Mussels on the Move

Mussels can be a dominate organism in many rocky intertidal communities. Unlike other permanent residents, like barnacles and algae that are permanently attached to the rocks, mussels can move short distances by creating byssal threads. The byssal threads are primarily used as a means of attachment, but by creating and attaching new byssal threads and cutting lose from old ones, mussels can pull themselves along rocky shores.

People who walk around in the intertidal may notice, like the authors of a cool article featured in Science, that mussels often cluster in interesting patterns on the rocks. The authors found that the size of the clusters and the pattern of the clusters is consistent among mussel beds (with similar mussel densities), set out to determine why.

They found that if you spread mussels out evenly in the laboratory, they will spontaneously form clusters similar to those seen in the field, even in the absence of the rocky substratum and wave action found in the field. They also found that if you place mussels in various-sized clusters at the start, there is a difference in movement based on initial cluster size. Mussels in small clusters (with 2-8 individuals) and mussels in large clusters (with 128 individuals) tended to move around and rearrange themselves a lot more than medium size clusters. So, mussels really want to be in a cluster of a certain size... but why?

Large clusters have may have a food limitation. With all of their neighbors filter feeding, having to many individuals around you may limit the amount of food you can capture. When the authors squirted food in the middle of large clusters, the individual mussels were pretty content to stay in the large cluster. But if clustering reduces food, why cluster? When placed back out in the field, individuals who were not in a cluster were more likely to be knocked off by wave action.

So, individual mussel will gather together in a cluster, but when the neighborhood gets too crowded they move. By arranging themselves in the rather interesting maze-like pattern of clusters and spaces often seen in the intertidal, individuals get protection from wave action, and have a higher growth rate than densely-packed mussel beds.

Van de Koppel, J., J. Gascoigne, G. Theraulaz, M. Rietkerk, M. Mooij, and P. Herman. 2008. Experimental evidence for spatial self-organization and its emergent effects in mussel bed ecosystems. Science. 322:739-742.

Playing dead

Just read a neat paper on the heritability of death feigning and how it may be selected for in the wild by Miyatake et al. (2004). Death feigning is when a creature pretends to be dead, either by falling off a twig and curling up its legs, or by freezing, presumably to escape predation. The gray death-feigning beetle pictured above can feign death for up to thirty minutes (according to the beetle dealer). What Miyatake and the others wanted to know was, is this death-feigning ability heritable, and does it actually help them escape predation?

So they took 200 red flour beetles and recorded how long they played dead for after touching them with a stick. The 10 males and 10 females who feigned death the longest were used to start a long line, and the 10 males and 10 females who feigned death the shortest were used to start a short line. They then repeated this procedure for ten generations, allowing only the 20 longest feigners and 20 shortest feigners to reproduce each generation.

After ten generations, they found that the long line feigned death for a longer period of time than the short line did. The long line feigned death for over a minute and a half, while the short line only feigned death for about 5 seconds. They also found a difference in the numbers of individuals who actually feigned death. What they saw was 86% of the long-line individuals feigned death, while only 7% of the short-line individuals feigned death.

So now they know that death feigning is heritable and can be selected for or against in nature, but does it actually work to help save them from predators? Will it actually be selected in nature? To find this out, they introduced a predator and recorded survivorship and behaviors of short-line and long-line individuals. What they found was that the jumper spider used as a predator would lose interest in the beetle if it feigned death. So most of the long-line individuals survived (64%), and most of the short-line beetles were eaten (73%).

So if the beetles have predators in an area that act like the jumper spider, you could expect that the beetles in those areas would have long death-feigning times, but in other areas without such predators, you may expect shorter feigning times. Now all you have to do is go out and test that!

Takahisa Miyatake, Kohji Katayamat, Yukari Takeda, Akiko Nakashima, Atsushi Sugita and Makoto Mizumoto. 2004. Is death-feigning adaptive? Heritable variation in fitness difference of death-feigning behaviour. Proceedings of the Royal Society B. 271: 2293-2296

Book review!

I just got finished reading Sean Carroll's Endless Forms Most Beautiful, and I thought I'd give my thoughts on it. This book is a little more complex than Your inner fish, so I would not recommend it to a complete science beginner. For this book, I think it is better to have a bit of a background in science, particularly in genetics. The book does try to use simplifying language to explain some of the genetic concepts, but I personally found it more confusing, and found my self having to stop and think about what the correct terminology was to understand it.



That being said, it is a fascinating book, that goes a bit more in depth about evo-devo. For me, I most enjoyed the talk about the genes behind wing formation, as one of the discussions I have with my students is about determining if insect wings are purely outgrowths of the exoskeleton. But even more fascinating was how all of these genes are regulated at the 'beginning'. Now the genes are turned on or off, by the presence or absences of certain proteins. These proteins are made be genes that are turned on by the presence or absence of other proteins, and so on and so forth. So what creates the gradient conditions of proteins to turn on the initial proteins?


Carroll puts for the idea that this may be due to uneven deposition of nutrients in the egg. I find this idea interesting and wonder if it has actually been looked at. Is this unevenness repeated in every egg laid down by the mother? Or caused by the first division or subsequent ones? Is there a difference in the 'unevenness' causation or how the initial genes are turned on between organisms which experience determinate vs. indeterminate cleavage?

So for me, the book was most interesting in the additional questions it raised. It also showcased just what one can do in the evo-devo field, and how that relates to variety of other disciples, including paleontology. So if you're a bit more curious about development and genetics, I would recommend this book.

Book review!

I just got finished reading "Your inner fish" by Neil Shubin, and I thought I'd give it a little review, for those of you interested in reading it.

Who it's good for: People with little to no biology background who are interested in how scientists come up with ideas and test them. Great for people who may not understand evolution and want to know a bit about the evidence for it and how that evidence was tested. Simplistic descriptions of fantastically elegant experiments, and the implications of the findings. This is an easy read, so much so that advanced junior high students would have no trouble following, but engaging enough to keep any one's interest.

For those of you with a little science background (or more than a little) this book can keep your interest too. It covers fossils, embryology and development, comparative morphology (fossil and current), as well as genetics. It integrates all of these disciplines nicely to tell the story of evolution. I found myself fascinated by the development chapter, so much so that I aim to take a course in it (or do some research, or both).

This would be a great book to use in introductory biology courses, and especially for non-major's biology courses to get at how the scientific process works, and the evidences for evolution. (Something which most non-major's courses cannot get across well)

Who it's NOT for: People who will be offended by the 'dumbing down' of science. If your picky about your word choices, or very exact in how you phrase things, don't pick up this book. Even I had to cringe over the fact that he referred to Amphioxus as a worm for an entire chapter. Every time I read it, I wanted to write in worm-like! But there is a trade-off between clarity and exactness, and for the most part I felt the sacrifices in the language is made up for by the idea that this will reach a broader audience.

The only other thing I feel compelled to note, was that this book, comprehensive as it was, did not go beyond tetrapods often. As he himself mentioned, the book could have been called 'your inner fly or your inner yeast', but rarely did he point out how invertebrate research fit in with examine the origins of our structures. But he did have an excellent list of further readings broken down by topic. I guess I'll have to look into "Endless forms most beautiful" next.