''Amphionides reynaudii'' is a species of

caridean shrimp The Caridea, commonly known as caridean shrimp or true shrimp, are an infraorder of shrimp within the order Decapoda. This infraorder contains all species of true shrimp. They are found widely around the world in both fresh and salt water. Many ot ...

, whose identity and position in the

crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapoda, decapods, ostracoda, seed shrimp, branchiopoda, branchiopods, argulidae, fish lice, krill, remipedes, isopoda, isopods, barnacles, copepods, ...

system A system is a group of interacting or interrelated elements that act according to a set of rules to form a unified whole. A system, surrounded and influenced by its environment, is described by its boundaries, structure and purpose and express ...

remained enigmatic for a long time. It is a small (less than one inch long)

plankton Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a cruc ...

found throughout the world's

tropical The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in the Southern Hemisphere at S. The tropics are also referred to ...

ocean The ocean (also the sea or the world ocean) is the body of salt water that covers approximately 70.8% of the surface of Earth and contains 97% of Earth's water. An ocean can also refer to any of the large bodies of water into which the wor ...

s, which until 2015 was considered the sole representative of the

order Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of ...

Amphionidacea, due to unusual morphological features. Molecular data however confirm it as a member of the caridean family

Pandalidae The family Pandalidae is a taxon of caridean shrimp. These species are commonly called pandalid shrimp. They are edible and have high economic value. They are characterised by the subdivided carpus of the second pereiopod and, mainly, by the lac ...

, and the confusion of morphology is because only larval phases have so far been studied.

Description

''Amphionides'' specimens observed have been up to long. In view of adult shrimp morphology, ''Amphionides'' appears unusual, with many body parts being reduced or absent. For example, it has only one pair of

mouthparts Mouthparts may refer to: * The parts of a mouth ** Arthropod mouthparts The mouthparts of arthropods have evolved into a number of forms, each adapted to a different style or mode of feeding. Most mouthparts represent modified, paired append ...

– the maxillae – the

mandible In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable bone ...

s and maxillules being

vestigial Vestigiality is the retention, during the process of evolution, of genetically determined structures or attributes that have lost some or all of the ancestral function in a given species. Assessment of the vestigiality must generally rely on co ...

. Males and females differ in the form of the antennae, and also by the presence in males of the eighth

thoracic The thorax or chest is a part of the anatomy of humans, mammals, and other tetrapod animals located between the neck and the abdomen. In insects, crustaceans, and the extinct trilobites, the thorax is one of the three main divisions of the crea ...

appendage An appendage (or outgrowth) is an external body part, or natural prolongation, that protrudes from an organism's body. In arthropods, an appendage refers to any of the homologous body parts that may extend from a body segment, including ante ...

, albeit in a reduced form. This is the site of the male

gonopore A gonopore, sometimes called a gonadopore, is a genital pore in many invertebrates. Hexapods, including insects have a single common gonopore, except mayflies, which have a pair of gonopores. More specifically, in the unmodified female it is t ...

(the female's gonopore is on the sixth thoracic appendage). The first

pleopod The decapod ( crustaceans such as a crab, lobster, shrimp or prawn) is made up of 20 body segments grouped into two main body parts: the cephalothorax and the pleon (abdomen). Each segment may possess one pair of appendages, although in variou ...

of the female is greatly enlarged and almost encloses the enlarged

carapace A carapace is a dorsal (upper) section of the exoskeleton or shell in a number of animal groups, including arthropods, such as crustaceans and arachnids, as well as vertebrates, such as turtles and tortoises. In turtles and tortoises, the unde ...

. This is assumed to be a chamber in which the

eggs Humans and human ancestors have scavenged and eaten animal eggs for millions of years. Humans in Southeast Asia had domesticated chickens and harvested their eggs for food by 1,500 BCE. The most widely consumed eggs are those of fowl, especial ...

are fertilised and retained until hatching. The more streamlined carapace and pleopods of the male make it more

hydrodynamic In physics and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids— liquids and gases. It has several subdisciplines, including ''aerodynamics'' (the study of air and other gases in motion) ...

, so fewer males are caught than females.

Distribution and ecology

''A. reynaudii'' has a

cosmopolitan distribution In biogeography, cosmopolitan distribution is the term for the range of a taxon that extends across all or most of the world in appropriate habitats. Such a taxon, usually a species, is said to exhibit cosmopolitanism or cosmopolitism. The extr ...

in the world's tropical oceans. It is

ic, inhabiting waters less than deep as a larva, but more often at depths of as an adult.

Classification

Originally described from earlier larval stages, ''Amphionides'' was actually first thought to be a shrimp. In 1969 it was connected to a supposed adult form described earlier by

Carl Zimmer Carl Zimmer (born 1966) is a popular science writer, blogger, columnist, and journalist who specializes in the topics of evolution, parasites, and heredity. The author of many books, he contributes science essays to publications such as ''The Ne ...

(1904), not recognisable as a caridean shrimp, and in 1973 was ''Amphionides'' placed in its own order Amphionidacea by Donald I. Williamson, i.e., at the same rank as the Decapoda and

Euphausiacea Krill are small crustaceans of the order Euphausiacea, and are found in all the world's oceans. The name "krill" comes from the Norwegian word ', meaning "small fry of fish", which is also often attributed to species of fish. Krill are consid ...

. The specific epithet ''reynaudii'' was given by Henri Milne-Edwards in honour of a friend of his, possibly Count François Dominique Reynaud de Montlosier. Based on molecular comparisons, the species was moved back to Decapoda and to the shrimp infraorder

Caridea The Caridea, commonly known as caridean shrimp or true shrimp, are an infraorder of shrimp within the order Decapoda. This infraorder contains all species of true shrimp. They are found widely around the world in both fresh and salt water. Many ...

in 2015, while the adult stage still remains undiscovered. It was speculated that observations from different oceans represent larval stages of different shrimp species. Broader analyses confirmed that ''Amphionides'' belongs to the family

, but it does not seem to be associated with any of the previously recognised pandalid genera, while it indeed has an inter-oceanic distribution.

References

External links

{{Taxonbar, from=Q1139268 Caridea Monotypic decapod genera Taxa named by Henri Milne-Edwards

Description

Distribution and ecology

Classification

References

Further reading

External links