New research suggests the generalist parasite Parvilucifera sinerae is forced to rely on old-fashioned luck when choosing a proper host.

The parasite is attracted to microscopic algae of the dinoflagellate group; but though they're equally attracted to all such algae, they're not always well received.

Some algal hosts are susceptible to the parasitic infection, while others are not. As researchers explain in their new paper on the unique relationship -- published in the journal Frontiers in Microbiology -- the game of choosing a host is similar to Russian roulette.

"If the host is susceptible, it would be as if the trigger has been pulled and no bullet is fired: therefore, the parasite wins, enters the host, reproduces and survives," Elisabet Alacid, a researcher at the Institute of Marine Sciences in Spain, said in a news release. "But if the host is not susceptible, it is like when the bullet is triggered: The host is resistant and the parasite dies."

Researchers say the host-selection strategies of generalist parasites can have a significant effect on local communities. In the case of Parvilucifera sinerae, which is equally attracted to all hosts, the strategy is not really a strategy at all, but the underappreciated ecological effect remains.

"For example, when the community is made up of two highly sensitive species competing for the same resource, the parasite can facilitate the coexistence of both," explained Esther Garcés, the study's director. "This happens if the infection reduces the population of the most competitive host, which can help the less abundant host to increase."

"On the other hand, when a resistant host dominates the community, the parasite population decreases," Garcés added. "The latter will survive at low concentrations, until its preferred hosts become abundant again."

Differences in genetics in both host and parasite can drive adaptive changes within communities.

"Within the same species, differences in genetics may lead to strains more or less resistant to a given parasite, similar to what happens in bacteria developing resistance against antibiotics," Alacid said. "Equally, different strains of the parasite may also show a greater or lesser virulence faced with the same host."

This complicates the use of generalists parasites to control toxic algae blooms. Their seemingly random infection strategy can end up harming beneficial algae species, much like non-specific chemotherapy can kill both cancer and healthy cells.