Crustacean

Crustaceans are a large group of animals that live mostly in water. Their name comes from the Latin word for "those with shells" because they have hard outer coverings called exoskeletons. They include familiar animals like shrimps, prawns, crabs, lobsters, and crayfish, as well as many others such as krill, barnacles, and copepods. These animals are a type of arthropod, which also includes insects and spiders, but they have special two-parted limbs and unique larval stages that set them apart.

Crustaceans vary greatly in size, from tiny species just a millimeter long to the huge Japanese spider crab that can stretch its legs over 12 feet. Most live in water, but some have adapted to live on land, like woodlice. Some crustaceans even live by attaching themselves to other animals or plants. They are very important in nature, forming a big part of the food chain, and many people enjoy eating them. Scientists who study these fascinating creatures are called carcinologists.

Anatomy

The body of a crustacean is made up of segments grouped into three main regions: the head, the thorax, and the abdomen. The head and thorax may fuse to form a cephalothorax, often covered by a hard shell called a carapace. Crustaceans have a hard exoskeleton that they must shed in order to grow.

Each body segment can have a pair of appendages. The head has antennae, mandibles, and maxillae, while the thorax has legs that may be used for walking or feeding. The abdomen often has appendages called pleopods and ends in a telson, which helps with movement. Crustaceans have an open circulatory system and a simple brain made of ganglia near the antennae.

Ecology

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

Krill play a vital role in the food chain in the Antarctic, and some crustaceans have become invasive species, such as 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 settled in the eastern Mediterranean, affecting local ecosystems.

Life cycle

Most crustaceans have separate sexes and reproduce sexually. Some, like barnacles, are hermaphrodites, meaning they have both male and female traits, and a few can even change their sex during their life. In many crustaceans, females can produce eggs without needing a male, a process called parthenogenesis.

Crustaceans go through different stages in their life cycle. One early stage is called the nauplius, which has simple appendages and a single eye. As they grow, they go through other stages like the zoea, which swims using different body parts than the nauplius. These young crustaceans develop special ways to hide from predators and improve their vision as they grow into adults.

Classification and phylogeny

The name "crustacean" comes from early scientists who described these animals. It was officially used in 1772 by Morten Thrane Brünnich in his work Zoologiæ Fundamenta.

Crustaceans are a large group with almost 67,000 known species, and scientists believe many more are still undiscovered. They range in size from the huge Japanese spider crab to tiny creatures like Stygotantulus stocki. All crustaceans share a special early life stage 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 may include many groups, with insects nested inside them. Modern classifications now recognize ten to twelve groups of crustaceans, including Thecostraca, Tantulocarida, Mystacocarida, Copepoda, Branchiura, and Pentastomida.

Fossil record

Crustaceans have a rich fossil record, with many groups appearing 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, which 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, followed by shrimp and crabs in the Jurassic. The greatest increase in crustacean diversity happened in the Cretaceous period, especially among crabs, possibly because of changes in their main predators, bony fish. The first true lobsters also appeared in this period.

Consumption by humans

Many crustaceans are eaten by people. In 2007, around 10,700,000 tons were caught, with most being decapod crustaceans like crabs, lobsters, shrimp, crawfish, and prawns. Over 60% of all crustaceans caught are shrimp and prawns, and 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 they are very numerous.