While there are many other tapeworms out there which infect vertebrate animals as their final hosts, what most people might not realise is that aside from the cyclophyllideans, most branches of the tapeworm trees are actually sleeping with the fishes (in particular, sharks), so to speak. Cyclophyllidean tapeworms are notable in that they have evolved to only use terrestrial vertebrate animals (tetrapods) as their final hosts...well, with one notable exception.
|Left: Scolex of Ichthyolepis, Right: Two of the host species, Mormyrus caschive (top), Marcusenius senegalensis (bottom)
Photo of Ichthyolepis scolex from Fig. 2 of the paper, Photos of elephantfishes by John P. Sullivan and Christian Fry
A recently published study described a very unusual species of cyclophyllidean tapeworm, which has made the switch from living in terrestrial vertebrates to living in the gut of a bony fish. To put things into perspective, finding an adult cyclophyllidean tapeworm in a teleost fish is like finding a pig living in the middle of the open ocean. The host of this special tapeworm are elephantfishes (Mormyridae) - a family of small electrical fish found in Africa, which are notable for their ability to generate electric fields, and their unusually large brains.
This species of intrepid parasite has been named Ichthyolepis africana, and the adult tapeworm dwells in the host's intestine, just behind the opening to the stomach, where it hangs in place using its formidable crown of hooks and four muscular suckers. Based on the phylogenetic analyses that scientists have conducted, the closest living relatives of this tapeworm are found in birds - specifically swifts, of all things.
And as if infecting a species of electric fish wasn't enough for this special tapeworm, I. africana was found in not just one, but SIX different species of elephantfishes, distributed across different parts of the African continent, including Senegal, Egypt, Sudan, and South Africa. And wherever they were found, they were present in between 36-63% of the elephantfish population that the scientists sampled. Its ubiquity and abundance shows that Ichthyolepis has had a long and well-established co-evolutionary relationship with this group of freshwater fish.
But how did it get there in the first place? Why and how did the ancestor of this tapeworm make the switch from living in a group of small birds to the gut of electric fishes - two lineages that have been separated by over 420 million years of divergent evolution?
A clue can be found with the animals that host this tapeworm's closest living relatives - which are swifts. Swifts belong to a family of birds called Apodidae, As their name implies, they are swift flyers with fantastic aerial manoeuvrability, which they use to snatch flying insects out of the sky. Tapeworms usually infect their vertebrate final host by having larval stages that develop in the bodies of prey animals that their final hosts feed on. So those insects would have served as marvellous vehicles for tapeworms which infect those birds.
But aerial hunting is not the only way for an animal to eat insects. Any insects that fell into a water body would have made a handy snack for many aquatic animals, and elephantfish - which usually feed on invertebrates such as small crustaceans and aquatic insects - would have eagerly hoovered up those morsels from above.
While most of those tapeworm larvae - which were adapted to the warm, cosy intestine of a bird - would have perish when they ended up in the gut of an elephantfish, an aberrant few might have had mutations which allow them to survive in such a unfamiliar environment, giving them a survival advantage. Over evolutionary time, surviving in an elephantfish's gut might have evolved into a viable alternative pathway to maturity, and the ancestors of Ichthyolepis might have found the conditions inside to be hospitable enough to abandon the bird host, and took up long-term residency in the gut of those electric fishes.
This type of host-switching or host-jumping across quite disparate host animal lineages has happened in other parasites too. In 2017, I wrote a post about a thorny-headed worm which has established itself in both seals and penguins - simply because they feed on the same prey (fish). Despite being in completely different classes of vertebrate animals, they were exposed to the same parasite via what they ate.
To some people, it may seem that spending your life living inside the body of another animal would relegate you to an evolutionary dead end. But the evolutionary histories of many different parasite lineages tell an entirely different story. It seems that when the right opportunities present themselves, parasites have often been ready to seize the moment, and make an evolutionary leap to take on new hosts, and beyond.
Scholz, T., Tavakol, S., & Luus-Powell, W. J. (2020). First adult cyclophyllidean tapeworm (Cestoda) from teleost fishes: host switching beyond tetrapods in Africa. International Journal for Parasitology 50: 561-568