Eye Didn’t See That

By Evan Hovey

A grandmother and her grandson watching the television.
The elder is straining to see while the young man is not having any troubles.
Photo by Evan Hovey.

It’s Thanksgiving and the family just finished stuffing their faces full of turkey and cranberry fluff. Everyone meanders into the living room to sit down, let the tryptophan sink in, and watch some football. As you sit there, you start to observe the older family members around you take out their glasses or bifocals and squint towards the television in attempts to see what is going on. You begin to ponder the thought, “am I going to start to lose my eye sight as well?” Well, as it turns out, as you age, the number of cells that respond to light and color (called photoreceptors) begins to decrease.

Songhomitra Panda-Jonas, Jost Jonas, and Martha Jakobczyk-Zmija at the University of Erlangen-Nurnberg, Germany, looked into the number of photoreceptors in the retina of the eye to determine whether there was a loss as you age. The retina is the thin layer of tissue that lines the back of the eye. It is the location where your eye transfers what you see to the brain. There are two different kinds of photoreceptors in your eyes: rods and cones. Rods are those that detect light at low levels, which is what helps us see at night. Cones, on the contrary, are those that take in high light levels and help decode color. The authors believed that there would be a decrease in both kinds of photoreceptors as the eye got older (the older the person, the fewer photoreceptors). They came up with this hypothesis in part because of prior knowledge of a loss of tissue associated with vision in other parts of the eye as you age.

The researchers approached this study by obtaining fifty-five eyes from human donors that died at ages ranging from 18-85. The eyes were removed from the bodies less than eleven hours after death. Then the eyes were cut open and tissue samples from the retina were obtained. To determine the amount of photoreceptors in the tissue samples, the researchers used an ultrasound to view the retina and counted the photoreceptors on a photograph taken with the ultrasound. The two different kinds of cells were distinguished by their sizes (the larger cells were the cones and the smaller cells were the rods).

The results they found were as expected: the older you get, the fewer photoreceptors you have and the worse your eyesight is. The decline of the number of photoreceptors was at a constant rate throughout all ages of life. However, the number of rods declined faster than the number of cones. The loss of these photoreceptors causes you to view things with more difficulty. As your rods die, you begin to develop night blindness (the inability to see well in poor lighting or darkness). When your cones die, you begin to lose more of your visual perception, which includes straining when looking at something from a distance, as well as affecting how you see fine detail such as reading a book or looking at a television. The combined loss of your rods and cones is part of what causes older individuals to have more vision problems.

As you progress through life, your photoreceptors decline, causing your vision to get worse. As you sit down after Thanksgiving to enjoy some good old-fashioned fall football and the elderly people strain to see the television, you now know that the oldest person in your family is most likely having the hardest time seeing that big touchdown.


If you would like to read the actual paper, the source is located below:

Panda-Jonas, S., Jonas J., Jakobczyk-Zmija, M. (1995). Retinal photoreceptor density decreases with age: Ophthalmology, 102 (12), 1853-1859

When The Going Gets Tough, The Tough Become Babies

Today I am giving new young life to a post from 2012. You can find the original here.

We celebrate the New Year as a time of rebirth, renewal, and do-overs. We join gyms, swear off our bad habits, and promise to be better people. This is especially true for those of us that have had a rough 2015... Our 2016-version-of-us has got to be better, right? But what if you could get a real do-over? What if you could be a kid again, grow up again, and become a brand new person? As far-fetched as it may sound, some animals do exactly that.

Cnidarians (the "C" is silent) are a huge group of aquatic animals that includes jellyfish, corals, and anemones (like the one Nemo lived in - Yeah, that tentacled home was a living animal). They are named after prickly plants known as nettles, or cnides in Greek, and if you touch one you will quickly know why. Cnidarians, armed with stinging cells called nematocysts, sting at the slightest touch.

Jellyfish make up many of the cnidarian species, and they have been found in every ocean and at every depth. Some even live in freshwater. The "typical" jellyfish life cycle starts when eggs and sperm are released into the water and find one another. When they do, they form larvae, which you can think of as baby jellyfish. The larvae sink and settle on a hard surface, where they mature into polyps. These polyps are jellyfish in a juvenile stage. The polyps elongate and begin to bud off adult medusa, which are the bell-shaped blobs with tentacles that most of us think of when we think of a jellyfish. Medusa mature to become reproductive adult jellyfish.


The jellyfish life cycle by Zina Deretsky at the National Science Foundation (NSF). Image available at Wikimedia.

Larval and polyp jellyfish are much more resistant to harsh conditions then are medusa jellyfish. When life gets hard for a jellyfish, perhaps because of starvation, physical damage, temperature changes or salinity changes, those that are in the larval or polyp stages can often shrink and rest in a hibernation-like state while they wait for more favorable conditions. But in some species, young adult medusa can even regress back to the juvenile polyp stage. By reverting back to a juvenile stage, they have more protection from the challenging world around them.

In most cases, this reversal to a juvenile state can only happen in young medusa that have not yet developed their gonads. Thus, the onset of sexual reproduction (puberty, if you will) might be regarded as the point of no return in development. However, one species, called the immortal jellyfish, has shown that this rule can be broken.

As an adult medusa, the immortal jellyfish is a pea-sized jellyfish with a round bell, bright red stomach and anywhere from 8 to 90 tentacles. It is currently the only known animal that can regress from a fully reproductively mature adult into a juvenile polyp. If exposed to dangerous conditions, immortal jellyfish medusae completely reduce all of their medusa-specific organs and tissues and develop new polyp-specific tissues, essentially becoming kids again!

This figure from the Piraino et al. 2004 paper at the Canadian Journal of Zoology shows the life stages of the immortal jellyfish. The adult medusa is in panel (a). Panels (b) and (c) show the medusa tranforming to a ball-like blob as it reverts to a juvenile stage. The green stain in these panels shows the cells initiating this transformation. Panel (d) shows the remnant of a medussa, and the black arrow shows the stalk that is common in the polyp stage. Panel (e) shows the resulting juvenile polyp.

But wait! It gets better! Theoretically, if an animal can revert to a juvenile stage at any point in its adult life, it could attain immortality. But if that were true, they would have the classic immortality problem: These animals would reach such high populations they would saturate the world's oceans...And this may actually be happening.

Immortal jellyfish are thought to originally be from the Caribbean, but they have since been discovered worldwide and their populations seem to be growing. Likely, they are hitching rides in the ballast water that is sucked into cargo ships to provide stability. If this is true, the immortal jellyfish polyps could be attaching to the ships' hulls and settling in for a long voyage to a new home.

We don't yet know if the immortal jellyfish are actually immortal, but it is fun to consider that they might be (although they can still be killed by predators or viruses, so they're not invincible). And we can take inspiration from them: When the going gets tough, try reverting to your more resilient juvenile self, but be thankful you don't have to go through middle school again!

Happy New Year!

To learn more, check these out:

1. Piraino, S., De Vito, D., Schmich, J., Bouillon, J., & Boero, F. (2004). Reverse development in Cnidaria Canadian Journal of Zoology, 82 (11), 1748-1754 DOI: 10.1139/z04-174vv

2. Miglietta, M., & Lessios, H. (2008). A silent invasion Biological Invasions, 11 (4), 825-834 DOI: 10.1007/s10530-008-9296-0

This Animal Looks Like a Penis With Teeth... But It's Even Stranger Than That

This is a naked mole rat.

Yes, this is arguably the freakiest-looking animal on Earth.
Photo by Roman Klementschitz at Wikimedia Commons.

Naked mole rats are rodents that live in underground tunnels under East African savannas and grasslands. There's nothing all that strange about that... but how they have adapted to this lifestyle is unique... and, quite frankly, amazing.

For one thing, to cope with the low oxygen levels of the subterranean environment, naked mole rats have very low metabolisms and breathing rates. One of the biggest uses of metabolic engines in mammals is to produce our own body heat. These little guys have cut this big expenditure by being what may be the only ectothermic mammals on the planet. Ectotherms are animals like most fish, amphibians and reptiles that get most of their body heat from their environment, rather than making it themselves. Because naked mole rats want to exchange heat with their environment, they want to eliminate insulation... giving them their hairless and fatless bodies. Now when they bask in the sun at their tunnel entrances or huddle with their family they can take in all that warmth without anything getting in the way.

There are some benefits to having low metabolisms and not using much oxygen: Naked mole rats live for nearly 30 years (compared to 1-3 years in regular rats). Oxygen creates free radicals, highly reactive chemicals that cause damage to DNA, leading to a wide range of diseases. Naked mole rats don't just use less oxygen, but they have special proteins that are resistant to these damaging chemicals. They also produce a specialized super-sugar that has essentially eliminated cancer in this long-living species. What's more, these elderly rodents have managed to avoid dementia and osteoporosis, traits we hope to learn more about through ongoing research.

Naked mole rats have also developed some unique sensory traits. Living with your entire extended family in underground burrows means that you live in high levels of carbon dioxide and walls saturated in pee. These little guys don't even have any body hair to protect their pink skin from all that burning ammonia. Their solution: get rid of pain. These guys have no pain receptors for noxious chemicals like acids or capsaicin (the stuff that makes hot peppers hot). Furthermore, they are lacking a specific neurotransmitter, called substance P, that other mammals use to send many pain signals. Since naked mole rats have less need for sensation in their skin, they have developed brains that have repurposed about 30% of the sematosensory cortex (the part of the brain that interprets touch sensations) to their digging teeth!

Perhaps the strangest quality of all for these animals is their behavior. Naked mole rats are one of only two known mammals that are eusocial (the other being the Damaraland mole rat). Eusociality is a social organization common among bees, wasps, ants and termites, in which the colony has castes that include queens, workers and soldiers. Among naked mole rats, there is a single queen in the colony that mates with a few dominant males; workers that dig the tunnels, gather food, and care for the young; and soldiers that protect the colony from predators. Workers and soldiers are all reproductively sterile with undeveloped gonads and low hormone levels. However, if the queen dies, one of the non-reproducing females will go through puberty and take on her role as the new queen.

Now that you know that naked mole rats are so much more than just a "freaky thing", enjoy this naked mole rat rap (or maybe even a whole episode of Disney's Kim Possible, which features Rufus, the naked mole rat):

Loving to Death

The brown antechinus may look like a
mouse - but that is where the similarities
end. Photo by Glen Fergus at Wikimedia.

Although most animal species breed multiple times throughout their lives, a few oddballs put everything they've got into a single reproductive season, after which they promptly die. This is a rare strategy (for obvious reasons), especially in mammals. One Australian mammal, the brown antechinus, is just odd enough to pull it off.

The brown antechinus is a small insectivorous mouse-sized critter from Australia that in fact is not a mouse at all. It is a marsupial; but unlike kangaroos and koalas, females do not carry their young in a pouch, but rather let them hang off their eight teats for four months. All males die when they are 11 months old (if not sooner) after a single 2-3 week long mating season during which they do little else than mate as often as possible. The mating season leaves all the males (whether mated or not) sterile, coursing with stress hormones, immunosuppressed, and riddled with microorganisms and parasites. Shortly thereafter all the males die, balding and bleeding messes.

The reproductive strategy of putting everything you've got into a single mating season and then dying is only an advantage if you can have many offspring in that single reproductive event. Male brown antechinuses can only succeed in this suicidal mating strategy if they father many of the young of many of the females. As a result, both male and female brown antechinuses are promiscuous (mate with many individuals).

Male brown antechinuses are generally bigger than females, and DNA testing has shown us that in the wild, larger males and males with bigger testes impregnate the most females. Diana Fisher and Andrew Cockburn from Australian National University tested whether larger male brown antechinuses were more likely to get the girls because females were more likely to choose them or because they were outcompeting other males.

Diana and Andrew trapped brown antechinuses and brought them into the lab. In one test, they placed three males in separate nest boxes next to one another in an arena and allowed females to choose among them and mate with whichever one she chose. Surprisingly, when presented with this choice, females did not consistently choose the largest males. They didn't even check them all out - The females mated with whatever male happened to be in the first nest box she entered.

When the researchers put three males into a single nest box and allowed the females to mate, she almost always immediately mated with one of the three males. The next day, the researchers put the female in a nest box with either the two losers from the day before or with two randomly chosen males she did not know. On this second day, females presented with two strangers immediately mated with one male, whereas females presented with the two losers from the day before were more likely to spend more time evading both males, but often eventually mated with one of them. On the third day, the researchers put the female in a nest box with either the loser from the previous two days or with another randomly chosen stranger. Nine out of ten females paired with a stranger mated with him on this third day, whereas only one female paired with a double-loser was willing to mate with him at all. Males that successfully mated on the first day were generally the largest of the three. Loser males that mated on the second day were generally the second-largest and unsuccessful males were generally the smallest.

Interestingly, when given a choice of males one at a time, female brown antechinuses do not seem to care at all about male size. But when males are directly competing with one another, the largest male seems to get the girl. It appears that body size plays a role in the dominance interactions among the males, and that females are paying attention to how the males relate to one another. Additionally, larger males that were more successful in mating also lived longer and had fewer parasites. This could be because it is more stressful to be a loser than to be a winner. Stress increases the production of stress hormones, which in turn reduces immune function. In all of these ways, bigger males are more likely to father more young, who in turn will be more likely to grow up to be big males too... but not for long...


Want to know more? Check these out:

Fisher, D., & Cockburn, A. (2005). The large-male advantage in brown antechinuses: female choice, male dominance, and delayed male death Behavioral Ecology, 17 (2), 164-171 DOI: 10.1093/beheco/arj012

Doing it to death: suicidal sex in "marsupial mice" at The Conversation

When The Going Gets Tough, The Tough Become Babies

We celebrate the New Year as a time of rebirth, renewal, and do-overs. We join gyms, swear off our bad habits, and promise to be better people. This is especially true for those of us that have had a rough 2012… Our 2013-version-of-us has got to be better, right? But what if you could get a real do-over? What if you could be a kid again, grow up again, and become a brand new person? As far-fetched as it may sound, some animals do exactly that.

Cnidarians (the “C” is silent) are a huge group of aquatic animals that includes jellyfish, corals, and anemones (like the one Nemo lived in – Yeah, that tentacled home was a living animal). They are named after prickly plants known as nettles, or cnides in Greek, and if you touch one you will quickly know why. Cnidarians, armed with stinging cells called nematocysts, sting at the slightest touch.

Jellyfish make up many of the cnidarian species, and they have been found in every ocean and at every depth. Some even live in freshwater. The “typical” jellyfish life cycle starts when eggs and sperm are released into the water and find one another. When they do, they form larvae, which you can think of as baby jellyfish. The larvae sink and settle on a hard surface, where they mature into polyps. These polyps are jellyfish in a juvenile stage. The polyps elongate and begin to bud off adult medusa, which are the bell-shaped blobs with tentacles that most of us think of when we think of a jellyfish. Medusa mature to become reproductive adult jellyfish.

The jellyfish life cycle by Zina Deretsky at the National Science Foundation (NSF). Image available at Wikimedia.

Larval and polyp jellyfish are much more resistant to harsh conditions then are medusa jellyfish. When life gets hard for a jellyfish, perhaps because of starvation, physical damage, temperature changes or salinity changes, those that are in the larval or polyp stages can often shrink and rest in a hibernation-like state while they wait for more favorable conditions. But in some species, young adult medusa can even regress back to the juvenile polyp stage. By reverting back to a juvenile stage, they have more protection from the challenging world around them.

In most cases, this reversal to a juvenile state can only happen in young medusa that have not yet developed their gonads. Thus, the onset of sexual reproduction (puberty, if you will) might be regarded as the point of no return in development. However, one species, called the immortal jellyfish, has shown that this rule can be broken.

As an adult medusa, the immortal jellyfish is a pea-sized jellyfish with a round bell, bright red stomach and anywhere from 8 to 90 tentacles. It is currently the only known animal that can regress from a fully reproductively mature adult into a juvenile polyp. If exposed to dangerous conditions, immortal jellyfish medusae completely reduce all of their medusa-specific organs and tissues and develop new polyp-specific tissues, essentially becoming kids again!


This figure from the Piraino et al. 2004 paper at the Canadian Journal of Zoology shows the life stages of the immortal jellyfish. The adult medusa is in panel (a). Panels (b) and (c) show the medusa tranforming to a ball-like blob as it reverts to a juvenile stage. The green stain in these panels shows the cells initiating this transformation. Panel (d) shows the remnant of a medussa, and the black arrow shows the stalk that is common in the polyp stage. Panel (e) shows the resulting juvenile polyp.

But wait! It gets better! Theoretically, if an animal can revert to a juvenile stage at any point in its adult life, it could attain immortality. But if that were true, they would have the classic immortality problem: These animals would reach such high populations they would saturate the world’s oceans…And this may actually be happening.

Immortal jellyfish are thought to originally be from the Caribbean, but they have since been discovered worldwide and their populations seem to be growing. Likely, they are hitching rides in the ballast water that is sucked into cargo ships to provide stability. If this is true, the immortal jellyfish polyps could be attaching to the ships’ hulls and settling in for a long voyage to a new home.

We don’t yet know if the immortal jellyfish are actually immortal, but it is fun to consider that they might be (although they can still be killed by predators or viruses, so they’re not invincible). And we can take inspiration from them: When the going gets tough, try reverting to your more resilient juvenile self, but be thankful you don’t have to go through middle school again!

Happy New Year!

Want to know more? Check these out:

1. Piraino, S., De Vito, D., Schmich, J., Bouillon, J., & Boero, F. (2004). Reverse development in Cnidaria Canadian Journal of Zoology, 82 (11), 1748-1754 DOI: 10.1139/z04-174

2. Miglietta, M., & Lessios, H. (2008). A silent invasion Biological Invasions, 11 (4), 825-834 DOI: 10.1007/s10530-008-9296-0