|Scientific name of the class|
|Sanders , 1955|
|Scientific name of the order|
|Birshtein , 1960|
|Scientific name of the family|
The Cephalocarida (horseshoe shrimp ) represent a rare class within the crustaceans . It includes the monotypical Brachypoda with the family of the Hutchinsoniellidae . So far, thirteen species are known from this group of animals, which was only discovered in the 1950s (Sanders (1955)).
Way of life of the Cephalocarida
The cephalocarida live in the loose transition area between the water and the fine mud of the seabed. Here the fine precipitate is not yet firmly sedimented, but is no longer present as a whirled suspension. Since this area is at almost every depth of the ocean, cephalocarids can be found from the intertidal zone to the deep sea. The search, however, is extremely difficult due to the sporadic occurrence and the low abundance of the animals. Cephalocarida are blind, their orientation takes place via the pronounced sense of smell of the 1st antennae and cuticula as well as via the sense of touch.
Construction of the Cephalocarida
The Cephalocarida reach a body length of about 3 millimeters. The body is elongated and, in addition to the head (cephalon), consists of a front body (thorax made up of nine segments), which carries the swimming legs, and a rear body without extremities ( abdomen made up of ten segments plus telson). The telson closes the abdomen and carries a pair of slender tail appendages ( furca ).
The head of the animals is covered by a horseshoe-shaped head shield that extends well above the bases of the head extremities. Eyes are completely absent. The 1st antenna is single-branched and relatively short, the 2nd antenna has two branches. The mandible (adult without palpus) and the 1st maxilla serve as the mouth parts . As a special feature among recent crustaceans, the 2nd maxilla of the cephalocarida is anatomically similar to the largely homonomous thoracic legs and, like these, serves as a floating leg.
The first seven pairs of legs of the thorax are leaf-like and flat. They consist of a base part ( protopodite ) to which essentially the two floating branches and several smaller, leaf-like extensions (endites) attach. The outer leg ( exopodit ) consists of two flat and wide parts, the inner branch ( endopodit ) is six-limbed and less wide. In the species Hutchinsoniella macracantha the eighth thoracic bone has no endopodites and only very small endites. The ninth pair of legs is very small and serves as an egg carrier. While the general physique of the thirteen cephalocarid species is the same, there are subtle differences in the structure and bristling of the thoracic legs. These differences are used for taxonomic determination (summarized by Olesen et al. (2011)).
The legs are not only used for locomotion, with their help also the acquisition of food takes place: With the species Hutchinsoniella macracantha one could determine two different beat speeds of the legs, of which only the fast one serves for locomotion. The slow leg strike is performed metachronically, that is, it moves in waves from one leg to the other. As a result, particles can get caught between the leg bases and are filtered out by bristles at the ends and swept into a central channel. The resulting pressure and the movement of the legs cause these particles to enter the oral cavity. In this way, food intake and locomotion are coupled, similar to gillipers and clawed tails .
The rope ladder nervous system of the cephalocarida consists of a brain, a sub-canal ganglion and an abdominal ganglion chain and shows a number of striking differences from other crustaceans. Cephalocarida are the only crustaceans whose brains have an olfactory system with real mushroom bodies . Since the complex structure of the cephalocarid fungal bodies has numerous homologies to that of centipedes and insects, its evolutionary origin can be traced back to the ancestor of all mandibular animals in the Cambrian (Stegner and Richter (2011)). Another neuroanatomical peculiarity of the Cephalocarida compared to other cancers is that segmental ganglia occur not only in the leg-bearing thorax, but also in the legless abdomen. The circulatory system consists only of an open heart , which, like other arthropods, is located in the back area. The heart runs as a simple tube through the first six thorax segments and has three inflow openings on each side (Göpel and Wirkner (2018)).
Reproduction and development
The Cephalocarida are hermaphrodites . They have a pair of ovaries that lie dorsally in the head and testicles in the 7th – 12th centuries. Fuselage segment. A fallopian tube extends from the ovary to the rear end of the body and there unites with the duct of the testicles. The common passage ends in the area of the 6th thoracic leg.
The animals lay a maximum of 2 large eggs at the same time, which have a rich supply of yolks and are carried on the ninth pair of thoracic legs. An advanced larva, known as Metanauplius, hatches from these (a nauplius larva does not occur). With several moults, with Hutchinsoniella 23 and with Lightiella 12, the animals reach their total number of segments when fully grown. The development of the nervous system begins extremely early. The hatching metanauplius already has a brain equipped with all nerves and sensory centers. In contrast to all other crustaceans, the thoracic ganglia of the cephalocarida are formed before the leg anlage.
Systematics of the Cephalocarida
The position of the cephalocarida within the crustacean system remains unclear, as all known features are either strongly derived or have been adopted as plesiomorphies from the last common ancestor of all crustaceans (summarized by Stegner (2014)). Sanders (1957) compared the cephalocarida as a sister group to all other crustaceans (at that time still without Remipedia ) and coined the notion that cephalocarida are " living fossils " , which is still widespread today . Due to the specific structure of their extremities ("turgor extremities") and above all due to the functional connection between food intake and locomotion, the cephalocarida according to Ax (1999) and Hessler (1992) were regarded as a sister group of the leaf crabs or the ostraca (leaf crabs and higher crustaceans ) with these combined as Thoracopoda compared to the remaining cancer groups. Walossek (1993, based on fossils) and Harzsch (2006, based on the nervous system) provided further alternatives. In a comprehensive phylogenomic analysis, Schwenter et al. (2017) the cephalocarida as a sister group of a taxon from leaf crabs, Remipedia and hexipedes , but also pointed out that the derived gene sequences of the cephalocarida are particularly susceptible to tree formation artifacts .
Internally, the thirteen species of Cephalocarida are divided into the following genera:
Knox & Fenwick , 1977
- Chiltoniella elongata Knox & Fenwick , 1977
Hampsonellus Hessler & Wakabara , 2000
- Hampsonellus brasiliensis Hessler & Wakabara , 2000
Hutchinsoniella Sanders , 1955
- Hutchinsoniella macracantha Sanders , 1955
- Lightiella Jones , 1961
- Sandersiella Shiino , 1965
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- Stegner and Richter (2015)