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Hooks can be deceiving


Every animal has its own parasites to worry about, but canivorous reptiles and amphibians have to deal with particularly gruesome ones. They can become infected with small, worm-like creatures called pentastomes that live inside their lungs, where they suck blood from ruptured blood vessels. Reptiles pick up the parasite when they eat infected prey.

Pentastomes are true escape artists. Once they realize they've entered a reptile stomach, they use their sharp hooks to claw themselves a way to the victim's lungs. In an experiment where pentastomes were implanted in a gecko's stomach, the parasites invaded the lungs in as little as four hours.

Pentastomes use their hooks to burrow from the stomach to the lungs.

Despite their interesting and somewhat disturbing life cycle, pentastomes have always been a bit obscure. This is not at all surprising, said parasitologist John Riley, if you consider the nature of some their hosts: crocodiles, monitor lizards and various venomous and constricting snakes. With a sense of self-humour, he quipped: "To employ these hosts as vehicles for pentastomes in long-term studies is a particularly esoteric branch of parasitological research, with relatively few (one?) adherents!"

Aside from their hosts, the pentastome themselves are also problematic to handle. When a living pentastome is even slightly punctured, it deflates and contracts. For these and other reasons, parasitologists identify pentastomes using differences in the size and shape of the hooks. What pleads for using the hooks is that they are rigid and tough, and can easily be removed and measured.

But hooks don't tell simple stories. Hook length and shape can vary within a single species, just like the length and shape of humans varies. Small differences in other body parts are also used for identification, such as male genitalia, but these measurements are more prone to errors if two specimens haven't been treated in the same way.

There are more than enough examples where pentastomes were misidentified. A parasite from Taiwan first described as a new pentastome for example, only to be reassigned to an pre-existing species one year later. Twenty years later it turned out this second assignment was wrong too: the parasite was a member of a different species altogether.

In a recent paper in PLoS ONE, parasitologists have uncovered another case of mistaken identities. In a parasite survey of cane toads, they came across pentastomes with two kinds of hooks. Some were sharp, others blunt. The pentastomes with sharp hooks were known to infect amphibians, but the blunt-hooked pentastomes had only been seen in lizard lungs before. Indeed, the team also found these parasites inside the Asian house gecko.

If the researchers had followed taxonomic guidelines, they should have classified the pentastomes as belonging to two different species, Raillietiella indica for the sharp-hooked parasites and Raillietiella frenatus for the blunt ones. But the team suspected that something else was going on. A DNA analysis confirmed their hunch. All the parasites, with sharp and blunt hooks, from toads and geckos, were genetically identical to each other. This suggested that these parasites are not two species, but one.

A pentastome that has retained its younger, sharper hook by accident.

One strange pentastome provided additional evidence for this idea. By chance, this parasite had retained the hooks of previous molts. The oldest of its hooks were small and sharp, while the youngest were broad and blunt. So as the pentastome matures, the size and shape of their hooks change. Raillietiella indica is just an early stage of Raillietiella frenatus.

The team also discovered that body size and hook shape were correlated. In other words, larger pentastomes also tend to have blunter hooks. Combined with the observation that pentastomes grow to larger sizes in geckos than they do in toads, and it becomes easy to see why these parasites appear to be of different forms.

John Riley already warned for potential confusion that hooks can sow in his 1986 overview of pentastome biology: "In practice hook data can only be meaningfully compared between fully adult specimens, and a major obstacle arises in deciding what constitutes the adult stage." He hit the hook on its head.


Pentastome images from reference.


Kelehear C, Spratt DM, Dubey S, Brown GP, & Shine R (2011). Using combined morphological, allometric and molecular approaches to identify species of the genus raillietiella (pentastomida). PloS one, 6 (9) PMID: 21949796

The views expressed are those of the author and are not necessarily those of Scientific American.

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