Wave Forces: Alternative Strategies For Survival

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’.

Littorina keenae, the study organism of Miller et al (2007). These snails tolerate wave forces by surviving dislodgement and returning to the shore.

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.

Excerpt from Puijalon et al (2011) showing drag against tenacity (breaking force). There is a clear negative correlation

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.

Excerpt from Miller et al (2007). Clearly dislodgement does not always mean death for littorine snails.

Works Cited:

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

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Funny little critters: Isopods and their importance

Isopods are the only crustacean to have evolved fully terrestrial species. These species are often seen as household pests. Photo credit: pestcontrolrx.com

It seems like forever ago that I, filled with the curiosity of a child, spent entire days in my back yard digging for “bugs”. With a bucket in one hand and a trowel in the other, I spent countless hours unearthing stones and flipping logs, terrorizing the thousands (if not hundreds of thousands) of sow bugs that called these places home. As it turns out, however, sow bugs are not bugs at all. They are, in fact, isopods: a clade (or group of organisms) more closely related to crabs and shrimp than beetles and fruit flies. Even though I have grown a lot since I was a child, my curiosity about these fascinating creatures has not wilted.

Although the isopods that are most common to the average person are land dwelling and often viewed as household pests, a majority of isopods are marine. In fact, it is in the marine environment that the Order Isopoda evolved at least 300 million years ago (Schram 1970). Marine isopods range in size from a couple millimeters to more than a foot in length and are important scavengers, detritivores and herbivores in near shore and deep water ecosystems alike. Although isopods are readily abundant worldwide, relatively little is known about them, particularly intertidal species that thrive in some of the world’s harshest environments.

The Giant Isopod – An important scavenger in deep parts of the Ocean. Able of growing to more than 36 cm, little is known about these “monsters of the deep”. Photo credit – NOAA

Species of one intertidal genus, Idotea, are important herbivores and detrivores in many intertidal ecosystems (eg. Gunnarsson & Berglund 2012; Orav-Kotta & Kotta 2004). Idotea have hooks that are assumed to improve survival in high wave velocities by allowing for strong attachment to rocks and seaweeds. This hypothesis, however, has never been directly tested. Nor has the effectiveness of these hooks on different substrata.

Idotea baltica – an important herbivore in eelgrass and Fucus communities of the Atlantic Ocean and Baltic Sea. Photo credit: Mark Blaxter

“How does the tenacity (force to dislodge) of Idotea differ between dissimilar seaweeds?” “Does the preferred habitat differ with wave exposure, as a result of this difference in tenacity?” These questions, and more, are currently under investigation by a colleague (see thetransientbiologist) and myself, at Bamfield Marine Sciences Centre in Bamfield, British Columbia.

Using a lab and field study, in concert, we plan to investigate the mechanisms by which one species, Idotea wosnesenskii, survives wave action while foraging effectively.

Follow my blog for more information and for preliminary results!! OR check out http://thetransientbiologist.wordpress.com!

Dislodging Idotea wosnesenskii from various substrata in the lab. Photo credit: Christina Smith (thetransientbiologist.wordpress.com)

Works Cited

Frederick R Schram (1970). Isopod from the Pennsylvanian of Illinois. Science 169 (3948): 854–855.

Gunnarsson, Karl, and Anders Berglund, 2012. The Brown Alga Fucus Radicans Suffers Heavy Grazing by the Isopod Idotea Baltica. Zooekologi

Orav-Kotta, Helen, and Jonne Kotta. 2004. Food and Habitat Choice of the Isopod Idotea Baltica in the Northeastern Baltic Sea. Hydrobiologia 514: 79-85.

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GET A GRIP: How waves affect distributions of intertidal organisms.

Many would agree that the intertidal zone is a beautiful place. However, few truly understand how impressively harsh it can be where the ocean meets the land. Plants and animals that live here are said to experience the worst of both worlds. These organisms constantly compete with each other for space on the shore, while having to cope with the stresses of life BOTH underwater and exposed to air. On top of this, a majority of these organisms live their lives avoiding other animals that actively hunt them, while trying to maintain a “grip” on the intertidal despite being constantly bombarded by waves, some of which apply forces far greater than that of the strongest hurricane. I am often astounded by the fact that ANYTHING can live here.

Wave velocities can reach up to 90 km/h (Denny and Gaylord, 2002). The forces associated with these waves can be immense.
Photo credit: Luke Miller

Have you ever driven down the highway and stuck your hand out of the window? The force you feel pushing your hand is what is known as drag and it increases with velocity, size of the object, streamlinedness and the density of the fluid. As a result, the force felt by an intertidal organism living on exposed shores of the outer coast may frequently experience forces a thousand times that of what they would experience in air at highway speeds.

In order to survive such immense wave velocities, organisms that live in the intertidal have evolved strategies to mitigate the potentially negative consequence of drag. In plants, such as macroalgae and surfgrass, and some animals, there is one very important strategy: BE FLEXIBLE (Koehl 1982)!  By being flexible, these organisms are capable of bending and folding in response to waves (Koehl, 1986; Carrington, 1990; Denny and Gaylord, 2002). This tends to decrease their size and streamline them such that the drag experienced on a regular basis is not enough to damage or dislodge them (Martone et al 2012). This strategy, however, has one key requirement: the organism must be attached, or cemented. This means that flexibility really can’t do much for organisms that move around.

Even seaweeds that produce calcium carbonate are flexible. They possess joints, called genicula, that allow for bending. Without them, these organisms would be completely rigid and likely would not survive without tremendous investment in supportive tissues Photo credit: P. Martone

The purpose of this blog is to report on research being conducted in Bamfield, British Columbia on a mobile herbivore, Idotea wosnesenskii. This peculiar animal is an isopod, similar to the common “wood louse” or “sal bug”. It, however, lives in the intertidal and often experiences the tremendous wave velocities of the exposed, outer coast. Idotea is an important herbivore in many systems and likely utilizes two main strategies to survive: hiding from flow and holding on for dear life. Over the next two months, we will be investigating the mechanisms of these strategies. In doing so, our research will help predict patterns of dislodgement, lend insight into possible consequences of climate change and shed light on the selective pressures of the intertidal zone.

Carrington, E. 1990. Drag and dislodgment of an intertidal macroalga: consequences of morphological variation in Mastocarpus papillatus Kützing. Journal of Experimental Marine Biology and Ecology 139: 185-200.

Denny, M. W. and B. Gaylord. 2002. The mechanics of wave-swept algae. Journal of Experimental Biology 205: 1355-1362.

Koehl, M. A. R. 1982. The interaction of moving water and sessile organisms. Scientific American 247: 124-132.

Koehl, M. A. R. 1986. Seaweeds in moving water: Form and mechanical function. pp. 603-634, In T. J. Givnish [ed.], On the Economy of Plant Form and Function. Cambridge University Press.

Martone, P.T., Kost, L., and M. Boller. (2012) Drag reduction in wave-swept macroalgae: alternative strategies and new predictions. Am. J. Bot. 99(5): 1-10

Idotea wosnesenskii

I. wosnesenskii
Photo credit : inaturalist.org

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