Crustacean

Crustaceans are a large group of animals that mostly live in water. Their name comes from the Latin word for "those with shells" because they have hard outer coverings called exoskeletons. Familiar crustaceans include crabs, lobsters, shrimps, prawns, crayfish, krill, barnacles, and copepods. They are a type of arthropod, like insects and spiders, but they have special two-parted limbs and unique young stages that make them different.

Crustaceans come in many sizes, from tiny species just a millimeter long to the very large Japanese spider crab that can stretch its legs over 12 feet. Most live in water, but some can live on land, like woodlice. Some crustaceans even attach themselves to other animals or plants. They are important in nature, as they are a big part of the food chain, and many people enjoy eating them. Scientists who study crustaceans are called carcinologists.

Anatomy

The body of a crustacean is divided into three main parts: the head, the thorax, and the abdomen. The head and thorax can join together to form a cephalothorax, which is often covered by a hard shell called a carapace. Crustaceans have a hard exoskeleton that they shed to grow bigger.

Each part of the body can have a pair of body parts that stick out. The head has antennae and parts for eating, while the thorax has legs for walking or getting food. The abdomen often has parts called pleopods and ends in a telson, which helps it move. Crustaceans have an open circulatory system and a simple brain close to the antennae.

Ecology

Most crustaceans live in water, like oceans and freshwater. But some live on land, like terrestrial crabs, terrestrial hermit crabs, and woodlice. Many can move around, but some live by attaching to other animals, such as sea lice, fish lice, whale lice, and tongue worms.

Krill are important in the food chain in the Antarctic. Some crustaceans have become invasive species, 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 the Red Sea and the Indo-Pacific have moved to the eastern Mediterranean, changing local ecosystems.

Life cycle

Most crustaceans have male and female sexes. Some, like barnacles, have both male and female traits.

Crustaceans grow through different stages. One early stage is called the nauplius. It has simple parts and one eye. As they grow, they go through stages like the zoea. The zoea swims using different body parts. These young crustaceans learn ways to stay safe and improve their vision as they grow into adults.

Classification and phylogeny

The name "crustacean" was first used by scientists to describe these animals. It was officially used in 1772 by Morten Thrane Brünnich in his work Zoologiæ Fundamenta.

Crustaceans are a big group with almost 67,000 known species. Scientists think there are many more still unknown. They range in size from the huge Japanese spider crab to very tiny creatures like Stygotantulus stocki. All crustaceans go through a special early life stage called the nauplius.

Scientists are still figuring out how crustaceans are related to other animals. Some studies show that crustaceans and insects are close relatives, grouped together as Pancrustacea. Other studies suggest crustaceans may include many groups, with insects inside them. Today, scientists recognize ten to twelve groups of crustaceans, including Thecostraca, Tantulocarida, Mystacocarida, Copepoda, Branchiura, and Pentastomida.

Fossil record

Crustaceans have a rich fossil record. Many types appeared before the end of the Cambrian period. These include Branchiopoda, Maxillopoda (such as barnacles and tongue worms), and Malacostraca. Some debate exists about whether certain Cambrian animals were true ostracods. Ostracods mostly appear later in the Ordovician.

Fossil crustaceans become more common from the Carboniferous period onward. Important groups like Cumacea and Isopoda appeared during this time. In the Decapoda group, prawns and polychelids first showed up in the Triassic. Shrimp and crabs came later in the Jurassic. The greatest increase in crustacean diversity happened in the Cretaceous period, especially among crabs. The first true lobsters also appeared in this period.

Consumption by humans

Many people eat crustaceans. In 2007, about 10,700,000 tons were caught. Most of these are decapod crustaceans such as crabs, lobsters, shrimp, crawfish, and prawns. Over 60% of all crustaceans caught are shrimp and prawns. Most of this comes from Asia, with China producing nearly half of the world's total. Other crustaceans, like krill, are not eaten as much even though there are many of them.