biology zoology blog benno meyer rochow lamprey parasitism

Something Terribly Fishy

But unquestionably true

Speak with a non-zoologist about “parasitic fishes” and the conversation will inevitably come to “murderous” piranhas. Yet, in spite of their reputation these South American freshwater fish are not parasitic, but represent predators with razor sharp teeth and a healthy appetite, which can indeed kill, but aren’t murderous. Predators, by definition, finish off their prey; yes, but parasites are animals that obtain nourishment from another living animal without killing it, like for instance blood-sucking hagfishes and lampreys.

The former are inhabitants of all oceans, grow to a length of almost one metre and attack other fish, occasionally actually “burrowing” into the bodies of their victims. The freshwater lamprey (called Neunauge = 9-eyed fish in German and Yatsumeunagi = 8-eyed eel in Japanese) also sucks blood, but it does not “dig in”. Certain nasty members of South American catfish are in the habit of attaching themselves to bigger fish in order to gorge themselves on the blood of their hosts. One species in particular, known as the Candiru in some areas, is greatly feared by human bathers as the slender fish has the unpleasant behaviour of mistaking urethra, anus, or vagina for the gill openings of a piscine host. Although the danger coming from this species is perhaps somewhat exaggerated, evidence that the fish can get stuck in a human is available here.

The European freshwater family of the bitterlings has developed an unusual form of parasitism. Spawning females deposit their eggs into the gills of a large pond bivalve and the developing fish larvae are being nourished by food particles siphoned by the shellfish from the surrounding water into its mantel cavity. Another peculiar form of fish parasitism occurs in the deep sea angler fishes. Permanently fused to a massive female, the tiny male angler fish obtains all its sustenance via the blood of its spouse to whom it is irreversibly connected and only the male’s reproductive organs remain in top-shape: the rest of its tiny body is allowed to degenerate once he becomes attached to a female. If as a young male he misses to meet up with his “Mrs.”, he is doomed and will die a lonely bachelor’s death.

I have had quite a bit of experience with a group of fishes that are often referred to by Australians as “ass fish” (although they also go by the politer name of “pearlfish”, especially in the USA). These slender, shiny and rather slippery, pencil-long fishes hide, congregate and possibly feed (it depends on the species) inside large starfish and sea-cucumbers. It has been suggested that they sometimes gather together inside their hosts for reproductive purposes, which led me to a remark like “what an orgy it must have been”, when I discovered 15 fish inside a single sea-cucumber! (Incidentally, my 1977 publication on this find in the journal “Copeia” got me into the Guinness Book of Animal Records!). Perhaps the sneakiest of all the fish with parasitic habits is the “False Cleaner”. This devilish fellow mimics in shape, coloration, size and behaviour true cleaner fish, a group of smallish species, which are permitted by bigger fish to enter their mouths and gill chambers in order to remove from there food remains, ectoparasites, etc. But instead of doing this useful “job”, the “False Cleaner” fish, however, take bites out of the host fish’s flesh and then make a quick getaway! Sneaky, nasty fellows, indeed.

© Dr V.B. Meyer-Rochow and, 2020.
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