Crustacean

Crustaceans are a large group of animals that mostly live in water. The name comes from the Latin word for "those with shells" because many of them have hard outer shells. They include animals like shrimps, prawns, crabs, lobsters, crayfish, and many others. These animals are part of a bigger group called arthropods, which also includes insects and spiders.

Crustaceans vary greatly in size, from tiny creatures smaller than a pinhead to the Japanese spider crab, which can have legs stretching nearly 4 meters across. Like other arthropods, crustaceans grow by shedding their old shell, a process called moult.

Most crustaceans live in oceans, rivers, and lakes, but some have adapted to live on land, like woodlice. Others live by attaching themselves to objects or even other animals. Crustaceans are very important for people because many are eaten around the world, especially shrimp and prawns. Scientists who study these animals are called carcinologists.

Anatomy

The body of a crustacean is made of segments grouped into three main parts: the head, the thorax, and the abdomen. The head and thorax may be joined together and covered by a hard shell called a carapace. Crustaceans have a tough exoskeleton that they shed to grow bigger.

Each body segment can have pairs of appendages, such as antennae on the head and legs on the thorax. Some crustaceans also have special abdominal appendages. These limbs help crustaceans move and eat. The body's main cavity is an open system where blood flows freely, and crustaceans have special pigments to carry oxygen. They also have a simple brain and structures that work like kidneys.

Ecology

Most crustaceans live in water, such as oceans and freshwater places, but some can live on land, like terrestrial crabs, terrestrial hermit crabs, and woodlice. Many crustaceans can move around, but some stay in one place or live on other animals, such as sea lice, fish lice, and whale lice.

Krill are important in the food chain, especially in Antarctic waters. Some crustaceans have moved to new places and changed where they live, like the Chinese mitten crab, Eriocheir sinensis, and the Asian shore crab, Hemigrapsus sanguineus. Since the opening of the Suez Canal, many crustacean species from faraway seas have settled in the Mediterranean Sea, changing the environment there.

Life cycle

Most crustaceans have males and females and reproduce by mating. Some can change their sex during life. A few can make eggs without mating.

Crustaceans grow through different stages. One early stage is called the nauplius, with simple eyes and few body parts. Later comes the zoea, which swims using parts on its chest. As they grow, these young animals change shape and become adults.

Classification and phylogeny

The name "crustacean" was first used by scientists who studied these animals. It was officially used in 1772 by Morten Thrane Brünnich in his book Zoologiæ Fundamenta.

Crustaceans are a large group with almost 67,000 known species. Scientists think there are many more that we haven’t discovered yet. They come in many sizes, from the very large Japanese spider crab — with legs stretching over 3.7 meters — to tiny creatures like Stygotantulus stocki that are only 100 micrometers long. All crustaceans share a special early stage of life called the nauplius.

Scientists are still learning how crustaceans are related to other animals. Some studies show that crustaceans and insects are close relatives, grouped together as Pancrustacea. Other studies suggest that crustaceans might include many groups, with insects inside them. Recent work has led to new ways of classifying crustaceans.

Fossil record

Crustaceans have a long fossil record. Many groups of crustaceans appeared in the fossil record before the end of the Cambrian period. These include the Branchiopoda, Maxillopoda (which includes barnacles and tongue worms), and Malacostraca. There is some debate about whether some Cambrian animals were true ostracods, which mostly appear later in the Ordovician.

Early crustaceans were not very common, but they became more numerous from the Carboniferous period onward. Some groups, like krill, have no known fossils, while others, such as mantis shrimp, still exist today alongside extinct groups. True crabs and lobsters first appeared much later, during the Cretaceous period.

Consumption by humans

Many people eat crustaceans, such as crabs, lobsters, shrimp, crawfish, and prawns. In 2007, people caught almost 10,700,000 ⁷ tons of these animals. Most of this comes from Asia, especially China, which catches nearly half of all the crustaceans in the world. While shrimp and prawns are the most common, other types like krill are eaten much less often.