It’s a blob. It’s an animal. It’s Trichoplax

The irresistibly attractive Trichoplax. Oliver Voigt, courtesy of Wikimedia Commons

Timeline, 2008: One of the greatest quests in animal biology is the search for the “ur-animal,” the proto-creature that lies at the base of the animal family tree. For many years, the sponge held the low spot as most primitive animal, a relatively simple cousin of ours consisting of a few tissues and a tube that filters water for nutrients.

An animal, simple and alone

Research now suggests, however, that there’s an even older cousin at the base of the tree, an animal-like organism with three cell layers and four cell types that moves by undulating and exists alone in its own taxon. This organism, with the species name Trichoplax adhaerens, is the sole living representative of the Placozoa, or “flat animals.” It has been a bit of a mystery creature ever since the amoeba-like things were first noted a century ago in a German aquarium. Today, new techniques in systematics, the study of how living things are related, have helped pinpoint its place on the tree of life. These techniques have, however, left us with a few complex questions about this “simple” animal.

The animal kingdom family tree gets complex fast, with an early division in the trunk. Whatever lies at its base—and we’re still not sure what the organism was—that ancestor yielded two basic animal lineages. One led to sponges (Porifera) and branched to Cnidarians—corals, hydras, and organisms like jellies that have primitive nervous systems. The other lineage is the Bilateria, which includes everything from flatworms to bugs to beluga whales to us. Obviously, this lineage also has developed a nervous system, in many cases one that is quite complex.

Why a sponge will never be nervous

Earlier thinking was that the sponges, in lacking a nervous system, represented some earliest ancestor from which the two lineages sprang. But recent molecular analysis of the sponge and placozoan genomes has left a few systematists scratching their heads. The big headline was that the sea sponge was no longer the most primitive or oldest taxon. That honor may now belong to the placozoan, although some analyses place it as having evolved after Porifera.

An ur-animal? Or just another ur-cousin?

It may go a ways back, but it’s not so far that scientists can say that T. adharens is the “ur-animal,” or “mother of all animals.” In fact, it appears to share a few things in common with Bilateria, such as special protein junctions for holding tissues together, but computer analysis places it squarely in the Cnidaria branch of the animal family tree. Thus, this “weird, wee beastie,” while possibly older than the sponge, is still not the proto-ancestral animal condition. Rather than being the “ur-animal” from which all other animals sprang, it’s more like an “ur-cousin” of Bilateria.

Nervous system genes but no nervous system?

T. adhaerens also represents another conundrum for evolutionary biologists and systematists. It and the sponge both carry coding in their genomes for neural proteins, yet neither have nervous systems. The Cnidarian nervous system itself may have evolved parallel to that of the Bilateria, an evolutionary phenomenon known as “convergent evolution.” When evolutionary processes occur in parallel, unrelated species may share adaptations—such as a nervous system—because of similar selection pressures. The results are what we call “analogous structures,” which have similar functions and may even seem quite similar. But they are shared because of similar evolutionary pressures, not because of a common ancestry.

Lost traits make evolutionary biologists tear out their hair

The primitive placozoan does not have a nervous system, although organisms that arose later in the Cnidarian lineage, such as jellies, do. Yet those neural genes in its genome leave an open question and a continuing debate: Is the placozoan an example of another common evolutionary phenomenon in which a trait arises, but then is lost? Some scientists have suggested that there may have been an even older version of a nervous system, predating the Cnidarian/Bilateria split. This trait then vanished, leaving behind only these traces in the primitive placozoan and sponge genomes. With this scenario, the two nervous systems would have a shared ancestry: instead of being analogous traits resulting from convergent evolution, they would represent homologous traits, shared because of a common neural ancestry.

Water bears go where no animal has gone before

A water bear (OMG, they're so cool!)

Timeline, 2008: They say that in space, no one can hear you scream (or at least that Alien movie said it). The reason for that is that sound waves require matter to be propagated, and in the vacuum of space, there is no matter. Hard as your vocal cords may push, they can’t make the sound travel through nothing.

That vacuum carries other complications for people beyond an inability to chat unprotected. Space is cold—temperatures run close to absolute zero, or -272 Celsius. With the vacuum, there is no oxygen and no pressure. And let’s not forget the bombardment with deadly cosmic rays and, if you’re hanging around just above Earth, UV rays 1000 times more powerful than those we experience on terra firma.

Boiling saliva and bubbling blood. Ewww.

Thus, if you send person or a dog or an ape unprotected into space, within minutes, the lack of pressure would lead to an uncomfortable death involving boiling saliva and bubbling blood. Yet, there are organisms known to survive the environment of space, primarily some bacteria and lichen, the symbiotic combination of algae and fungus. Now, we’ve learned that a little critter that lives on lichen also can be quite the intrepid space traveler.

Water bears–not even remotely like bears

The space animals—the first animals, in fact, known to pull off unprotected space travel—are representatives of two species of tardigrades. There are up to 1000 species of these little animals, more familiarly known as “water bears” because of their rotund, bear-like appearance. Researchers had noted their hardiness under earthbound conditions, observing that although the animals thrive in a damp environment, when conditions go dry, they can shut down for as long as 10 years until the environment moistens up again. Some accounts have compared the water bears to “Sea Monkeys,” the brine shrimp of comics advertisements that come to you dried up in a little packet, only to “miraculously” come to life when you add water.

On a September 2007 European Space Agency mission, scientists decided to test the tardigrades in the most hostile environment available—in space just above planet Earth. They tested two species of water bear, Richtersius coronifer and Milnesium tardigradum, both entering space as dried out versions of their usual selves. The 120 animals from each species were divided into four groups, one that remained on Earth as a control, two that were exposed to the vacuum of space and different combinations of UV rays, and a fourth that experienced only the vacuum of space without the radiation.

Vacuum OK, UV bad

The animals spent 10 days traveling unprotected far above the earth before returning to Earth, where they were hydrated in the lab. Amazingly enough, all three groups of space-traveling tardigrades initially perked right up and lived for a few days. After that, however, only the vacuum-alone group maintained that rate of survival. In the UV-exposed groups, animals that had experienced the vacuum of space and exposure to UV-A and UV-B survived at rates between 10% and 15%. Animals that had been exposed to all three types of UV—A, B, and C—all died.

Nevertheless, the animals that did survive appeared to thrive, reproducing heartily and generally living the usual life of the unusual water bear. Researchers find their ability to withstand radiation particularly intriguing, given that the bombardment would normally shred the DNA of most organisms. Investigators hope to find out more about how the bears  resist the perils of space, seeking perhaps to co-opt some of the tardigrade’s techniques to use in protecting astronauts.

Space, schmace. How about 4000 m deep?

Tardigrades didn’t really need to travel into outer space—or really, inner space—to prove their toughness. They are known to live as high as 6000 meters up in the Himalayas and as deep as 4000 meters down in ocean trenches. Water bears have also been found living in apparent ease in hot springs above boiling temperature.

Nematode may trick birds with berry-bellied ants

Comparison of normal worker ants (top) and ants infected with a nematode. When the ant Cephalotes atratus is infected with a parasitic nematode, its normally black abdomen turns red, resembling the many red berries in the tropical forest canopy. According to researchers, this is a strategy concocted by nematodes to entice birds to eat the normally unpalatable ant and spread the parasite in their droppings. (Credit: Steve Yanoviak/University of Arkansas)

Timeline, 2008: Host-parasite relationships can be some of the most interesting studies in biology. In some cases, a parasite requires more than one host to complete its life cycle, undergoing early development in one host, adult existence in another host, and egg-laying in still another. There’s the hairworm that turns grasshoppers into zombies as part of its life cycle, and the toxoplasma parasite, which may alter the behavior of humans and animals alike. Often, the infection ends with the host engaging in life-threatening behaviors that lead the parasite to the next step in the cycle.

A recent discovery of a most unusual host-parasite relationship, however, results in changes not only in host behavior but also in host appearance. The infected host, an ant living in the forest canopy in Panama and Peru, actually takes on the look of a luscious, ripe fruit.

Berry-butted ants

Researchers had traveled to the Peruvian forest on a quest to learn more about the airborne acrobatics of these ants, Cephalotes atratus. This ant is a true entomological artist, adjusting itself in midair if knocked from its perch. Re-orienting its body, it can glide back to the tree trunk, grabbing on and climbing to where it belongs, avoiding the dangers of the forest floor.

As the investigators monitored the colony, they became aware of some odd-looking members of the group. These ants had large red abdomens that shimmered and glowed and looked for all the world like one of the tropical berries dotting the forest around them. Curious about these odd ants, the scientists took some to the lab for further investigation. Ant researchers are an obsessive breed, and they had even placed a bet over whether or not these berry-bellied ants were a new species.

A belly full of another species’ eggs

When they sliced open one of the bellies under a microscope, what they found surprised them. Inside, a female nematode had packed the ant’s abdomen full of her eggs. The bright red belly was an incubator and, the researchers surmised, a way station on the nematode’s route to the next step in its life cycle. This was the same old C. atratus with a brand new look.

Tropical birds would normally ignore these ants, which are black, bitter, and well defended with a tough, crunchy armor. But any tropical bird would go for a bright, red, beautiful berry just waiting to be plucked. The scientists found that in addition to triggering changes to make the ant belly look like a berry, the nematode also, in the time-honored manner of parasites, altered its host’s behavior: the berry-bellied ants, perched on their trees, would hold their burgeoning abdomens aloft, a typical sign of alarm in ants. A bird would easily be tricked into thinking that the bug was a berry. One quick snap, and that belly full of nematode eggs would be inside the belly of a bird.

Poop: A life cycle completed

And then the eggs would exit the bird the usual way, ending up in the bird’s feces. The ants enter the picture again, this time collecting the feces and their contents as food for their colony’s larvae. The eggs hatch in the larvae and the new nematodes make their way to the ant belly to start the cycle anew.

The nematode itself is a new find, a new species dubbed Myrmeconema neotropicum. And it seems that earlier discoverers of the berry-bellied ants also thought they had a new species on their hands: the researchers turned up a few previous berry-bellied specimens in museums and other collections labeled with new species names. No one had thought that the difference in appearance might be the result of a parasitic infection: this relationship is the first known example of a parasite causing its host to mimic a fruit.

Birds remain the missing link

There is one hitch to the newly discovered nematode-ant-bird association: the researchers never actually saw a tropical bird snap up a juicy, fruit-mimicking ant. They report seeing different species of birds scan the bushes where such ants sheltered, but there were never any witnessed ant consumptions. Thus, this inferred piece of the puzzle—the involvement of birds and their droppings in the life cycle of this nematode—remains to be proven.

Think the eye defies evolutionary theory? Think again

The compound lens of the insect eye

Win for Darwin

When Darwin proposed his theory of evolution by natural selection, he recognized at the time that the eye might be a problem. In fact, he even said it was “absurd” to think that the complex human eye could have evolved as a result of random mutations and natural selection. Although evolution remains a fact, and natural selection remains a theory, the human eye now has some solid evolutionary precedence. A group of scientists that has established a primitive marine worm, Platynereis dumerilii, as a developmental biology model has found that it provides the key to the evolution of the human—and insect—eye.

Multiple events in eye evolution, or only one?

The divide over the eye occurred because the insects have the familiar compound-lens—think how fly eyesight is depicted—and vertebrates have a single lens. Additionally, insects use rhabdomeric photoreceptors, and vertebrates have a type known as ciliary receptors. The rhabdomeric receptors increase surface area in the manner of our small intestine—by having finger-like extensions of the cell. The ciliary cells have a hairy appearance because of cilia that pop outward from the cell. A burning question in evolutionary biology was how these two very different kinds of eyes with different types of photoreceptors evolved. Were there multiple events of eye evolution, or just one?

Just once?

P. dumerilii work indicates a single evolutionary event, although the usual scientific caveats in the absence of an eyewitness still apply. This little polychaete worm, a living fossil, hasn’t changed in about 600 million years, and part of its prototypical insect brain responds to light. In this system is a complex of cells that forms three pairs of eyes and has two types of photoreceptor cells. Yep, those two types are the ciliary and the rhabdomeric. This little marine worm has both kinds of receptors, using the rhabdomeric receptors in its little eyes and the ciliary receptors in its brain. Researchers speculate that the light receptors in the brain serve to regulate the animal’s circadian rhythm.

How could the existence of these two types of receptors simultaneously lead to the evolution of two very different kinds of eyes? An ancestral form could have had duplicate copies of one or both genes present. Ultimately, if the second copy of the rhabdomeric receptor gene were recruited to an eye-like structure, evolution continued down the insect path. But, if the second copy of a ciliary cell’s photoreceiving gene were co-opted for another function, and the cells were ultimately recruited from the brain for use in the eye, then evolution marched in the vertebrate direction.

All of the above is completely speculation, although this worm’s light-sensitive molecule, or opsin, is very much like the opsin our own rods and cones make, and the molecular biology strongly indicates a relationship. It doesn’t completely rule out multiple eye-evolution events, but it certainly provides some nice evidence for a common eye ancestor for insects and vertebrates.

Note: This work appeared in 2004 and got a detailed writeup at Pharyngula.

Tricky little orchids

Orchids attract collectors all over the world. One of the things that draws us to these unusual plants is their Machiavellian approach to life. They unfeelingly employ deception to their benefit, usually practicing their art on unsuspecting members of the insect community. Research has revealed that one species of orchid, Anacamptis morio (or Orchis morio), or the green-winged orchid, lays its bold insect trap in an attempt to avoid a trap itself.

Inbreeding avoidance: not just for royalty

Although plants can do many things that most members of the animal kingdom cannot—self-fertilize or increase chromosome numbers in a generation—they’re still better off when reproductive measures result in an increase in genetic variation. As with most organisms, inbreeding is not a healthy thing for a plant, and many plants have mechanisms to avoid it.

The idea of inbreeding avoidance led researchers to a theory to explain the remarkable behavior of many orchids. These beautiful, much-coveted flowers attract humans and insects with their alluring fragrances and colors. For insects, some orchids add to the attraction by mimicking the female of the insect species, or wafting the scent of eau d’ dung for insects that prefer laying their eggs in such places. But of the 30,000 known orchid species, about 10,000 have nothing to offer the hapless insect in return: their flowers have no nectar.

Why keep coming back for nothing?

Researchers have sought to explain why insects would continue to visit such a stingy plant, and why the plants continue to get away with and employ their nectar-free strategy. The strategy itself seems in violation of so much of our understanding of the natural world, a place typically characterized by tradeoffs. In fact, orchids without nectar are not wildly popular among insects—it is difficult in many cases to witness a bee pollinating a green-winged orchid in the wild—but they still do manage to get pollinated.

Scientists investigated wild-growing green-winged orchids on a Swedish island and figured out why this species cheats insects so mercilessly. It’s about genetic variation. The flowers attract the bugs, but offer the foraging insects nothing, driving them on to explore other plants. Although the orchids have not provided food, they have given the unsuspecting insect a payload of a different kind: pollen. The bug—still on a quest for nectar—forages in other plants, pollinating as it goes along. Voila! No self-pollination. Plants that result from self-pollination are usually weak and unhealthy, and self-pollinating can be a waste of precious pollen.

Interviewing bees

Scientists detected this self-pollination avoidance by interviewing bees. They queried specific bees with plants that had been artificially dosed with nectar or with plants in their natural nectar-free state. The researchers found that bees stayed around the nectar-ful plants twice as long and investigated twice as many flowers on the same plant, which would promote self-pollination. Bees that found no nectar moved along to other plants, promoting cross-pollination.

One thing that could confound the interpretation of these results is that bees can remember how a plant smells. If a bee strikes out with one orchid, it will remember that orchid’s smell and not waste its time foraging around in other flowers that smell the same.

In separate research performed by a team in Switzerland, scientists found that the flowers of a nectar-producing orchid species all smell very much the same. But flowers on different plants of the green-winged orchid all smell different. A bee might have failure at one green-winged orchid and remember the smell, but then fly straight into another green-winged orchid plant because its smell is different. The unhappy bee falls into the orchid’s trap and gets nothing, but the deceitful orchid itself has had a great success: avoiding the trap of self-pollination.

Going to Hawaii? Watch out for the flesh-eating caterpillars

Flesh-eating caterpillars lurk in Hawaii’s rainforests

Islands can produce some of the strangest evolutionary novelties on the planet. Island-living elephants shrink to tiny sizes, while tortoises grow gigantic. The fate of species on islands is its own specialized study because the only way species can arrive on an island is over the water. Scientists, in the study of island biogeography, focus on how plants, animals, and microbiota end up on the islands where they occur.

What happens after they arrive is apparently anybody’s guess. Islands are unusual because they can lack the stiff competition of mainland ecosystems. Common factors in our daily lives, like ants, can be completely lacking. Because so many pieces of an ecological puzzle are missing on an island, niches remain open for the organisms that do arrive and get a foothold. Animals and plants end up doing things on islands that their kindred are not known to do anywhere else in the world. A recently discovered example is a caterpillar that has broken all the rules of caterpillardom. It eats meat. It hunts its prey. It uses its silk as a weapon. It deliberately camouflages itself with non-caterpillar components. And it’s a brutal killer.

Like a wolf that dives for clams

This particular capterpillar and its four just-discovered relatives reside on one of the most isolated island chains in the world, the Hawaiian archipelago. These islands are well known for evolutionary novelties, and these new species of the genus Hyposmocoma are no different. Well, actually, they’re very different. One scientist has said that discovering the behavior of these larval moths is like discovering a wolf species that dives for clams.

This caterpillar, a tiny, brutal, sneaky killer, creeps up on its prey, an unsuspecting snail resting on a leaf in the Hawaiian rainforest. The caterpillar itself is bound in silk, and it proceeds to spend almost a half hour anchoring the hapless snail to the leaf with more silk. The silk, made of gelatinous proteins, pins the snail by its shell as tightly as a spider wraps its threads around prey.

Once the caterpillar has immobilized its target, preventing the snail from escaping through a fall off of the leaf, the nascent moth emerges from its own silk casing. The snail retreats into its shell, and the caterpillar follows, beginning to feed on the trapped snail, starting with the head. It literally eats the snail alive.

This behavior is extraordinarily unusual for a caterpillar, the juvenile form of moths and butterflies. The vast majority of caterpillar species are vegetarian; of the 150,000 known species, only 200 have been identified as flesh eaters and predators. These few do not use their silks to trap their food, and they don’t eat snails, which are mollusks, targeting instead soft-bodied insects.

Caterpillar divers and adaptive radiation

But the genus Hyposmocoma is known for its diversity. Some of its members dive underwater for food. The interesting thing about the snail-eating caterpillars is that they seem to have radiated through almost all of the Hawaiian islands. The first species was identified on Maui, but since its discovery, researchers have found species on most of the other islands. Evolutionary biologists are intrigued by the many novel aspects of this caterpillar’s life history because it is so unusual for this many unique factors—novel food source, novel hunting technique, novel eating technique—to have evolved in the same species.

Wearing the spoils of capture as camouflage

One other unique thing about this caterpillar’s approach to dinner is its use of decoration. Once the mollusk-eating caterpillar has spent the day dining on escargot, it will attach the snail’s empty shell to its silken casing, along with bits of lichen and other materials, in an apparent attempt to camouflage itself.

With clones like these, who needs anemones?

Finding Nemo makes marine biologists of us all

I once lived a block away from a beach in Northern California, and when my sons and I wandered the sands at low tide, we often saw sea anemones attached to the rocks, closed up and looking much like rocks themselves, waiting for the water to return. My sons, fans of Finding Nemo, still find these animals intriguing because of their association with a cartoon clownfish, but as it turns out, these brainless organisms have a few lessons to teach the grownups about the art of war.

Attack of the clones

Anemones, which look like plants that open and close with the rise and fall of the tides, are really animals from the phylum Cnidaria, which makes them close relatives of corals and jellyfish. Although they do provide a home for clownfish in a mutualistic relationship, where both the clownfish and the anemone benefit from the association, anemones are predators. They consist primarily of their stinging tentacles and a central mouth that allows them to eat fish, mussels, plankton, and marine worms.

Although anemones seem to be adhered permanently to rocks, they can, in fact, move around. Anemones have a “foot” that they use to attach to objects, but they also can be free-swimming, which comes in handy in the art of sea anemone warfare. (To see them in action, click on video, above.)

Sea anemone warfare could well be characterized as an attack of the clones. These animals reproduce by a process called lateral fission, in which new anemones grow by mitosis from an existing anemone, although they can engage in sexual reproduction when necessary. But when a colony of anemones is engaged in a battle, it consists entirely of genetically identical clones.

Yet even though they are identical, these clones, like the genetically identical cells in your liver and your heart, have different jobs to do in anemone warfare. Scientists have known that anemones can be aggressive with one another, tossing around stinging cells as their weapons of choice in battle. But observing groups of anemones in their natural environment is almost impossible because the creatures only fight at high tide, masked by the waves.

To solve this problem, a group of California researchers took a rock with two clone tribes of anemones on it into the lab and created their own, controlled high and low tides. What they saw astonished them. The clones, although identical, appeared to have different jobs and assorted themselves in different positions depending on their role in the colony.

Battle arms, or “acrorhagi”

The warring groups had a clearly marked demilitarized zone on the rock, a border region that researchers say can be maintained for long periods in the wild. When the tide is high, though, one group of clones will send out scouts, anemones that venture into the border area in an apparent bid to expand the territory for the colony. When the opposition colony senses the presence of the scouts, its warriors go into action, puffing up large specialized battle arms called acrorhagi, tripling their body length, and firing off salvos of stinging cells at the adventuresome scouts. Even warriors as far as four rows back get into the action, rearing up the toss cells and defend their territory.

In the midst of this battle, the reproductive clones hunker down in the center of the colony, protected and able to produce more clones. Clones differentiate into warriors or scouts or reproducers based on environmental signals interacting with their genes; every clonal group has a different response to these signals and arranges its armies in different permutations.

Poor Stumpy

Warriors very rarely win a battle, and typically, the anemones maintained their territories rather than achieving any major expansions. The scouts appear to run the greatest risk; one hapless scout from the lab studies, whom the researchers nicknamed Stumpy, was so aggressive in its explorations that when it returned to its home colony, it was attacked by its own clones. Researchers speculated that it bore far too many foreign stinging cells sustained in the attacks, thus resulting in a case of mistaken identity for poor Stumpy.

%d bloggers like this: