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. They are special because their limbs are usually divided into two parts, and they go through interesting stages as they grow.

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 up of segments grouped into three main parts: the head, the thorax, and the abdomen. The head and thorax may sometimes be joined together and covered by a hard shell called a carapace. Crustaceans have a tough exoskeleton that they must 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, eat, and even transfer sperm during mating. 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 on their own, but some stay in one place or live on other animals, such as sea lice, fish lice, and whale lice.

Krill play an important role in the food chain, especially in Antarctic waters. Some crustaceans have traveled to new places and changed local ecosystems, 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, affecting the environment there.

Life cycle

Most crustaceans have separate male and female sexes and reproduce by mating. Some types can change their sex during their lives, and a few can produce eggs without needing to mate with a male.

Crustaceans go through different stages as they grow. One early stage is called the nauplius, which has simple eyes and a few body parts. Later stages include the zoea, which swims using body parts on its chest. As they grow, these young animals change shape and develop into adults.

Classification and phylogeny

The name "crustacean" comes from early 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, and 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. Even though they look different, all crustaceans share a special early stage of life called the nauplius.

Scientists are still learning exactly how crustaceans are related to other animals. Some studies show that crustaceans and insects are very close relatives, grouped together as Pancrustacea. Other studies suggest that crustaceans might include many groups, with insects nested inside them. Recent work has led to new ways of classifying crustaceans, recognizing ten to twelve different classes instead of the older six.

Fossil record

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

Early crustaceans were rare, but they became more common 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 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 make up most of what is caught, other types like krill are eaten much less often, even though krill are very plentiful in the oceans.