SUBPHYLUM CRUSTACEA (erustacea, hard shelled)
The familiar members of the subphylum Crustacea are crayfish, shrimp, lobsters and crabs. Many others are lesser known but very common taxa. These include copepods, cladocerans fairy shrimp, lsopod, amphipods and barnacles. Crustaceans are all aquatic except some Isopods and crabs.
Crustaceans differ from other living arthropods in two ways.
1. They have two pairs of antennae. All other arthropods have one pair or none.
2. Crustaceans possess biramous appendages. Each appendage is composed of three segments:
(a) Protopodite: It a basal segment It has two rami. Ramus is a distal process that gives the appendage a Y shape.
(b) Endopodite: It is medial remus.
(c) Exopodite: It is lateral ramus.
Trilobites also have similar structures. It is evidence that trilobites are ancestors of the crustaceans.
CLASS MALACOSTRACA (malakos, soft + ostreion, shell)
Malacostraca is the largest class of crustacean. It includes, crabs, lobsters. crayfish, shrimp, mysids, shrimplike krill, isopods and amphipods.
The order Decapoda is the largest order of crustacean.It includes shrimp, crayfish, lobsters, and crabs. Shrimp have a laterally compressed muscular abdomen. They have pleopods for swimming. Lobsters, crabs and crayfish are adapted to crawling on the surface of the substrate. The abdomen of crabs is greatly reduced. It is kept flexed beneath the cephalothorax.
The body of crayfish is divided into two regions: Cephalothorax and abdomen.
(a)Cephalothorax: It is formed by fusion of sensory and feeding tagma (the head) with the locomotor tagma (the thorax). The exoskeleton of the cephfilothorax extends laterally and ventrally .It forms a shield like carapace.
(b) Abdomen: The abdomen is posterior to the cephalothorax. It has locomotory and visceral functions. It take the form of muscular tail in crayfish.
Paired appendages are present in both body regions.
(a) Antennae: These are First two pairs of cephalothoracic appendages These are the first and second antennae.
(b) Mandibles: The third to fifth pairs of appendages are associated with the mouth. The third pair of appendages is mandibles. It is chewing or grinding structures.
(c) Maxillae : the fourth and fifth pairs of appendages are called maxillae. they are used for food handling. The second maxilla hears a gill. It also has thin bladelike structure called a scaphognathite ( gill bailer). It is used or circulating water over the gills.
(d) Maxillipeds: The sixth to eighth cephalothoracic appendages are called maxillipeds. These are derived from the thoracic tagma. They are accessory sensory and food handling appendages. The last to pairs of maxillipeds also bear gills.
(e) Periopods (walking legs): The thoracic appendages 9 to 13 arc called periopods (walking legs). The first periopod is called cheliped. It is enlarged and chelates (pincher like) It is used in defense and capturing food.
(f) Pleopods: All except last pair or appendages of the abdomen are called pleopods. These are used for swimming. In females, developing eggs attach to pleopods. The embryos are brooded in hatching. In males, the first two pairs of arthropods are modified into gonopods (claspers). These are used for sperm transfer during copulation.
(g) Uropods: The abdomen ends n a median extension called telson. The telson bears the anus. Flattened biramous appendages are attached to of the last segment of telson. These arc called uropods. The telson and uropods make a flipper like structure. It is used in swimming and in escape responses.
Evolution of appendages: All crustacean appendages except the first antennae evolved from an ancestral biramous form. Their embryological development is evidence of it. They arise as simple two branched structures during development. The appendages of crayfish are homologous structure.
Cray fish prey on other invertebrates. It eats plant matter. It also scavenges on dead and dying animals. The foregut has enlarged stomach. Its part is specialized for grinding. Digestive gland secretes digestive enzymes. It also absorbs products of digestion. The midgut extends from the stomach. It is often called the intestine. A short hindgut ends in an anus. Hind gut is important for water and salt regulation.
The gills of a cray fish attach to the bases of cephalothoracic appendage. Gills are present in a bronchial (gill) chamber. It is space between the carapace and lateral body wall. The scaphognathite of the second maxilla beat. It drives water anteriorly through the bronchial chamber. Oxygen and carbon dioxide are exchanged between blood and water across the gill surfaces. It is respiratory pigment hemocyanin carries oxygen in blood plasma.
Circulation in cryayfish is similar to most arthropods. Dorsal, anterior and positerior arteries start from a muscular heart. Branches of these vessels open into sinuses or the hemoeoel. Blood is collected in ventral sinus. It enters the gills and return to the pericardial sinus surrounding tile heart.
Crustacean nervous system is similar to annelids and arachnids. Primitive crustaceans have ladder like ventral nervous system. Higher crustaceans show a tendency toward centralization and cephalization. Crayfish have supraesophageal and subesophageal ganglia. These ganglia receive sensor input from receptors in the head and control the head appendages. The ventral nerves and segmental ganglia fuse. Thus giant neuron in the ventral coral cord functions in escape responses. Nerve impulses are conducted posteriorly along giant nurse fibers of a crayfish. As a result, powerful abdominal flexor muscles of the abdomen contract alternately with the weaker extensor muscles. It causes the abdomen to flex(the propulsive stroke) and then extend (the recovery stroke). The telson and uropods form a paddle like tail. It propels the crayfish posteriorly.
Sense organs; Sense organs of crayfish are antennae, compound eyes, simple eyes, statocysts; chemoreceptors, proprioceptors and tactile setae.
1. Chemical receptors: Chemical receptors are widely distributed over the appendages and head. Many of the setae covering the mouthparts and antennae are chemorceeptors. They are used in sampling and detecting pheromones.
2. Statocvst: A single pair of statocyst is present at the bases of the first antennae. A statocyst is a pitlike invagination of the exoskeleton. It contains setae and a group or cemented sand grains called a statolith. Cray fish movements move the statolith and displace setae. Statocysts provides information about movement, orientation with respect to the pull of gravity and the vibrations of the substrate. The statocyst is cuticular. Therefore, it is replaced with each molt. Sand is incorporated into the statocyst when the crustacean is buried in sand.
3.Tactile receptors: Tactile receptors are present on the appendages and at joints. They are also involved in equilibrium, balance, and position senses. Stretch receptors at the joints are stimulated during crawling or resting. Crustaceans detect tilting from Changing patterns of stimulation. These receptors are important to crustaceans that lack statocyst
4. Compound eye: Cray–fish have compound eyes. It is mounted on movable eye stalks. The lens system consists of 25 to 14, 000 units called ommatidia.
5.Ocelli: Larval crustaceans have a single, median photoreceptor. It consists of a few sensilla. These simple eyes are called ocelli. These allow larval crustaceans to orient toward or away from the light. But they do not form images. Many larvae are planktonic. Thev use their ocelli to orient toward surthee waters.
The endocrine system of a crayfish controls ecdysis, sex determination and color change. Endocrine glands release hormones into the blood. Blood circulate them. Hormones cause responses at certain targets tissues. In crustaceans, endocrine functions are closely tied o nervous functions. Nervous tissues produce and release hormones. These tissues are cal led neurosecretory tissues. There are following endocrine glands and neurosecretory cells in crustaceans:
(a) X-organs: X-organs are neuro-secretory tissues in the eye stalks of crayfish.
(b) Sinus gland: It is associated with each X-organ. It accumulates and releases the . ecretions of the X organ.
(c) Y-organs: Y organs are not directly associated with nervous system. They are present near the bases of the maxillae.
Both the X-organ and the Y organs control ecdvsis. The X-organ produces molt-inhibiting hormone. The sinus gland releases it. The target of this hormone is Y-organ. This hormone inactivates the Y-organ. Certain conditions prevent the release of molt-Inhibiting hormone. In the absence of these conditions, the Y organ release ; ecdysone hormone. It causes molting. Some other hormones also involved in molting. One of these is a molt-accelerating hormone.
(d) Androgenic glands: These are present in the cephalothorax of males. The androgenic hormones promote the development of testes and male characteristics like gonopods. The removal of androgenic glands from males causes development of female sex haracteristics. In one experiment androgenic glands were implanted into a female. She develops testes and gonopods.
Hormones regulate following functions in crustacean functions:
1. They control molting or ecdysis.
2. They control development of sex in males and females.
3. They control the development of female brooding structures in response to ovarian hormones.
4. They control the seasonal regulation of ovarian function.
5. They regulate the heart rate and color changes by eve stalk hormones.
Excretion and osmoregulation
The excretory organs of crayfish are called antennal glands. They are present at bases of the second antennae and are green in living crayfish. In other crustaceans, they are called maxillary glands. They are present at the bases of the second maxillae in them.
Antennal glands are structurally siniilar to the coxal glands of arachnids. Excretory products form by the filtration of blood. Ions, sugars and amino acids are reabsorbed in the tubule. Ammonia is the primary excretory product. Ammonia also diffuses across thin parts of the exoskeleton. Ammonia is water soluble. Water rapidly dilutes it. All freshwater crustaceans face a continual influx or freshwater and loss of ions. Thus, the elimination of excess water and the reabsorption of ions are important functions in them.
Gill surfaces are also important in ammonia excretion and water and ion regulation (osmoregulation).
Crayfish and all other crustacean except the barnacles are dioceious. Gonads are present in the dorsal portion of the thorax. The gonoducts open at the base of the third (females) or fifth (males) pleopods. Mating occurs after molting of female. The male turns the female onto her back. It deposits non-flagellated sperm near the openings of the female’s gonoducts. Fertilization occurs. Eggs are shed after copulation. The eggs are sticky. They are attached to the female’s pleopcids. Fanning movements of the pleopods over the eggs aerated.
Development: The development of crayfish embryos is direct. The young hatches as miniature adults. Many other crustaceans develop a planktonic free-swimming larva. It is called a nauplius. In some, the nauplius develops into a miniature adult.
Crabs and their relatives develop a second larval stage called a zoea. In some cases, all adult features are present in larva, except sexual maturity. It is called the postlarva.
CLASS BRANCHIOPODA (branchio, gill + podos, foot)
Members of the class Branchiopoda live in freshwater. All branchiopods possess flattened leaf like appendages. These appendages are used in respiration, filter feeding and locomotion.
Fairy shrimp and brine shrimp
Fairy shrimp and brine shrimp belong to order Anostraea. Fairy shrimp live in temporary ponds formed from rains. Eggs are brooded. Then the female dies. The temporary pond becomes dry. The embryos become dormant in a resistant capsule. Embryos lay on the forest floor. Then the pond fills again the following spring. Now the embryo hatch into nauplius larvae. Animals, wind or water currents carry the embryos to other locations. Thus they have short and uncertain life cycle. It is an adaptation to living in mnds that dry up. These are slows swimming and defenseless crustaceans.Therefore, they live primarily in temporary ponds. Such pond contains few larger predators. Brine shrimp also form resistant embryos. They live in salt lakes and ponds. e.g the Great Salt Lake in Utah).
Water fleas (Daphnia)
Members of the order Cladocera are called water fleas. A large carapace covers their bodies. They swim by repeatedly thrusting their second antennae downward. It creates a jerky upward locomotion.
Life cycle: Females reproduce parthenogenetically (without fertilization) in spring and summer. They can rapidly populate a pond or lake. Eggs are brooded in an egg case beneath the carapace. The egg case is released at the next molt. Now this egg can float or sink to the bottom of the pond or lake. Females produce eggs in response to decreasing temperature, changing photoperiod or decreasing food supply. These eggs develop parthenogenetically into males. Sexual reproduction produces resistant winter eggs. These eggs pass winter and hatch in the spring.
CLASS COPEPODA (kope, oar + podos, foot) Members of the class Copepoda inclucde some of the most abundant crustaceans. These are both marine and freshwater species.
1. Copepods have a cylindrical body and a median ocellus. Ocellus develops in the Ilalupl i us stage and persists into the adult stage.
2. The first antennae are modified for swimming.
3. The abdomen is free or appendages.
4. Most copepods are planktonic. A few copepods live on the substrate. A fievs. arc predatory. Others are commensals or parasites of marine invertebrates. fishes, or marine mammals.
5. They use their second maxillae or filter feeding.
Members of the class Cirripedia are the barnacles. They are sessile and highly moth lied as adults. They are exclusively marine. They include about one thousand species.
Life cycle: Most barnacles are monoecious. The planktonic nauplius of barnacles is changed into planktonic larval stage called cypris larva. It has a bivalved carapace. Cypris larvae attach to the substrate by their first antennae. They are metamorphosed to adults. The abdomen is reduced during metamorphosis. The gut tract becomes U-shaped. Thoracic appendages are modified for flittering and moving food into the mouth. Calcareous plates cover the larval carapace in the adult stage.
Habitat: Barnacles attach to a variety of substrates. These are rock outcroppings, shipbottoms, whales. and other animals. Some barnacles attach to their substrate by a stalk. Other are non-stalked. These are called acorn barnacles. Barnacles reduce ship speed and fuel efficiency. Some barnacles are high modified parasites.