To gape or not to gape? Some mussels’ choices influence their place in a habitat
The segregation of habitat between native and invasive species often comes down to a competition between their physiological and behavioral abilities. This is especially true in habitats prone to frequent change; as both indigenous and invasive species respond to environmental variations in a habitat, it’s the difference in their responses that can determine their success or failure.
In South Africa, the indigenous mussel Perna perna (below, left) seems to have the odds stacked against it. Its coastal ecosystem is under heavy fire from invasive species, it’s subjected to variable, extreme environmental conditions in its intertidal home and its behavioral repertoire is more than a little limited. What’s a mussel to do? Really, the only thing it can do: open and close its shell (“gaping”). Turns out that this simple behavior has a strong influence on the outcome of the mussels’ turf war.
The Mediterranean mussel Mytilus galloprovincialis (below, right) is one of the world’s most widespread marine invasive species andcan be found all over the northern and southern hemispheres’ temperate zones. Having found its way to South Africa in the late 1970s, it slowly branched out along the entire west coast and has now spread along 800-900 km of the south coast, too. There, it shows partial habitat segregation with the P. perna in the lower eulittoral zone, or mussel zone, where P. perna typically dominates the lower zone and M. galloprovincialis dominates the higher mussel zone, with some overlap.
The bivalves are regularly covered and uncovered by the changing tide and endurea steady rhythm of wet and dry conditions. When the outgoing tide leaves them high and dry, the mussels have two choices. They can keep their valves closed, which minimizes water loss, but requires them to use anaerobic metabolism (a way for an organism to produce usable energy in the form of ATP without the involvement of oxygen; it’s basically respiration without oxygen). Alternately, they can open and close their valves, which maintains a more efficient aerobic metabolism (energy creation that uses oxygen), but opens them up to (no pun intended) to water loss and the risk of drying out.

Mussels of each species and from each zone were exposed to air at two different temperatures by Katy Nicastro, Gerardo Zardi (CCMAR, CIMAR-Laboratorio Associado at Universidade do Algarve in Portugal), Christopher McQuaid (Department of Zoology & Entomology at Rhodes University in South Africa), Linda Stephens, Gregory Blatch (Department of Biochemistry, Microbiology & Biotechnology at Rhodes University) and Sarah Radlof (Department of Statistics at Rhodes University) in three experiments conducted to observe gaping behavior, water loss and mortality due to dessication. The two species took very different approaches to air exposure. M. galloprovincialis did not show gaping behavior at either temperature, while P. perna showed gaping at both temperatures, with an increased number of gaping individuals and of number of gapes per hour at the higher temperature. Consequently, water loss rates were higher for P. perna than for M. galloprovincialis (average loss of 21% and 4% of total body water, respectively) and while water loss was greater for both species at the higher temperature, P. perna’s water loss rate was much steeper when the temperature was increased. P. perna likewise had higher mortality rates in the desiccation experiment than M. galloprovincialis, but the invasive mussels did show a greater production of stress proteins related to anoxic stress.
Gaping, as simple as it seems, has a profound effect on the segregation of habitat between the native and invasive mussels. While gaping may relegate P. perna to the lower area of the mussel zone, it doesn’t exactly get stuck with a raw deal. It’s greater attachment strength allows it to withstand greater hydrodynamic stress than the invasive mussels that might venture into the zone. P. perna initially aids the survival of M. galloprovincialis in the lower zone by providing protection against waves, but eventually excludes it competitively in the long run and takes the lower zone all for itself. Meanwhile, keeping their traps shut condemns the invasive M. galloprovincialis to more stress and a less efficient metabolism (the end products of which can be toxic or lethal if left to accumulate), but minimizes water loss and allows it to make itself at home in the upper mussel zone, where gaping P. perna can’t survive or compete with it. Territory gets divvied up and both invaders and natives find a niche for themselves based on the simple act of opening up, or not.
Reference: Nicastro KR, Zardi GI, McQuaid CD, Stephens L, Radloff S, & Blatch GL (2010). The role of gaping behaviour in habitat partitioning between coexisting intertidal mussels. BMC ecology, 10 PMID: 20624310
Images: Mytilus galloprovincialis with Symplegma reptans living on it, by Flikr user Jay Vavra. Perna perna from Collection Georges Declercq, via the World Register of Marine Species. Both used under a Creative Commons license.






The final experiment tested whether or not passive touch experiences could affect decision-making like active manipulation of objects had. Eighty-six participants were “primed by the seat of their pants” and sat in either hard wooden chairs or soft cushioned one while completing an impression formation task similar to the previous experiment and a negotiation task. This negotiation had participants pretending to shop for a new car (sticker price $16,500) and making two offers on the car (the second assuming that the dealer rejected the first offer). Comparable to the previous experiment, people who sat in the hard chairs said the employee was more stable than did participants who sat in the soft chairs. In the negotiation, hard chair participants changed their price between the two offers by a lesser amount than the soft chair participants did, suggesting that a haptic mindset can be triggered even when touch occur in body parts beside the hands and even when an object is not being actively manipulated.
Now, the best part of all of this is that only a few days before the Gourmet news broke, I received a sample issue of Cook’s Illustrated in the mail. If you want thoughtful, considered editorial of the type that Kimball talks about, I suggest you run screaming in the other direction. Keith Dresser’s (obviously an “expert created from the top down and with a lifetime of experience”, otherwise he would not have made it onto Mr. Kimball’s hallowed pages) “How to Pan-Sear Shrimp,” insists that shrimp can be caramelized. This is wrong and happens to be a pet peeve of mine. The browning that happens when you pan sear shrimp, or a burger, or grill a steak, etc. isn’t caramelization at work, but the Maillard reaction, a complex series of chemical reactions that occur when the carbonyl group of a reducing sugar reacts with the amino group of an amino acid, usually in the presence of heat. This non-enzymatic browning results in an array of molecules and compounds responsible for positive and negative flavors and odors. In layman’s terms, it’s the chemical reaction that gives your meat that wonderful brown, flavorful crust.
The results of the Maillard reaction (named after Louis-Camille Maillard, the French physician and chemist who was the first person to describe it) often look and taste the same as those of caramelization, but they’re two very different processes. The Maillard reaction involves both reducing sugars and amino acids, while caramelization involves only sugars undergoing various chemical reactions (among them, sucrose inversion, intramolecular bonding, isomerization and dehydration, condensation, fragmentation and polymerization reactions).
