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Posts Tagged ‘invertebrate’

Parasites are organisms (living things) that require a host organism to survive and reproduce, usually at the host’s expense. Examples are many and include those that live on the outside of their host (ectoparasites e.g. fleas, ticks), and those that live inside their hosts (endoparasites e.g. tapeworms, liver flukes), ranging in size from microscopic viruses that hide inside our cells to large multicellular animals such as parasitic wasps and botflies (if you like gory horror stories, read about the latter here – science is often weirder than fiction!).  

Researchers at Liverpool and Glasgow Universities (1) investigated the nematode Heterorhabditis bacteriophora’s infection of greater waxmoth larvae (nematodes form the Nematoda phylum and are essentially tube-shaped worms, hence their alternative name “roundworms”). Heterorhabditis is an obligate parasite – meaning it cannot survive without a host. As a larval worm, it lives in the soil until it finds an insect larva host to enter. Once inside the insect, Heterorhabditis releases a bacteria species, Photorhabdus luminescens, that kills the insect host and digests it to form a nutrient-rich soup that Heterorhabditis eats. Living off the pre-digested insect, Heterorhabditis matures and reproduces hermaphroditically (i.e. the nematode is both male and female) and the new nematode larvae mature within the dead insect host. Eventually the insect host is devoured and it splits, releasing thousands of new Heterorhabditis larvae into the environment ready to infect another insect host and restart the life-cycle.

While this macabre scene is taking place inside the dead waxmoth larvae, the waxmoth remains potentially attractive to predators, such as birds, because, unlike after a normal death, the larva doesn’t dry out and shrivel. While birds are not affected by Heterorhabditis,  being eaten by a bird would be a big problem for the nematode as it will be killed by the bird’s digestive system.

Robin (thanks to Smudge9000)Heterorhabditis has an ingenious way to avoid this early demise. A few days after Heterorhabditis infects the waxmoth larva, the larva changes colour – it becomes bioluminescent (glows) for a short while, but it also permanently changes to bright pink in colour. Birds have good colour vision, and the research team demonstrated that European robins, Erithacus rubecula, were significantly more likely to choose to eat uninfected waxmoth larvae over infected ones. The team also noticed that if birds did peck at or eat a pink, infected larvae they would later be more likely to choose uninfected larvae, leading the team to suspect that the nematode also makes the waxmoth taste unpleasant. By changing its hosts colour, and reinforcing this colour warning with a foul taste, Heterorhabditis persuades potential avian predators not to eat infected larvae, allowing the parasite to continue its lifecycle in the waxmoth without interference.

References

1. Fenton A et al. 2011. Parasite-induced warning coloration: a novel form of host manipulation. Animal Behaviour 81: 417-422

Further Information

Parasites 
Daily Parasite
Wikipedia
The Life Tree
Aberystwyth University
Nematodes
University of Nebraska-Lincoln
Earthlife
Berkeley University
Wikipedia
Bioluminescence
San Diego Natural History Museum
– National Geographic: why deep-sea creatures glow

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