Crustaceans (Meaning and Explanation)

We explain what crustaceans are and how they are classified. Also, its feeding, reproduction and other characteristics.

What are crustaceans?

Crustaceans (from Latin crust which means “bark”) are arthropods that have an articulated exoskeleton and its size can vary between one millimeter and four meters in length. There are about six thousand seven hundred living species. Like all arthropods, crustaceans are invertebrate animals.

Most are aquatic species and they can live in salt water (like krill), fresh water (like crayfish), or they can even live in both types of water (like shrimp). On a smaller scale, there are terrestrial crustaceans such as the cochineal (also known as the “little bug”).

In general, crustaceans live and move freely. Some are isopod-type parasites that attack fish and other crustaceans, so they depend on their hosts to move around (such as Artystone trysibia).

There is a minority of crustaceans that do not have limbs and they do not move throughout their lives, but remain clinging to the rocks of the sea (like barnacles or sea acorns).

Characteristics of crustaceans

Crustaceans have an exoskeleton that covers a large part of their body and protects them from both predators and extreme weather conditions. Its physiognomy is divided into two sections:

Cephalothorax It covers the head and thorax.
Abdomen It is composed of articulated segments.

However, in some species three sections are recognized due to the separation of the head and thorax. have biramous limbs (i.e. legs that branch in two), they usually have five pairs of legs and two pairs of antennae.

Crustaceans have a simple digestive system and an excretion system through the “antennal glands” (appendages located under the antennae), which collect waste and expel it through the pores.

Examples of crustaceans

crustaceans barnacles — Barnacles remain clinging to stones throughout their lives.

Some examples of crustaceans are:

The remipedios They are the most primitive and live in the depths of the sea, in areas such as Australia, the Caribbean Sea, the Indian Ocean and the Canary Islands. They are characterized by being blind, swimming on their backs and being hermaphrodites.
The crabs They are of the banchiopoda type and live on the coasts of almost the entire world. Its diet is omnivorous and its size varies greatly depending on its species (from a few millimeters to more than a meter wide). During the growth stage, if they lose a limb, they can regenerate it.
The barnacles. They are those that live on rocks on the seashore. They are characterized by not having limbs and by remaining clinging to stones throughout their lives. They feed on plankton and detritus (solid remains in the form of particles, which come from organic matter) which they consume through filtration thanks to the food brought by the waves.
The artystone trysibia. They are parasitic crustaceans of the isopod type that during their youth swim through fresh waters and as adults stay in the mouths of fish until they manage to reach and occupy the abdominal cavity.
The mealybugs. Also known as “little bugs”, they are crustaceans that, despite having adapted to the terrestrial habitat, breathe through gills (which is why they need to live in humid places). They usually walk at night and have a life expectancy of three years.
The krills. They are very curious crustaceans that have luminous organs called “photophores” (near the mouth and genitals) that emit blue light. There are several theories regarding the function of this light, but the most consistent is that they fulfill reproductive functions.
The copepods. They are parasitic crustaceans of the maxillopod type and are very small in size (they reach ten millimeters in length). They do not have a shell, but they do have a brain shield and a single eye. Their diet is based on plankton.

Types of crustaceans

Crustaceans are classified into six types:

Banchiopoda. They are those that live in fresh water. They are characterized by being small and having external gills on the back of the head. They can have a shell-shaped shell (such as clam shrimp, from the order laevicaudata), in the form of a brain shield (such as notostraceans) or not have one (such as brine shrimp, from the order anostraceans).
Remipedia They are those that do not have eyes, usually have an elongated body (similar to that of the worm) and have a first pair of antennae that they use as a smell sensor (to detect dead animals). They live in caves in the depths of the sea, in areas such as Australia, the Caribbean Sea, the Canary Islands and the Indian Ocean.
Cephalocarida. They are among the most primitive. They live in the mud and sand of the deepest soils of the sea. They are very small in size (between two and four millimeters), they do not have eyes and they are hermaphrodites. They feed on detritus.
Maxillopoda Its name comes from Greek maxilla which means “jaw” and pous which means “foot”. Its body is very small and the jaw and lower limbs are very close. They have a very primitive and simple nervous system. They feed on bacteria from suspended organic matter and the blood of other individuals (in the case of parasite-type species).
Ostracode. They are those that live in salty and fresh waters. They have a shell similar in appearance to oysters, of the bivalve type (divided into two valves). The sensitivity of their eyes varies in each species and they can be photo-receptors, thermo-receptors and mechano-receptors.
Malacostraca. They are crustaceans that live in fresh and salt waters, with some exceptions of terrestrial species (that live on tropical coasts). They have a semi-soft shell and sensory hairs that cover their legs. They are carnivores and predators, feeding on small mollusks and fish.

Crustacean feeding

camel shrimp crutaceans — The camel shrimp is a carnivorous crustacean.

Crustacean feeding varies according to different species. The simplest type of feeding is filter feeding (like copepods and small shrimp). Others are scavengers and feed on the remains of creatures, or are active predators, such as crabs and lobsters that come out at night to hunt.

Its digestive system is simple. It is usually made up of a straight duct and, through an organ that functions as a food grinder, they carry out digestion. In other cases, they usually have spiral-shaped digestive glands that absorb nutrients.

Reproduction of crustaceans

crustaceans have a sexual and oviparous reproductive system (that is, through external fertilization of the egg). Some species present direct development (an already formed individual is born from the egg). However, most crustaceans have indirect development (they are born as larvae and evolve into their maturity stage).

Most crustaceans have separate sexes although there are a few species that are hermaphrodites (that is, the same individual has both sexes: the female and the male), such as barnacles. Other species change their sex as they mature.