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

PHYLUM ARTHROPODA

SUBPHYLUM BRANCHIATA, OR CRUSTACEA

CLASS BRANCHIOPODA

Branchiopods inhabit various continental Water bodies, including cave waters, puddles, saline lakes, and estuaries. All of them lead a free-living lifestyle. Over 800 species have been described.

The degree of HEAD cephalization varies among different representatives, ranging from a protocephalon to a fully segmented head; however, the head never fuses with the thoracic segments. The tagmata differ relatively little from one another; the thoracic appendages are of uniform Structure, leaf-like, often unsegmented, and alongside swimming, they perform filtration and respiratory Functions.

The Heart appears as a longitudinal vessel located on the dorsal side, extending from the end of the abdomen almost to the head, and possesses up to 18 pairs of ostia. It lacks separate chambers,

yet it contains several segmentally arranged Valves that ensure the movement of hemolymph in only one direction—towards the head. The Nervous system is ladder-like, with widely spaced ventral nerve cords.

Development proceeds with metamorphosis: a nauplius hatches from the egg.

The Class Branchiopoda comprises 4 orders, which we will examine below.

Order Notostraca. Tadpole shrimp are inhabitants of shallow, standing freshwater bodies, particularly temporary or ephemeral pools. Of the 15 known species, two have been found in Ukraine. Body sizes do not exceed 4—5 cm.

The body is elongated; the head, Thorax, and Base of the abdomen are covered by a dorsal shield (carapace), which is an outgrowth of the maxillary segments and is not fused with the trunk (Fig. 69). The head is fused. Both pairs of antennae are shortened. There are up to 50 trunk segments in total. The first 10 segments bear pairs of biramous appendages, with the first and, to a lesser extent, the second pairs of legs differing sharply from the others by possessing long thread-like appendages that perform a sensory function. The remaining segments bear 4 to 6 pairs of legs, even though each segment has a single pair of nerve ganglia. This phenomenon (polypody) is unknown in other Arthropods. The posterior trunk segments lack appendages. The telson terminates in a long furca. The bases of the legs feature gnathobases (mastigatory outgrowths), which tadpole shrimp use to push food (detritus, small animals, fish and amphibian spawn) toward the Mouth. They may also prey on larger animals (tadpoles, fry, other tadpole shrimp).

When water bodies dry up, tadpole shrimp burrow into the sediment, where they remain viable for some time; their eggs can remain dry yet undamaged for up to 15 years and are easily carried by the wind, which facilitates the dispersal of these animals.

They reproduce primarily parthenogenetically; males are scarce and entirely absent in many species. A metanauplius hatches from the egg. Development takes 2—3 weeks.

In Ukraine, widespread species include the spring tadpole shrimp (Lepidurus apus), which lives mainly in temporary water bodies from April to June, and the summer tadpole shrimp (Triops cancriformis), more frequently encountered in small permanent water bodies During the first half of summer.

Fig. 69. Tadpole shrimp Triops cancriformis (a - dorsal view; b - ventral view):

1 - eye; 2 - thread-like appendages of the first pair of thoracic legs; 3 - abdomen; 4 - furca; 5 - thoracic legs

Fossil tadpole shrimp are known from the late Carboniferous period. Interestingly, many specimens of *T. cancriformis* found in fossilized mud from the early Triassic period are practically indistinguishable from modern ones. Consequently, this species is one of the most ancient, having existed for at least 200 million years!

Order Conchostraca (Clam shrimp). About 200 species of conchostracans (only 5 in Ukraine) inhabit small permanent or temporary water bodies with fresh or brackish water. These are mostly benthic animals. Sizes range from 1—2 to 30 mm.

The body is laterally compressed, consisting of a head and a trunk that terminates in a telson with a short furca. It is covered by a chitinous bivalve shell, sometimes impregnated with calcium carbonate (Fig. 70). Concentric growth rings are visible along its periphery, delineating areas added after each molt; there may be up to 90 such rings. On the dorsal side, the valves are connected by an elastic ligament composed of organic matter and are closed by means of a single adductor Muscle located near the head. There is a pair of sessile compound eyes, a naupliar eye, small uniramous antennules, and elongated biramous antennae adapted for swimming. The number of trunk segments ranges from 10 to 32; each bears a pair of legs with well-developed lobes. Most legs direct food toward the mouth, but in males, the first 1—2 pairs are modified into specialized claspers for holding the female during copulation, whereas in females, the exopodites of several posterior legs are modified into filamentous outgrowths for retaining eggs within the shell. The abdomen is rudimentary.

Fig. 70. Clam shrimp:

a - shell; b - internal structure; 1 - naupliar eye; 2 - compound eye; 3 - Mandible; 4 - adductor muscle; 5 - ligament; 6 - midgut; 7 - filamentous outgrowths; 8 - telson; 9 - furca; 10 - trunk legs; 11 - modified trunk legs; 12 - antenna; 13 - antennule

Conchostracans are dioecious, and some species exhibit obligate parthenogenesis. Females carry fertilized eggs beneath their carapace. These eggs retain their viability for a long time even under desiccation, freezing, or similar conditions. They hatch into a nauplius or a carapace-covered metanauplius.

All conchostracans are filter feeders, consuming detritus and small organisms. The largest species (Estheria, Limnadia) generally rest on their sides, while smaller ones burrow in the substrate, stirring up silt with movements of their abdomen and legs. Occasionally (Lynceus), they swim upside down with their valves open, and they very rarely descend to the bottom.

Fig. 71. Cladocerans:

a - diagram of The structure of Daphnia; b - Leptodora kindtii; 1 - antennule; 2 - eye; 3 - compound eye; 4 - antenna; 5 - hepatic cecum; 6 - Brain; 7 - intestine; 8 - heart; 9 - Ovary; 10 - embryos in the brood pouch; 11 - abdomen; 12 - thoracic legs

140 species have been found, with over a dozen inhabiting the Black and Azov Seas.

The body is laterally compressed and consists of a head, thorax, and abdomen with a small furca. Most species possess a carapace that is folded along the back but not divided into valves (Fig. 71). The head, uncovered by the carapace, often features a rostrum at the anterior end; antennules are very short and unbranched, while antennae are greatly elongated, branched, with several lateral processes covered in setae, serving for swimming. Mandibles and the first pair of maxillae are well developed, whereas the second pair of maxillae is considerably reduced. There is an unpaired compound eye and a naupliar ocellus.

The thorax consists of 4–6 segments with a corresponding number of legs. The thoracic legs are leaf-like, bearing gill-like processes used for Respiration and filtration. In predatory forms, gills are absent, and the endopods bear hooks or setae for capturing prey. Females often have a cavity between the body and the carapace—the brood chamber—where eggs develop. The abdomen is typically unsegmented, terminating in a short furca or a long spine (in predatory species).

Cladocera are characterized by heterogony. Under favorable conditions, only females exist, laying parthenogenetic diploid (amictic) eggs. In various species, the brood pouch contains anywhere from 2 to 100 eggs. After 1–2 days, young crustaceans (all female) hatch, and the process repeats; consequently, cladocerans can multiply very rapidly in huge numbers. Typically, young crustaceans molt upon hatching. Following multiple molts over 2–6 days, they reach sexual maturity. Under unfavorable conditions (drop in Temperature, food scarcity, changes in The chemical composition of the water, etc.), males hatch from eggs, and haploid (mictic) eggs form in the female reproductive tracts, which can develop only after Fertilization by males. Mictic eggs are rich in yolk; after fertilization, they become encased in thick shells. In species with a brood chamber (Daphnia), fertilized eggs are deposited there. They become surrounded by a layer of Cells that develop a tough cuticle. Often, such a shell forms around two eggs, creating a so-called "saddle"—an ephippium. During the female's molt, the ephippium is released into the water. Because it contains an air-filled layer, it can be transported over long distances. Mictic eggs must undergo a dormant stage, during which they are highly resistant to freezing and desiccation. Under favorable conditions, they hatch into females that resume parthenogenetic reproduction.

In Daphnia and other genera, seasonal intraspecific Variability—cytomorphosis—occurs throughout the year depending on water temperature. The higher the temperature, the longer their carapace outgrowths and antennae become. This is because warmer water has a lower density, meaning a larger surface area of support is required for the crustacean to maintain its position in the water Column.

Most cladocerans are filter feeders, consuming very small unicellular organisms (Bacteria, Algae, Protozoa). Filter feeders include freshwater species of the genera Daphnia (D. pulex, D. magna) and Bosmina (B. longirostris). Predatory species (Leptodora, Bythotrephes) prey on crustaceans, rotifers, and occasionally consume fish fry. They locate prey visually, pursue the victim, capture it with their thoracic legs, and chew it with their mandibles. A single individual destroys 25–50 crustaceans per day. Although predatory species can wipe out up to 40% of other cladocerans in certain water bodies, they themselves serve as food for fish, so the Damage caused by such predation is negligible. The Nutritional Value of cladocerans, particularly daphniids, is very high: they contain about 50% protein and nearly 11% fat. Cladocera are a primary food source for the fry of various commercial fish species (common carp, other cyprinids, pikeperch fry, sturgeons, salmonids, etc.). Daphnia are cultured in special pools at fish hatcheries.

Order Anostraca (Fairy shrimp). Representatives of the order Anostraca inhabit various continental water bodies, including cave waters, temporary pools, saline lakes, and estuaries (limans). Over 200 species have been described, 12 of which are found in Ukraine. The size of fairy shrimp does not exceed 2 cm.

Fig. 72. Fairy shrimp (Artemia salina, inhabiting water of varying salinity):

1 - antenna; 2 - naupliar eye; 3 - compound eye; 4 - thoracic legs; 5 - furca

The head consists of the protocerebrum, which bears a pair of stalked compound eyes and an unpaired naupliar eye, short unsegmented antennules, and longer antennae, as well as the gnathocephalon (mandibular and two maxillary segments with jaws). Each of the 11–12 thoracic segments bears a pair of leaf-like legs. The abdomen consists of 7–8 segments and a telson with a furca, lacking appendages. The first two abdominal segments are fused to form the so-called genital region: in females, this is where the egg sac is located, while in males, it houses the paired copulatory organ. Development includes metamorphosis: a nauplius hatches from the egg.

Anostraca are primarily filter feeders, though they are also capable of predation. They swim upside down, using rhythmic movements of their thoracic legs to swim while simultaneously creating a current of water toward the mouth.

One of the best-studied species of this order is Artemia salina (Fig. 72), which is globally distributed in estuaries, lakes, and other water bodies with varying salinity: from nearly fresh to hypersaline. Studies of the PHYSIOLOGICAL CHARACTERISTICS OF Artemia have shown that it maintains a constant Blood salinity regardless of fluctuations in the salinity of the surrounding water, which is linked, as mentioned earlier, to the specific functioning of its excretory system.

The primary food of Artemia consists of several species of microscopic diatoms and green algae, and when these are scarce, bacteria inhabiting the bottom of water bodies. The body proportions and appendages of Artemia change depending on salinity. Previously, different forms of Artemia were described as separate species.

In addition to populations with both males and females, parthenogenetic forms of Artemia are known. An interesting feature of Artemia salina is the existence of polyploid races—a phenomenon very rare among animals. There are di-, tri-, penta-, and octoploid races, some of which are capable of both sexual reproduction and parthenogenesis, while remaining indistinguishable in size or external Morphology.

Artemia eggs are remarkably resilient—they can withstand complete desiccation (in experiments, they were heated to 80°C), temperatures dropping to -194°C, and other extremes. Nauplii hatch from the eggs and develop into sexually mature individuals within 20–35 days.

High reproductive rates and strong ecological plasticity make Artemia a promising subject for cultivation as live fish food. Artemia cysts are harvested in large quantities for feeding fish fry: when placed in water, nauplii emerge almost immediately. In Ukraine, Artemia occurs in massive numbers in the Syvash.