Arthropoda- Crustacea

By: Azman Rashid



General Characteristics of Crustaceans:

Crustaceans are found in marine and freshwater environments with 40,000 different species found so far and new ones found every day (see pictures). Most Crustaceans are marine with the exception of crayfish in freshwater, and tropical crabs that live on land. The Crustaceans have multiple appendages are specialized based on their function. Appendages are any external body part that comes off of from the body. The appendages become specialized to do a variety of different functions: walking, feeding, sensory reception, copulation, and defense. Well-known examples of Crustaceans are crabs, lobsters, crayfish, and shrimp. There are three main groups of Crustaceans:
Isopods: these are a large group of small marine organisms. They are plentiful in the bottom of deep oceans. Some uncommon members of this group are pill bugs and wood lice. These crustaceans live in moist logs and leaves.
Copepods: Small crustaceans that are plentiful in number. Copepods are part of marine and freshwater plankton communities eating protists and bacteria. There predators are many fishes found in freshwater bodies of water.
Decapods: This group composes of the typical large crustaceans like lobster, crayfish, and crabs. They have a cuticle hardened by calcium carbonate and a carapace (defined under self-protection). Most decapods are marine except for some exceptions like freshwater crayfish, and terrestrial tropical crabs. Even though Decapods are very large, their larvae or young are planktonic, microscopic, drifting organisms

All crustaceans have antennae at some point in their lives, and all their bodies have three distinct parts: the head, the thorax, and the abdomen. When looking at a crustacean, one may observe between sixteen to sixty body segments, but the external shell is one continuous layer, which thins where one body segment meets another. (TB)
A giant Antarctic amphipod belonging to the genus Eusirus
A giant Antarctic amphipod belonging to the genus Eusirus

Out of all the arthropod classes, the crustacean is the only class that is primarily aquatic. (ZJ- source 7)

A tree diagram which helps to differentiate between the different orders and families of crustaceans; (SP)
A tree diagram which helps to differentiate between the different orders and families of crustaceans; (SP)

A diagram of a lobster, a type of crustacean, detailing the type of body plan characteristic of crustaceans. This includes the three major parts of the crustancean: the head, the thorax, and the abdomen, as well as different appendages. (MR; Source 22)

Acquiring and digesting food:

Crustaceans have a mandible, a jaw-like mouth, for feeding on other organisms. The "mouth" of the Crustacean is formed by three or more specialized appendages. The mouth appendages are used to chew the food for digestion. Examples of food for a Crustacean is protists and bacteria more commonly eaten by Copepods.Crustaceans use setae to filter feed. Setae are very fine combs existing on some thoracic appendages (legs on the segments of the thorax - middle part of body). The animal flutters or beats any other appendages it has, securing sufficient water currents for filter feeding. Filter feeding evolved independently in Crustaceans: various appendages are used by different members, and not all are the same.(SP)

external image EvolutionDigestiveSystemCrustaceanCrayfish%20F12_07.gif

Sensing the environment:

These animals have organs for all five senses. They have well-developed sensory organs including eyes, olfactory receptors for smell, and two antennas for touch and smell. Crustaceans are the only arthropods with two antennas.
Nervous System and Senses

The movement of Crustaceans is coordinated by elaborate sensory and nervous systems. This includes some chemical and touch receptors in the antennae, compound eyes, statocysts (a small organ of balance and orientation consisting of a sensory vesicle) on the head, and sensory hairs embedded in the exoskeleton throughout the body. The nervous system of Crustaceans is similar to that of annelids, except that the ganglia’s of Crustaceans (a mass of nerve tissue existing outside the central nervous system) are bigger and more complex, allowing a finer level of motor control (VN).

Arthropoda employ compound eyes in combination with simple eyes. The compound eyes are made up of many ommatidia, which consist of a lens and a light detecting cell. Although these do not provide great detail, they can detect movement well. (PS Source 17)


Locomotion is the active travel of an organism from place to place. Depending on their ecosystem, Crustaceans either have three or more pairs of legs attached to the thorax, swimming, tail-like appendages or both. Either mode of transportation requires a modified appendage on the organism. In water, most Crustaceans float in water, not limited by the force of gravity. However, water is denser than air allowing friction to be a major force that prevents movement of aquatic organisms. To overcome friction, animals must have a sleek, torpedo-like shape. On land, gravity is the major force preventing movement. Crustaceans developed the exoskeleton to support the organism or the organism would sag from its own weight. An exoskeleton is a hardened layer over the animal's flesh. The exoskeleton allows movement to happen as the exoskeleton provides a place where the muscles are connected to move appendages. Although the exoskeleton is very hard and rigid, the joints of legs have thinner and more flexible exoskeleton layer.

The claws on crustaceans not only help for eating but also help them move by crawling. (CP source 18)


Respiration occurs differently in land, and water Crustaceans. The land Crustaceans have internal surfaces specialized for gas exchange through the exoskeleton. The aquatic or water Crustaceans have feathery extensions to their gills that allow for a greater surface area in contact with the nearby water. Specialized, paddle-like appendages drive a current of water over the gills where osmosis will occur. Larger crustaceans usually have gills where smaller Crustaceans may not have gills and have gas exchange through thin areas of the cuticle.

Gills developed in large crustaceans out of the first segment of legs on the thorax. In decapods, the gills are housed underneath the carapace (shell) where they receive oxygenated water that is funneled into the branchial chamber. Decapods that live on land have larger branchial chambers with more vessels than aquatic decapods do. Land decapods also have specialized chambers designed to store water to keep the gills moist. Isopods and copepods do not have carapaces to protect the gills. (JS 21)

Metabolic waste removal:

The organism excretes nitrogenous waste by diffusion through thin areas of the cuticle. For aquatic organisms, the nitrogenous waste would most likely be ammonia because of its solubility in water. The ammonia molecules can easily pass through membranes of the organism, diffusing into the surrounding water with ease.
Crustaceans mainly use active transport, which is a type of diffusion, to get rid of metabolic waste. Active transport is the way molecules diffuse from areas of low concentrations to areas of high concentrations using energy to push the molecules against the concentration gradient that is created by this difference in concentrations. Active transport requires energy because the molecules don't go with the concentration gradient. (MM)

Terrestrial crustacians have an excretory system that is specifically designed to conserve water. These arthropoda have malphigian tubules that collect waste from blood and reabsorb water so that the only material that is excreted is dry. Marine crustacia have an excretory system that consists of a pair of green glands that are on the part of the head that is anterior to the esophagus. Each organ has a glandular region for waste removal. These include a bladder and a matching duct that opens at the base of the crustacian antenna. (JP)


Arthropods have open circulatory systems. This is type of system has no difference between "blood" and interstitial fluid, but a general body fluid called hemolymph. In Crustaceans, the hemolymph is pumped by the hearth through short arteries, blood vessels moving away from the heart to organs. The arteries in Crustaceans go to the sinuses. Sinuses are areas in the organism that surround the tissue and organ. From the embryonic stage of Crustaceans, the coelom is the main body cavity formed within the mesoderm. But, as the Crustacean approaches adulthood, the hemocoel, all of the body sinuses, becomes the main cavity.
The hemocoel exists when blood and coelomic fluid are not physically separated by blood-vessel walls. Therefore blood, not coelomic fluid, occupies the cavity. (IL - source 19)

Self protection:

Crustaceans would be formless mass without the exoskeleton becoming easy prey for predators. The cuticle provides protection for the organism over the body. A cuticle is the exoskeleton of the arthropod made from layers of protein, chitin, a carbohydrate, and (in decapods) Calcium carbonate. Some Crustaceans have pincers (specialized appendages) used to defend the organism. Lost or wounded appendages can be regenerated using the organism's energy. Decapods have a carapace covering the upper side of the cephalthorax,the head and thorax fused together. The thorax is the
extends from the neck and the diaphragm, a sheet of skeletal muscle that extends in the bottom of the rib cage. In many organisms the carapace is known as shells on many crabs. The carapace is very strong and can often take a lot of impact. Yet, Crustaceans aren't always well protected under their comfy exoskeletons as exoskeletons do not grow with the Crustacean. This forces the Crustacean to under go a process called Ecdysis or molting. Ecdysis is when an arthropod has to shed its old exoskeleton to secrete a new one. The lack of exoskeleton, and large amount of energy needed leaves the organism vulnerable to predators and dangers.

Osmotic balance:

A new species of Epimeria found in the Antarctic peninsula
A new species of Epimeria found in the Antarctic peninsula

The exoskeleton of the Crustacean prevents large amounts of water loss. Yet, the Crustaceans have pores that allow osmosis to occur, and glands that regulate the salt balance of the hemolymph. The hemolymph is the "blood" in Crustaceans along with nutrients. Yet, some Crustaceans like Spider crabs are osmoconformers: organisms with little ability to regulate internal salt concentration. These organisms tend to live in environments with stable salt concentrations in water, but if exposed to different concentrations, the organism will die.

Most crustaceans use gills in order to regulate the amount of salt that enters and exits their bodies. In fact in most crustaceans, these gills are the chief organs involved in the maintaining of a salt equilibrium. In environments that lack a large salt concentration, the gills of a crustacean actively absorb the salt in the environment. Conversely, in environments of extreme salt concentration the gills deter the absorbance of excess salt. (ZJ- source 7)

Temperature balance:

The aquatic Crustaceans are mostly thermoconformers, organisms with little control over internal body temperature. The few terrestrial or land Crustaceans can adjust their internal temperature using the similar behavior of lizard ectotherms. Ectotherms or cold-blooded organisms do not regulate their own temperature through metabolism, but they do regulate body temperature through basking in the sunlight when the the air is cool, and finding shade when the air is too warm.

Although thermoreceptors, which are sensory neurons which detect changes in temperature, are not present in Crustacea, certain studies have shown that they are more prominent in temperatures between .2 and 2 decrees Celsius. This is done through orthokinesis and klinokinesis (which are changes in movement and speed respectively caused by the change in environment). Although this is the case, there is still not enough information to seperate the Crustacea into further groups based on the differences in environmental temperature (such as the case with fishes). (RK - source 20)


There are different sexes or Crustaceans: a male and female. To copulate, a female crustacean first produces eggs in her ovaries, and passes them through oviducts (a passageway from the ovaries to outside the body). (MB - Source 11) Then, the male uses specialized appendages to transfer sperm into the reproductive pore of female during copulation. Once fertilized, the eggs will take some time (ranging from days to weeks, depending on the species of crustacean) before hatching. (MB - Source 11) From this point onward, the young crustacean is on its own in terms of feeding, growth, and survival. (MB - Source 11).
crustaceans, such as water fleas that live in temporarey ponds, reproduce by cyclic or obligate parthenogenesis, where males are unknown or rare. Females in parthenogenetic species produce eggs which do not require fertilization to develop. Feshwater crustaceans have two types of eggs: one which develops immediately, while the other which may take up to several hundred years (mp-source 13).
Fossil Record
Crustaceans have fossil records that can be traced back to the early Cambrian era almost 600 million years ago. In Russia archaelogists discovered a fossil record of a twelve legged sea spider called Karagassiema. They predicted the fossil to have come from rocks close to 650 million years old. (AP- source 12)

Most major groups of crustaceans have fossil record that starts in the Cambrian era, but have a most abundant record starting in the Carboniferous era and more recent eras. Fossils are more common for certain types of crustaceans, such as shrimps and crabs, and rarer for others, such as krill. (CM)

Review Questions
What is the "blood" of a crutsian?(RJ)
What are the three main types of crustaceans, and what are the defining aspects of each group? (PS)
Why is it logical that the crustacian has an open circulatory system? (LPE)
What main characteristics of crustacean differentiates it from an insect or an arachnid in the arthropoda phylum? (MS)