As I have previously mentioned (GET A GRIP), organisms that live in the rocky intertidal are affected substantially by wave action. While many organisms have evolved tactics to avoid drag and dislodgement, this is likely not the only strategy. Instead, organisms can tolerate wave forces, regardless of how strong!! It is a long-standing belief among evolutionary biologists that redundant strategies tend to be negatively correlated. This is presumed to be due to the fact that survival strategies are often costly to the organism (metabolically or some other way) and, so, it would be impractical to invest in unnecessary strategies. In wave or current swept habitats, this concept translates to what has been described as an ‘avoidance-tolerance continuum’, in which organisms do not engage heavily in both avoidance and tolerance strategies at the same time. In this blog post, I will be discussing the results of two somewhat recent studies that address this phenomenon of drag or dislodgement ‘tolerance’.
Research recently conducted in France, on aquatic plants, examined this avoidance-tolerance continuum (Puijalon et al 2011). They measured drag (the force experienced at a given velocity, see GET A GRIP), and tenacity (the force required to dislodge the plant from the substrata) of a variety of current-swept species. They then plotted drag at one velocity against tenacity and found that strategies were negatively correlated. This lends support to the avoidance-tolerance continuum and is consistent with previous studies on herbivory that also demonstrate the existence of this continuum. Drag tolerance, however, is not the only means of tolerating wave forces.
In a study conducted in 2007, researchers at Stanford University (Miller et al 2007) asked the question “Does dislodgement always mean death?” This is an interesting thought and somewhat of a unique one. To address this question, the authors used a small intertidal snail, Littorina keenae. In this study, snails were collected, painted with brightly coloured nail polish and released. Snails were then either left as a control, or artificially dislodged. The researchers found that, depending on wave-exposure, 54-90% of snails survived dislodgement and tended to return to their original tidal height prior to dislodgement. This is fascinating but offers many more interesting questions:
Are mobile species that are not killed by dislodgement less likely to invest in adaptations that increase tenacity? Does the avoidance-tolerance continuum exist at all in mobile species? Clearly there is still a substantial lack of knowledge about how mobile invertebrates survive wave action.
Miller, L. P., O’Donnell, M. J., & Mach, K. J. (2007). Dislodged but not dead: survivorship of a high intertidal snail following wave dislodgement. Journal of the Marine Biological Association of the UK, 87(03), 735. doi:10.1017/S0025315407055221
Puijalon, S., Bouma, T. J., Douady, C. J., van Groenendael, J., Anten, N. P. R., Martel, E., & Bornette, G. (2011). Plant resistance to mechanical stress: evidence of an avoidance-tolerance trade-off. The New phytologist, 191(4), 1141–9. doi:10.1111/j.1469-8137.2011.03763.x