A tale of cuckoo(fish)

by Gauri Gharpure – We all have heard of the famous cuckoo bird (koel in Hindi). The male is jet black and is famous for singing at the advent of spring. The female is infamous for laying eggs in others birds’ nest (especially the house crow). The cuckoo’s eggs then develop quickly and hatch before the host’s and the chicks compete (often aggressively) with the host’s chicks for food. They eventually fully develop and leave the nest big and healthy, without so much as a thank you to the host. This phenomenon is known as brood parasitism, in which an animal parasitizes the nest of a host individual by depositing egg(s) in its nest/brood. The host, often unknowingly, incubates the eggs and raises the parasite’s offspring, many times at the cost of its own offspring(s) as the parasite’s offspring develop faster and aggressively compete for parental care. The host shows some behaviours to avoid being parasitized (like increasing vigilance, removing suspicious eggs, etc.), but like most host-parasite interactions, it is a co-evolutionary arms race mostly skewed towards the parasite.

There have been reports of several birds (cuckoo, cowbirds etc.) and insects (some beetles, butterflies, etc.) that exhibit brood parasitism. I recently came across an interesting case in which a fish species exhibits this behaviour. It is aptly name cuckoo catfish (Synodontis multipunctatus) and it parasitizes a species of cichlid fish (Simochromis diagramma) in Lake Tanganyika in Africa. It is the only known non-avian vertebrate brood parasite and has a really interesting way of going cuckoo.

The cichlid species that are parasitized here perform mouth brooding, in which both sexes first release their gametes in the water during courtship and the females pick up the fertilized eggs from the water into her mouth, incubating them there until they hatch. They thus show parental care of the brood and this is exactly what the cuckoo catfish, the parasite, exploits. The catfish have been known to “hang around” where the cichlid fish are courting and wait for them to spawn (release their gametes in the water). The cichlid fish swim in circles doing so and the catfish use this chaos to slip in some of their eggs in the arena. Thus, when the female cichlid goes around quickly collecting her brood of eggs, she inadvertently picks up the parasite catfish’s eggs too. The catfish eggs then develop inside the mouthbrooding females at a much faster rate than the host eggs and hatch into young catfish. The juvenile catfish feed on the yolk sac in the eggs of the host cichlid’s eggs, and that often results in the death of the entire brood of the host eggs.

A study by Polacik, Reichard, Smith and Blazek in 2019 showed another trick that this parasitic catfish has up its sleeve. When the female cichlid fish swims to pick up the fertilized eggs as we saw before, she takes in some catfish eggs too. Not all catfish eggs are picked up in one go but they have been shown to remain on the substrate, and the eggs continue to develop. When the first brood in the female cichlid’s mouth completes the development and she releases them back in the water, the female again goes around looking for stray eggs and this is where the catfish eggs show a remarkable trick: the ability to reinfect the host. The eggs which were developing earlier tend to show attraction towards the female cichlid and she picks up these eggs for brooding. Thus, the single batch of catfish eggs can infect the host cichlid multiple times and thus ensure maximal survival.

We are just unravelling the amazing and bizarre ways these catfish exploit their host cichlid fish and the host’s efforts to prevent them from succeeding. This study system offers a myriad of interesting questions about the choice of host by the parasite, host recognition, accelerated development in the parasite eggs, the spatial nature of this interaction and so on. Whatever the findings are, I am sure they will make for a very interesting tale about the cuckoo catfish and the cichlid fish and their arms race over space and time to get the upper hand.

References:

  1. Reichard, M. (2019). Cuckoo catfish. Current Biology, 29(15), R722-R723.
  2. Polačik, M., Reichard, M., Smith, C., & Blažek, R. (2019). Parasitic cuckoo catfish exploit
    parental responses to stray offspring. Philosophical Transactions of the Royal Society B,
    374(1769), 20180412.
  3. https://www.nationalgeographic.com/animals/article/fish-parasites-cuckoo-catfish-cichlids-
    africa-news

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