Breaking the Size Barrier – Giant Bacteria, part 1

If you double the size of a cell in each direction, the volume 

increases eight fold. This makes take eight times longer 

for a molecule to diffuse through the whole cell.

Last week we talked about how the reactions that must take place inside cells often limit the maximum size of bacteria. Because important molecules can reach every part of the bacterial cell only by diffusion, the organism can’t have too large a volume. At the same time, the bacterium needs as much surface area as possible for important molecules to diffuse into the cell. This means that they need a high surface area: volume ratio. We showed last time that if you double (2x) the length of a bacterium in three directions, then the volume is increased eight fold (8x). This would result in cubing (2³=8) the mixing rate and traffic time as well. If the size of a bacterium was increased from a typical size of 1 µm to a theoretical 100 µm bacteria, it could take almost a day for two molecules to find one another (traffic time). It wouldn’t seem plausible that bacteria this size could remain alive.

HOWEVER, I want to show you two bacteria that have found ways around this size limitation. Even more impressive (and a sign of how inventive nature can be), each of these organisms has found a different way to beat the system. Our two examples are the two largest prokaryotes known, and can be seen by the naked eye. This is really something considering that we can’t see our own cells without a microscope.

Our first size offender is called Thiomargarita namibiensis (T. namibiensis). The thio- part of the name means that this is a sulfur oxidizing bacterium, while the last part of its name records that it was first found on the ocean floor just off the coast of the African country, Namibia. Sulfur bacteria change elemental sulfur (S₀) into sulfur oxides (SO_2-4). These reactions release enough energy to make ATP (the chemical energy of the cell). In order to carry out these oxidation reactions, some sulfur bacteria use nitrate as an electron acceptor during ATP production. This works out just fine when there is a lot of nitrogen present in the immediate environment, but at the bottom of the ocean this is not always the case. Most of the nitrogen comes within reach of the bacterium only after a storm disturbs the ocean floor.

Our “sulfur pearl of Namibia” bacterium (arrow) is as big 

as the head of the fruit fly. To compare, each 

eye of the fruit fly contains over 16,000 cells!

Therefore, T. namibiensis must scavenge as much nitrogen as possible and store it within a large central vacuole (a membrane bound sac) for the lean times. It also stores sulfur in smaller granules, leading to a speckled pearl-like appearance over the clear nitrogen vacuole (which explains the middle part of name, margarita = pearl. Often, these bacteria stick together in a line and look like a string of pearls).

T. namibiensis is a spherical bacterium. Round cells are least well equipped for good mixing and traffic times; the center is far from any cell surface. But if the cell was flattened out or narrow in one dimension the traffic times could be reduced, even if the organism was larger. For this reason, many bacteria are not round, but perhaps rod-shaped or flattened rhomboids. Here we see that T. namibiensis is huge (up to 750 µm) while still spherical. That size makes it just about the size of the period at the end of this sentence; not much compared to a beach ball, but 3 million times the volume of a typical spherical bacterium.

T. namibiensis usually occurs in chains of ten or so bacteria, with pearlescent sulfur granules as shown in the left image. In cross-section on the left, you can see both the thin band of cytoplasm and the large nitrogen-containing vacuole.

The first key to Thiomargarita’s size is that large central vacuole of nitrogen. As shown in the righthand photomicrograph (courtesy Woods Hole Oceanographic Institute), there is only a thin layer of cytoplasm (the essential, viscous, water-based medium that fills the cell) between the vacuole and the cell membrane. The vacuole itself consumes almost 98% of the total cell volume. This small layer of cytoplasm means that all the important molecules are close to the surface through which they diffuse; therefore, the large size of the cell does not violate any limitations placed on its mixing rates or traffic times. While the size of the bacterium is huge, the distance any one molecule has to travel is still small. In fact, the amount of cytoplasm in T. namibiensis is just about the same as in a normal sized bacterium.

The large diameter of T. namibiensis also helps it survive in two ways that are less evident. One advantage has to do with the diffusive boundary layer. Because of the natural friction between all molecules, there is always an area next to any surface where the flow of liquid is reduced to near zero. Reduced flow means reduced numbers of important molecules can be picked and carried; therefore, the concentration of important molecules is reduced, a bad thing for bacteria trying to survive. However, because of the huge size of T. namibiensis, much of the cell sticks up above the sea floor’s diffusive boundary layer, into the area where diffusion can be more productive.

The second survival advantage is slightly more straightforward. T. namibienisis and other megabacteria are just too big to be bothered by predators. T. namibiensis doesn’t have to worry about being eaten, because no bacterial predator is big enough to “swallow” it. This is similar to the ancient sauropod species, like Brachiosuarus or Diplodicus, which had no predators once they grew to adult

Just like a T. Rex couldn’t bring down or swallow

a brachiosaur, a normal bacterium (the white dot

in the top right hand corner) can’t eat T. namibiensis.

size – a healthy sense of self-preservation would keep any T. Rex from trying to eat an adult brachiosaur.

We have seen that limitations on bacterial size imposed by diffusion can be overcome if natural selection results in some advantageous characteristic and if there is a reproductive advantage to be being big. The development of a central vacuole permitted T. namibiensis to become bigger, and being bigger provided an advantage for survival on the sea floor. It seemed designed to end up just so, but remember that evolution is not purposeful. It is merely a series of random changes and random environmental changes that render some characteristic advantageous, disadvantageous, or moot.

Next time we will look at another giant bacterium. This second rule-breaker has a completely different solution to the diffusion/size limitation. Just as we highlighted with the nylon metabolizing bacteria a few weeks ago, nature can find an infinite number of ways to overcome a single problem. It just takes random mutation (a change), environmental pressure (a need for the change) and time (for the reproductive advantage afforded by the change to have an effect on the population).


Schulz, H., & Jørgensen, B. (2001). Big Bacteria Annual Review of Microbiology, 55 (1), 105-137 DOI: 10.1146/annurev.micro.55.1.105

Girnth, A., Grünke, S., Lichtschlag, A., Felden, J., Knittel, K., Wenzhöfer, F., de Beer, D., & Boetius, A. (2011). A novel, mat-forming Thiomargarita population associated with a sulfidic fluid flow from a deep-sea mud volcano Environmental Microbiology, 13 (2), 495-505 DOI: 10.1111/j.1462-2920.2010.02353.x



For more information on surface area: volume, sulfur bacteria, and T. namibiensis, please see below:


Cell surface:volume laboratories:
http://www.oocities.org/capecanaveral/Hall/1410/lab-B-24.html
www.nnin.org/doc/SurfaceVolumeRatioB_TG.pdf
http://illuminations.nctm.org/LessonDetail.aspx?id=L609
http://www.neiljohan.com/projects/biology/sa-vol.htm


sulfur bacteria:
http://www.moldbacteria.com/bacteria_testing.html
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/E/Eubacteria.html
http://filebox.vt.edu/users/chagedor/biol_4684/Microbes/greensul.html
http://bmb-it-services.bmb.psu.edu/bryant/lab/Project/GSB/index.html
http://m.biotecharticles.com/Biology-Article/Green-and-Purple-Sulfur-Bacteria-705.html
http://filebox.vt.edu/users/chagedor/biol_4684/Microbes/purprnb.html


Thiomargarita:
http://web.mst.edu/~microbio/BIO221_2005/T_namibiensis.htm
http://www.sciencenews.org/sn_arc99/4_17_99/fob5.htm

Why Do Males And Females Look Different?

Biology concepts – sexual dimorphism, phenotype, evolution, sexual selection, secondary sex characteristics, reproductive success, natural selection

Elephants are an animal that we can picture easily in

our head. But is this a male or a female? Don’t answer

quickly, in African elephants both the males and

females have tusks, but in the Asian elephants, it’s only

the males (usually).

We all know what a hippo looks like, an elephant, a duck. In most cases, if you name a species, you can picture the animal in your head. But are you picturing a male or a female? Sometimes they look the same, we can only tell the males from the females if we get close enough and are socially rude enough.

But in some cases, it’s much easier to tell the guys from the gals, so much so that sometimes scientists misidentify them as different species. The differences between how males and females look and how they look is called sexual dimorphism (di = two, and morph = form) and it can range from the subtle to the fantastic.

We have been talking about bilateral asymmetry in the past few weeks, and our next examples of bilateral asymmetry require a discussion of sexual dimorphism – a subject full of its own exceptions.

The mildest form of sexual dimorphism is when the difference lies in just in reproductive organs.  This may or may not be visible to the naked eye. Take the American white pelican (Pelicanus erythrorhynochos). On average, the males are just slightly larger than the females, but you couldn’t tell this by looking at them. Only their reproductive organs tell them apart and the external portion of the cloaca of a male looks just like that of a female. Maybe you could separate them another way – I hear only guys like the Three Stooges.

A better example would be the spotted hyena (Crocuta crocuta). The females probably like the Stooges more than the males, because this species has females that are extremely masculinized. Many studies have been done on just how this species is unique among mammals in its lack of sexual dimorphism.

Is this a male or female spotted hyena? Even experts

can’t tell. The females are just as aggressive as the males

and they could easily chase off a cheetah. Males and

females look exactly alike, but I’m betting a female

wouldn’t let a meal get away so easily.

A 2014 review discusses how the female external reproductive tissues look just like the males. Scientists have studied hyena individuals for years assuming they were males until all of suddenly they give birth to a litter of pups! The review goes over the data that shows that much of the external genitalia are masculinized before the reproductive organs can even start producing hormones, so much of the similarities between males and females is genetically driven. But not all – certain aspects could be stopped with anti-androgen drugs.

A 2012 study showed that spotted hyenas have 5x lower levels of SHBG (sex hormone binding globulin). This protein binds up estrogens and androgens and regulates how available they are to the tissues. The spotted hyena has a slight mutation in the gene. The result is that lower overall levels of that gene product (protein) are made. With less regulating protein, the androgens are free to strongly masculinize both the tissues and the behaviors of the females. They are bigger, stronger, and more aggressive than the males. This, along with their external reproductive organs looking so similar to males makes them a complete exception in the mammals.

But it isn’t always so hard to tell boy from girl. There are several external body features that may help if you find yourself needing to tell, say, a boy wombat from a girl wombat.

Size (mass, length, height, muscularity) is a common sexually dimorphic trait. In mammals and birds, the males are most often larger than the females, but our talk of spotted hyenas from above tells you that isn’t always the case. The exceptions carry over to birds as well. When the gender that is normally smaller in most species of a phylum turns out to be bigger, this is called reversed sexual size dimorphism or just reversed size dimorphism (RSD).

These are southern elephants seas, a mating pair. No, he’s

not a cradle robber, the males are just that much bigger

than the females. The penguins let you know just how

far south we are. Does she look scared to you?

Hawks, owls, and falcons (all raptors) show this RSD, which was investigated in 2005. The study found that the small-male hypothesis was supported – that males got smaller to become better foragers, while the females remained large or got larger as prey for their chicks got larger. The study concluded that RSD was a results of natural selection for resource and niche management rather than a selection based on who to mate with (sexual selection).

Amongst the mammals that follow the rule of larger males, the biggest size dimorphism is seen in the southern elephant seal (Mirounga leonina). The males weigh 8-10x more than the females, and they have a huge proboscis that the females don’t have. When hanging out together, they are often mistaken for an adult and a juvenile....unless she’s a trophy wife and he’s 50 years older than her. Then it’s completely believable.

Outside of mammals and birds, phyla generally have females that are larger than males. That’s if there is a difference in size between the sexes at all - many species don’t have sexual size dimorphism. One that does is the golden silk spider (Nephila clavipes) has a female that 35-70x the mass of the male and is 7-8x longer than he is. Many spiders have larger females.

On the left is the golden silk spider that lives in North

America, from NC to TX. The intruder above is the male,

while the female is hogging most of the picture. On the

right is A. aquatica where the male is bigger and both

males and females live underwater their entire lives.

But even in spiders there is an exception. The water spider (Argyroneta aquatica) is one of the few spiders where the male is larger than the female, but that’s not the weird part. It spins a web under water that acts as a diving bell. The spider pulls down air and holds it under the bell of the web. A 2013 study showed that the web contains a biogel that holds the air in the web. It can pull oxygen out of the water and replenish the air in the bell, so the spider can live and hunt under water without ever coming to the surface again.

Often, male and female animals have differences in secondary sex characteristics – traits that distinguish the two genders but are not related directly to the reproductive organs. Colors or ornaments (like wattles, antlers, etc.) can be used to tell the differences between males and females. These are phenotypic (pheno = observed and type = characteristic) differences; they make the two animals look different, not just be of different size.

Color is a good example of a phenotypic sexual dimorphism (sexual dichromatism). Cardinals are red (male) or kind of grayish-brown (female), while male and female Eclectus parrots (Eclectus roratus) are both colorful, they just have completely different coloration patterns (see picture below). Mandrill (a type of primate) males have coloration on their face and bums, while the females are basically all one color.

The Eclectus parrots on the left are also a mating pair.

The male is green and the female is red and blue. Why

might this sexual dimorphism have developed. Both are

bright and could be spotted easily, although in a forest

the male is probably hidden better. The right image is the

triplewart seadevil female. I superimposed a male about

the right size and where he would attach (see arrow).

Secondary sex characteristics often work in combination with differences in size. Perhaps one of the most dramatic examples is the triplewart seadevil (Cryptopsaras couseii), a type of anglerfish. The female is huge, up to 10 kg, with a bioluminescent lure and a gaping mouth. But the male is 1/25th her size and only 150 g at most; he looks nothing like her. He exists only as a parasite that attaches to her side and gets nourishment from her body. He is there when it is time to mate because he’s always there, just hanging on.

Why would it be advantageous for species to show a sexual dimorphism – like size, phenotype, or even behavior? There are sexual dimorphic behaviors, like male penguins presenting pebbles to prospective mates or male manakin birds dancing for females. Some birds dance better than others – at least according to the females, so this is a selection criterion just like other sexual dimorphisms, but these are beyond our discussion today.

Sexual selection (mate selection) criteria are good reasons for sexual dimorphisms. If a male (or every once in a while a female) has enough energy to make ornaments (or even better, larger ornaments like horns, wattles, etc.), then they must be good at finding food or have good genes. This would give those with larger ornaments a reproductive advantage and would select for genes that promote larger ornaments. Over time, there would be greater and greater separation between males and females.

Likewise, larger tusks or antlers would allow a male to compete better against other males. This would again help separate those with supposedly stronger genes. Winning a battle might reflect bigger muscles, again a sign of better energy procurement or the ability to resist disease. All in all, he’d be a better mate for a female looking for physical survival traits. The more a species starts to control its environment (ie. humans), the less these survival or strength genes matter.

This is a form of sexual behavior dimorphism in the

manakin bird. The male dances for the female. Now we

know where Michael Jackson got the idea for the

moonwalk.

Then again, sexual dimorphism may be a survival advantage. If the two genders are put together somewhat differently, then perhaps they will exploit different food sources in the same area. This would allow both males and females to get enough energy and more of each gender would survive to reproduce because they aren’t competing with each other for resources. This is what happens with some hummingbird species where the males and females have different bill shapes and lengths that allow them to drink from different types of flowers.

A new paper shows that plumage color in birds is often related to survival advantage - not mate selection advantage. Plumage can be used for camouflage, when males live in slightly different environments than females. The alternative - if they don’t survive, they probably won’t mate. On the other hand, sexual size dimorphisms can promote stronger mate selection if the males are bigger (sexual selection), or may allow for the mothers to hunt better and find more food for offspring if it is they that are larger (natural selection).

In some arthropods, there is often a sexual size dimorphism where the female is larger. This would allow them to lay more eggs – more eggs means more potential offspring might survive to reproduce themselves. Likewise, female humans have a wider pelvis to allow for passage of the baby through the birth canal - a dimorphism not associated with mate selection. Males don’t need that – thank goodness.

We see here that the point of sexual dimorphisms can be for reproductive success or survival advantage. These are what keeps a species living generation after generation. However, evolution has deemed reproductive success even more important than individual life span. In pheasants, the females live much longer, so the males have to make themselves stand out so that they will mate as often as possible in their shorter lives. Therefore, they are colored much more brightly.

Next week, sexual dimorphism isn’t just an animal thing. There are genders in plants too. Sometimes they different sexes have very different characteristics so that they can mate as well, but do plants select mates?

Dunn, P., Armenta, J., & Whittingham, L. (2015). Natural and sexual selection act on different axes of variation in avian plumage color Science Advances, 1 (2) DOI: 10.1126/sciadv.1400155

Neumann, D., & Kureck, A. (2013). Composite structure of silken threads and a proteinaceous hydrogel which form the diving bell wall of the water spider Agyroneta aquatica SpringerPlus, 2 (1) DOI: 10.1186/2193-1801-2-223

Cunha, G., Risbridger, G., Wang, H., Place, N., Grumbach, M., Cunha, T., Weldele, M., Conley, A., Barcellos, D., Agarwal, S., Bhargava, A., Drea, C., Hammond, G., Siiteri, P., Coscia, E., McPhaul, M., Baskin, L., & Glickman, S. (2014). Development of the external genitalia: Perspectives from the spotted hyena (Crocuta crocuta) Differentiation, 87 (1-2), 4-22 DOI: 10.1016/j.diff.2013.12.003

Hammond, G., Miguel-Queralt, S., Yalcinkaya, T., Underhill, C., Place, N., Glickman, S., Drea, C., Wagner, A., & Siiteri, P. (2012). Phylogenetic Comparisons Implicate Sex Hormone-Binding Globulin in “Masculinization” of the Female Spotted Hyena Endocrinology, 153 (3), 1435-1443 DOI: 10.1210/en.2011-1837

Krüger, O. (2005). The Evolution of Reversed Sexual Size Dimorphism in Hawks, Falcons and Owls: A Comparative Study Evolutionary Ecology, 19 (5), 467-486 DOI: 10.1007/s10682-005-0293-9

For more information or classroom activities, see:

Sexual dimorphism –

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CB4QFjAA&url=http%3A%2F%2Fwww.bio.indiana.edu%2Fcommunity%2Ffaculty%2FICE_files%2FSelection-All.pdf&ei=eIwZVf_6ItL8oASA-YKoAg&usg=AFQjCNFD9Si4w9hvZnrD3gYgqn1DSfCmzA&sig2=MZxaCeNSFqWqmUi51MKYfA&bvm=bv.89381419,d.aWw

http://www2.nau.edu/~bio372-c/class/behavior/lesson1-1-1.htm

https://animaldiversity.ummz.umich.edu/quaardvark/examples/

http://explorers.neaq.org/2012/03/fiji-sexual-dimorphism-here-there.html

http://www.bbc.co.uk/nature/adaptations/Sexual_dimorphism

http://www2.nau.edu/~gaud/bio300b/sexdi.htm

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=9&ved=0CFwQFjAI&url=http%3A%2F%2Fcetus.ucsd.edu%2Fsio133%2FPDF%2FSexual%2520Dimorphism.pdf&ei=XY0ZVcf9Jcf1oASE0IKACA&usg=AFQjCNEq5OflKYjaNLnSDagQ4OhlhjE9ZQ&sig2=XMGwRtjt_WqQVvGxfBVMbg

http://www.sciencedaily.com/releases/2014/02/140219160410.htm

http://animals.about.com/od/zoology12/f/sexualdimorphis.htm

http://www.learnaboutbutterflies.com/Survival%20Strategies%205.htm

http://www.aaos.org/news/aaosnow/jun14/research1.asp

sexual selection –

http://bivalves.teacherfriendlyguide.org/index.php?option=com_content&view=article&id=42:types-of-natural-selection&catid=27&Itemid=124

https://sites.google.com/site/theclassroommatinggame/home

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&ved=0CDEQFjAC&url=http%3A%2F%2Fwww.bio.indiana.edu%2Fcommunity%2Ffaculty%2FICE_files%2FSelection-All.pdf&ei=v44ZVe7rCNKtogTfjoLgBA&usg=AFQjCNFD9Si4w9hvZnrD3gYgqn1DSfCmzA&sig2=uwrukJTsL5nNIjJ_wzVEGQ

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=4&ved=0CDgQFjAD&url=http%3A%2F%2Fdownloads.arkive.org%2Fdocuments%2Feducation%2F11-14yrs_-_Sexual_Selection_-_The_Dating_Game_-_Teachers_Notes.doc&ei=v44ZVe7rCNKtogTfjoLgBA&usg=AFQjCNFYaQLdDv87M68K_e19cDA943D_IA&sig2=7YKmQbYAMuErygw6ie8DLA

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3763015/

http://education.nationalgeographic.com/education/media/sexual-selection/?ar_a=1

http://evolution.about.com/od/teaching/a/Sexual-Selection-Lab.htm

http://evolution.berkeley.edu/evosite/evo101/IIIE3Sexualselection2.shtml

https://www.questia.com/library/journal/1G1-294903551/the-guppy-game-understanding-the-big-ideas-of-natural

http://evolution.berkeley.edu/evosite/evo101/IIIE3Sexualselection.shtml

http://darwin200.christs.cam.ac.uk/pages/index.php?page_id=d4

Looking Sideways In The Mirror

Biology Concepts – platyhelminthes, asymmetry, bilateral symmetry, evolution, cephalization, natural selection, fish, lepidophagy

What is the largest living structure on Earth? No, it’s not the 2200 acre Armillaria ostoyae fungus in Oregon that we talked about previously. That is the largest single organism, but there is something bigger.

The Great Barrier Reef houses more species of

coral than any other place on earth, more than 600

species call the reef home. You see how many shapes

they can take. Does this mean they are

asymmetric animals?

The Great Barrier Reef off the northeast coast of Australia is alive. Reefs are made of the exoskeletons of coral polyps, with the new corals growing next to and on top of the older ones. With all the nooks and crannies available, coral reefs are some of the most diverse ecosystems on Earth, with thousands of species per square mile.

This can add up quickly, because the Great Barrier Reef is more than 132,973 sq. miles (344,400 sq. km) in area. And if you still don’t believe it is a living structure, get this; it’s moving south! Climate change is warming the waters off the coast, and corals and tropical fish are moving south with the warmer temperature. Meanwhile, the northern edge recedes as the waters get too hot for corals.

Corals take all sorts of shapes (see picture above), and despite what we talked about last week, they don’t seem to be either bilaterally or radial symmetric. Sure, brain corals look radial, but most corals don’t have a repetitive shape. Are these asymmetric animals?

Nope - remember that the coral you see is the exoskeleton of the polyp, not the animal itself. Just because the apartment building isn’t symmetric, it doesn’t mean the animal is as well. Coral polyps are definitely radially symmetric, so our discussion of last week still holds.

Corals and other radial animals are exceptions, since 99% of animals are actually bilaterally symmetric. But there are exceptions with the bilateral animals as well. Some species that have been bilateral for millions of years then evolved a tweak to the system. Some part them became asymmetric in order to give them an advantage. Their stories are exceptional and we should explore some of them.

Coral polyps are cnidarian animals. They live inside

the calcium shells they produce, and it is the shells

that seem asymmetric in many cases. But the polyps

show radial symmetry. Each stalk and white “flower”

is an individual polyp.

But we begin with a challenge –as we discuss the different animals that break symmetry in the next few posts, see if you can find something else that is common to many of them.

Let’s start with the first animals that became bilaterally symmetric– the platyhelminthes, or flatworms. Most flatworms are small, just barely visible with the human eye, and most swim in the water. There are free-living versions and parasitic species and it is in the parasites that we find our first animal that has decided that completely bilateral isn’t necessary.

Polyopsithocotylea monogenea is a group of flatworms that live on and feed on fish gills. About 0.05-1 mm long, these platyhelminthes attach themselves to one of the gill ridges and take up residence there for life. The attachment they use is called a haptor, and they come in different shapes and sizes.

The oncomiracidium stage of the worm is directly after the egg stage and before it becomes an adult. This stage is completely bilaterally symmetric. But when the adult stage is reached and it’s time to settle down and starting feeding on some fish’s gills, they become asymmetric via their haptor attachment.

Opisthaptors, or just haptors, are the attachment

organs for many parasites, including parasitic

flatworms. They can use suckers or clamps and hooks

in order to anchor the worm in its preferred habitat.

You can see that these haptors maintain animal

symmetry, but not all do.

Different host species fish have different gill anatomy, so the attachment point and position will be different. The haptor(s) have to be located where attachment is possible. This means that they may be on one side of the body or the other, or two on one side and one on the other, etc.

The initial haptor is usually located on the posterior end, on one side only. So much for bilateral symmetry. In some species this haptor has suckers, in others it has hard clamps, and yet other species have both.

As the adult grows, more haptors may develop, just where the animal touches the gill. Some may have 50 or more haptors arranged around their posterior, becoming more and more asymmetric. But there is a plan, they only grow where attachment is possible; some signal is generated by contact and this stimulates growth of more attachments.

Here we have a family of parasitic worms that aren’t symmetric living within a phylum of worms that were the first to be bilaterally symmetric - exceptional. But, take one step up the chain and you see the fish they live on. It just so happens that at least one group of fish parasitized by P. monogean worms are asymmetric themselves.

These are the profiles of some monogean flatworms. The

haptors of these parasites grown in odd places and

destroy the bilateral symmetry of the animal. But it is

necessary for the worm to attach to the gills of their

prey fish.

Cichlid fish are one of the most diverse family of animals known, with more than 1700 known species. They are found in the Old World and the New. They exhibit some amazing adaptations, especially when many are found in one place, Lake Tanganyika on the border of Tanzania and the Democratic Republic of the Congo.

The cichlids are successful because different species have developed different feeding niches, and this is where we meet our asymmetric cichlids – they eat the scales off other fish! One genus, Perissodus, has at least six species that eat scales, all endemic to Lake Tangayiki (although there are also other scale eaters in other locations).

Scale ripping and eating is called lepidophagy (lepido = scale, and phagy = eat). Scales are an unexpectedly good source of nutrition. They chock full of protein and calcium phosphate, and their attachments are both cartilaginous, fatty, and come with some carbohydrate. Remember that the next time you order fish in a fancy restaurant. Tell the chef not to scale it – you’ll be quite the topic of conversation in the kitchen.

Fish scales have many uses. Some, especially from

herrings, are used to make the pearlescent cosmetics

that are sold today. Gals, your putting fish scales on your

eyes and lips. Fish scales can also be turned into artificial

bone, or they can be food for lepidophageous fish.

Mind you, this isn’t eating the scales of dead fish, or eating the scales that drop off live fish. Lepidophagy means eating the scales that are still attached to live fish. It’s a fish smorgasbord. This is both good and bad. Scales on live fish grow back fast, so there is always a ready supply of food.

But, you can imagine that the fish being unfrocked don’t appreciate it very much and fight back or swim away quickly. That doesn’t even take into account how hard it is to bite the scales off a swimming fish. Therefore, lepidophages must evolve anatomies and behaviors that give them a chance to succeed. Or perhaps it would be better to say, they acquired characteristics that made being a lepidophage an advantage.

Here are two P. microlepis scale eating fish. One is right

mouthed and the other is left mouthed. You can see how

the way their mouth develops breaks their bilateral

symmetry. The right-mouthed version (on the left) will

only eat scales from the left side of fish. How is it that both

versions can be maintained in a population?

The species P. microlepis has developed one particularly amazing way to help it eat the scales off of neighbor fish. It’s mouth and jaws have evolved so that they bend sideways. There are right-mouthed P. microlepis and left-mouthed individuals. The difference is obvious, right-mouthed individuals will feed only from the left side of their prey and vice versa. They have an asymmetry, or lateralization, of both anatomy and behavior

A 2012 video study showed that right-mouthed individuals almost always attack prey from the left, and their strikes are more powerful and successful when coming from the preferred side. One could ask, why are their both types? How did right- and left-mouthed individuals come to evolve and why are there still both types?

A different 2012 study shows that juveniles prefer one side or the other, even before their mouth bend has become pronounced, so it is a deeply penetrating characteristic, both heritable and perhaps partially acquired. There’s evolution and genetics at work here.

The prevailing model is that at any one time, right or left-mouthed individuals will predominate in a population. Let’s say that right now, right-mouthed feeders are the majority. The prey fish will learn to pay more attention to their left side, as this side is more vulnerable.

This makes it harder for right-mouthed individuals to feed, but easier for left-mouthed fish, because the prey fish ignore their right side relative to the left. In time, the right-mouthed individuals breed less well and the numbers in the population will shift. The left-mouthed feeders will become the majority. This is an example of negative frequency-dependent selection, where as a trait becomes more common it becomes less advantageous, and there is a balancing selection.

The flu virus comes with one of many hemagglutin proteins

and one of many neuraminidase proteins. If one genetic

version is too successful, most people will develop

immunity to it and it becomes less fit in the population of

hosts. Its success is its downfall and a more rare version

will rise up. This is negative frequency-dependent selection.

The cycle will begin again after left-mouthed individuals come to dominant in the population. Back and forth the population will go. If the population stayed 50/50, nobody would gain an advantage, and overall, they would all suffer. The system only works if a small number develop opposite to the majority. The gene regulatory complex that controls if an individual will be right- or left-mouthed must be very complex if it can take into account that a few need to be lateralized the other way.

However, a 2012 study throws this into question. They found equal distributions of right-and left mouthed individuals in five populations they studied. Also, left-mouthed and right-mouthed individuals mated with each other just as often as they mate with same-sided individuals (called disassortative mating). Perhaps the mouth bend is not a true dimorphism (di = two, and morph = shape).

Or, as a 2008 study suggests, negative frequency-dependent selection works best when there is disassortative mating. This may be necessary since a 2007 study showed that lefty:lefty matings give 2:1 lefty offspring, right:lefty mates give equal righty and lefty offspring, but righty:righty pairs ONLY give righty kids. Figure out the genetics of that. The authors proposed two possibilities – mendelian genetics with lefty being dominant and dominant homozygous being lethal, or cross-incompatibility that is predominant in lefty:lefty homozygotes, (meaning lefty homozygotes can’t mate successfully mate with the other types).

Today we have seen a swimming flatworm that feeds on some fish, and some fish that feed on the scales of other fish. And both of them achieve this only because they have adapted their bilateral symmetry to become just a bit asymmetric.

Next week, there are other animals that break symmetry to survive. Flatfish lay on their sides at the bottom of lakes and oceans, yet they still use binocular vision. How can that be?

Takeuchi, Y., Hori, M., & Oda, Y. (2012). Lateralized Kinematics of Predation Behavior in a Lake Tanganyika Scale-Eating Cichlid Fish PLoS ONE, 7 (1) DOI: 10.1371/journal.pone.0029272

Lee, H., Kusche, H., & Meyer, A. (2012). Handed Foraging Behavior in Scale-Eating Cichlid Fish: Its Potential Role in Shaping Morphological Asymmetry PLoS ONE, 7 (9) DOI: 10.1371/journal.pone.0044670

Kusche, H., Lee, H., & Meyer, A. (2012). Mouth asymmetry in the textbook example of scale-eating cichlid fish is not a discrete dimorphism after all Proceedings of the Royal Society B: Biological Sciences, 279 (1748), 4715-4723 DOI: 10.1098/rspb.2012.2082

Takahashi, T., & Hori, M. (2008). Evidence of disassortative mating in a Tanganyikan cichlid fish and its role in the maintenance of intrapopulation dimorphism Biology Letters, 4 (5), 497-499 DOI: 10.1098/rsbl.2008.0244

Hori, M., Ochi, H., & Kohda, M. (2007). Inheritance Pattern of Lateral Dimorphism in Two Cichlids (a Scale Eater, Perissodus microlepis, and an Herbivore, Neolamprologus moorii) in Lake Tanganyika Zoological Science, 24 (5), 486-492 DOI: 10.2108/zsj.24.486

For more information or classroom activities, see:

Great Barrier Reef –

http://www.greatbarrierreef.org/

http://whc.unesco.org/en/list/154

http://video.nationalgeographic.com/video/oceans-narrated-by-sylvia-earle/oceans-barrier-reef

http://www.gbrmpa.gov.au/about-the-reef/facts-about-the-great-barrier-reef

http://www.pbs.org/wgbh/nova/education/activities/2215_reef.html

http://geography.mrdonn.org/greatbarrierreef.html

http://www.reefteach.com.au/about-the-reef/links/eduresources/

http://education.nationalgeographic.com/archive/xpeditions/lessons/14/g912/geoactreef.html?ar_a=1

http://www.reefcheckaustralia.org/for-teachers.html

http://www.mesa.edu.au/resources/ourgbr06.asp

http://science.uniserve.edu.au/school/curric/stage4_5/coralreefs.htm

http://video.nationalgeographic.com/video/oceans-narrated-by-sylvia-earle/oceans-barrier-reef

http://www.discovery.com/tv-shows/other-shows/videos/discovery-atlas-australia-revealed-great-barrier-reef/

http://www.sciencekids.co.nz/videos/nature/greatbarrierreef.html

http://www.smithsonianchannel.com/shows/secrets-of-the-great-barrier-reef/0/134605

https://www.youtube.com/watch?v=2T2i29M92RQ

http://www.animalplanet.com/video-topics/wild-animals/great-barrier-reef-playlist/

Coral polyps –

http://www.calacademy.org/educators/lesson-plans/build-a-coral-polyp

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=7&ved=0CDkQFjAG&url=http%3A%2F%2Fcoast.noaa.gov%2Fpsc%2Fseamedia%2FLessons%2FG3U3L2%2520Coral%2520Structure%2520and%2520Function.pdf%3Fredirect%3D301ocm&ei=WeXsVI3NFI2PyASq44LgCw&usg=AFQjCNEhqIwraaKbhPdUaAw_EHjlwmOQZA&sig2=uWH06Am2pFOOWUv_hRZLQA&bvm=bv.86475890,d.aWw

http://www.teachoceanscience.net/teaching_resources/education_modules/coral_reefs_and_climate_change/access_classroom_resources/

http://www.educationworld.com/a_lesson/00-2/lp2134.shtml

http://www.cpalms.org/Public/PreviewResource/Preview/25461

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=12&ved=0CCMQFjABOAo&url=http%3A%2F%2Fwww.si.edu%2FContent%2FSE%2FEducator%2520Guides%2FCRA-Educators-Guide.pdf&ei=weXsVI36CsL2yQTgtoDoBg&usg=AFQjCNHY3JSYr7ODQc4DJoihRH9e2j9E7w&sig2=kpBDF8NsWAYbDdVyLheTvQ&bvm=bv.86475890,d.aWw

https://www.youtube.com/watch?v=_zpiEY77Ujk

https://www.youtube.com/watch?v=11KV00yDnbY

http://www.discovery.com/tv-shows/other-shows/videos/short-attention-what-is-a-coral/

http://www.wri.org/resources/videos/coral-reefs-polyps-peril

http://oceanexplorer.noaa.gov/explorations/03mountains/logs/jul17/media/coralpolyp.html

http://animal-world.com/Aquarium-Coral-Reefs/Stony-Corals-LPS

http://www.coralscience.org/main/articles/biochemistry-2/how-reefs-grow

http://animals.howstuffworks.com/marine-life/coral-reef1.htm

Cichlids –

http://www.cichlid-forum.com/articles/lake_tanganyika_diverse.php http://www.news.leiden.edu/news/super-fast-evolution.html

http://www.amazon.com/The-Cichlid-Fishes-Experiment-Evolution/dp/0738205281

http://video.nationalgeographic.com/video/cichlid_mouthfighting

http://news.stanford.edu/news/2014/september/fish-genome-fernald-092214.html

http://malawicichlids.com/mw01100.htm

http://www.futurity.org/cichlids-vertebrate-evolution-759392/

http://animalguide.georgiaaquarium.org/home/galleries/river-scout/gallery-animals/cichlid

http://www.aquaticcommunity.com/cichlid/tang.php

http://www.biodiversityscience.com/tag/adaptation-2/