Swim bladder worm
| Swim bladder worm | ||||||||||||
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Anguillicoloides crassus |
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| Scientific name | ||||||||||||
| Anguillicoloides crassus | ||||||||||||
| Kuwahara, Niimi & Hagaki , 1974 |
The swim bladder worm ( Anguillicoloides crassus ) is a blood-sucking parasitic nematode that parasitizes in the swim bladder of eels . Originally A. crassus only lived in Japan and only attacked the Japanese eel there . The introduction of the swim bladder worm to Europe makes it a so-called neozoon , which has meanwhile also been held responsible for the decline in the European eel population .
features
The spindle-shaped parasites reach body sizes of up to 4.5 cm in length and 5 mm in diameter. The eel blood in the worm's intestine gives it its typical dark color. Its mouth consists of a ring of teeth that allows it to penetrate the epithelial tissue of the swim bladder wall. The integument is thin, fragile and surrounded by a mucous covering.
distribution
Originally restricted to the Japanese island, A. crassus spread steadily in northern Germany and the Netherlands in the early 1980s due to Japanese eel imports . Further stations were: 1985 the Camargue ( France ); 1986 Po Valley ( Italy ), Denmark and Norway ; 1987 England , southern Sweden and Spain ( Ebro ); 1989 Poland , Greece and Egypt . In 1991 A. crassus was reported in Aalen from the Hungarian Balaton . Since even the Danube , which is not part of the natural range of the European eel, was populated with eels, A. crassus is now also widespread there. From America there are currently no reports of A. crassus infestation in American eel populations , although the European eel and the American eel have their spawning grounds in the same area. Apparently, salt water represents a barrier to spread, also because of the lack of intermediate hosts for A. crassus .
Life cycle
Postembryonic development takes place in four juvenile stages :
1st larval stage
A female produces tens of thousands of eggs. Within approximately 105 microns long eggs and still within the uterus of the female adult animal , the developing embryo , after the segmentation of the egg, first instar. The larvae have a thickened front end. The eggs float freely in the swim bladder lumen after being deposited. During active swimming phases of the eel, the eggs reach the digestive tract of the eel through the process of pressure equalization by pumping air bubbles via the ductus pneumaticus . There they are covered with a mucous membrane.
2nd larval stage
Also in the egg and sometimes in the uterus of the adult, molting takes place in the 2nd larval stage. The exuvia from the 1st larval stage remains as a protective cover. The animals are now around 275–280 µm long. The tail is relatively long, the cuticle has a transverse curl at the base . This ensures good mobility and supports the whip-like snaking movements. Packed in the eel 's droppings, the larvae slip out of the egg shell and are released into the open water with the droppings. Finally, through ingestion, they reach the first intermediate host, the so-called freshwater copepods . There, when the intestinal wall is penetrated and the haemocoel of the intermediate host is penetrated , the exuvia is also stripped off. In the Haemocoel, the animals grow to around 710 µm and moult to the third larval stage after 10–12 days.
3rd larval stage
The infected copepods enter the second intermediate host (e.g. rudders and juvenile fish of various species) through ingestion , so there is a so-called host change . The now approx. 1 mm large parasite has a thorn-like appendage at the mouth opening, which apparently serves for food intake and for locomotion in the tissue of the intermediate host. The eels feed on the second intermediate hosts and are now the ultimate host of the parasite. In the target organ , the swim bladder , the swim bladder worm reaches a maximum body length of 1.35 mm. This is also where molting to larval stage 4 takes place.
4th larval stage
In the last larval stage, the swim bladder worm still has the thorn-like extension. The esophagus , which in the 3rd larval stage makes up 30% of the total body length, is now shortened to 10-15% of the total body length, which is now about 5 mm. now it comes to the first moult.
Pre-adult and adult stages
After the first mature moult (pre-adult stage), which takes place in the swim bladder lumen, the young nematodes reach body sizes of approx. 4.49 mm, their gonads are not yet fully developed and can be recognized as undifferentiated tubes. After further moulting and from a size of 10 mm one speaks of the adult stage. The gonads mature completely until they reach full body size. Male animals reach body sizes of 3.5 cm in length and 2 mm in diameter, female animals 4.5 cm in length and 5 mm in diameter.
Symptoms in the final host
In its natural range, A. crassus is a common parasite that does not have a major impact on the eel populations there. Infestation rates of 10 to 40% are common. In Europe, however, A. crassus has much higher infestation rates of up to 100% and infestation densities of 200 adult nematodes per eel.
Since the second larval stages of A. crassus can theoretically reach the ultimate host, different adaptations took place depending on the stage. The larvae then mostly live in the tissue of the swim bladder and feed on it. It is only in the fully grown ( adult ) stage that A. crassus enters the swim bladder and feeds on hemophagus - i.e. blood . Affected eels show loss of appetite and emaciation. Scar tissue that forms on the skin of the swim bladder can seriously impair its function. As bottom fish, eels are less dependent on the swim bladder in fresh water. In the open sea, on the way to the spawning area in the Sargasso Sea , the eels need the swim bladder as a pressure compensation organ. Due to the scarring, balancing and floating can be so disturbed or completely switched off that the infested eels lose too much energy through active swimming on their long migration. Since they live exclusively on their fat reserves during the migration, they can starve to death before they have even reached the Sargasso Sea, or the energy reserves are no longer sufficient for spawning.
swell
- F. Hartmann (1993): Studies on the biology, epidemiology and harmful effects of Anguillicola crassus Kuwahara, Niimi and Itagaki 1974 (Nematoda), a blood-sucking parasite in the swim bladder of the European eel (Anguilla anguilla L.) . Dissertation at the University of Hamburg, Department of Biology ( full text ; PDF; 1.8 MB)