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The creative power of nature is incomprehensible. For every way of living there seems to be a creature, and every creature is molded, shaped and polished to fit in its niche in the most optimal way. A swimming and jumping dolphin gives us an impression of that power of adaption: an elegant mammal that shoots through the water and is able to catch the evenly well adapted fast fishes.
The fish louse Argulus presented here is another example of beautiful adaption to its niche, which is: to parasitise on fishes. Yet we shudder with disgust as we take a closer look: this not a pleasing animal. To begin with: all humans have an instinctive repulsion against parasites. And than it has a flat body. And frightening hooks to attach itself to its prey. Even more horrifying: the feeding proboscis with its razor sharp needle.
On the right picture you see the ventral hooks with which give the creature a good grip that prevents sliding backwards. Finally, for more grip it has two strong suckerdiscs.
A dish formed shell covers its body, and with the hooks and sucker discs the attach him brick fast on the fish, like a false scale. This with a preference for a place just behind the side fins, to prevent the unfortunate fish from scraping him off. By the alternating release of suction power on either disc, the creature may shamble over its victim and stab the snout in whatever spot looks favourable. There it pumps in digestive enzymes in the fish meat, and after that the nourishing fluids are sucked in, much in the same way that predator bugs do this. So you could call the fish louse a predator with a prey which is much larger then itself and kept alive. But animals with that kind of feeding habits are called parasites, which in a strict sense is not correct, because this word comes from the Greek: para=beside, together en sito=food, feeding, so a parasite should be something that feeds on (part of) the food which is gathered by something else. The tapeworm is such a real parasite, because it uses part of our food for himself. The tapeworm is an example of an endoparasite, or internal parasite, while the fish louse is an exoparasite, or external parasite. Besides, the fish-louse is of course no louse, but a highly adapted crustacean.
None of this knowledge will have even the slightest interest from owners of expensive fish, who only want to know how to kill it as quick as possible. In the summer, Argulus can be a real plague, fish have been found that carried several hundred of them. Of course in fish farms they can multiply very fast and lead to great damage. The streamlined shape shell of the parasite makes it almost impossible for the fish to scrape it off, but it also seems reponsible for the fact that Argulus infected fish are not hampered in swimming. The fish do swim slower though, but that's because of the loss in power and possibly infections on the perforated skin.
WEBLINK:
The cost of infection:
Argulus foliaceus and its impact on the swimming performance of the three-spined stickleback
(Gasterosteus aculeatus).
Stewart A, Hunt R, Mitchell R, Muhawenimana V, Wilson CAME, Jackson JA, Cable J. 2018. Data
from: The cost of infection: Argulus foliaceus and its impact on the swimming performance of
the three-spined stickleback (Gasterosteus aculeatus) Dryad Digital Repository.
(10.5061/dryad.g69c080) doi:
10.1098/rsif.2018.0571