 40,000 species have been described.

Sizes range from fractions of a millimeter to 80 cm. Crustaceans are characterized by a high diversity in the External Structure of the body and appendages, whereas their internal Organization is relatively typical, despite certain differences among representatives of various classes.

Structure. The body of crustaceans is typically segmented, with the number of segments varying widely. The body is divided into three tagmata: the HEAD, Thorax, and abdomen. The segments of each tagma can be clearly demarcated from those of another or fused to varying degrees. The head bears a pre-oral lobe (acron) and several segments (recent data indicate six). The number of thoracic and abdominal segments varies, except for members of the Class Malacostraca, in which the thoracic tagma has eight segments and the abdominal has six to seven. The abdomen typically terminates in an anal lobe (telson), which often bears a pair of appendages (the furca, or fork).

The head may be undivided or consist of two distinct regions: the protocephalon and the gnathocephalon, which are articulated with each other. The protocephalon is the anterior part of the head bearing the antennules and antennae; the gnathocephalon results from the fusion of the last three (jaw-bearing) head segments, carrying the mandibles and both pairs of maxillae. In many malacostracans, such as the crayfish, the gnathocephalon fuses with the thoracic region to form a gnathothorax (maxilliped-thorax complex), which may be covered by a dorsal carapace. In many forms, all head segments fuse with one or two anterior thoracic segments to form a tagma known as the syncaphalon (Fig. 310). The head and all thoracic segments together constitute the cephalothorax. In some crustaceans, several posterior abdominal segments fuse with the telson to form the so-called pleotelson.

Fig. 310. STRUCTURE OF THE crustacean head (after Snodgrass):

a — fairy shrimp Eubranchipus vernalis (protocephalon demarcated); b — shrimp Spirontocaris polaris (protocephalon dissected out);

c — amphipod Orchestoidea californica, complex head (syncaphalon):

1 — antennule; 2 — antenna; 3 — labrum; 4 — compound eye; 5 — Mandible; 6 — thoracic legs; 7 — protocephalon;

8, 9 — free head segments; 10, 11 — First and Second thoracic segments, respectively; 12 — maxillula (Maxilla I); 13 — maxilla (maxilla II);

14 — maxilliped belonging to the first thoracic segment incorporated into the head; 15 — carapace

Body segments bear a pair of appendages each. A typical crustacean appendage consists of a basal portion, the protopodite, comprising one to three segments, from which two branches diverge: the outer exopodite and the inner endopodite. In many crustaceans, certain body segments also bear a gill appendage, the epipodite, on the protopodite (Fig. 311). The exopodite is often reduced, rendering the appendage uniramous. The head appendages are represented by five pairs. The first are the antennules, the second are the antennae, followed by the mandibles, with the next two pairs being the lower jaws, or maxillae. The thoracic appendages are either all similar or the anterior ones (one to three pairs) are modified into maxillipeds. These lose their locomotory function and participate in capturing and crushing food. The function of thoracic appendages free from feeding is primarily locomotory (swimming, walking) or grasping, respiratory, etc. Abdominal appendages are present only in higher crustaceans (Malacostraca). They are typically biramous; most often they serve non-locomotory Functions, such as respiration or acting as copulatory Organs, while the last pair of abdominal appendages in many decapod crustaceans is modified into powerful swimming plates (Fig. 312). In some parasitic crustaceans, appendages lose their segmentation or disappear entirely in the adult stage.

Fig. 311. Structural diagram of a primitive crustacean appendage (after Snodgrass):

1 — protopodite; 2 — endopodite; 3 — exopodite; 4 — epipodites (respiratory outgrowths)

Fig. 312. Appendages of a male crayfish:

1 — antenna I (antennule); 2 — antenna II; 3 — mandible; 4 — maxillula (maxilla I); 5 — maxilla (maxilla II); 6–8 — maxillipeds; 9–13 — walking legs;

14-19 - abdominal appendages (14 - copulatory, 19 - swimming - uropod)

Integument. Externally, the body is covered with a cuticle which, unlike that of other arthropods, lacks the waterproof epicuticle layer; consequently, water freely evaporates through their integument on land. The pigments responsible for the diverse coloration are located in the relatively thin exocuticle. The endocuticle is quite thick (Fig. 313). In many small crustaceans, the integument is soft and transparent; in others, the outer layers of the endocuticle are impregnated with calcium carbonate, transforming the cuticle into a rigid carapace.

Fig. 313. Diagram of the crustacean integument structure (from Giesbrecht): 1 - seta; 2 - pore canal opening; 3 - pigment layer;

4 - calcareous layer; 5 - non-calcareous layer; 6 - hypodermis; 7 - basement membrane; 8 - endocuticle; 9 - exocuticle

In many crustaceans, the cuticle forms outgrowths—immovable or movably articulated spines, setae, and hairs, often of bizarre appearance. They undergo particular development in swimming forms (designed to increase body surface area). Underlying the cuticle is a layer of hypodermis consisting of well-differentiated Cells. Among them are chromatophore cells containing various pigments.

Digestive System. The Mouth is located on the ventral side of the head, bounded anteriorly and posteriorly by unpaired cuticular folds—

the upper and lower Lips, while mandibles and two pairs of maxillae, adapted for crushing food, are situated on either side. The ectodermal foregut consists of a long Esophagus, the posterior part of which may expand into a crop, while the lining cuticle can become significantly thickened and form spinules and setae that serve for additional food grinding or function as a filter preventing large food pieces from entering the midgut. In higher crustaceans, the posterior part of the esophagus is differentiated into a gastric mill (Fig. 314) armed with setae or robust cuticular plates.

Fig. 314. Structure of the digestive system in the crayfish (from Dogel):

1 - Stomach; 2 - pyloric region of The Stomach; 3 - blind dorsal caecum of the midgut; 4 - midgut; 5 - ridge separating the midgut from the hindgut; 6 - hindgut; 7 - hepatic duct; 8, 9 - ridges on the stomach wall

The endodermal midgut, in its simplest form, is a straight, long tube without appendages or outgrowths, as seen in copepods (Copepoda) and some ostracods (Ostracoda). In most species, however, it bears one or several pairs of extensions called the digestive gland, or Liver. An inverse correlation exists between The Development of the midgut and the liver. The liver produces Enzymes that break down fats, Proteins, and CARBOHYDRATES. Absorption also takes place here. Intracellular Digestion is absent in the liver. The hindgut is generally short, reaching significant proportions only in certain species. In some parasitic crustaceans (Sacculina), the digestive system is reduced, and they absorb host nutrients through their entire body surface.

Crustaceans feed on various foods: some filter organic debris and small organisms—Bacteria, unicellular Algae, and various invertebrates—from the water; others actively tear off pieces of living or dead animals and plants using their mandibles.

The excretory system is represented by two pairs of excretory organs located at the Base of the antennae (antennal glands) and the second pair of maxillae (maxillary glands). Typically, an excretory gland consists of an end sac (coelom remnant) and an excretory duct (a modified coelomoduct), which may become more complex (Fig. 315) by dividing into a labyrinth and a bladder. The excretory glands of crustaceans also function as osmoregulatory organs. Some crustaceans can tolerate sharp salinity fluctuations (euryhaline organisms), but the majority are stenohaline (marine or freshwater).

Fig. 315. Diagram of the antennal gland structure in the crayfish (from Dogel):

1 - bladder; 2 - excretory pore; 3 - coelomic sac; 4 - labyrinth; 5 - nephridial canal

The fat body also participates in excretion, structured similarly to that of other arthropods (discussed in detail using insects as an example).

Respiratory organs. Most crustaceans possess dermal gills closely associated with the appendages. Thin-walled sac-like, lamellar, or feather-like outgrowths—epipodites—are located on the protopodites of the appendages, into which the body cavity extends and hemolymph circulates (Fig. 316, a). Typically, gills are situated on the thoracic appendages, and only in mantis shrimp (e.g., Squilla) and isopods (e.g., water slaters) are the abdominal legs modified into gills. In decapods, such as the crayfish, the gills are located within two branchial cavities beneath the lateral folds of the cephalothorax shield. Water is moved through the branchial cavity by specialized outgrowths of the second pair of maxillae, which beat up to 200 times per minute.

Fig. 316. Diagram of the crustacean Respiratory system:

a - crayfish (cross-section diagram through a thoracic segment (from Matveyev); b - section through the respiratory appendage of a woodlouse (from Natali)

1 - branchial cover - lateral margin of the carapace; 2 - base of the leg; 3 - gills; 4 - Heart; 5 - liver; 6 - gut; 7 - descending artery;

8 - nerve cord; 9 - subneural longitudinal artery; 10 - pseudotracheae; 11 - spiracle; 12 - air cavity

Respiration in many small crustaceans occurs through the thin body integument, and specialized respiratory organs are absent (classes Ostracoda, Maxillopoda, etc.). The respiratory apparatus has undergone significant changes in crustaceans that transitioned to a terrestrial lifestyle. For instance, in the coconut crab (Birgus latro), which still lives on land on Pacific islands, numerous botryoidal Skin folds with a dense network of Blood Vessels have developed on the inner walls of the branchial cavities, while the gills are reduced. Thus, the branchial cavities are transformed into peculiar pulmonary cavities, and respiration becomes aerial. In woodlice (order Isopoda), the lamellar abdominal legs bear deep, branching cuticular invaginations—pseudotracheae—where gas exchange occurs; these are tracheal Lungs that closely resemble insect tracheae (Fig. 316, b).

Circulatory system. The structure of the circulatory system in crustaceans largely depends on the level of development of their respiratory organs. It is reduced or completely absent in forms lacking respiratory organs. The better the gills are developed, the more blood vessels are present, which is associated with The Need for a highly efficient circulatory system to transport oxygen from the respiratory organs to the Tissues.

Typically, crustaceans possess a central pulsating organ—The Heart—located above the gut, near the gills (Fig. 317). In most crustaceans, it resides in the thoracic region, whereas in Isopoda, whose gills are associated with the abdominal appendages, it is situated in the abdomen. The sides of the heart feature valved openings (ostia) through which blood enters. Anterior and, less frequently, posterior aortae branch off from the heart, and these may give rise to additional Arteries. Ultimately, hemolymph from the arteries and their branches enters the body cavity, from where it flows via specialized channels (sinuses) to the gills. There it is oxygenated and, through specialized "branchio-cardiac channels," enters the Pericardium—the region of the mixocoel surrounding the heart—and then passes through the ostia back into the heart itself. All of these mixocoel compartments (sinuses, pericardium) are separated by delicate membranes formed from ground substance that lack cellular structure.

Fig. 317. Diagram of the CIRCULATORY SYSTEM OF the crayfish (after Dogiel):

1 — antennal artery; 2 - anterior aorta; 3 - heart; 4 - pericardium; 5 - branchio-cardiac channels; 6 - descending artery;

6 - posterior dorsal artery; 7 - posterior (upper abdominal) artery; 8 - subneural artery; 9 - abdominal venous sinus

Oxygen dissolves in the plasma; in some Decapoda, the hemolymph has a blue color because its plasma contains a dissolved pigment—hemocyanin—capable of transporting oxygen. Its structure is similar to Hemoglobin, but instead of iron, it contains divalent copper. The hemolymph contains cells, predominantly amoeboid in shape, capable of phagocytosis.

The Nervous system has a typical arthropod structure. The supraesophageal ganglion consists of the Forebrain (protocerebrum), which innervates the eyes; the Midbrain (deutocerebrum), from which nerves extend to the antennules; and the Hindbrain (tritocerebrum) (Fig. 318). Nerves to the antennae originate from the tritocerebrum or the circumesophageal connectives. The ventral nerve cords may be widely spaced, with their ganglia connected by commissures to form a ladder-like nervous system (in Branchiopoda). In most crustaceans, however, they are closely apposed, and the paired ganglia fuse to form a ventral nerve cord (Fig. 319).

Fig. 318. Supraesophageal ganglion, circumesophageal connectives, and vegetative (stomatogastric) nervous System of the crayfish in ventral view (after Dogiel): 1 - protocerebrum; 2 - Optic nerve; 3 - cephalothoracic receptor nerves;

4 - deutocerebrum; 5 - antenna II nerves; 6 - circumesophageal connective; 7 - vegetative (stomatogastric) nervous system;

8 - connective ganglion; 9 - pharyngeal (esophageal) ganglion; 10 - tritocerebrum; 11 - antenna I nerve

Fig. 319. Structure of the nervous system in crustaceans (after Giesbrecht):

a - Anostraca; b - Euphausiacea; c - Stomatopoda; d, e - Decapoda; f - Copepoda; g - Ostracoda

Furthermore, within the class, There is a trend toward the fusion of ganglia from several adjacent segments, leading to a shortening of the nerve cord and, in some cases—such as in crabs—to The formation of a single synganglion. The ganglia of the ventral nerve cord or ladder innervate the organs of their respective segment (receptors, limb Muscles), and when they fuse, nerves persist that extend from the composite ganglion to the corresponding segment.

In the protocerebrum, tritocerebrum, and the ganglia of the ventral nerve cord, alongside neural elements, there are groups of neurosecretory cells. They secrete Hormones that regulate METABOLISM, molting, and developmental processes.

In addition, specialized Endocrine glands are present. A pair of Y-organs is located in the head near the eyes, producing the molting hormone ecdysterone, alongside the X-organ, whose hormones enter a specialized sinus gland and subsequently pass into the hemolymph (Fig. 320). The X-organ produces an antagonist hormone to ecdysterone that inhibits molting. The hormone of the X-organ also influences body coloration. The hypodermis of many crabs, shrimp, etc., contains specialized stellate cells—chromatophores—filled with pigment granules. When the pigment granules are evenly dispersed throughout The Cell, the color is visible, whereas when they concentrate in the center, the color disappears. Under The Influence of nerve impulses from the eyes, the X-organ releases a hormone that affects the Distribution of a specific pigment, enabling the crustacean to change color (for camouflage, signaling, or other purposes).

Fig. 320. Diagram of the neurosecretory system and sinus gland of a crab (after Dogiel):

1 - ventral ganglionic mass; 2 - Brain; 3 - neurosecretory cells; 4 - optic nerve; 5 - eyestalk; 6 - processes of neurosecretory cells; 7 - compound eye; 8 - sinus gland; 9 - endocrine gland

Like other arthropods, crustaceans possess an Autonomic (vegetative) nervous system In addition to the central one, which is closely linked with the Central Nervous System. The anterior (stomatogastric) section of the Autonomic nervous system has been studied most thoroughly (Fig. 318). In the crayfish, it consists of the pharyngeal and gastric ganglia, as well as a pair of ganglia lying on the circumesophageal connectives.

Nerves extend from the ganglia to the Internal Organs.

Sense Organs are well developed. Tactile sensilla are located primarily on the antennules, antennae, and other appendages. In many crustaceans, the antennules bear chemical sense organs (gustatory receptors) shaped like a comb made of specialized thin-walled setae (Fig. 321).

Fig. 321. Sensory setae on the antennule of the crab Geryon affinis (after Dogiel):

1 - comb of sensory setae; 2 - sensory cells; 3 - nerve

Organs of equilibrium are known only in some higher crustaceans. In Decapoda, the basal segment of the antennules contains a special chamber—the statocyst—the inner walls of which bear Hair-like receptors (Fig. 322). The statocyst communicates with the external environment via an opening through which sand grains enter to function as statoliths. During molting, these are shed, and the crustacean restores its supply of statoliths by picking them up with its appendages or by burying its head in the sand. In mysids, statocystes are located in the last pair of abdominal appendages (uropods).

Fig. 322. Statocysts (after Schimkewitsch): a - in the antennule of the crayfish; b - in the uropods of mysids

1, 2 - antennular flagella; 3 - basal segments; 4 - opening of the statocyst pit;

5 - sensory hairs on the floor of the statocyst; 6 - sand grains; 7 - statocyst nerve; 8 - exopodite; 9 - endopodite

Visual organs are of two types. Many crustaceans possess an unpaired nauplius eye located between the bases of the antennules, which is characteristic both of larvae (the nauplius) and of many adult crustaceans (branchiopods, copepods, etc.). It consists of three or four fused cup-like structures (Fig. 323). Each cup consists of a special transparent lenticular Cell Functioning as a lens, and a layer of several photosensitive (retinal) cells located beneath it. Below and on the sides, it is surrounded by pigment cells. In different groups, the structure of nauplius eyes varies significantly in the number of cups and cellular composition; the lens is often absent. Such eyes belong to the so-called inverted type: nerve fibers emerge from the retinal cells on the lens side. It is believed that nauplius eyes are capable of reacting only to light intensity.

Fig. 323. Diagram of crustacean eye structure (after Dogiel):

a - nauplius eye of Ostracoda; b - compound eye of Branchipus; c - cross-section of a specific area of the eye (after Dogiel):

1 - optic cup; 2 - retinal cells; 3 - pigment cells; 4 - optic nerve; 5 - lens; 6 - layer of retinal and pigment cells;

7 - crystalline cones; 8 - ganglionic clusters of Nerve Cells; 9 - brain; 10 - Muscle of the eye stalk; 11 - nerve fibers; 12 - transparent cuticle; 13 - ommatidium; 14 - pigment layers between individual ommatidia; 15 - basement membrane

Most adult crustaceans are capable of distinguishing objects using compound (facetted) eyes. A compound eye contains from several up to 4,000 visual units (ommatidia) (Fig. 323, b, c).

An ommatidium consists of a light-refracting apparatus, a group of sensitive retinal cells giving rise to nerves, and pigment cells. Nerves emerge from the retinal cells at the base (non-inverted eye). Each ommatidium is isolated from the others by a parietal layer of pigment that absorbs most light rays. Consequently, only a fraction of the light rays striking the ommatidium surface at an angle close to a right angle reaches the retinal cells. Thus, each ommatidium perceives only a portion of the image. The complete image is assembled from many fragments, much like pieces of a mosaic picture (mosaic Vision).

Reproductive System. Most crustaceans are dioecious, with the exception of some hermaphroditic sedentary forms. Distinct Sexual Dimorphism is frequently observed. For instance, certain appendages of the male are modified into clasping organs to hold the female (antennules of Copepoda) or be

come copulatory organs. In some groups, males are significantly smaller than females; in barnacles and certain parasitic isopods, the males are dwarfed. Heterogony—the alternation of parthenogenetic generations with bisexual ones—is observed in barnacles.

The Gonads are paired, often partially or completely fused, yet the genital ducts and openings remain paired (Fig. 324, a, b). Oviducts with glandular walls, which secrete a dense shell around the eggs, extend from the Ovary. Females often possess seminal receptacles. In males, the walls of the sperm ducts leading from the Testes contain glandular cells that secrete a sticky substance cementing the sperm into spermatophores. During mating, the male attaches a spermatophore to the female's body or introduces it into her genital organs.

Fig. 324. Reproductive system of the crayfish (after Dogiel): a - female; b - male; c - spermatozoa of Astacus; d - Galathea:

1 - ovary; 2 - oviduct; 3, 4 - paired and unpaired PARTS OF THE Testis, respectively; 5 - sperm duct; 6 - vas deferens;

7 - genital opening; 8 - protopodite of the posterior thoracic leg; 9 - tail capsule; 10 - neck with three processes

In most crustaceans, spermatozoa have a typical shape, but (for example, in Ostracoda) they can reach enormous lengths of up to 5–6 mm, representing the longest spermatozoa known in animals. In many higher crustaceans, spermatozoa have a rather bizarre appearance and are completely immotile (Fig. 324, c, d).

Reproduction. Crustacean fecundity varies: the crayfish lays about 200 eggs, whereas some crabs lay up to 2 million. Eggs are covered by a tough membrane—the chorion—and are usually carried on the female's appendages or in special brood chambers; sometimes they are glued into egg sacs attached to the body.

Most crustaceans develop through metamorphosis. A planktonic larva—the nauplius—hatches from the egg (Fig. 325, a). It has an oval shape and three pairs of appendages: antennules, antennae, and mandibles, the latter two pairs being biramous. Unlike in adult forms, the antennae of the nauplius serve for swimming and food gathering, while the mandibles function solely for swimming. An anal lobe (telson) is located at the posterior end of the larva's body. The nauplius possesses an unpaired nauplius eye.

Fig. 325. Larval stages of crustaceans (from Ivanova-Kazas):

a — nauplius of the cyclops Cyclops; b — metanauplius of the tadpole shrimp Apus; c — zoea of the crab Maja; d - mysis larva of the shrimp Pandalus (from Dogiel);

e — megalopa of the crab Portunus; f — phyllosoma of the spiny lobster Palinurus (from Ivanova-Kazas):

1 — antennae; 2 — antennules; 3 — naupliar eye; 4 — antennal gland; 5 — gut; 6 — mandible; 7 — thoracic segments; 8 — compound eye; 9 — dorsal spine; 10 — abdominal segments; 11 — maxilliped; 12 — rostrum; 13 — pleopods; 14 — pereiopods; 15 — chela

At the posterior end of the body, just anterior to the telson, lies the growth zone where new body segments are formed: first, the segments bearing the first and second pairs of maxillae, followed by the anterior thoracic segments. A larva possessing all head and anterior thoracic segments is called a metanauplius. Higher crustaceans exhibit additional larval stages.

Postembryonic development involves molting. After each molt, new segments appear and new appendages are formed, meaning development proceeds via anamorphosis. In crayfish, a miniature copy of the adult animal with a full Complement of segments hatches directly from the egg, meaning their development proceeds via epimorphosis.

According to various Classification systems, the subphylum Branchiopoda, or Crustacea, comprises several dozen classes. The most important of these are discussed below.