A Tale of Two Parasites

Parasites are fascinating. We might tend to think of them as cunning free-loaders, slowly sucking the life out of their unsuspecting victims but the truth is often far more nuanced and bizarre. Modern parasites can permanently alter minds, castrate their victims to make them more useful, even replace their host’s tongues, all just to get ahead. And they’ve been doing this for a very, very long time. For example we now have a record 300 million years of interaction between species of extinct crinoids and two different parasites that preyed on them. The relationship revealed is intriguing and represents one of the longest parasitic interactions ever discovered in the fossil record. It also proves that parasites have always been a vital driving force in the evolution of life on this planet.

Modern crinoids off the Channel Islands in Southern California. Source: NOAA

To set the scene we first need to discuss what ‘crinoids’ are. Sometimes called ‘Sea Lilies’ this odd group of flower-like organisms are actually echinoderms, close relatives of sea urchins and starfish. They live in the world’s seas, spanning a whole range of habitats from the deepest oceans to coral reefs. Deep in the past they were far more common and diverse then they are today but unfortunately their success has waned over the eons and now there are only 600 species. If that still seems a large number then consider that over 5,000 species are known from the fossil record, mostly from before the time of the dinosaurs. The crinoids at the centre of our story span a time period from the mid-Ordovician to the mid-Jurassic, covering around 300 million years and they seem to have suffered from two distinct parasitic infections.

The first of these is a snail. Its earliest appearance is on Silurian aged fossils and initially it seems to have been fairly harmless. The snails were coprophages, meaning that they ate faeces, and they positioned themselves over the top of the crinoid’s waste chute in order to hoover up any available waste products. Arguably gross but not especially sinister right? Well previous research on these fossils has show that over time the snails adapted their habits and instead of just taking advantage of the crinoids own systems they began actively drilling through the crinoid’s body to tap directly into the animal’s gut and extract nutrients that way. This alone is quite fascinating but now new research, led by the University of Cincinnati, has discovered an additional layer of complexity. During the Devonian (around 360 million years ago) something very strange started to happen to the snails and also to the crinoids they exploited; both began developing pronounced spines. This development only occurred in the species of crinoids parasitized by the snails, over 40 non-host species carried on as usual, sans spines. So what was it about being parasitized by these snails that was causing their hosts to evolve this spiky protection? The answer probably lies in the same reason any animal evolves spines, predation. Making yourself inedible is what sharp spikes are all about and the Devonian period indeed saw a sudden proliferation of fish and sharks which might have been looking for easy meals. It seems though that it wasn’t the crinoids themselves that they were interested in. After all not all crinoid species developed spines at this time. Instead the researchers speculated that it was the snails themselves that predators were feeding on. In some ways the crinoids were merely collateral damage but nevertheless they had to evolve a whole new, elaborate defence system to protect themselves. So here we have whole species sent down novel evolutionary paths by a parasite, but it turns out that these snails might not have been all bad. Enter parasite number two.

Fossil crinoids showing damage from boring worms. Credit: Carlton Brett

This second discovery is a top competitor for the illustrious title of ‘longest parasite-host interaction where the parasite is now extinct’ but superficially it might seem less impressive then the snail. For a start we aren’t entirely sure what the parasite was, because we have no body fossils of it. The only evidence of its existence comes from fossilised crinoids, some of which are covered in tiny holes and pits. In some cases up to 40% of the skeleton has been lost in this way probably as the parasite bored its way through the skeleton either in search of food or a safe home. Although the exact culprit isn’t known there is speculation that it was probably a worm-like organism and it seems to have been infecting the crinoids since the Ordovician (around 480 million years ago). The really interesting thing though is that crinoids parasitized by the snails never have evidence of the worms. It is always one or the other but never, ever both. Since the worm infections appear to have been very common (with some population suffering 50% infection rates) it is hard to interpret this is any way other then that the snails offered some kind of protection to their crinoid hosts. This suggests that far from being an unending misery for the crinoids having the snails might have had an advantage given the obvious destructive power of the worms. Exactly how they might have done this isn’t clear but the connection seems undeniable.

Neither parasite apparently ever endangered its host enough to kill it, or certainly not often enough to effect the survival of the various crinoid species. Instead this demonstrates a long term stalemate between hosts and parasites which enabled the relationships to endure for hundreds of millions of years. The hosts and their attendant parasites also survived several of the major ecological crises which wiped out much of life, although eventually of course both species of parasite went extinct. Hopefully this discovery will open up new avenues of research into the historic importance of parasites and at the very least give us a greater appreciation of these intriguing and complex organisms.

Source: University of Cincinnati press release. Research presented at the Geological Society of America’s annual meeting 2015.

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