Timing is everything — parasite survival strategies

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A live monogenean flatworm, Branchotenthes octohamatus, parasitic on the gills of species of shovelnose rays.

Like humans, most parasites do their best to maximise offspring survival and wellbeing. Parasites select hosts that provide adequate food and shelter to allow juveniles to reach adulthood and reproduce. Successful reproduction ensures species survival.

A research project conducted by the Museum’s Parasitology Section and the University of Adelaide involving Dr Leslie Chisholm and Associate Professor Ian Whittington, identified that different parasite species display unique characteristics to ensure their progeny make it through life. The results challenge prior assumptions that parasites only employ simple and limited strategies for survival.

The study investigated survival and reproductive strategies in flatworm parasites living on the outer body surfaces (skin, gills, and cloaca) of southern fiddler rays and shovelnose rays living in waters off Adelaide and other southern Australian regions.

Monogenean parasite from the cloaca of a ray. Image: Leslie Chisholm.

Calicotyle australis, a monogenean parasite from the cloaca of several species of shovelnose rays.

The flatworms are known as Monogenea, meaning ‘one generation’. This name reflects the fact that these fish parasites have a single host lifecycle: adult worms lay eggs that hatch to release an infective larva. The larva must then find and attach to their specific host species within a defined ‘window of opportunity’ to complete the cycle. Otherwise, the larva perishes.

The research was conducted using parasite-infected rays held in experimental aquaria. Three different parasite invasive strategies were identified.

One parasite species laid eggs that hatched only in response to disturbance by movement: a ‘sit and wait’ strategy.

Another species displayed a ‘hedge your bets’ egg hatching strategy, where some eggs hatched spontaneously in response to light periodicity.

A third species laid eggs that hatched with a strong 24-hour rhythm, also referred to as a circadian rhythm.

These different and complex egg-hatching strategies probably evolved over millions of years of parasitism by flatworms of rays.

 

Whittington, I. D. (2005–2007). Marine flatworm parasites of elasmobranchs: a unique model for experiments exploring invasion strategies, biology and specificity to help understand parasitism. Funded by an Australian Research Council Discovery Grant.