BRACHYURA: CHARACTERISTICS, TAXONOMY, NUTRITION, REPRODUCTION - BIOLOGY - 2025

The brachyuran are a group of animals belonging to the phylum Arthropoda, being known as crabs. Like all arthropods, they present a great variety of articulated appendages, with different functions.

They were described for the first time by the famous Swedish naturalist Carlos Linnaeus in 1758. They are mainly found in marine-type ecosystems, distributed at all depths, being more frequent on the coasts.

Braquiuro specimen. Source: Lieutenant Elizabeth Crapo, NOAA Corps

These animals are very important, economically speaking, in the gastronomy industry, since they are part of a wide variety of dishes, many of which are of great international renown.

characteristics

The members of the Brachyura infraorder are multicellular eukaryotic organisms that are made up of various types of cells, which perform specific functions such as nutrient absorption, gamete production, and muscle contracture, among others.

They are triblastic and coelominated animals. This means that during its embryonic development, the three germ layers are observed: ectoderm, mesoderm and endoderm. A cavity known as a coelom is formed from the mesoderm.

Likewise, crabs are characterized by having bilateral symmetry, which implies that they are made up of two exactly equal halves, taking the longitudinal axis of the animal as a reference.

These reproduce sexually, have internal fertilization, are oviparous and their development is indirect, since when they hatch from the egg, they do so in the form of larvae.

Taxonomy

The taxonomic classification of brachyurans is as follows:

-Domain: Eukarya

-Animalia Kingdom

-Filo: Arthropoda

-Subphylum: Crustacea

-Class: Malacostraca

-Order: Decapoda

-Suborder: Pleocyemata

-Infraorder: Brachyura

Morphology

The body of brachyurans, like that of all arthropods, is divided into several zones or regions: cephalothorax and abdomen. The latter is extremely small, so it is not very significant.

One of the most characteristic elements of crabs is the resistant shell that covers their entire body. This is an exoskeleton made up mainly of chitin and calcium carbonate. This shell extends to the sides and forms a cavity at the ends in which the gills are located.

About 13 pairs of appendages are detached from the brachyury cephalothorax, which are distributed as follows: 5 pairs of legs, antennae and anténules, maxillae, mandibles, 3 pairs of maxilli and maxillae. Each of these fulfills specific functions.

Braquiuro specimen. Its appendages are clearly appreciated. Source: Yale Peabody Museum of Natural History

As expected, the legs fulfill a locomotion function, while the antennas and anténules mainly have a sensory function. The rest of the appendages are involved in the food handling and grinding process, as well as in respiration.

These organisms are sexually dimorphic. In the case of males, the chelas are larger than those of the females. Likewise, quite noticeable differences can be observed in the abdomen.

The abdomen of males is pointed and narrow and that of females is much wider. Similarly, the female pleopods are much more developed than the two pairs of pleopods that the males have.

Nutrition

Brachyurs are heterotrophic organisms. This means that they are not able to synthesize their nutrients, so they must feed on other living beings or the substances made by them. In this sense, the diet of crabs is quite varied, with a very diverse diet.

Feeding type

Within the brachyurans, most are omnivores. However, there are also some that are purely herbivores, carnivores, detritivores or scavengers.

In the case of herbivores, they are those that essentially feed on plants, such as algae that predominate in certain aquatic habitats and plankton.

On the other hand, carnivores are those that prefer to feed on other animals. For the most part, they are predators of small crustaceans and mollusks such as clams and mussels. Likewise, they also tend to feed on some worms that may be in the ecosystems in which they develop.

Detritivore crabs basically feed on decomposing organic matter.

Among the brachyurus there are very unique and curious cases in relation to food. For example, there are species that are able to accommodate their food preferences to the habitat in which they are found.

If they are in one where algae abound, they behave as herbivores, while if they are in a muddy place they act as sedimentivores. A classic example of this type of behavior is Neohelice granulata.

Food travel

As with many arthropods, crabs have several articulated appendages, among which those that participate in their feeding process are the chelas and the maxillipeds. These are at the level of the head, very close to the mouth opening.

With the help of these appendages, the food is directed towards the mouth and it is the strong jaws that are in charge of grinding and chopping the food. This enters the digestive tract, passing the esophagus and then the cardiac stomach, where it undergoes the action of the gastric grinder and is demolished.

From there it passes to the pyloric stomach, where it comes into contact with digestive enzymes synthesized in the hepatopancreas, which contribute to the degradation and processing of nutrients. Here a kind of filtration occurs, in which the liquid content is sent to the hepatopancreas, where it is absorbed.

The rest of the food continues its transit through the digestive system, and reaches the cecum where the absorption of nutrients occurs. Finally, the material that was not assimilated by the body, is sent to the posterior intestine, to cross the rectum and be expelled through the anus.

Reproduction

Brachyurans are dioecious animals, which means that the sexes are separated. There are female and male specimens. Likewise, among these organisms there is the so-called sexual dimorphism, thanks to which there are marked differences between females and males.

Mating ritual

The reproduction process in brachyurans is a bit complex, since it involves, in the first place, a mating ritual. This is varied and diverse and depends on the species.

For example, there are some who make a wide display of movements with their appendages, there are others that use chemical substances such as pheromones, which attract individuals of the opposite sex, and many others emit characteristic sounds using their appendages.

The courtship rituals of crabs have always drawn the attention of specialists, so they still continue to study them to elucidate them correctly.

Fertilization and development

The fertilization process observed in brachyurans is internal, that is, it occurs inside the female's body. Mating occurs "belly to belly", especially when the female has shed her shell. The male makes use of his copulating pleopods and deposits the sperm in the gonopore of the female.

However, it is possible that fertilization does not occur immediately, since the female has the ability to store the sperm and use them later to fertilize her eggs.

Once the ovules are fertilized, they are not released to the external environment, but are transferred to the female's abdomen, where they remain fixed thanks to the intervention of a substance that has the ability to keep them attached there.

The fertilized eggs are kept in that place, until embryonic development is complete. Larvae hatch from the eggs and are released into the water, where they remain freely, undergoing various molts. It is important to note that the larvae of the brachyurans are called zoeas.

Each zoea goes through a specific number of molts, which depends exclusively on the species. Finally they reach the adult stage, where it is possible to reproduce.

References

1. Boschi, E. (2016). The Argentine sea and its fishing resources. National Institute for Fisheries Research and Development.
2. Brusca, RC & Brusca, GJ, (2005). Invertebrates, 2nd edition. McGraw-Hill-Interamericana, Madrid
3. Curtis, H., Barnes, S., Schneck, A. and Massarini, A. (2008). Biology. Editorial Médica Panamericana. 7th edition.
4. Hayward, P. and Ryland, J. (1995). Handbook of the marine fauna of north - west Europe. Oxford University Press
5. Hickman, CP, Roberts, LS, Larson, A., Ober, WC, & Garrison, C. (2001). Integrated principles of zoology (Vol. 15). McGraw-Hill
6. Thatje, S. and Calcagno, J. (2014). Brachyuros. Chapter of the book "Marine invertebrates". Vásquez Mazzini Editores.