Why no brood-parasitic amphibian?
(writing in progress)
Brood-parasitism occurs in birds and fishes, but not in mammals or amphibians.
I am unsure how many fishes use brood-parasitism. However, in this Post I would like to provide a few details of one well-known example.
In Lake Tanganyika, there is a radiation of Cichlidae (https://www.inaturalist.org/observations?place_id=any&taxon_id=54196&view=species): freshwater fishes that, in many cases, mouth-brood.
Mouth-brooding means that shortly after being spawned and fertilised the eggs are taken into the mouth of a parent, where they are protected and aerated until hatching. Since cichlids show parental care in this way, there is an opportunity for brood-parasitism.
The spotted catfish (Siluriformes: Mochokidae: Synodontis multipunctatus, https://www.inaturalist.org/observations?place_id=any&taxon_id=188522&view=species) is a fish paralleling the cuckoos among birds.
This is because it shows at least three behaviours detrimental to the reproduction of the cichlid host but beneficial to its own reproduction at the expense of its hosts. It brood-parasitises at least six species of cichlids naturally, particularly Ctenochromis horei (https://www.inaturalist.org/taxa/98741-Ctenochromis-horei) and Simochromis babaulti (https://www.inaturalist.org/taxa/611661-Pseudosimochromis-babaulti), and can artificially (in the aquarium) be made to brood-parasitise others, e.g. species from Lake Nyasa where the spotted catfish does not occur.
The original reference for this brood-parasitism appeared in Nature, vol. 323, page 58, in 1986. The Wikipedia site for Synodontis multipunctatus (https://en.wikipedia.org/wiki/Synodontis_multipunctatus) provides a useful summary.
The spotted catfish grows up to about 15 cm. When the host cichlid spawns, the parent spotted catfish eats some or many of the eggs, thus directly killing progeny of its host at first emergence of these progeny. Then the spotted catfish spawns and fertilises its own eggs, near the host fish. The host fish takes up the eggs, obviously intending to take up only its own eggs, into the mouth.
However, the two actions performed by the spotted catfish by this point ensure that many of the eggs taken up by the host are actually eggs of the spotted catfish. The eggs of the spotted catfish tend to hatch early inside the mouth of the mouth-brooding host species of cichlid, and the baby catfish predate the remaining eggs or hatchlings of the cichlid.
The parent cichlid does not reject the eggs or babies of the spotted catfish, despite the facts that
- the spotted catfish parents have eaten cichlid eggs,
- the spotted catfish parents have spawned their own eggs in full sensory ‘view’ of the cichlid parent,
- the spotted catfish hatchlings kill the cichlid offspring inside the cichlid’s own mouth, and
- the spotted catfish offspring are different from the cichlid’s own young.
These sequential failures by the cichlid parent, to detect or to react or both, ensure that much or most of the subsequent parental care by the cichlid is devoted to the catfish babies, not its own babies. The actual number of spotted catfish babies remaining in the cichlid’s care may be as few as one.
Whatever their number, they use the host’s mouth for shelter. They venture out to forage for themselves, as would the cichlid’s own offspring, had they survived.
Compared with birds, there is some analogy between the bird nest and the brooding mouth, as a prerequisite for brood-parasitism. There is also analogy in that
- in neither the bird nor the fish does the host reject the brood-parasite’s young; instead, the host allows its reproductive efforts to be hijacked, and
- a number of parasitic offspring are cared for at once.
(writing in progress)
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