Crustacea: hard skin, soft inside!

Crustacea are the most common Arthropods living in the marine environment. They include shrimps, crabs, lobsters, krill, barnacles and many more, which are colonizing also fresh waters and terrestrial environments.

Phylum ARTHROPODA (insects, spiders, crustaceans)

Arthropoda are an extremely large group of hard friends, which includes also insects and spiders. There are close to 1.000.000 species of arthropods which are found in a wide variety of habitat. They rank so as the most successful of the animal phyla. Arthropods are segmented animals, with limbs adapted to various functions.

Moreover an important characteristic is the presence of an exoskeleton made of chitin. The exoskeleton can not increase in size, so when the animal grows it must discard the old skeleton and secrete a new one in a process called molting.

Another feature in arthropods is a complex and evolutes eye, capable of image formation. It is called compound eye because it is made of a large number or light sensitive units, called ommatidia, each one has cornea, lens, light sensitive cells and neural connector.

Arthropoda is a very diverse phylum with 3 existing subphylum: Chelicerata (spiders and horseshoe crabs), Uniramia (insects, centipedes and millipedes) and Crustacea (crabs, shrimps and lobsters). Of course we are going to focus only on the marine forms of Chelicerata and Crustacea.


This group is divided in 3 classes, 2 marine and one terrestrial:

Aracnida (spider, ticks and mites) it is an exclusively terrestrial class; we are not going to describe it.

Merostomata (horseshoe crabs) they are all marine species. They live in sandy and muddy bottoms, where they burrow actively under the substate. They feed on polychetes. Unfortunately they live only along the Asian and the North American coasts.

Pycnogonida (sea spiders) they are quite common animals, well distributed in all the latitudes. In spite of this it is not so easy meet them during diving because they live in hydroids or bryozoans colonies, feeding on polyps. In fact we don’t have photo, yet.


This is a large group of animals including many common and commercial species. They count 31.000 species widely distributed in the marine environment, but colonizing also fresh waters and terrestrial environment.

The body of crustaceans is divided in a head, presenting well developed sensory structures, a thorax and an abdomen presenting appendages specialized for locomotion, feeding and reproduction.

This subphylum is divided in 8 classes, most of them very common or even exclusive of marine environment. We are going to describe, as always, that ones that are visible by divers or snorkellers.

Cephalocarida rare in marine environment.

Branchiopoda (brine shrimps) common in fresh water environment.

Ostracoda (clam shrimps) important members of deep sea zooplankton.

Copepoda (copepods) important members of zooplankton, both in surface waters and in deep sea.

Mystacocarida rare in marine environment.

Branchiura rare in marine environment.

Cirripeda (barnacles) 900 species of marine animals permanently attached to some sort of substrate, both rocks or every floating body, even sea turtles and whales. They present well developed calcareous skeleton from which the thoracic appendages, called cirri, are extended in the water to collect food particles from the water.

Malacostraca (crabs, shrimps, lobsters, krill) this is the largest class of crustaceans that includes many different Orders, among them:

Euphausacea (krill) 90 species of very important organisms in the pelagic environment. They are part of the zooplankton, they feed on phytoplankton and they are the most important food source for baleen whales.

Decapoda (shrimps, crabs, lobsters) it is the most large group, presenting 8.500 species, belonging mostly to the benthonic organisms. They present the head and the thoracic segments covered by a carapace. They present 5 pairs of swimming legs and developed feeding appendages (decapods= 10 legs). Shrimps have a well developed abdomen with appendages (pleopods) adapted for swimming. Crabs have reduced abdomen and are dorso-ventrally flattened with walking legs. Lobsters have a well developed abdomen without pleopods; they can move backwards by tail propulsion.

Mysidacea they are small shrimp with long and flat body. They live in large number between the algae and the anemones or inside sea caves, where they find protection.

Isopoda (pill bugs) 4.500 species of marine, fresh water and terrestrial organisms. They are flattened dorso-ventrally and they are scavengers and omnivorous. Some species are fish ecto-parasite and are visible on the fish body or gills.

Amphipoda (beach hoppers) 4.000 species of marine and freshwaters crustaceans, flattened laterally that move by hoppering, crawling or swimming. Many of them are pelagic, even if the majority is bottom dwellers. They are very common in the intertidal zone. They are scavengers or detritus feeders.

... it continues!...

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