Wednesday, March 25, 2015

This Nose Knows

Biology concepts – evolution, asymmetry, bilateral symmetry, phonic lips, whales, echolocation, encephalization quotient, density

This picture gives you a good idea of just how big
a spermaceti whale is. Captain Ahab wanted to take
this guy on mano y mano. He was nuts.
Captain Ahab had an obsession for the white whale in Moby Dick. It was a killer, but not a killer whale. It swamped boats, rammed ships, and generally made a nuisance of itself. But it seemed to be intelligent as well, the way it planned attacks and how it looked at him sometimes. Is that weird for a whale?

Not for Moby Dick; he was a spermaceti whale. The fact that he was white and revengeful is nothing compared to how evolution has fashioned the real-life spermaceti whales. They have a head that is so weird - I just can’t wrap my head around it.

First off, the spermaceti whale is often known by its shorter name, the one that sounds like the cells in the male reproductive fluid. I have learned that using that word gets my posts blocked from most schools, so lets use the scientific name, Physeter macrocephalus, where macro = large and cephalus = head.

P. macrocephalus is the largest of the toothed whales. It is the fifth largest whale in the world, behind the blue, right, fin, and bowhead whales – all baleen whales that eat krill and plankton. P. macrocephalus averages 25,000-55,000 kg (60 US tons) and can be 70 ft (21.5 m) in length.

Fully one third of that length is head - and it gets weirder. The spermaceti whale’s head is very much an exception in the animal world. As it just so happens, its head is asymmetric, which brings P. macrocephalus into our growing list of asymmetric animals – the flatworm parasites of fish gills, the scale-eating cichlid fish, the flatfish, and the narwhals. See a pattern here?

A big head suggests a big brain, and P. macrocephalus has the biggest brain on the planet, averaging over 18 pounds (8 kg). So maybe Ahab was right when he and the other sailors described Moby Dick as having “intelligent malignity.”

The model is of an 18 lb. spermaceti whale brain. I
can’t vouch for the color. Notice how it’s as big as
your head! Bigger! The model of the left is named
Frank and he is available for lingerie and catalog
Nope, it takes more than a big brain to have big thoughts. A more telling statistic for mammals is brain size relative to the predicted brain mass based on body size, something called the encephalization quotient (EQ). Brain size does usually increase with body size, but the increase isn’t linear, so scientists include a cephalization factor (C).

To compare relative cognitive power between mammal species, EQ is the ratio of C over the expected value for C of an animal of given mass (S), EQ = CSr. Humans have the highest EQ (7.4-7.8) but dolphins are pretty hefty as well (4.14). Whales of different types have different EQs,; P. macrocephalus’ value is very high (~3.8). But if he wanted to have a brain like humans, it would have to weigh several hundred pounds! The blue whale’s EQ is much lower (~1.0); in general the toothed whales (Odontocetes) have much higher EQs than the baleen whales. In fact, the toothed whales and dolphins as a group are pretty much second only to the humans in EQ.

A 2012 study sort of links humans and toothed whales like P. macrocephalus when it comes to EQ. Their paper suggests that the greatest variance in EQ occurs in primates and toothed whales, and suggests that the evolutionary constraints have been relaxed in these two groups of animals.

A 2013 study suggests that during evolution, the toothed whales underwent a body mass decrease, while baleen whales underwent a mass increase – each without changing the brain size much. This led to toothed whales having higher EQs, closer to humans than even some primates (lemurs are often below a 1.0).

But EQ doesn’t mean everything. A new study comparing killer whale brains and P. macrocephalus brains suggests that the much smaller killer whale has a brain about the same size as P. macrocephalus. While it gives them a bigger EQ in general, the main difference in the brains of these two cetaceans (whales and dolphins) is in their cerebellums.

In this case, the killer whale is the exception – in all other mammals, the cerebellum size scales directly with overall brain size. The results suggest to the authors that the differences relate to what they eat and how they dive – the killer whales have to be much more agile, and this is one thing in which the cerebellum functions.

A great illustration of the P. macrocephalus head. The skull is in tan, the spermaceti and junk in yellow, brain fits in 
the little triangle made by the jaw bone, the frontal sac, and where the nasal passages go down to the lungs. You
can see the two nasal passages and their different paths in the transverse cut. See how the blowhole is so far
front? I put an arrow where it exits in the transverse cut. The right nasal passage goes to the phonic lips.
All this talk of big brains in the spermaceti whale may give you the wrong idea. Look at the picture above and you get a better appreciation for the size of this animal. And again, the head is just so weird. The vast majority of P. macrocephalus’ head is outside his/her skull (the tan portion)!

The biggest part of the whale’s head is devoted to the spermaceti organ and the junk organ (or melon). The brain is in the little case toward the back and behind the jaw. The real question is what all those compartments and tissues above the skull are for.
P. macrocephalus is one of the whales that uses clicks and rolls as well as echolocation. Lots of research has been done on the vocalizations of whales so let me explain….. no, is too long, let me sum up.

Echolocation uses high frequency short clicks, and they’re loud - over 230 decibels. We’ve talked about these before. The lower frequency coda (long rolls) are for gabbing, and slow clicks can be heard for 60 km so they are for males keeping track of other females during breeding season, according to a 2013 paper. These clicks and codas can be highly directional and are very powerful. This is what all that equipment is for.

Here’s how it happens. A vibration is produced just south of the blowhole (more on this below). That vibration is projected backward, through the spermaceti organ. This organ is filled with a whitish, waxy, fatty material. Sailors thought it was the whale’s male reproductive fluid (it isn’t) – and that’s how the organ and the whale got their common name. It is about 1900 L (502 gal) of very useful spermaceti oil for lamps. This is why they were hunted almost to extinction.

The left image is the surface of the frontal air sac
where the clicks and codas are reflected back through
the melon. The right image is the phonic lips of a
spermaceti whale. Made form a nostril, they act much
like our vocal chords, but I know people who can make
a heck of a noise with their nostril and a Kleenex.
At the back of the spermaceti organ is a knobbed plate in the frontal air sac. This reflects, focuses, and amplifies the vibrations. They bounces back toward the front of the whale head, through the junk (melon). This organ is also filled with waxes and oils, but the sailors didn’t think it was worth any money, so they called it the junk. This organ is made of many vertical compartments of spermaceti. When it leaves the front of the whale, it is one powerful click.

When the echolocation returns from the target, it vibrates the lower jaw and a fat pad at the back of the jaw. This connects directly to the auditory part of the brain, so the return click is processed to give a distance and direction to the prey. The slow clicks and social communication are made about the same way, but some are so powerful that they can stun or even kill nearby prey so they can be eaten easily.

Now you know another way that P. macrocephalus is an exception, few other animals can echolocate, although dolphins do have a much smaller melon for the purpose. We still need to talk about how that vibration is created.

The upper respiratory portion of the spermaceti whale is a thing to behold. There are two nasal passages as you would expect, but they take very different paths. The left nasal passage travels to the left of the spermaceti organ, while the right flattens out and travels between the spermaceti organ and the junk.

Add to this that while the left nasal passage ends in the left nostril – the blowhole, the right nostril doesn’t communicate with the outside world! It ends at the phonic lips, the source of the vibration. As a result, the spermaceti whales have one nostril while all other whales have two, and the one they have is set way off to the left side of the head. This arrangement makes the whale asymmetric.

This is not P. macrocephalus; it’s a blue whale. You can
see the difference easily. The blowhole is way back on
the head, and there are two holes in the blowhole, one
for each nostril.
The position of the blowhole is way up front. All other whales have their blowholes behind the jaw, as the nasal passages go almost straight up. The blowhole being set way off to the left helps make spermaceti whales easy to identify when they surface.

To explain the phonic lips, think of the honk and rumble when some people blow their nose. That’s from vibration of their nostrils. Well, P. macrocephalus does the same thing, although the nostril is inside its head, only located on the right side, and has been modified to look more like our vocal folds.

On first examination, the phonic lips looked like the lips of a monkey, so the French name is museau de singe (see picture above). This makes the P. macrocephalus the only whale with one set of phonic lips, all others have two - and this exception leads to another. Since the two nasal passages are quite separate, a 2005 study found that the spermaceti whale is the only whale that can breath and click at the same time!

In late 2014, seven sperm whales beached themselves
in Australia. This presents a problem because they have
to be cleaned up. As they decay, gas builds up inside.
Somebody (least seniority) has to release that gas.
World’s – worst – job.
Lest all of this hasn’t been impressive enough, the spermaceti organ may have another amazing function. P. macrocephalus dives deeper than any other animal, 3000 m or more. To swim down that far is hard, and if you sink easily, then staying on the surface would be hard. Scientists think P. macrocephalus conserves muscular energy by changing the density of the spermaceti fluid.

When diving, the whale can suck water in through the blowhole. This cools the waxy fluid in the spermaceti organ. The density goes up and helps the whale dive. When it wants to surface, it can increase the blood flow around the organ. This brings more heat and melts the spermaceti. Its lower density makes the whale more buoyant and helps it to surface! Evolution is amazing.

Next week, let’s leave the water and check out some asymmetric flying animals.

Ridgway, S., & Hanson, A. (2014). Sperm Whales and Killer Whales with the Largest Brains of All Toothed Whales Show Extreme Differences in Cerebellum Brain, Behavior and Evolution, 83 (4), 266-274 DOI: 10.1159/000360519

Oliveira, C., Wahlberg, M., Johnson, M., Miller, P., & Madsen, P. (2013). The function of male sperm whale slow clicks in a high latitude habitat: Communication, echolocation, or prey debilitation? The Journal of the Acoustical Society of America, 133 (5) DOI: 10.1121/1.4795798

BODDY, A., McGOWEN, M., SHERWOOD, C., GROSSMAN, L., GOODMAN, M., & WILDMAN, D. (2012). Comparative analysis of encephalization in mammals reveals relaxed constraints on anthropoid primate and cetacean brain scaling Journal of Evolutionary Biology, 25 (5), 981-994 DOI: 10.1111/j.1420-9101.2012.02491.x

Montgomery, S., Geisler, J., McGowen, M., Fox, C., Marino, L., & Gatesy, J. (2013). THE EVOLUTIONARY HISTORY OF CETACEAN BRAIN AND BODY SIZE Evolution, 67 (11), 3339-3353 DOI: 10.1111/evo.12197

For more information or classroom activities, see:

Spermaceti whales –

Encephalization quotient –

Echolocation in whales –

Wednesday, March 18, 2015

The Search For The Unicorn - Slightly Off Center

Biology concepts – teeth, narwhals, unicorns, bilateral symmetry, evolution, mechanosensing, asymmetry

The movie Legend starred Tom Cruise and Mia Sara,
as well as a bunch of little people – you know, actors
that were small, not small actors. The unicorn pair
represented light and goodness, and kept the devil
at bay. Until Mia got cocky and touched one. Then
Cruise had to save the day.
It’s no secret that some pretty odd and awful stories have come out of North Korea in the past few years. Kim Jung Un and his recent ancestors have done some amazing things….. supposedly. Un’s father, Kim Jung Il apparently invented the hamburger, and he shot 11 hole-in-ones in his first round of golf.

Not to be outdone, Kim Jung Un made an amazing announcement in 2012. He and his archeologists discovered a unicorn lair. Yep, North Korea’s twenty-something leader proved the existence of unicorns. The lair was supposedly the resting place of the unicorn ridden by the great King Dongmyeong, around the year 0 CE.

The earliest writings that describe unicorns were those of the Greek, Ctesias, in the late 5th century BCE. He described the Indian Ass, an animal with a white, strong body and perhaps a red head from which sprung a long single horn of red, white, and black. It was said that a cup made from the horn could neutralize any poison.

There are real animals with one horn, like the
unicorn leatherjacket fish in the top left, and the
Indian rhinoceros at the bottom left. The rhinoceros
beetle has one big horn and fairly large part of his
jaw below, so I don’t know if he counts. On the top
right is the Meller’s chameleon. They say he a has a
horn on his nose, but you have to look close and
want to see it.
Four hundred and fifty years later, Pliny the Elder, historian of Rome, also wrote about a very strong animal with a single horn protruding from its forehead. He described an oryx (an antelope with a single horn), an Indian Ox (probably a rhinoceros – rhino = nose and ceros = horn), and the same Indian Ass with a horse-like build and a single horn.

Pliny wrote, “The unicorn (uni = one, and ceros = horn) is the fiercest animal, and it is said that it is impossible to capture one alive. It has the body of a horse, the head of a stag, the feet of an elephant, the tail of a boar, and a single black horn three feet long in the middle of its forehead. Its cry is a deep bellow.” Uh-huh. That doesn’t sound much like an antelope or a rhino, so I guess he meant the Indian Ass.

Soon, Romans were trading long spiral tusks, but no one was telling where exactly they had come from. These “unicorn” horns were snow white with a tight spiral. As a result of these horns, the unicorn in the West settled down to be a pure white horse with a very long, pure white, spiraled horn. This is the image we generally see in tapestries and illustrations.

Kirin Beer from Japan uses a unicorn (kirin) as its
logo. Look closely and you can see the single
horn on its head.
In the Far East there were unicorns as well. Known as the qilin (pronounced chee-lin) in China, there was a version in Japan too, the kirin. This was a benevolent animal, with shiny scales like a dragon and one or perhaps two horns. It avoided fighting and walked so softly that it would not disturb or harm a blade of grass. An animal like this (perhaps the saola) is most likely the one referred to in the North Korea stories.

But what about real life? Most likely, those horns in the Roman markets were really narwhal tusks, as discussed in a 2011 paper. It is very likely that the narwhal played into the unicorn legend, as their tusks could be offered as concrete proof of unicorn existence.

The narwhal (Monodon monoceros) is an amazing animal, and fits into our recent theme of animals that abandon bilateral symmetry. Monodon means one tooth, and monoceros means one horn; a pretty accurate name, all in all.

Our post today uncovers many of the problems
with these cartoon narwhals. Yes, they love where
there is ice. But they don’t have all those teeth, the
tusk isn’t centered and doesn’t come out of their
forehead, and they don’t have a dorsal fin
to speak of.
Narwhals are a species of whale, meaning that they are mammals. They live way up north. From Baffin Bay, around Greenland, to the north of Russian, they swim in pods of 10-100, but you’ll rarely see them even if you live near there. There are perhaps 45,000-50,000 narwhals today.

This is a steady number because it’s so hard to get to where they live. Consequently, narwhals haven’t been hunted into extinction. They spend a lot of their time on deep dives under the ice floes, so they aren’t seen often. No narwhal has ever been seen feeding; we only know what they eat from examining stomach contents.

Their most distinctive feature is the long (up to 10 ft/3 m) tusk on the males. Just one tusk, mind you, like a unicorn horn. The narwhal tusk - like elephant, walrus or warthog tusks - is a tooth.

Very young narwhals have six maxillary (upper jaw) tooth buds and two pairs of tooth buds in the lower jaw (mandible). However, only one pair develops any further. A tooth bud is what you find on an X-ray of a child (see picture).

You can see the teeth developing from crown to
root in the darker tooth buds. The pulp is usually
dark, but the middle tooth has had a root canal
and a filling has been placed in the whole pulp
chamber. The large tooth to eh left is the first
molar. It doesn’t have a baby tooth to push out
of its way.
Teeth form in the jawbones as tooth buds. Most narwhal teeth never go past the tooth bud stage, but occasionally a tooth will erupt where one shouldn’t. These are often misshapen or caught between the bone and the palate, or in the wrong place. This is all good evidence that the teeth are vestigial; they serve no functional purpose for the normal narwhal.

Just one tooth, almost always the left cuspid (most people call it a canine), does develop. Hold on though, it isn’t that simple. Instead of developing in a vertically directed tooth bud and erupting down through the jaw, the left canine stays horizontal and erupt right through the front of the jaw and through the narwhals lip!

Since the tusk is derived from the left cuspid, it erupts left of center, making the narwhal bilaterally asymmetric! A 2012 study showed that the bony attachment and length proves that the narwhal tusk is a canine, not an incisor as so many people think. But, it’s not just the location that makes the narwhal tusk amazing, it’s how it’s made and what it can do.

A 1988 study suggests that the tight spiral as it grows keep the tusk from curving. A curved tusk would make it hard of the narwhal to swim in a straight line. Whatever the reason, the spiral is an iconic image for both narwhals and unicorns.

The top image shows how the narwhal tusk is off
center. The bottom image is my analogy. The tusk
is offset like a knight with his jousting lance. This
is Heath Ledger in A Knight’s Tale. Um….why isn’t
he wearing armor?
Despite being a tooth, the tusk is quite flexible. It can bend up to a foot (0.3 m) in any direction without breaking. It’s awfully long, we said 10 ft. above, but most are in the 8-9 foot range. This is huge when you think that most male narwhals are only about 15 foot long in the body.

Teeth are normally built with the hard enamel on the outside. Enamel is harder than bone and protects the teeth from breakage when chewing. The mouth is a rough environment and teeth have to put up with a lot of abuse.

Deep to the enamel is a material called dentin. This stuff has a lot of similarity to bone, although it isn’t quite as hard and doesn’t have living cells within it (like osteocytes – see this post). The dentin does contain millions of tubules that go from the enamel junction all the way to the pulp in the center. The pulp has a nerve and blood vessels.

The dentinal tubules have fluid and small processes of the neuron in them. When you eat something cold or have a cavity, the fluid in these tubules moves and changes the pressure in the pulp chamber. The single neuron in the tooth is a pain neuron, so any pressure change is interpreted by your brain as pain. It teaches you to take care of your teeth, but it ain’t the most pleasant of all evolutionary adaptations.

The cartoon on the left shows the enamel crown
covering the dentin and the dentinal tubules.
Inside the tubules are the odontoblasts that lay
down dentin all during the life of the tooth and the
nerves that go into the tubules. The right image is
an electron photomicrograph of the tubules.
The narwhal tusk is different. It is the only tooth known that has the dentin on the outside, although a 1987 study showed that it has no enamel, so it isn’t really an inside out tooth. The dentin is covered by a thin layer of cementum. This is what normally covers the roots of the teeth and helps attach them to bone. The dentin of the narwhal tusk has about 10 million of those tubules, but it is different from human dentin.

A 1990 study compared calcium content and hardness between human teeth and narwhals. The narwhal cementum was more mineralized than human, but the dentin of narwhals was less mineralized than human dentin and was softer. This may be why the narwhal tusk is so flexible.

The tubules of the narwhal tusk dentin connect to channels in the cementum, so there is a communication to the outside. A group in 2014 showed this and used the information to hypothesize that the tusk is a mechanosensor. Experiments showed that their heart rate changed when the water touching the tusk was switched from freshwater to salt water. They hypothesize that the tusk senses temperature, salinity, pressure, and perhaps touch to help in navigation and hunting.

But if that’s the case, why do only males have them? Females have to hunt too. The group from the 2014 paper offers that males and females have sexually dimorphic foraging techniques – they eat different things and hunt differently, so females don’t need horns. This is not well-supported. Many scientists believe the long tusk is a sign of health and genes and is therefore an ornament for mate selection.

The dorsal fin of the narwhal is greatly reduced. It
has notches that scientists hop to use to identify
individuals. The lack of a dorsal fin is believed to
be so they don’t injure it on the under side of the
ice floes when they surface, but it could also be so
they don’t run it into the ocean floor as they feed
upside down.
Occasionally, one will see females with a tusk, but like with many tusked females (elephants, etc), they are usually shorter. You can also find narwhal males with two tusks. But two tusks doesn’t mean that they are returned to bilateral symmetry. Both tusks spiral to the left! There must be some strong left-hand genes at work.

One last thing. The offset tusk lead to another weird narwhal behavior. A group in 2007 put cameras and positional monitors on some narwhals and found that they tend to swim upside down a lot. Almost 70% of their time on the ocean floor was spent in the supine position. Since the tusk points down just slightly, scientists believe they hunt upside down so that the tusk won’t get stuck in the ocean floor and break! The tusk must be pretty important - or they just like lounging on their backs.

Next week – another whale has become asymmetric, but in a completely different way. This time, it’s the nose that goes.

Christen AG, & Christen JA (2011). The unicorn and the narwhal: a tale of the tooth. Journal of the history of dentistry, 59 (3), 135-42 PMID: 22372187

Kingsley, M., & Ramsay, M. (1988). The Spiral in the Tusk of the Narwhal ARCTIC, 41 (3) DOI: 10.14430/arctic1723

Nweeia, M., Eichmiller, F., Hauschka, P., Donahue, G., Orr, J., Ferguson, S., Watt, C., Mead, J., Potter, C., Dietz, R., Giuseppetti, A., Black, S., Trachtenberg, A., & Kuo, W. (2014). Sensory ability in the narwhal tooth organ system The Anatomical Record, 297 (4), 599-617 DOI: 10.1002/ar.22886

Dietz, R., Shapiro, A., Bakhtiari, M., Orr, J., Tyack, P., Richard, P., Eskesen, I., & Marshall, G. (2007). Upside-down swimming behaviour of free-ranging narwhals BMC Ecology, 7 (1) DOI: 10.1186/1472-6785-7-14

For more information or classroom activities, see:

Narwhals –

Tooth structure –

Wednesday, March 11, 2015

The Eyes Have It

Biology concepts – asymmetry, lateral polymorphism, flatfish, evolution, copepod, ecology, niche

Ray Harryhausen was the most famous of the stop
motion artists in the movies. This version of the
Cyclops was his creation for the 1958 movie, The 7th
Voyage of Sinbad. I can’t see how the Cyclops could
catch anything with just the one eye – he had no
depth perception.
We have been talking about bilateral symmetry in the past few weeks, and this would include having two eyes, one on each half of your face. Two eyes must be a pretty important evolutionary adaptation; can you think of an animal that has just one eye – other than a cyclops, that is? (the answer is somewhere in post) Some protists have a single eyespot for sensing light – but they aren’t animals.

Predators need to catch their food, so they need depth perception. For this you need two overlapping images. You don’t do the math consciously, but your brain uses the differences in each image to tell you how far away the target prey is. To get two images simultaneously, you need both eyes to be on the front of your face.

Prey animals, usually herbivores, have to worry about being chased down by a predator. Prey animals are usually quick, but they need clues to get a good start before the predator gets too close. Their eyes are designed to pick up motion; it doesn’t matter how far away it is. If something moves in their line of sight, they assume it’s a predator and they bolt. By having their eyes on the sides of their head, they have a maximal range of vision which gives them the best chance to see that lioness coming.

Just to show you how important evolution thinks it is to have at least two eyes, let’s discuss a group of animals that have found a way to make two drastic changes work for them. Flatfish have evolved to lie on their side, but they’re predators so they can’t afford to have one eye constantly seeing nothing but the sandy ocean bottom. Consequently, they have moved one eye to the other side of their head!

This is a winter flounder (Pseudopleuronectes
americanus). Notice how it blends in to the floor. It
does more by flapping and tossing sand on its back.
The mouth points up when the fish is vertical, so the
left most eye is the one that migrated. Makes sense,
you wouldn’t want to move an eye under your chin –
that would be silly.
Flatfish are all from the order Pleuronectiformes (pleuro = toward the side), sometimes called the Heterosomata (hetero = differently, and soma = bodied). There are some 715 species of flatfish in 11 families, and they include the turbots, sole, flounder, plaice, and halibut. They are all predators that lie in wait on the ocean-, lake-, sea-, or riverbed. But they don’t start out that way.

All the Pleuronectifromes start out as fry that swim upright. Their top is at the top, and the bottom is toward the bottom. They live nearer the top of the water than the bottom, have pigment coloration on both sides of their body, and feed on phyto- and zooplankton. But the surface isn’t the safest place to live; bigger fish are always around to eat the small fry.

So, as they start to develop into adults, many changes take place. They swim down to the floor of whatever body of water they call home. One eye starts to move! It travels over the top of their head and onto the other side, like the Mr. Potato Head of some deranged child.

As you can imagine, moving an eye isn’t an easy thing to do. Their brain has to move, as do the cranial and facial bones. Their mouth has to make room for the eye coming its way. All in all, it’s a tough piece of work.

The top image is the visible side of a rock sole, the
bottom image is the lower side of the same fish. As the
fish matures and lies on its side, the coloring changes.
Pigments are energetically expensive to make, so why
waste them. The middle image shows the line where it
goes from pigmented to unpigmented. What control!
Moving an eye from one side of the head to the other seems illogical, why not just develop with both eyes on one side, or make do with one eye? Creationists have used the flatfishes as an argument against evolution. We have fish with eyes on each side of their head, and fish with both eyes on one side of their head. They argued that if natural selection was responsible for the change in flatfish, there should be fossils or fish that are intermediates.

Well, there are evolutionary intermediates – even some living examples. A paper in 2008 introduced us to two extinct species of flatfish. In each (Amphisitium and Heteronectes), one eye had migrated, making the fish asymmetric, but it had not crossed the crown of the skull and made it to the other side. These are definitely intermediate species, but we can go one better.

The Psettodes genus of flatfish (the turbots - yummy by the way) have one eye that is located right at the crown of their skull. Perhaps technically you could say that is has migrated to the other side, but just barely. And this brings us to another question, which eye moves?

Some flatfish are right-eyed (dextral) that swim left side down. Others are left-eyed (sinistral) and swim with their right side down. Some species are strict and some are more likely to have reversants (individuals that lay on their other side). A 2005 paper stated that only about 7 of the 700 species of flatfish show lateral polymorphism (lateral = side, poly = more than one, and morph = shape), ie. some right-eyed and some left-eyed individuals.

The bottom image has on the left side depicts the two
extinct species of flatfish where the migrating eye hadn’t
quite made it to its destination. The middle cartoon is
the extant Psettodidae, like the Indian Halibut on the top.
The eye has just made it to the crest of the cranium. The
right cartoon on bottom shows a species where the eye
has completely migrated. Looks like great support
for evolution.
For example, a 2009 paper describes reversants for two right-eyed flounder species which are the first left-eyed individuals ever seen in these two species (Microstomus achne and Cleisthenes pinetorum). It goes the other way too – a 2013 study describes the first right-eyed individual ever seen in a megrim (sometimes called a whiff, Lepidorhombus whiffiagonis). But in the left-eyed California halibut (Paralicythys californicus), up to 40% of the individuals are right-eyed. Perhaps right-eyed species are stricter than left-eyed species.

The exception to that rule is the starry flounder (Platichthys stellatus). It’s a member of a right-eyed family of flounders, but in some cases half of the individuals are left-eyed. In this case, there seems to be more to the story - the lateral polymorphism occurs only in populations of specific geographic areas.

A paper from 2007 looked into the mystery that 7 species that show lateral polymorphism, but only two (starry flounder and P. fleusus) show a geographical distribution in their polymorphs. Off the coasts of Japan and Russian, 100% of the starry flounders are left-eyed, but near Alaska they are only 75% sinistral and from Washington state to central California the populations are about 50/50.

The researchers looked into several questions. Was there more to being left-eyed or right eyed than just which side the eye went to? Does the side make a difference that could account for the geographic distribution? They found out – yes, and yes.

They saw that the right and left-eyed individuals have more asymmetries between them than just the side of the body that the eye migrates to. They have differences in mouth size and angle, as well as tail size of all things. Right-eyed individuals had significantly longer and wider tails than did left-eyed individuals!

The research also shows that in areas where the two groups compete the most, the differences between the dextral and sinistral individuals are the greatest. This suggested to them that the differences allow them to compete for different prey – to fill different ecological niches. The hypothesis is that the polymorphic asymmetries give them different advantages which they then exploit and this is why there is a stable geographic distribution in populations.

A very complicated but informative chart from the paper
on pitx2 reactivation during metamorphosis. Note which
side is right and left and then follow what happens for
the two species, one right-eyed and one left-eyed. The
right side shows what can happen if you block pitx2. In
the reversed individual, the left habenula (dd. l) enlarges
instead of the right.
Well, that’s cool, but how is it controlled – what makes a fish right- or left-eyed? A 2009 paper started to investigate this. During development of the embryo, certain internal asymmetries develop (will talk more about this in a few weeks). In fish and other vertebrates, this is controlled by expression of certain genes in the habenula of the brain. One of the gene products (proteins) in the habenula is called pitx2.

There are actually two habenulae, one in each hemisphere of the brain, as part of the thalamus. The researchers looked at the brains of right-eyed and left-eyed flounder species and detected some things that were the same and some things that were different.

After the pitx2 did its job in the embryo, it was turned off. But right before metamorphosis – when the eye migrates and the fish lies down on one side, pitx2 was turned back on – only in the left habenula, regardless of which way the fish’s eye was going to migrate and which way it was going to lie down.

Not only that, but the right habenula started to grow bigger than the left habenula in both dextral and sinistral species. The only thing that was different was the rotation of the brain, with the left habenula moving forward in the right-eyed species and the opposite occurring in the left-eyed species. The same changes occurred in fish no matter which eye migrated!

This copepod is about to swim into the top border of
this image. The reason for this is that he only has one eye
(the light spot between the antennae), so he has no depth
perception! By the way, he’s only 2 mm long, so he
probably will just bounce off the edge.
However, in those fish that they experimentally blocked the second round of pitx2 activity, you could get a normally turned fish or a reversant. The only difference - when you got a reversant, it was the left habenula that enlarged, not the right. I think we still have some more to learn.

By the way – at the beginning of today’s post I asked if there were any real animals with one eye. I made it sound like there aren’t but in fact there are exceptions. In some small crustaceans (arthropods) called copepods, the majority of species have a single eye right in the middle of their head! What’s more, a 1994 study showed that the holdfast of a particular copepod parasite is asymmetric (like we saw last week) and this particular copepod is a parasite of only flatfishes! How symmetric this tale of asymmetry turned out to be!

Next week - can a single tooth render an animal asymmetric? Well, that depends on the tooth, doesn't it.

MacDonald P (2013). A rare occurrence of reversal in the common megrim Lepidorhombus whiffiagonis (Pleuronectiformes: Scophthalmidae) in the northern North Sea. Journal of fish biology, 83 (3), 691-4 PMID: 23991885

Suzuki, T., Washio, Y., Aritaki, M., Fujinami, Y., Shimizu, D., Uji, S., & Hashimoto, H. (2009). Metamorphic pitx2 expression in the left habenula correlated with lateralization of eye-sidedness in flounder Development, Growth & Differentiation, 51 (9), 797-808 DOI: 10.1111/j.1440-169X.2009.01139.x

Goto T (2009). Reversals in two dextral flounder species, Microstomus achne and Cleisthenes pinetorum (Pleuronectida; Teleostei), from Japan. Journal of fish biology, 74 (3), 669-73 PMID: 20735586

BERGSTROM, C. (2007). Morphological evidence of correlational selection and ecological segregation between dextral and sinistral forms in a polymorphic flatfish, Platichthys stellatus Journal of Evolutionary Biology, 20 (3), 1104-1114 DOI: 10.1111/j.1420-9101.2006.01290.x

Friedman, M. (2008). The evolutionary origin of flatfish asymmetry Nature, 454 (7201), 209-212 DOI: 10.1038/nature07108

For more information or classroom activities, see:

Flatfish –

Copepods –

Habenula -