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Crustaceans are a diverse group of organisms. They are invertebrates, meaning they do not have a backbone, and they are protected by an outer shell called a carapace. The carapace acts as an exoskeleton, providing the rigidity and structure that bones of an internal skeleton would normally provide. All crustaceans have segmented bodies with radial symmetry, meaning if you drew an imaginary line down the middle of their back, the two sides would be approximate mirror images.
The general crustacean body type includes segments grouped within three sections: a head, a thorax, and an abdomen. The head contains the sensory structures, like eyes and antennae. The thorax usually contains the legs primarily used for locomotion, and the abdomen usually contains appendages that can be used for swimming, like a tail or swimmerets. Swimmerets are structures similar to truncated legs that help the organism swim.
Crustaceans live in all types of ecosystems; most are aquatic, but some are terrestrial (meaning they live on land). As we've learned, the group is diverse, but here's a list of different crustacean groups you may have heard of:
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Respiration strategy obviously varies with the type of environment the organism lives in. Remember, the goal of respiration is to bring oxygen into the body and expel waste gases out of the body. Very small aquatic crustaceans exchange gases through the process of diffusion directly across the body surface called the integument. The blood is close enough to the surface to directly exchange gases at the surface. This is feasible mainly because the oxygen requirement of tiny organisms is relatively low, and because the small organisms have a great deal of surface area compared to body mass (meaning more gas can be exchanged relative to body size).
Larger aquatic crustaceans primarily use gills for respiration. These feathered surfaces contain membranes that bind to dissolved oxygen in the water as water passes over. The oxygen then moves from the gills into the haemolymph (the blood of crustaceans) in the circulatory system so that the oxygen can be delivered to the rest of the body.
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Like humans, some crustaceans have the respiratory pigment hemoglobin in the blood to bind to oxygen, giving the blood a red color. Some members of the Malacostraca crustaceans have hemocyanin to bind to oxygen. This gives crustacean blood a blue tint instead of the red color provided by hemoglobin. Hemocyanin is a copper-based respiratory pigment. As waste materials are collected by the circulatory system, some are brought back to the gills so that they can be expelled from the organism. In other words, gases can be diffused both ways: from the water into the body and from the body out into the water.
Interestingly, even the crustaceans found in terrestrial environments have modified gill structures with additional structures that help keep the gills moist. Land crustaceans have more rigid gill surfaces, as well as a specialized set of branchial chambers inside the body to help deliver oxygen efficiently. This enables them to breathe air out of water, rather than relying on dissolved oxygen in an aquatic environment. Additionally, without the more rigid gill surfaces, the gills would collapse out of water, making breathing impossible. These internal branchial chambers resemble lungs but still function in the presence of gills.
This lesson provided a brief introduction to the diverse world of crustaceans. These invertebrates are usually found in water, but some have adapted to life on land. There are four major groups of crustaceans: Branchiopods, Maxillopods, Ostracods, and Malacostraca. Though there are many differences among these groups, but they all use similar forms of respiration. The smallest organisms diffuse gases across their body surface, and the larger organisms rely on gills to acquire oxygen from the surrounding environment. Most crustaceans use the respiratory pigment hemoglobin to bind to oxygen, but some members of Malacostraca use hemocyanin instead. Crustaceans that live in terrestrial habitats have modified gills that enable them to breathe air, and their internal branchial chambers are more complex, making life on land possible.
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