Olabarria, C., Gestoso, L., Lima, F. P., Vázquez, E., Comeau, L. A., Gomes, F., … Babarro, J. M. F. (2016). Response of two mytilids to a heatwave: The complex interplay of physiology, behaviour and ecological interactions. PLoS ONE, 11(10), e0164330. doi:10.1371/journal.pone.0164330
Invasive species are often touted as being destructive to environments; they barge in uninvited, set up shop, and often thrive to the detriment of other organisms. As with most “problems” though, subtle and unexpected interactions can often be overlooked. This is exactly what could be playing out between two species of mussel off the coast of Spain.
Mussels live in the intertidal zone—an extreme environment where inhabitants have to cope with large temperature fluctuations, desiccation (water loss), and variable salinity, among other physiological challenges. For colonial organisms like mussels, living in close quarters adds another complication to overcoming these challenges. At the center of an aggregate, temperatures can spike beyond lethal limits, nutrients can be limited due to uptake by the animals on the outskirts, and local water chemistries can change as stressed organisms expel waste. These conditions make it challenging enough to survive, but what happens when a resilient invader shows up?
Celia Olabarria and her colleagues decided to investigate, using a series of experiments to monitor respiration, heart rate, and heat-shock protein levels in mussels during high and low tide scenarios. The key players were Xenostrobus securis, the invasive Australian mussel, and Mytilus galloprovincialis, a commercially important species native to Spanish coastlines. Mytilus’ susceptibility to higher temperatures was already causing concern given global warming, but the addition of X. securis increased uncertainty about the commercial species’ future.
Olabarria approached these questions using traditional and futuristic approaches. To monitor the temperature fluctuations in the intertidal zone, her team used “robo-mussels”—basically data loggers housed within mussel shells and planted within different areas of a mussel aggregation. The variability was immense: 16º C when submerged and up to 45º C when exposed during low tide (that’s 61-113º F for those not metrically inclined)! With this data in hand, laboratory experiments were then designed where collected mussels were arranged into single and mixed-species aggregates and subjected to simulated heat waves during low tides. Throughout the lab experiments, mortality, respiration, heart rate, water loss, production of heat-shock proteins, and gaping behavior (intermittent opening of the shell) were monitored.
Results and Big Picture
There were three physical processes or behaviors that were incredibly important: water loss, gaping behavior, and overall mortality. Gaping serves to help stabilize oxygen levels within the mussel, but can also increase water loss during periods of exposure to the air. This behavior, in particular, was of specific interest since Mytilus had never before been described as a gaping species; instead, it had always been listed as one that relied on inefficient anaerobic respiration during low tide. The combination of water loss and gaping behavior is thought to be what contributed to the overall mortality seen in the mussel aggregates. Specifically, single-species aggregates of native Mytilus showed the highest mortality during heat waves, while X. securis coasted through relatively unscathed. However, in mixed-species colonies, Mytilus’ mortality dropped by half (Fig. 1).
But why would a mixed colony with an invasive species result in a drop in mortality for the native animal? Olabarria and her team are speculating it has to do with the gaping behavior. X. securis was known to gape on a regular basis, opening its shells frequently, both during high and low tide scenarios. Even though Mytilus was observed gaping occasionally, the frequency may have been too low to sufficiently change the local environment. In mixed aggregates, the new hypothesis is that X. securis’ gaping results in heat exchange and increases in humidity. This effectively cools the immediate surroundings, increasing Mytilus’ fitness.
There are many more nuanced interactions between these physiological processes, and only the most important results are described here, but this should serve as a reminder that not all “problems” are cut and dry. While invasive species are still a large threat to ecosystem stability, we don’t always know how physiological flexibility will collide with ecological factors. So, what other invasive species do you think might be lending a helping hand as they move into new neighborhoods?