Everyone's Got a Crush on Crustaceans

Arthropoda- CrustaceaDonna McDermott


Diagnostic Characteristics that Define Crustaceans:

Crustaceans are animals that mostly dwell in salt water , though some live on land and some live in freshwater. This sub-phylum is thought to have initially evolved in the oceans.

Crustaceans are distinguishable by the exoskeleton that covers their entire body, including their several specialized and jointed appendages. Their bodies are generally segmented into two or three parts. These include the cephalothorax (the head and thorax) and abdomen. Crustaceans also have bilateral symmetry, meaning that if you cut the lobster in the photo above from its head to its swimming appendages, it would look the same on both sides. They also have a coelom, which is a cavity inside the mesoderm filled with fluid.

All crustaceans have four antennae, a pair of mandibles (Jaw bones), and two compounded eyes. Most crustaceans have segmented bodies consisting of a head, thorax, and abdomen. Off of each segment there is a pair of appendages. Crustaceans use gills for respiration. Also all crustaceans, like all other arthropods, have a hard, flexible exoskeleton.

Most crustaceans have separate sexes and are distinguished by appendages on the abdomen called swimmerets. Terrestrial crabs such as the Christmas Island red crab mate seasonally and return to the sea to release eggs. Crabs' eggs are retained by the females until they hatch into free-swimming larvae.

Crustaceans are split into three main groups-
Isopods consist of small marine species that can dwell at the bottom of oceans or on land. They include pill bugs.
Isopods are the most diverse group of the crustaceans, including approximately 10,000 species. In addition to pillbugs, isopods also include sowbugs, and their marine relatives such as gribbles and slaters. Isopods are common inhabitatants of nearly all environments.

Copepods are mostly freshwater plankton. They eat protists and bacteria and are eaten by fish. There are some copepods that are parasitic. They attach themselves to sharks, fish, mollusks, corals, and marine mammals.

Decapods are the largest and most well-known crustaceans. They include lobsters, crabs, crayfish, shrimp, and barnacles. These crustaceans usually live in saltwater, though crayfish and some crabs can be found living in and around fresh water. They usually have an exoskeleton made mainly of calcium carbonate. One of their five pairs of legs is a large pair of pincers, called the cheliped.
external image anatomy-lobster.jpg

Acquiring and Digesting Food:

Most crustaceans consume food with mouth parts that chew. They often have a mandible( a jaw-like structure.) Their mouthparts also include the maxillae and maxillipeds, which are modified appendages. (An appendage would be something like an arm or a leg). Crustaceans have a direct digestive tract consisting of a mouth, an esophagus, a three-part gut cavity, and a rectum. Food enters through the mouth, and travels down the esophagus into the foregut. The foregut contains a gastric mill, which as many teeth and hairs to help break down the food into small chunks. Once the food particles have been broken down sufficiently, they go into the midgut. The midgut contains many pouches called diverticula, which take part in the digestive process that allow food particles to be absorbed by body cells. The hindgut, which is short, is the exit for the undigested food via the anus. Barnacles are able to strain food from the water and bring it back into their circular, protruding exoskeletons. As crustaceans eat they grow, and they are forced to shed their exoskeleton. This process is called molting, and crustaceans do it throughout their entire lives, but they do it the most when they are changing from larvae to adults.

Sensing the Environment:

Crustaceans sense the environment with two pairs of antennae. These antennae are jointed, like most of the rest of the crustacean's body. The first pair is uniramous, meaning that it is a single series of segments attached end to end, and the second pair is biramous, meaning that each antennae branch off into two single series of segments attached end to end. In addition to sensing, the antennae also assist in copulation, swimming and feeding.


Most crustaceans have the ability to walk on legs that protrude from their thorax and/or abdomen. They generally have three or more pairs of legs. Most crustaceans live underwater, so they don't really need to support much weight on a regular basis. Also, marine crustaceans generally have appendages to help them swim (like fins in the style of a mermaid tail). Some crustaceans move front to back, like lobsters, while others move side to side, like crabs.


Small crustaceans will have a small area for exchanging gasses located on a very thin part of their cuticle(exoskeleton). This inner wall of the carapace (dorsal section of the exoskeleton) is filled with blood vessels and is occasionally the only respiratory organ in these smaller crustaceans. Larger crustaceans generally have gills that are protected but not covered entirely by their exoskeleton. They use a bailer, small paddles, to move the water across their feather like gills to maximize gas exchange efficiency.

Metabolic Waste Removal:

Similar to how gasses are exchanged, nitrogenous wastes (wastes containing nitrogen. In mammals, this is urine) are dispersed through a thin area of the cuticle. Crustaceans have two different excretory organs: the antennal gland and the maxillary gland. Both have an end sac and a convulated duct that may expand into a bladder before having an opening to the outside. In adult crustaceans, only one of these glands is usually functional. Both glands primarily regulate ionic balance. The balance of salts and water is also partly controlled by the gut, which is able to absorb both. Another function of the antennal gland is that it is able to absorb glucose. The end product of nitrogen metabolism in most crustaceans is ammonia, which is excreted through the gills. Some of the terrestrial crustaceans are able to produce the less toxic urea or uric acid, which may be stored in special cells near the bases of the legs or excreted without the loss of much water.


Crustaceans have an open circulatory system, meaning all of their “blood” swims around and bathes their organs in one big, organized pile. Their tube-shaped hearts are along the top parts of their bodies. These hearts pump hemolymph (a mix of blood and the fluid that fills up the space between cells (interstitial fluid)) into the sinuses of the organism ( a sinus is the space around an organ). The nutrients in the hemolymph are exchanged directly through the membranes of the organs. Crustaceans use either hemoglobin or hemocyanin (a protein containing copper) to transport oxygen through the hemolymph. Although most have hearts, some smaller crustaceans can circulate their hemolymph with body movements .

The following diagram is not of a crustacean, but illustrates the concept of an open circulatory system.


Crustacean Circulatory System (say that five times fast) (BY)
Crustacean Circulatory System (say that five times fast) (BY)

Self Protection:

Crustaceans are covered by an extensive exoskeleton. This exoskeleton can be made of CaCO3, protein, and chitin (a rigid polysaccharide.) The exoskeleton works as 'armor' to protect the organism from outside harm. The exoskeleton can be fused in certain areas or have joints connecting the armor together, but the exoskeleton must be shed/ molted for the crustation to be able to grow.

Crustaceans' appendages are also coated by their exoskeleton, and to increase mobility they are jointed. Crustaceans have several jointed legs that can regenerate when removed.

Some crustaceans, such as crabs and lobsters, also have relatively large pincers to be used to defend themselves. These pincers are covered by hard exoskeleton and usually have two long and point- covered edges that can squeeze together.

Osmotic Balance:

Crustaceans have a pair of specialized glands for the purpose of regulating the salt balance of their hemolymph. Hemolymph is made of blood and various other nutrients, and surrounds the crustacean's organs.

Temperature Balance:

Crustaceans are thermoconformers, meaning their body temperature changes with any change in temperature of their outside environment. They have little to no control over this.

Review Questions:
1) What are the three main categories crustaceans are split into and what are their main characteristics?(Rabya Saraf)
2) How do crustaceans' circulatory systems work? How is this unique to that of other organisms? (Sarah Fleming)
3) How are antenna used by crustaceans to sense the environment around them? ( JJF)
4) List the steps of acquiring and digesting food, beginning with how it is taken into the body and concluding with how it exits the body. Explain each step. (DG)
5) What mechanisms of defense do crustaceans use to protect themselves against predators? (APS)


Campbell, Neil A., and Jane B. Reece. Biology. Sixth Edition. Boston: Benjamin-Cummings Company, 2002.
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http://www.britannica.com/EBchecked/topic/163169/digestion (Ali Kirsch)
http://www.nilesbio.com/subcat293.html (Katey Duchin)
http://tolweb.org/isopoda (Sara Waugh)
http://www.ucmp.berkeley.edu/arthropoda/crustacea/crustaceamorphalh.html (Jesse Landy)
http://www.eoearth.org/article/Crustacea (Jackson Murphy)
http://www.britannica.com/EBchecked/topic/144848/crustacean (Leo Schwartz)
http://www.oceaninn.com/guides/crustacea.htm (KNS)
http://www.britannica.com/EBchecked/topic/144848/crustacean/33810/ (Liz Daley)
Sparknotes AP Biology Book (Walker K.)
http://biology.clc.uc.edu/courses/bio106/carter.htm(Alyssa Zisk)
http://www.mcsuk.org/marineworld/species/crustaceans(Alysse Steward)
http://cas.bellarmine.edu/tietjen/images/cdad.gif [BY]
www.globalheartbeat.org/cd/Crab_physiology_version_2.pdf (JAF)