Origins and Nature of Parasitism

Collinearity

Two or more variables which have an influence on each other, e.g. length and weight

What is the difference between parasitism

commensalism, and symbiosis?, Parasitism benefits one organism at the cost of another; commensalism benefits one with no effect on the other; symbiosis benefits both.

What is an example of a commensal relationship in marine animals?

The sea anemone and the hermit crab — the anemone benefits by feeding near the crab, while the crab is unaffected.

What is the pea crab's relationship with mussels?

Generally commensal — it lives inside mussels with little impact unless the mussel is stressed, at which point it may become parasitic.

Why are lice on damselflies not parasitic in some contexts?

They use the fly for transport (phoresy), not feeding — so they act as commensals or hitchhikers, not parasites in that phase.

How do ocellaris clownfish and Ritteri sea anemones demonstrate mutual symbiosis?

The clownfish protects the anemone from predators, while the anemone's stinging tentacles protect the clownfish.

Why doesn't the clownfish get stung by the anemone?

It has a mucus coating that mimics the chemical signature of the anemone, preventing the firing of nematocysts.

Why is the fig-wasp relationship not always mutualistic?

Female figs do not provide egg-laying sites for the wasps — the wasps still pollinate but don't benefit reproductively.

What do fossil records reveal about the ancient origins of parasitism?

Fossils show preserved evidence of parasitism — such as nematodes emerging from fossilised ants and isopods attached to ancient crustaceans.

What is significant about fossilised Bopyridae isopods?

They show a transitional parasitic form: embedded between the exoskeleton and soft tissue, revealing an intermediate between ecto- and endoparasitism.

What is an example of a parasite with both free-living and parasitic stages?

Some fly larvae parasitise humans or animals, while the adults live freely in air or water.

What are gordian worms and how do they manipulate their hosts?

They infect terrestrial insects, then manipulate their behaviour to jump into water, where the worm emerges and continues its life cycle.

How do gordian worms exploit the food web?

After escaping their insect host in water, they can survive being eaten by a frog and re-emerge, continuing their life cycle.

How does the zebra stripe experiment relate to parasitism?

Striped coats deter biting flies, showing how visual patterning may evolve as a defence against ectoparasites.

What role does the MC1R gene play in human evolution?

It regulates pigmentation, and studies suggest it may have evolved in response to parasite pressure, UV light, or thermoregulation.

Why might humans have lost body hair?

To reduce parasite load — hairless skin is less attractive to lice and other ectoparasites, especially in warm environments.

Why is sexual reproduction favoured despite its cost?

It generates genetic diversity, helping hosts keep pace with rapidly evolving parasites.

What is the Red Queen hypothesis?

An evolutionary model where hosts and parasites are locked in a constant arms race, each evolving rapidly to survive.

Where does the term "Red Queen" come from?

From Through the Looking-Glass, where the Red Queen says, "It takes all the running you can do to stay in the same place."

Why do parasites accelerate host evolution?

They evolve quickly due to short generation times, forcing hosts to adapt just to maintain relative fitness.

How does the reproductive mode of a host affect parasite resistance?

Sexually reproducing hosts can reshuffle genes, giving an edge in resisting parasites compared to clonal lineages.

What is Paedocypris progenetica and why is it significant?

It's the world's smallest vertebrate, with sexual dimorphism and reproductive streamlining that may reflect environmental and parasitic pressures.

What kind of environments does Paedocypris live in?

Highly acidic, oxygen-poor peat swamps in Southeast Asia, where ecological constraints may influence parasite exposure and reproductive adaptations.

What is a phenocopy in the context of parasitism?

A phenotype triggered by environmental conditions (e.g., infection) rather than genetic change — mimicking an evolved trait.

Why is it difficult to define parasitism strictly?

Because many relationships shift contextually between parasitism, commensalism, and mutualism depending on life stage, host health, and environment.

What does the concept of host manipulation imply about parasite evolution?

That parasites often evolve specific strategies to control host behaviour in ways that improve their own fitness and transmission.

How can the same species exhibit both commensal and parasitic behaviour?

Depending on host condition or environmental stress, organisms like pea crabs may shift from harmless to harmful relationships.

What is phoresy?

A form of commensalism where one organism uses another purely for transport, without feeding on or harming it.

Why is phoresy not considered parasitism?

The phoretic organism doesn't derive nutrients or cause harm — it only benefits from locomotion.

How does mutualism differ from obligate symbiosis?

Mutualism involves benefit to both but can be facultative; obligate symbiosis means at least one species cannot survive without the relationship.

How do parasites challenge the definition of individuality?

Some parasites manipulate or control host behaviour so completely that the host becomes an extension of the parasite's life cycle.

What is kleptoparasitism?

A form of parasitism where one organism steals food or resources gathered by another, rather than feeding directly on its tissues.

What is an example of kleptoparasitism?

Some birds (e.g. frigatebirds) steal food mid-air from other seabirds rather than hunting themselves.

Why are some parasitic isopods considered "intermediate" forms?

They embed inside host tissue but remain partially external — blurring the line between ecto- and endoparasitism.

What is an endoparasite?

A parasite that lives inside the host's body, often in tissues or organs, and typically has complex life stages.

What is an ectoparasite?

A parasite that lives on the external surface of the host, such as lice, fleas, or ticks.

Why is rapid parasite evolution a threat to conservation?

Hosts in fragmented or stressed ecosystems may not be able to adapt quickly enough to new or shifting parasite pressures.

What role do parasites play in ecosystem dynamics?

They influence population size, host behaviour, and energy flow — sometimes even regulating dominant species.

Why are parasites often overlooked in biodiversity studies?

They are small, cryptic, and under-described — yet may outnumber free-living species and be crucial for ecological balance.

What does the presence of parasitism in fossils suggest about its evolutionary history?

That parasitism is ancient and has evolved multiple times independently across lineages.

Why might parasitism evolve repeatedly across taxa?

It offers a stable energy source (living tissue) and can reduce the need for competitive foraging — providing a consistent evolutionary advantage.