Episode 102 – Parasites

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It may not sound appealing, but without a doubt one of the most successful of all life strategies is to siphon, steal, and exploit resources from other living things. In this episode, we discuss the evolution, deep history, and extraordinary diversity of Parasites.

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The oldest chemical evidence for animal life might not be so clear-cut

Parasitism: Mooching for a Living

Parasitism is a type of symbiosis. But it’s not the friendly kind of symbiosis like bees pollinating flowers or remoras cleaning off sharks; parasites specifically harm their hosts. They’re not quite predators, which kill and eat prey, and they’re not quite pathogens, which cause disease (although, as usual, the boundaries of these terms can get blurry). Parasites get by on siphoning or stealing resources from other organisms.

Top left: leeches are blood-sucking ectoparasites. Image: GlebK, CC BY-SA 3.0
Top right: parasitoid wasps like this one lay their eggs inside living hosts. Image: CSIRO, CC BY 3.0
Bottom left: this insect is infested with Ophiocordyceps fungus. Image: Erich G. Vallery, CC BY 3.0
Bottom right: Earth’s largest flowers, corpse flowers are parasites of grape vines. Image: Henrik Ishihara Globaljuggler CC BY-SA 3.0

It may seem a bit foreign to us humans, but parasitism is an extremely successful life strategy. By some estimates, around half of all living organisms could be considered parasitic. Parasites might live inside their hosts (think roundworms) or outside (think ticks); they might spend most of their time on one host (like lice) or require multiple hosts throughout their lifetime (like lancet flukes); some are parasitic all the time, while others only dabble occasionally; parasites can be animals, plants, fungi, protozoan, bacteria … even viruses could be considered cellular parasites.

Many parasites take nutrients directly from the host, such as by sucking blood, while others might make use of a host’s nest or food stores (like dewdrop spiders). Some parasites just outright steal things, like the gulls that swipe your fries on the beach (these are called kleptoparasites), and others even parasitize their host’s behavior, like the barnacles that force crabs to care for their baby barnacles or the cuckoo birds that force other birds to raise their chicks. And of course, there are parasitoids, most famously the many species of wasps that lay their eggs inside living hosts that serve as babies’ first breakfast when they hatch.

Paleo-Parasites

Most parasites are small and squishy, and most parasitism is very behavior-based, so identifying parasitism in the fossil record can be tricky. But it’s not impossible. Fossil parasites might be identified by their morphological adaptations for parasitism, by their association with their host, or by evidence left behind on the host itself. You might remember news from not-too-long ago of the discovery of the ancient lice caught in amber along the dinosaur feathers they were feeding upon or the fossilized fly pupae with parasitoid wasps inside or older news like the tapeworm eggs in ancient shark poop.

Unsurprisingly, evidence of parasitism goes back about as far as evidence of ecosystems themselves. Perhaps the oldest known parasites are tubeworms from the Early Cambrian that seem to have lived on the shells of brachiopods, stealing their food as they sucked it in.

Path to Parasitism

Parasitic behavior has evolved innumerable times in life history, and there are no doubt innumerable evolutionary paths to parasitism. Some, like parasitoid wasps and dewdrop spiders, might have begun as predators before turning to parasitizing their prey instead. Others might have begun as scavengers and later evolved to eating pieces off of living prey.

One common evolutionary feature of parasites is co-speciation, a phenomenon where parasites evolve new species at the same rate and timing that their hosts do. After all, species evolve as their environment changes, and for many parasites, their host is their environment. This pattern has been seen, for example, in chewing lice whose evolutionary tree looks very much like the evolutionary tree of the pocket gophers they live on.

So Much More

Parasite Evolution: Here’s How Some Animals Became Moochers

Some technical references:
Parasitoid evolution in insects
Evolution of trophic transmission – how do parasites come to rely on their hosts being eaten by predators?
Evolution of parasitism in plants

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If you enjoyed this topic and want more like it, check out these related episodes:

• Episode 59 – Bats
• Episode 133 – Symbiosis
• Episode 70 – Convergent Evolution
• Episode 61 – Behavior in the Fossil Record

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