Salpidae

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Salpidae
A blastozoid chain

A blastozoid chain

Systematics
Over trunk : Neumünder (Deuterostomia)
Trunk : Chordates (chordata)
Sub-stem : Tunicates (urochordata)
Class : Salps (Thaliacea)
Order : Salpida
Family : Salpidae
Scientific name
Salpidae
Lahille , 1888

The Salpidae are tunicates that live in the open ocean . At first glance, they have a jellyfish-like appearance, but belong to the chordates (Chordata) and are thus related to the vertebrates (Vertebrata).

Their main distribution area are the tropical and subtropical seas between 40 ° north and south latitude . By ocean currents, e.g. B. the Gulf Stream , but they can also be drifted far north to 60 ° north latitude. Salpa thompsoni lives in the cold seas around Antarctica. Salpa fusiformis and Thalia democratica are occasionally found in the North Sea . They are mainly found near the surface of the sea. The maximum depth is around 400 meters.

features

Structure of a salpe

Salpidae are barrel-shaped, mostly colorless, transparent organisms. The individual organisms reach a length of 8 mm to 19 cm (oozoids from Thetys vagina ). One end of the body serves as an ingestion opening (mouth), the other as an egestion opening (excretion). The cellulose-containing coat ( tunica ) surrounding the body is relatively thick. The mantle surface, especially the rear end, is often provided with comb-like extensions and ridges. The body is divided into two hollow halves by the inclined gill, the actual gill intestine and the cloacal space . The gill consists of a large beam that supports the two gill slits and is provided with numerous ciliate transverse ribs on the inside.

A ventrally open, more rarely closed body muscle made up of four to eight rings surrounds the body. With its contraction, the Salpidae can actively swim, but have no way of moving against a current. The generated water flow is filtered through the gill intestine (pharynx) and is therefore also used for food intake. The food particles are held in place by mucus that is formed by the endostyle and then transported into the esophagus through eyelashes . The short intestine lies on the ventral side and is bundled in a loop into a dense knot called the nucleus. The intestine opens into the cloacal space. The nucleus is often vividly colored and can glow at times. The genus Cyclosalpa has an additional, paired luminous organ in the body cavity.

The nervous system of the Salpidae consists of a dorsal ganglion node located near the ingestion opening , the brain, from which several nerve cords run through the body. There is also a single, horseshoe-shaped eye nearby.

Salpids grow extremely quickly and can double their weight in the course of a day, and in an hour their body length can increase by 10%.

Life cycle

The Salpidae carry out a generation change and alternate between solitary oozoids that have emerged from a fertilized egg, which have no sexual organs and reproduce vegetatively, and colonial blastozoids, which form chains of many individual animals, which reproduce sexually. Oozoids and blastozoids are fully developed organisms, the basic structure of which is largely the same. One difference is the lack of sexual organs in the oozoids. In addition, blastozoids are usually pointed at the rear, they lack the posterior processes of the oozoids, their egestion opening is asymmetrical and they have three longitudinal ridges on their ventral side.

The generation change was first described by the German naturalist Adelbert von Chamisso in 1819, who discovered this life cycle during the circumnavigation of the Russian ship Rurik . Previously, oozoids and blastozoids were described as different species.

The oozoids form numerous buds on the ventral side of the endostyle, on the so-called stolo prolifer, between the second and third third of their body, which eventually break through the mantle and become blastozoids. These are small chain alps that are initially connected to each other via the intestinal canals of the individual animals. The blastozoids separate in linear chains. The initially single-row chains rearrange themselves later to two symmetrical rows, or, in the case of the genus Cyclosalpa , also form ring-shaped connections. The chains can be several meters long. With increasing maturity, the connecting intestinal canals are reduced. In some species, the so-called cyclosalps, the blastozoids remain connected to one another via stalks or plate-shaped adhesive processes.

The blastozoids form gonads from a size of a few millimeters . Sexually mature blastozoids are then called gonozoids. You always have both female and male reproductive organs. First, an ovary is formed, which is usually located on the right in the cloacal wall and usually only forms one egg. The testicles are tufted, sit in the nucleus and release the sperm into the cloacal space. The eggs always mature earlier than the sperm and can therefore only be fertilized by the sperm of older gonozoids. The embryo develops in a placenta in the area of ​​the cloacal space. The cells surrounding the egg divide very quickly and form an embryo sac or dummy embryo in which the actual embryo is embedded. A tissue can be detected in it that corresponds to the notochord . On the ventral side, the embryo is connected to the mother's bloodstream through the placenta. With increasing growth, the embryo can first penetrate the embryo sac, then also the cloaca wall and is finally born as a solitary oozoid through the cloaca. He takes the placenta with him, a rudiment of which remains for life. Blastozoids are therefore viviparous (give birth to life). There is no larval stage.

At Thalia democratica , the lifespan of a generation, depending on the food supply, is between 50 hours and two weeks.

Ecological importance

Salpids can occur in very large numbers. Swarms over 100 km in length have often been found. Up to 7,000 individual animals could be counted in one cubic meter of water. Since the Salpidae are transparent, they are relatively safe from predators. Their hollow body provides a habitat for some crustaceans , e.g. B. female copepods of the genus Sapphirina and amphipods of the family Hyperiidae . Phronima sedentaria from the last family eats up the cavity and then uses it as a shelter for its clutch.

Systematics

The Salpidae are a family of the Salps (Thaliacea). Their sister group are the fire rollers (Pyrosomida). The taxon formed by both is the sister group of the barrel alps (Doliolida), which differ from the Salpidae, among other things, by a ventrally closed body muscle. All salps are probably derived from the sessile sea ​​squirts (Ascidiae), which are therefore paraphyletic .

Genera and species

Pegea confederata on Azerbaijani postage stamp

There are 14 valid genera with almost 50 species:

literature

Individual evidence

  1. ↑ a b c d Wilfried Westheide & Reinhard Rieger: Special Zoology Part 1: Protozoa and Invertebrates , 1st Edition, Spectrum Academic Publishing Heidelberg • Berlin, 1996, page 851. 852, ISBN 3-437-20515-3
  2. ↑ Thomas Stach & JM Turbeville. Phylogeny of Tunicata inferred from molecular and morphological characters. Molecular Phylogenetics and Evolution 25 (2002) 408-428, doi : 10.1016 / S1055-7903 (02) 00305-6

Web links

Commons : Salpidae  - Collection of images, videos and audio files
  • World Register of Marine Species: Salpidae