Astacus

This article is about the genus of crayfish. For the city in Greece, see Astakos. For the ancient Greek city now in Turkey, see Astacus (Bithynia). For the mythical figure, see Astacus (mythology).

The genus Astacus (derived from the Greek word αστακός, or astacós which translates to "lobster" or "crayfish") consists of three extant (living) species and three somewhat contested extinct species. These crayfish are found across Europe and live exclusively in freshwater, mostly inhabiting the bottom of lakes, ponds, and streams.

"A. astacus have a dark brown or black coloration on their dorsal side, and an olive-brown or reddish-brown coloration on their ventral side. Their carapace is smooth, with small granular tubercles along the edges. The rostrum is flat and has two spines near its base. The claws are broad with well-defined bases, reddish-brown on the ventral side, and rough on the dorsal side."

Taxonomic information

The genus Astacus belongs to the family Astacidae which is a part of the superfamily Astacoidea consisting of three groups of

Astacus Taxonomy
Astacus astacus
Conservation status
Vulnerable
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Arthropoda
Subphylum:	Crustacea
Superclass:	Multicrustacea
Class:	Malacostraca
Subclass:	Eumalacostraca
Superorder:	Eucarida
Order:	Decapoda
Suborder:	Pleocyemata
Infraorder:	Astacidea
Superfamily:	Astacoidea
Family:	Astacidea
Genus:	Astacus Fabricius, 1775
Species:	Astacus astacus (Linnaeus, 1758) Astacus balcanicus (Karaman, 1929) Astacus colchicus Kessler, 1876 †Astacus edwardsii Van Straelen, 1928 †Astacus laevissimus Fritsch & Kafka, 1887 †Astacus multicavatus Bell, 1863

freshwater crayfish that are found in the Northern Hemisphere. This was the first named genus of freshwater crayfish, with many members often being reexamined and placed in different groups, this is what happened to three of the fossils that were previously placed in the Astacus genus.

Astacidae	Pacifastacus Astacus Astacus astacus A. balcanicus A. colchicus Pontastacus Austropotamobius

This image shows an adult A. astacus specimen. The dark colouring works helps to camouflage them against their dark and sandy environment.

The genus Astacus is a family of crayfish that exists all across Europe. They have been found to span the entire continent and prefer temperate water ranging from 12°C to 20°C but they are also able to survive during colder periods when temperatures are closer to freezing.

The genus has been known to eat almost anything, typically crawling along the bottom of a body of water searching for another meal. They can be defined as detritovores since they persist mostly on decaying plant matter as well as dead or dying animals. This unique diet makes these crayfish essential for maintaining clean and healthy waterways as they clear out decaying matter and consume fish that are affected by illness or disease.

As is the case with all arthropods/ecdysozoans, the genus Astacus must molt their cuticle layer in order to grow and develop.

Reproduction

The Astacus genus exhibits sexual dimorphism and external reproduction. Reproduction is heavily based on the environmental conditions such as temperature and photoperiod, when ideal conditions are met fertilization can occur. The male testis produce sperm which then passes through the vas defrens which packages the sperm into a spermatophore and then passes it through to the gonopods. The spermatophore is then released onto the ventral surface of a female who then takes it into their annulus ventralis. The spermatophore can then be stored for up to several months before ovulation occurs within the female when the desired conditions are met. Once ovulation is complete, a special gland on the female (Glair gland) secretes an enzyme that breaks down the spermatophore and releases the sperm across the ventral surface of the body. The eggs are then released through the gonopore and are subsequently fertilized by the sperm. Fertilized eggs are then kept under the female's pleopods and undergo centrolecithal cleavage until they have developed enough to be released as crayfish larvae.

Due to the similar nature of the Astacus species, it is not uncommon to see hybridization occur both in a lab environment as well as in the wild. Since they follow the same reproductive procedure and have considerable genetic similarities, the species can interbreed with varying levels of success. To date there are no known populations of hybrid Astacus crayfish as the process of hybridization often leads to non-viable or sterile offspring.

Economic significance

Historically, crayfish of the species Astacus have been known to hold economic significance as an object of trade dating back to Europe in the Middle Ages. Initially, Astacus was considered a delicacy, but as demand grew so did industrial harvesting of the crayfish in the nineteenth century.

There exists a homeopathic remedy called Astacus Fluviatilis. This remedy seems to be first described in the book Materia Medica by homeopath John Henry Clarke in 1902 and purported benefits are discussed. One study examining the effects of Astacus Fluviatilis on Urticaria (hives) reported that treatment with this homeopathic remedy showed a decrease in symptoms.

Decline and Conservation

The population of A. astacus has seen significant decline due to the crayfish plague, a water mold disease Aphanomyces astaci, carried by non-native signal crayfish, Pacifastacus leniusculus. The introduction of signal crayfish to compensate for economic losses furthered the species decline by furthering the spread of the plague Additionally, human activities such as transporting crayfish between water bodies and using infected equipment have facilitated the further spread of the disease.

Some noble crayfish populations have developed partial resistance to the plague, surviving for longer than others. Researchers are hopeful that selective breeding programs could create new lineages of noble crayfish that are more resistant or immune to the plague. Current conservation efforts are focused on breeding and reintroducing A. astacus back into their native habitats.

Species

Generally, the genus Astacus consists of relatively similar species of crayfish, with only minor differences in physical traits and geographic ranges. These similarities can make proper classification difficult. Similarly, inconsistencies are present in literature that describe the morphological differences between sister taxa Astacus and Pontastacus.

There were previously considered to be 6 species of crayfish within the Astacus genus but new research has shown that the fossils depicting extinct members of the genus

Extant species

Astacus astacus (Linnaeus, 1775)

This is the most widespread and it is the most well studied of all the extant and extinct species in the genus. It is commonly known as the "Noble crayfish", the "Broad-fingered crayfish", or the "European crayfish".

Astacus balcanicus (Karaman, 1929)

This species of Astacus is defined by their unique geological location. All members of this species live only in one of three bodies of water. This includes the Vardar river basin and Lake Ohrid in North Macedonia, and Lake Pamvotida in Greece. The separation from A. astacus likely occurred due to Pleistocene glaciations as well as human impacts such as pollution or translocation of species.

Astacus colchius (Kessler, 1876)

This species is the only crayfish species native to Georgia, and is specifically limited to the upper basins of the Rioni river. There is a high degree of morphological similarity between Astacus astacus and Astacus colchius, however, molecular studies of the two species indicate that phylogenetic differences are present.

Extinct Species

†Astacus edwardsii (Van Straelen, 1928)

Astacus edwardsii was first identified in 1932 from a plaster cast of a now destroyed holotype fossil that was discovered in France. Although initially being classified as a member of the Astacus genus, more recent research has shown that the specimen possessed traits that were not seen anywhere else in the genus. This has lead to the creation of a new genus within the Astacidae family known as the Emplastron. The species is now known as Emplastron edwardsii as of 2021.

?†Astacus laevissimus (Fritsch & Kafka, 1887)

Astacus laevissimus was the name given to a fossil specimen in 1887. The specimen has recently fallen under scrutiny due to the fact that the fossil was found in marine sediment. This has caused researchers to believe that the A. laevissimus fossil is likely a marine lobster rather than a crayfish. The name Astacus laevissimus is mostly considered a nomen dubium, but an appropriate replacement has not been found due to poor fossil records.

†Astacus multicavitus (Bell, 1863)

Astacus multicavitus was first identified from a fossil found in marine sediment. The fossil was placed within the genus Astacus based on initial visual analysis but further research suggests that it is likely not a member of this genus. The fossil has been more accurately classified as a marine lobster within the Erymidae family.