An Inconvenient Hagfish

On the importance of intermediates.

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1280px-eptatretus_stoutii

We think of scientific progress as working like building blocks constantly being added to a growing structure, but sometimes a scientific discovery can actually lead us to realize that we know less than we thought we did. Take vision, for instance. Vertebrates (animals with backbones) have complex, highly-developed “camera” eyes, which include a lens and an image-forming retina, while our invertebrate evolutionary ancestors had only eye spots, which are comparatively very simple and can only sense changes in light level.

At some point between vertebrates and their invertebrate ancestors, primitive patches of light sensitive cells which served only to alert their owners to day/night cycles and perhaps the passing of dangerous shadows, evolved into an incredibly intricate organ capable of forming clear, sharp images; distinguishing minute movements; and detecting minor shifts in light intensity.

584px-diagram_of_eye_evolution-svg
Schematic of how the vertebrate eye is hypothesized to have evolved, by Matticus78

In order for evolutionary biologists to fully understand when and how this massive leap in complexity was made, we need an intermediate stage. Intermediates usually come in the form of transitional fossils; that is, remains of organisms that are early examples of a new lineage, and don’t yet possess all of the features that would later evolve in that group. An intriguing and relatively recent example is Tiktaalik, a creature discovered on Ellesmere Island (Canada) in 2004, which appears to be an ancestor of all terrestrial vertebrates, and which possesses intermediate characteristics between fish and tetrapods (animals with four limbs, the earliest of which still lived in the water), such as wrist joints and primitive lungs. The discovery of this fossil has enabled biologists to see what key innovations allowed vertebrates to move onto land, and to precisely date when it happened.

There are also species which are referred to as “living fossils”, organisms which bear a striking resemblance to their ancient ancestors, and which are believed to have physically changed little since that time. (We’ve actually covered a number of interesting living fossils on this blog, including lungfish, Welwitschia, aardvarks, the platypus, and horseshoe crabs.) In the absence of the right fossil, or in the case of soft body parts that aren’t usually well-preserved in fossils, these species can sometimes answer important questions. While we can’t be certain that an ancient ancestor was similar in every respect to a living fossil, assuming so can be a good starting point until better (and possibly contradictory) evidence comes along.

So where does that leave us with the evolution of eyes? Well, eyes being made of soft tissue, they are rarely well preserved in the fossil record, so this was one case in which looking at a living fossil was both possible and made sense.

Hagfish, which look like a cross between a snake and an eel, sit at the base of the vertebrate family tree (although they are not quite vertebrates themselves), a sort of “proto-vertebrate.” Hagfish are considered to be a living fossil of their ancient, jawless fish ancestors, appearing remarkably similar to those examined from fossils. They also have primitive eyes. Assuming that contemporary hagfishes were representative of their ancient progenitors, this indicated that the first proto-vertebrates did not yet have complex eyes, and gave scientists an earliest possible date for the development of this feature. If proto-vertebrates didn’t have them, but all later, true vertebrates did, then complex eyes were no more than 530 million years old, corresponding to the time of the common ancestor of hagfish and vertebrates. Or so we believed.

hagfish
The hagfish (ancestors) in question.  Taken from: Gabbott et al. (2016) Proc. R. Soc. B. 283: 20161151

This past summer, a new piece of research was published which upended our assumptions. A detailed electron microscope and spectral analysis of fossilized Mayomyzon (the hagfish ancestor) has indicated the presence of pigment-bearing organelles called melanosomes, which are themselves indicative of a retina. Previously, these melanosomes, which appear in the fossil as dark spots, had been interpreted as either microbes or a decay-resistant material such as cartilage.

This new finding suggests that the simple eyes of living hagfish are not a trait passed down unchanged through the ages, but the result of degeneration over time, perhaps due to their no longer being needed for survival (much like the sense of smell in primates). What’s more, science has now lost its anchor point for the beginning of vertebrate-type eyes. If an organism with pigmented cells and a retina existed 530 million years ago, then these structures must have begun to develop significantly earlier, although until a fossil is discovered that shows an intermediate stage between Mayomyzon and primitive invertebrate eyes, we can only speculate as to how much earlier.

This discovery is intriguing because it shows how new evidence can sometimes remove some of those already-placed building blocks of knowledge, and how something as apparently minor as tiny dark spots on a fossil can cause us to have to reevaluate long-held assumptions.

Sources

  • Gabbott et al. (2016) Proc. R. Soc. B. 283: 20161151
  • Lamb et al. (2007) Nature Rev. Neuroscience 8: 960-975

*The image at the top of the page is of Pacific hagfish at 150 m depth, California, Cordell Bank National Marine Sanctuary, taken and placed in the public domain by Linda Snook.

The Life and Times of the Last Earthpig

(Via:)
(Via: National Geographic)

Common Name: The Aardvark

A.K.A.: Orycteropus afer, Family Orycteropodidae

Vital Stats:

  • Also referred to as the “antbear” or “earthpig”
  • Common name derives from Afrikaans words meaning ‘earth’ and ‘pig’
  • Habitats include savannas, grasslands, and woodlands
  • Weighs 40-65kg (88-140lbs.) and can grow up to 2.2m (7’3”) long
  • Can live up to 24 years in captivity
  • Nocturnal, feeding only during the evening and at night

Found: Sub-Saharan Africa

Aardvark Map

It Does What?!

Like the platypus and several other creatures we’ve looked at, aardvarks are considered “living fossils,” organisms which have changed little from the way they looked millions of years ago (around 20 million, in this case).

Aardvarks don’t look much like most mammals of today, other than a passing similarity to the South American anteater, to which it isn’t closely related. In fact, aardvarks aren’t particularly closely related to anything. Not only are they the sole species in their genus, but they have their own family and order as well. This is because everything else that used to inhabit these ranks has since become extinct. At one time, there were at least 14 different species in the aardvark family, spread over parts of Africa, Europe, and Asia; but today, there’s just our friend the earthpig. Strangely, among the aardvark’s closest living relatives are manatees and elephants (all part of the motley superorder, Afrotheria), which suggests just what distant cousins they must be.

Okay, so aside from having outlived its family members, what’s so interesting about these things? Well, one look at them will tell you they must have evolved to fit some unusual lifestyle. Aardvarks are myrmecophagous, meaning they specialise in eating ants and termites, and nearly everything about that odd little body is geared to this task. First, finding their insect food means digging into large anthills and termite mounds, so aardvarks have become prodigious diggers, tunnelling at rates of up to two feet in 15 seconds with their heavily clawed feet. They use this skill in creating their underground burrows as well, excavating tunnels up to 13m (43’) long and even changing their home’s layout from time to time. Because, you know, you get tired of the same old thing…

caption(Via:)
Clark the Aardvark, fresh from finishing his new ensuite bathroom with walk-in closet.
(By: Frans Lanting, Via: Posterlouge)

Moving further up, the aardvark’s narrow, elongated head and long, snake-like tongue are perfect for dipping into the minute passages made by ants and termites. They even have a special sticky saliva that adheres to ants at a touch. In a single night of feeding sessions lasting from five seconds to two minutes per stop, an aardvark can attack 200 hills, consuming as many as 50,000 insects. The ants and termites try to fight back, of course, but the aardvark has thick, tough skin and can seal its nostrils shut, making bites and stings ineffective.

There’s just one feature of the aardvark that doesn’t make a lot of sense for its insect-eating lifestyle, and that’s a set of back teeth. (In fact, they’re are born with front teeth as well, but lose them at maturity.) No other myrmecophage on Earth has a functional set of teeth… you just don’t need ‘em to eat ants. So why do aardvarks have them? A little thing called the Aardvark Cucumber!

In a bonus piece of evolutionary weirdness, aardvarks supplement their diet with a single type of fruit, a cucumber which has now become entirely reliant on hungry aardvarks for its continued existence. The plant flowers above ground – as plants do – but then pushes itself into the earth as it sets fruit, resulting in a subterranean fruit. These cucumbers are dug up by aardvarks and eaten as a source of moisture, while the seeds go undigested and are conveniently deposited elsewhere with a ready source of fertiliser for germination. Without the aardvark, seed dispersal would be impossible, and new plants would be unable to obtain enough water and nutrients to survive.

So there you have the life of the lonely aardvark… enemy of the ants, saviour of the cucumber, brother to no one.

caption(Via:)
“I laugh in the face of probable extinction… and nail clippers.”
(Via: Zooborns)

[Fun Fact: If pursued into its burrow, an aardvark will protect itself by sealing off the tunnel behind itself and digging further into the ground in the other direction.]

[Also… On their front feet, aardvarks have lost their equivalent to our thumb, retaining only four digits.]

Says Who?

  • Endo et al. (2003) Annals of Anatomy 185: 367-372
  • Lehmann et al. (2004) Journal of African Earth Sciences 40: 201-217
  • Lehmann (2008) Fossil Record 11(2): 67-81
  • Taylor et al. (2002) Journal of Arid Environments 50: 135-152
  • Taylor & Skinner (2003) Journal of the Zoological Society of London 261: 291-297

The Plant That Time Forgot (Welwitschia mirabilis)

(Via: Wikimedia Commons)

Common Name: Welwitschia mirabilis

A.K.A.: Welwitschia

Vital Stats:

  • Welwitschia is a gymnosperm, like pines or firs, and thus reproduces via male and female cones
  • Considered a “living fossil”
  • Named after one of its discoverers, Austrian botanist Friedrich Welwitsch
  • In mature specimens, the woody stem can grow up to one metre (3.3’) across

Found: In the Namib desert, along the west coast of Namibia and Angola

It Does What?!

Restricted to a tiny, arid swath of African desert, Welwitschia mirabilis represents the last remaining species of a very unusual lineage of plants. Close relatives met with extinction over the aeons, while welwitschia, tucked away in its remote and harsh desert range with little competition, just kept going. The fact that the species is alone, not just in its genus, but also in its family and order (the two ranks above genus in plant systematics), speaks to just how distantly related to any other living plant it is. For the sake of comparison, the Rosales, the order to which roses, apples, and pears belong, contains around 7700 species in 9 families and 260 genera. So original and captivating is welwitschia among plants that it has been the subject of more than 250 scientific articles since it was first described in 1863.

A mere infant. But probably still older than you are.
(Via: Lizworld.com)

So what makes this thing so weird? Well, plants typically have what’s called an apical meristem at the tips of their stems and/or branches. You can think of this as a clump of stem cells that keeps dividing, throwing off new leaves and buds in its wake. If you cut off the apical meristem, the plant must either develop a new one elsewhere, or stop producing new tissue.

In welwitschia, this isn’t the case. At the beginning of the plant’s life, the apical meristem produces just two leaves, and then dies. The plant will never grow another leaf, which is much more surprising when you consider that it may well live for more than a thousand years. How do you get through a millennium with only two leaves?! The answer is, these aren’t ordinary leaves. Uniquely, welwitschia’s two strap-like leaves have a band of meristematic tissue built into their base, which means they can continue to elongate outward indefinitely. The leaves will continue to grow at a rate of around half a millimetre (0.02”) per day for as long as the plant lives. If you’re thinking that this must mean leaves that are several hundred metres long, unfortunately, no, they aren’t. The leaves are abraded away by sand storms and eaten by passing animals. Even in the best case scenario, the cells at the leaf tips have a maximum lifetime of about ten years (still pretty good for a leaf…). What’s more, the leaves tend to get frayed and split over time, and end up looking like a lot more than just two leaves. Despite all the punishment, though, each leaf can reach a length of up to four metres (13’), giving a mature welwitschia a width of up to eight metres (26’) across.

Welwitschia’s answer to the pinecone.
(Image by Friedrich A. Lohmuller)

As you might expect from a long-lived relic of the past, there aren’t a lot of these plants around. For once, this has less to do with human disturbance than natural circumstances. Over millions of years, the range where welwitschia grows has dried out considerably, and in fact continues to get drier even now. Today, the plant relies largely on fog to meet its water needs, restricting its range to a thin strip of desert coastline where fogs occur regularly. Unlike cactuses or succulents, welwitschia has never evolved the ability to store water. Also problematic is a fungus, Aspergillus niger, which frequently infects and destroys germinating seeds. These factors together can mean that a welwitschia colony can sometimes go many years without successfully reproducing.

And of course, no threatened species would be complete without some human interference. In recent decades, unscrupulous collectors have removed plants from already small breeding populations, making it even more difficult to sustain their numbers. Interestingly, it’s noted in Wikipedia that plants in Angola are actually better protected from collecting than those in Namibia due to the higher concentration of landmines there.

So… landmines: bad for humans, good for endangered plants.

You think you have problems with split ends?
(Via: Natural History Museum)

Says Who?

  • The Gymnosperm Database
  • Dilcher et al. (2005) American Journal of Botany 92(8):1294-1310
  • Henschel & Seely (2000) Plant Ecology 150:7-26
  • Jacobson & Lester (2003) Journal of Heredity 94(3):212-217
  • Rodin (1958) American Journal of Botany 45(2):96-103

Thank a Horseshoe Crab

(Via: reefguide.org)

Common Name: Horseshoe Crab

A.K.A.: Family Limulidae

Vital Stats:

  • Four extant species of horseshoe crab in three genera (Limulus, Carcinoscorpius, and Tachypleus)
  • Females are larger than males, and can reach up to 60cm (24”) long in some species
  • Believed to live between 20 and 40 years

Found: Coastal waters of southeast Asia, Oceania, and eastern North America

It Does What?!

Like the platypus and the lungfish, horseshoe crabs are what biologists refer to as “living fossils,” meaning their basic form has gone essentially unchanged for many millions of years. In the case of horseshoe crabs, fossils as old as 445 million years have been found that are quite similar to the extant species of today.

Despite their common name, the Limulidae aren’t true crabs. They’re arthropods, like crabs, but are actually more closely related to spiders and scorpions. In fact, beneath that tough shell, they do look quite spider-like. If spiders had tails, that is.

Basically a tarantula in combat gear.
(Via: Wikimedia Commons)

Horseshoe crabs live in shallow coastal waters, feeding off worms and molluscs from the ocean floor. They are able to feed in near complete darkness at night due to a remarkable visual system. The creatures have three different types of eyes – compound, median, and rudimentary – located to both sides and to the front of their shell. What’s more, their compound eyes become a million times more sensitive to light at night than they are during the day. Since that’s roughly how much less light they have to work with at night, the crabs are able to see equally well at night and during the day.

Most people who have observed horseshoe crabs know them from their unusual breeding habits. Each spring and early summer, male crabs will search out a mate and attach themselves to the female’s shell using a special modified leg. Then, during the highest tides of the year, usually at night, the females crawl up onto shore by the hundreds, carrying their male cargo. Having picked a spot that’s moist, but not so low as to be washed away with the tide, they dig a nest into the sand and lay their eggs. The attached males get first dibs at fertilising the pre-laid eggs, but must share the task with numerous mate-less onlookers who rush in to get their shot at fatherhood as well (crabs are so uncouth). Since eggs number in the tens of thousands per female, many will probably be successful. Most of these thousands of eggs, however, will become food for migratory birds, who appreciate the extra protein snack on their long journeys. After a month or so, the uneaten eggs will hatch into larvae, which remain on the beach in groups for a couple of weeks before moulting into juvenile horseshoe crabs and finally moving into the water.

Horseshoe crabs, making more horseshoe crabs.
(Via: Wikimedia Commons)

Now you might be thinking, “That’s all well and good, but what can horseshoe crabs do for me?” Well, as it turns out, these creatures are some of the most prolific blood donors on Earth (whether they like it or not). Like our friend Mr. Spock, horseshoe crabs have copper-based blood, rather than the iron-based concoction favoured by humans. They are literally blue-blooded. And instead of white blood cells to fight off infection, they have amebocytes. These amebocytes are so valuable in detecting certain types of bacterial infections in humans that a quart of horseshoe crab blood is worth approximately $15,000 US. Crabs are caught, transported to a lab, and drained of about 30% of their blood before being released. The company behind this 50 million dollar per year industry states that only about 3% of the quarter million crabs die from the procedure annually, while other studies have found the number to be nearer to 15% (read more about it here). Knowing who’s right may become very important, as horseshoe crab populations are declining worldwide, additionally affecting the migratory birds that feed on their eggs. Either way, next time you survive an E. coli infection, thank a horseshoe crab.

No, no… we don’t mind. Really.
(Via: TYWKIWDBI)

[Fun Fact: Horseshoe crabs are thought to be the closest living relative of the extinct trilobite.]

[Also, here’s a cool video of (who else?) Sir David Attenborough explaining the mating habits of horseshoe crabs.]

Says Who?

Randomly Assembled and Surprisingly Dangerous: The Platypus

(Via: National Geographic)

Common Name: The Duck-Billed Platypus

A.K.A.: Ornithorhynchus anatinus

Vital Stats:

  • Only species of Family Ornithorhynchidae
  • Males average 50cm (20”) long, females 43cm (17”)
  • Weigh between 0.7 and 2.4kg (1.5 – 5.3lbs.)
  • Body temperature of 32 degrees Celcius; five degrees lower than placental mammals
  • Live up to 17 years in captivity
  • Eat freshwater crustaceans, worms, and insect larvae

Found: Eastern Australia and Tasmania

It Does What?!

Besides looking like it was assembled from spare parts? We’ve all seen pictures of platypuses (yes, “platypuses”, not “platypi”) before, and everyone knows what total oddities they are: the duck-like bill, the beaver-esque tail, the fact that they lay eggs, despite being mammals; but behind these weird traits lie… even more weird traits! So let’s take a moment to appreciate the lesser-known eccentricities of the platypus, shall we?

First off, these cuddly looking freaks are actually dangerous. Male platypuses have a spur on each hind foot which is filled with a venom powerful enough to kill a large dog. While it isn’t enough to take out a human, it does cause severe, incapacitating pain whose after-effects can last for months. One of only a very few venomous mammals, the male’s venom production increases during the breeding season, suggesting its purpose may lie in competition with other males.

Why your dog and your platypus shouldn’t play together.
(By Jason Edwards, via: How Stuff Works)

And speaking of breeding, reproduction in platypuses isn’t exactly ‘mammal standard’, either. Unlike all other mammals, which have two sex chromosomes (X and Y; XX for females, XY for males, with rare exceptions), the platypus has ten. Talk about evolutionary overkill. A male platypus has the pattern XYXYXYXYXY, while a female has ten Xs. Researchers have found that the actual genetic structure of these sex chromosomes is actually more similar to birds than mammals, although 80% of platypus genes are common to other mammals.

After this alphabet soup of chromosomes arranges itself, up to three fertilised eggs mature in utero for about four weeks; much longer than in most other egg-laying species (in birds, this may be only a day or two). Once laid, the eggs are only about the size of a thumbnail, and hatch in around ten days. While platypuses produce milk, they don’t actually have proper teats to suckle their babies- the fluid is released from pores in the skin. A small channel on the mother’s abdomen collects the milk, which is then lapped up by the young. Strangely, the babies are actually born with teeth, but lose them before adulthood. Such is the impracticality of platypus design…

Adorably impractical.
(Via: noahbrier.com)

Finally, let’s explore platypus hunting methods. Platypuses are the only mammals with the sixth sense of electroreception. Those leathery duck bills of theirs are actually precision receptors that can detect the electric fields created in the water by the contractions of muscles in their prey. Considering the prey in question is largely worms and insect larvae, we’re talking big-time sensitivity here. The bill is also very receptive to changes in pressure, so a movement in still water can be picked up in this way as well. Researchers have suggested that by interpreting the difference in arrival time of the pressure and electrical signals, the hunter may even be able to determine the distance of the prey. This would be especially useful, given that platypuses close both their eyes and ears when hunting. In fact, they won’t even eat underwater; captured food is stored in cheek pouches and brought to land to be consumed.

So there you have it. The platypus: even weirder than you thought.

[Fun Fact:The female platypus has two ovaries, but only the left one works.]

Intelligent Design’s Worst Nightmare
(Via: Animal Planet)

Says Who?

  • Brown (2008) Nature 453: 138-139
  • Grant & Fanning (2007) Platypus. CSIRO Publishing.
  • Graves (2008) Annual Review of Genetics 42: 565-586
  • Moyal (2002) Platypus: The Extraordinary Story of How a Curious Creature Baffled the World. Smithsonian Press.

How to Stay Cool the Lungfish Way

Via: Science News for Kids

Common Name: The Lungfish

A.K.A.: Subclass Dipnoi

Vital Stats:

  • 6 species; 4 in Africa, 1 in South America, 1 in Australia
  • Some species can reach up to 2m (6.6’) long and weigh 43kg (95lbs.)
  • Omnivorous, eating plants, insects, crustaceans, worms, fish, and frogs
  • Largest genome of all terrestrial vertebrates at ~133 billion base pairs

Found: Slow-moving freshwater bodies in South America, Africa, and Australia

It Does What?!

Well, they’re not much to look at, but in the “quietly carrying on while everything drops dead around you” department, the lungfishes are tops. These large, eel-looking creatures are what biologists refer to as “living fossils”, species which have existed in more or less their present form for a very, very long time. In the case of the lungfishes, around 400 million years. For the sake of comparison, this was around the same period that plants developed roots and leaves. That long ago. In fact, researchers believe that the lungfishes are the closest living relatives of the terrestrial vertebrates (that is, anything with a spinal column that lives on land).

These will probably outlast humanity.
Via: One More Generation

So what makes these things so interesting, besides being old? First off, they breathe air, as you might have guessed from their name. Australian lungfishes have a single lung, and, while they normally breathe through their gills, are able to supplement their oxygen intake with air during times of high exertion or when their water gets stale (Fun side note: During mating, Australian lungfishes make loud burping noises at the surface of the water which are thought to be part of the courtship ritual. I’ll refrain from making any Aussie jokes here… ). African and South American lungfishes, on the other hand, have two lungs and breathe nothing but air. Their gills are completely atrophied, such that they could actually drown if kept under for much longer than their usual 5-8 minutes between breaths.

“Hey! I’m trying to aestivate in here!”
Photo by: Tobias Musschoot

This ability to breathe without water results in the other fantastic ability of subclass Dipnoi. South American and African lungfish live in habitats which often dry up completely during the hottest part of the year. The fishes’ gross but brilliant answer to this is to burrow up to half a metre down into the soft mud and excrete a huge amount of mucous. As the surrounding mud dries up, the mucous forms a hard shell which keeps the curled up lungfish moist and cool. A small hole at the top of this snot-cocoon allows the fish to breathe. It’s metabolism slowed to only a small fraction of the normal rate, the creature will aestivate (like ‘hibernate’, but without the cold) like this for several months until the rains return. Laboratory experiments have shown that an African lungfish can remain alive under these conditions for as long as six years.

“Granddad”: probably older than your Granddad
Via: Shedd Aquarium

Aside from their amazing survival abilities, these fish have unusual lives, as fish go. They are extraordinarily long-lived. The Shedd Aquarium in Chicago holds an Australian lungfish known as “Granddad” which arrived there as an adult in 1933, making him at least 80 years old. Females of this species don’t even mate until they’re at least 22 years old (or so they tell their parents). What’s more, some species actually care for their young. The mother and father build an underwater nest for their offspring, which can only breathe via their semi-atrophied gills for the first seven weeks, and the father uses his body to release additional oxygen into the surrounding water, helping them to breathe. So, dual childcare: not such a new idea after all.

[Extra Credit –  Here’s a short video of a lungfish being stalked by a pelican. Spoiler: It ends badly for the lungfish.]

Says Who?

  • Brinkmann et al. (2004) Journal of Molecular Evolution 59: 834-848
  • Fishman et al. (1992) Proceedings of the American Philosophical Society 136(1): 61-72
  • Glass (2008) Respiratory Physiology & Neurobiology 160: 18-20
  • Joss (2006) General and Comparative Endocrinology 148: 285-289
  • Lee et al. (2006) General and Comparative Endocrinology 148: 306-314
  • www.fishbase.org