Mr. Krabs, a quintessential example of a crustacean

Arthropoda - Crustacea

Crustaceans belong to the phylum Arthropoda, the most diverse in the animal kingdom. Crustaceans share this phylum with arachnids and insects, which are all characterized by an exoskeleton, molting, jointed appendages, and a body made up of distinct segments. These distinct body segments are known as tagmata, and include the head, thorax, and abdomen which are created through a process called tagmosis (14 AN).
The phylum Arthropoda is made up of 5 subclasses:

  1. Malacostraca, Consisting of around 29,000 species, malacostraca is the most extensive and successful of classes of crustaceans and contains approximately 2/3 of the crustacean species. Lobsetrs, crabs, and shrimp are all examples of malacostacans. Most species have a fixed, segmented body plan consisting of a head, thorax, and abdomen. The head supports a pair of compounded eyes and two antennae (one of which is used for elevation and swimming balance). (4 KA)
  2. Branchiopoda
  3. Ostracoda
  4. Copepoda
  5. Cirripedia (12 HL)
    Cirripedia anotomy (20 AL)
    Cirripedia anotomy (20 AL)

Diagnostic Characteristics

Crustaceans (Crabs, Lobsters, Crayfish, and Shrimp) are found in aquatic environments and evolved in the ocean, although they can also be found in freshwater habitats. Some species of crustaceans are at the base of extremely important food chains (krill) while others are important in recycling nutrients trapped in the bodies of dead organisms. (16 VK) They are also recognizable by their multifaceted, compound eyes and biramous appendages (each section has one pair), and two separate pairs of sensory antennae. All crustaceans are enclosed in protective exoskeletons made of chitin, which moult (shed) in order to accomodate growth (4 MB). Most arthropods exhibit all five of these characteristics: 1) bilateral symmetry, 2) segmented body, 3) hard exoskeleton, 4) jointed legs, and 5) many pairs of legs (CH 3).

Acquiring and Digesting Food
All crustaceans feed using their appendages to strain food from the water. Barnacles, which are sessile crustaceans with parts of the cuticles hardened by calcium carbonate, also feed using this method and Copepods, or small crustaceans, feed on protists and bacteria. Many crustaceans also have claws that help with crawling and eating. (16 VK)
gastric mill

Sensing the Environment
Two pairs of sensory antennae and a pair of compound eyes give crustaceans the ability not only to feel around their environment but also to see what is around them. Crustaceans have a central nervous system composed of a dorsal brain, a paired ventral nerve cord, and ganglia. They have two types of eyes that allow them to sense the environment. (J. Sun 18) These compound eyes allow for a broad yet dull sense of sight. They also allow crustaceans to see well in dim light.(JE)
Swimming appendages, walking legs on the thorax, and appendages on the abdomen allow for movement through the water and also along the bottom of the ocean floor. This locomotion is controlled by neuroactive steroids. (EK)
Small crustaceans exchange gases across areas of the cuticle whereas larger crustaceans have gills. Despite the exoskeleton, all crustaceans are able to perform diffusion of respiratory gases. Gas exchange takes place through the entire shell. (23 DC). Certain Land crabs have evolved lung-like enlarged brachial chambers as an adaption to aerial breathing (AR 13).
Metabolic Waste Removal
Malocostracans generally excrete the majority of their liquid waste through the ducts of their nephridial glands, found on the body segments of the second antennae and the maxillae (EG)

Crustaceans have an open circulatory system, meaning hemolymph (fluid) is propelled by a heart through short arteries into sinus cavities surrounding the tissues and organs. Hemolymph then reenters the heart through pores equipped with valves.
Many Arthopods even have no heart. The blood is kept in motion by a blood pump or movement within the body. Instead of Hemoglobin, some Arthropods have Hemocyanin, which uses copper instead of iron to carry oxygen (13t2).

Self Protection

To defend themselves against predators, crustaceans have a hard exoskeleton and defense pincers. In addition, when a crustacean loses an appendage, it has the ability to regenerate the lost part. The Shell is made of calcium and chitin and it duplicates the crustacean's skeletal system; this surrounds the truck of the animal, which houses its abdomen and throat. (14 SC)

Osmotic Balance
Four factors that influence osmotic balance in crustaceans include the brain factor, which regulates the antennal glands, intestine, and gills, the thoracic ganglion factor, which regulates the stomach, intestine, and gills, the eyestalk complex, which regulates both the antennal glands and the gills, and the pericardial organs, which regulates salt and water metabolism by the heart muscle and gills (7 J. Stein).

Temperature Balance
In an experiment done on a species of lobster, the biologists discovered that in response to temperature changes (brackish water vs. freshwater, etc.), the crustacean underwent a change in its hemolymph pH. Hemolymph is the blood analogue for crustaceans and anything else with an open circulatory system. The change in hemolymph pH has an effect on the strength and frequency of the heartbeat, as well as respiration and other physiological functions, and so the crustacean can adapt to the temperature of the water. (AS 15)

Review Questions:
1. Most scientists belive the crustacean clade is paraphyletic. Are arthropods considered crustaceans based on genetic or physiological characteristics?(15RM)
2. Give two examples of how crustaceans of any subclass contribute to their environment. (AW)
3. How does hemolymph pH help maintain temperature balance? (EK16)
4. What evolutionary advantages does the shell provide crustaceans, and what drawbacks does it provide? (DPOD 15)

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