CRUSTACEANS: CHARACTERISTICS, REPRODUCTION, RESPIRATION - BIOLOGY - 2025

The crustaceans are very abundant subphylum of arthropods, most water. They include the well-known lobsters, crabs, prawns, among others. They also contain a heterogeneous series of very abundant but poorly understood microscopic organisms.

They have an articulated exoskeleton, whose composition is rich in chitin, mainly. One of the characteristics of the group is the presence of two pairs of antennae and the larval stage, known as the nauplius larva. They have a shedding of the cuticle and usually have separate sexes, with a few exceptions.

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General characteristics

Crustaceans differ from the rest of the arthropods in different characteristics, but the most outstanding ones are: the presence of two pairs of antennae, two pairs of maxillae on the head, followed by the body segments, with a pair of appendages on each one.

All the appendages of the body - with the exception of the first antennae - are of the biremean type.

Birrameian appendages are characteristic of crustaceans, and other aquatic arthropods, such as the now extinct trilobites. The structure comprises an appendage with two axes - in contrast to the unirrámeos, which only have one axis.

Number of body segments

The body of crustaceans is divided into a large number of segments, from 16 to 20 on average, although in some species there may be more than 60 segments. The characteristic of having a high number of body segments is considered ancestral.

In most crustaceans there is a fusion of the thoracic segments with the head, in a structure called cephalothorax.

Cuticle

In these animals, a dorsal cuticle extends from the head to the posterior region and to the sides of the individual. This covering is the shell of the organism and can vary in structure, depending on the group. The cuticle is secreted and its composition includes molecules of protein, chitin and calcareous material.

Like other arthropods, crustaceans experience molting or ecdysis events. This is a physiological process by which organisms secrete a complete new integument, with the removal of the previous cuticle.

In other words, arthropods do not grow continuously, they have an intermittent development that occurs in the following way: the animal loses the old cuticle, then the increase in size occurs and ends with the synthesis of the new cuticle. Between the molting processes, the animal does not grow.

The ecdysis mechanism is activated by a series of environmental stimuli. Once started, it is under the control of the animal's hormones.

Taxonomy and classes

Relationship with other arthropods

Crustaceans are part of the arthropods. This phylum is divided into four living subphyla, where crustaceans and hexapods are grouped into a clade called Pancrustacea. This phylogenetic hypothesis is widely accepted.

However, there is evidence that hexapods arise within the crustacean lineage. If this proposed divergence pattern is true, it would be phylogenetically correct to refer to insects as terrestrial crustaceans.

Crustaceans comprise a fairly large group, with some 67,000 species distributed worldwide, colonizing a significant number of habitats with various ways of life. The size range goes from microscopic shapes to shapes much larger than the well-known river crabs.

Lessons

They are divided into six classes, although preliminary studies using molecular evidence do not support the monophyly of the group.

Remipedia class

This class is made up of small individuals. So far, ten species have been described, found in caves that have contact with bodies of sea water. As is typical of cave-dwelling animals, these crustaceans do not have eyes.

These organisms are believed to possess the characteristics of the hypothetical ancestor of crustaceans. They present from 25 to 38 body segments that include the thorax and abdomen. These segments contain pairs of appendages similar to each other and suitable for movement in water.

They do not exhibit sexual dimorphism - differences between males and females of the same species. They are hermaphrodites, with the female gonopores located in segment number 7, and the male in segment number 14. They present the typical larvae of crustaceans.

Species of this class have been described in the Caribbean basin, the Indian Ocean, the Canary Islands and even Australia.

Class Cephalocarida

In terms of diversity and number of species, the Cephalocarida class resembles the previous group. Only nine or ten benthic and very small species are known (the number varies depending on the author consulted). They are also suspected of having primitive features.

The appendages of the thorax are very similar to each other, they do not have eyes or abdominal appendages.

Regarding reproduction, they are hermaphrodites. Their peculiar characteristic is that both male and female gametes discharge into the same duct.

Geographically, the presence of these animals has been reported on the coasts of the United States, in India and in Japan.

Class Branchiopoda

Brachiopods include an immense number of organisms, approximately 10,000 species. There are three orders within the group: Anostraca, Notostraca and Diplostraca. They include small and medium-sized organisms.

Its most prominent feature is a series of sheet-like appendages, each divided into lobes with a branchial sheet in the outer region.

Most species inhabit freshwater bodies, although some have been reported living in saltwater. A peculiar feature of the group is its ability to swim with its back down.

Their development includes the nauplius larvae, and through a series of transformations they reach the final form of the adult. However, some individuals have direct development.

Ostracoda classes

The representatives of this group of organisms are very small, in some cases even microscopic. They are diverse, with more than 13,000 species described so far. They are very abundant in the fossil record.

They are distributed worldwide, both in fresh waters and in the seas and oceans. They play a crucial role in the trophic networks of aquatic ecosystems. They feed on a wide range of nutritional material, and a few species are parasitic.

Regarding the design of their body, they show a considerable fusion of the trunk segments. It has one to three pairs of limbs, with a reduced number of thoracic appendages.

Class Maxillopoda

This class of crustaceans includes more than 10,000 species distributed throughout the world. They are characterized by having a reduction in the number of segments of the abdomen and also in the appendages.

The body is generally organized into five head segments, six thoracic segments, and four abdominal segments. In some species this distribution is not fulfilled, with reductions being common.

There are six subclasses called Thecostraca, Tantulocarida, Branchiura, Pentastomida, Mystacocarida and Copepoda.

Malacostraca class

They are the largest group of crustaceans, with more than 20,000 species, where the most famous representatives of the group are located. They include decapods, stomatopods, and krill.

Individuals assigned to this class usually have six segments in the thorax, and all segments are provided with appendages.

Reproduction

In most crutáceans, the sexes are separated and present a series of adaptations for copulation, specific to each group.

In some members of the Cirripedia infraclass, individuals are monoecious, but there is cross fertilization. In other groups, where males are "rare" (they exist in very low density within populations), parthenogenesis is a common event.

In most crustaceans, development involves a larval stage, which through the process of metamorphosis finally transforms into the adult. The most common larva of the group is the nauplius or nauplius larva. However, there are organisms whose development is direct; a miniature version of the adult emerges from the egg.

Breathing

Gas exchange in the smallest individuals in the group occurs easily. In these organisms there is no specialized structure for this process.

In this way, it occurs through the finest regions of the cuticle, for example in the area located in the appendages. It can also occur throughout the body, depending on the species.

On the other hand, in the larger animals of the group, the process is more complicated and there must be specialized organs that are in charge of mediating gas exchange. Between these organs we have the gills, a series of projections that resemble a feather.

Circulation

Crustaceans, like the other organisms that belong to arthropods, have an open circulatory system. This means that there are no veins or blood separations from the interstitial fluid, as occurs in animals that have a closed circulatory system, as in mammals, for example.

The blood of these organisms is called hemolymph, a substance that leaves the heart through the system of arteries and circulates through the hemocele. On return, the hemoliph reaches the pericardial sinus. From the heart, hemolymph can enter through one or more arteries.

The valves present in each artery have the function of preventing the hemolymph from entering again.

The afferent channels of the sinuses carry the hemolymph to the gills, where the exchange of oxygen and carbon dioxide takes place. Fluid returns to the pericardial sinus through efferent channels.

Pigments in the hemolymph

Unlike mammals, in crustaceans and other arthropods, blood can take on a series of colors and shades, depending on the species. It can be transparent, reddish or bluish.

Hemocyanin is a pigment that contains two copper atoms in its structure - remember that the respiratory pigment hemoglobin has one iron atom. The copper gives it a blue hue.

Coagulation

The arthropod hemolymph has the property of forming clots, to prevent certain wounds from causing a significant loss of fluids.

Excretion

In adult crustaceans, excretion occurs through a series of tubes located in the ventral region. If the ducts open at the base of the jaws, they are called maxillary glands, while if the pore is located at the base of the antennae they are called antennal glands.

The types of glands mentioned are not mutually exclusive. Although it is not very common, there are species of adult crustaceans that present both.

In some species of crustaceans, such as the river crab, the antennal glands are very folded and are of significant size. In these cases, it is called a green gland.

The excretion of nitrogenous wastes - mainly ammonia - occurs mainly by simple diffusion processes, in areas where the cuticle is not thickened, generally in the gills.

Function of the excretory organs

The excretory organs participate in ion regulation and in the osmotic composition of body fluids. This fact is particularly important in crustaceans that inhabit freshwater bodies.

Many organisms are constantly threatened by the dilution of their fluids. If we think about the principles of diffusion and osmosis, the water tends to enter the animal. The antennal glands form a dilute, low-salt substance that acts as a flow controller.

Importantly, crustaceans lack Malpighi tubes. These structures are in charge of excretory functions in other groups of arthropods, such as spiders and insects.

Feeding

Feeding habits vary widely between groups of crustaceans. In fact, some forms are capable of changing from one form to another depending on environmental stimuli and the availability of food at the time, using the same set of mouthparts.

A significant number of crustaceans have adaptations at the level of the mouthpart system that allow active hunting of potential prey.

Others consume the nutrients that are suspended in the water, such as plankton and bacteria. These organisms are responsible for creating a current in the water to promote the entry of nutritional particles.

Predators consume larvae, worms, other crustaceans, and some fish. Some are also capable of feeding on dead animals and decaying organic matter.

Habitat and distribution

Crustaceans are animals that inhabit marine ecosystems to a greater extent. However, there are species that live in bodies of fresh water. They are distributed worldwide.

References

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