Multi-coloured livestock

Thanks to tending ants, mixed aphid colonies persist

mutualism of ants and aphids: protection for honeydew

The aphid milking ant Lasius japonicus ensures long-lasting coexistence of two colour morphs of the mugwort aphid, from which it harvests honeydew, Saori Watanabe and colleagues write. Without intervention, its favourite colour would be displaced.

Like many other ants, the Asian ant Lasius japonicus has a mutualistic relationship with aphids. The aphids suck sap from their host plant and excrete excess sugars, dissolved in a liquid: honeydew. The ant fights off their natural enemies and harvests (‘milks’) the sugary honeydew. One of its mutualistic partners is the Japanese mugwort aphid, Macrosiphoniella yomogicola, which lives on mugwort, a common plant of Europe and Asia. The protection of the ant is of crucial importance to the aphids; each colony will fall victim to its enemies if not protected.

The mugwort aphid occurs in different colours, with red and green as the most common types; large green specimens will turn black. The ant has a preference for the green morph, Saori Watanabe and colleagues show, because it excretes a higher quality honeydew. But as a consequence, the red morph, which retains a larger proportion of the sugars that it obtains from the host plant, can reproduce at a higher rate. All aphids are females that reproduce parthenogenetically, their young being clones of their mother. Red aphids produce red daughters, green aphids green daughters – and the green morph runs a risk to be displaced by the red one.

But, as it turns out, the ants prevent this from happening. The researchers show that the red aphids indeed are able to multiply faster than the green ones. As a consequence, in laboratory experiments, the proportion of green aphids in a mixed colony decreased, but only if the researchers withheld attending ants. If, however, ants were allowed to join the aphids, the reproduction rate of the green morph increased, and the green aphids now reproduced as fast as the red aphids. Thus, in the presence of ants, the proportion between green and red morphs was stable.

It is not clear how the ants improve the reproduction rate of the green aphids, but it saves the green morph from local extinction.

In the field, almost all colonies are mixed. It is understandable that no pure red colonies are to be found. No ant would be interested in such colony, which produces only low quality honeydew, so it would be lost. But why don’t green colonies exist? Why wouldn’t the ants remove the red aphids from a mixed colony by eating them, so that only high quality honeydew would be produced?

Apparently, the presence of red aphids is advantageous for some reason. That has to do with the winter period, the researchers suggest. At the end of the season, the aphids give birth to daughters and sons, which mate and produce fertilized eggs that can overwinter if the host plant survives. However, after flowering in autumn, mugwort dies off. The researchers hypothesize that red aphids may suppress flowering, so that the plant persists. They are now going to test that idea.

It would mean that the ant needs both aphid morphs, the green one for high quality honeydew, the red one to maintain the colony to the next season. It would also mean that the two types of aphids need each other. The red morph cannot do without the green one, which attracts attending ants, and the green morph cannot do without the red one, which prevents the host plant from dying off in winter.

But as dependent the aphid morphs may be on each other, they cannot live together for a long time without the ant interfering.

Willy van Strien

Photo: aphid tending ant, not Lasius japonicus. Caramallo (Wikimedia Commons, Creative Commons CC BY-SA 3.0)

Watanabe, S., J. Yoshimura & E. Hasegawa, 2018. Ants improve the reproduction of inferior morphs to maintain a polymorphism in symbiont aphids. Scientific Reports 8: 2313. Doi: 10.1038/s41598-018-20159-w
Watanabe, S., T. Murakami, J. Yoshimura & E. Hasegawa, 2016. Color polymorphism in an aphid is maintained by attending ants. Science Advances 2: e1600606. Doi: 10.1126/sciadv.1600606

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Hidden beauty

Chameleons are characterized by glowing bony tubercles

Calumma crypticum possesses glowing bony tubercles

Many chameleon species are discovered to possess bony bumps that emit blue light through the skin. The glowing bumps enable the animals to recognize conspecifics and males probably show them off to females. We don’t perceive them in natural light, but the animals do, David Prötzel and colleagues think.

There are many ways in which an animal can make itself stand out, for instance with scent, colour, song, dance, decorative feathers or eyes on stalks. But here is a new one: chameleons appear to have small bumps on their skull that emit blue light, David Prötzel and colleagues report.

The glowing tubercles of Calumma crypticum become visible under UV lightThat may sound like horror, but there is nothing ghostly about it. Bone tissue is fluorescent: if it is hit by ultraviolet light, it is excited and will emit blue light. Chameleons use this natural phenomenon. The bumps, or tubercles, on their skull protrude through most of the skin and are covered only by a thin, transparent layer of cells, which is like a window. If ultraviolet passes that window and falls on the tubercles, they will glow blue.

Normally we don’t see it, as the amount of ultraviolet (UV) in natural light is too small. Therefore, the phenomenon was only discovered when the researchers illuminated heads of chameleons with a UV lamp. But natural light does contain enough ultraviolet to make the glowing bumps visible to the chameleons themselves, the researchers suspect, as their eyes are more sensitive to blue light than ours. Many chameleon species in Madagascar and in Africa have such glowing bumps, especially species that inhabit humid forests, where the component of ambient UV light is relatively high; the emitted blue light contrasts well with the dark background.

The pattern of bony bumps is species specific; most bumps are seen around and behind the eyes, but some species have such blue glowing tubercles not only on the skull, but on the entire body. Chameleons will recognize their conspecifics by the distinctive pattern of tubercles, which is a stable signal for these animals with their colour changing behaviour.

As in the nearly all species males posses on average more tubercles than females, the researchers assume that males display them to seduce a female.

So, chameleons possess glowing bone bumps as ornamentations used for recognition and display; it is possible that other lizards and snakes will be discovered to show fluorescencent bony bumps.

Willy van Strien

Large: Calumma crypticum, male. Axel Strauβ (Wikimedia Commons, Creative Commons CC BY-SA 3.0)
Small: head of Calumma crypticum, preserved specimen from the Zoologische Staatssammlung München (Germany), under ultraviolet light. Copied from David Prötzel et al. (Creative Commons, CC BY 4.0) and mirrored, so that head is oriented in the same direction as in the large picture

On this YouTube video, the researchers illuminate Furcifer pardalis and two Brookesia species to elicit the blue glow. And on this video you see the knobby skull of Calumma globifer.

Prötzel, D., M. Heß, M.D. Scherz, M. Schwager, A. vant Padje & F. Glaw, 2018. Widespread bone-based fluorescence in chameleons. Scientific Reports 8: 698. Doi: 10.1038 / s41598-017-19070-7

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Family ties

Caring treehopper mother recognizes her offspring

female treehopper, Alchisme grossa, guarding her eggs

Females of the treehopper Alchisme grossa exhibit complex maternal care. If a mother has been separated from her offspring, she can localize and recognize them, as Daniel Torrico-Bazoberry and colleagues show.

The treehopper Alchisme grossa occurs in Central and South America, where it lives on a number of host plants, feeding on plant sap. The females lay their eggs on the host plants; they cut a slit in the midrib at the underside of a leaf, deposit the eggs in it and cover the egg mass with a frothy secretion.

Alchisme grossa females exhibit extensive brood careThey then will exhibit extensive brood care for a few months, Daniel Torrico-Bazoberry and colleagues write. The caring behaviour is unique for such a small creature. A female positions herself over the eggs and shields them with an enlarged pronotum (dorsal plate of the thorax) which bears two horns at the front side. If parasites or predators, such as spiders or predatory bugs, try to reach the eggs of a guarding mother, she will behave like a hero. She moves her body, fans her wings and kicks with strong legs to scare off the enemies. A batch of eggs is certainly lost without its mother; if it does not fall prey to enemies, it will desiccate.

The offspring rely on maternal care until they have completed development. The treehoppers undergo an incomplete metamorphosis. The nymphs that hatch from the eggs resemble adults, but are smaller. They undergo five instar stages before they are fully grown. Just like adult treehoppers, nymphs are sap feeders. Shortly before they hatch, the mother cuts a number of small holes in the midrib near the batch of eggs, so that the tiny nymphs can easily puncture the vein to tap the sap flow. And she stays with them. If the nymphs feel threatened, they drum on the leaf with their legs and the mother will come.

Now Torrico-Bazoberry shows that a female can localize and recognize her offspring after having been separated from them. This is useful, because often several females start a family on a single host plant, each on her own leaf. Torrico-Bazoberry put ten to fifteen nymphs from a single family on a host plant in the lab and a female on 20 centimetre distance on the same plant; in some cases she was the mother, in others she was not.

Separated from their mother, the nymphs often started to trample; one or a few began, the rest joined in producing a wave-like synchronized behaviour. Upon this rocking behaviour, the nymphs gathered. If the female that was put on the plant was their mother, they aggregated more closely; apparently they detected her presence. The female responded to the rocking behaviour if she was the mother: each mother approached the nymphs. Some non-mothers also did, but not all of them.

Nymphs of treehopper Alchisme grossa receive maternal care throughout developmentApparently, mother and nymphs discriminate kin from non-kin, probably on the basis of the composition of the chemical compounds of the outer skin layer, the researchers suggest. Chemical analyses revealed that this composition differs between individuals, differences between nymphs of a single family being much smaller than differences between nymphs of different families. Because the nymphs aggregate more closely in presence of their mother, it is easier for her to defend them and prevent them from desiccation.

Sometimes, the nymphs stay on their natal leaf until reaching maturity, but sometimes they disperse over the plant stem before that time. The mothers follow their young and together they form mixed aggregations with nymphs of other families and their mothers.

Willy van Strien

Photos: Treehopper Alchisme grossa. Andreas Kay (via Flickr. Creative Commons CC BY-NC-SA 2.0)
Large: female with eggs on midrib of leaf
Small, first: female
Small, second: older nymphs on plant stem

Torrico-Bazoberry, D., L. Caceres-Sanchez, L. Flores-Prado, D. Aguilera-Olivares, F.E. Fontúrbel, H.M. Niemeyer & C.F. Pinto, 2018. Kin recognition in a subsocial treehopper (Hemiptera: Membracidae). Ecological Entomology, online Jan. 23. Doi: 10.1111/een.12506
Torrico-Bazoberry, D., C.F. Pinto, L. Flores-Prado, F.E. Fontúrbel & H.M. Niemeyer, 2016. Natural selection in the tropical treehopper Alchisme grossa (Hemiptera: Membracidae) on two sympatric host-plants. Arthropod-Plant Interactions 10: 229-235. Doi: 10.1007/s11829-016-9427-y
Torrico-Bazoberry, D., L. Caceres-Sanchez, D. Saavedra-Ulloa, L. Flores-Prado, H.M. Niemeyer & C.F. Pinto, 2014. Biology and ecology of Alchisme grossa in a cloud forest of the Bolivian Yungas. Journal of Insect Science 14: 196. Doi: 10.1093/jisesa/ieu031
Camacho, L., C. Keil & O. Dangles, 2014. Factors influencing egg parasitism in sub-social insects: insights from the treehopper Alchisme grossa (Hemiptera, Auchenorrhyncha, Membracidae). Ecological Entomology 39: 58–65. Doi: 10.1111/een.12060

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Idea of lenses abandoned

Brittle star senses light with network of photosensitive cells

Ophiocoma wendtii possesses network of light-sensitive cells

A network of thousands of photosensitive cells allows brittle stars to detect dark places where they can hide from predators, Lauren Sumner-Rooney and colleagues write. No lenses are involved, as has been hypothesized.

The brittle star Ophiocoma wendtii, which lives on coral reefs in the Caribbean, has a strong aversion to light and during the day it retreats into dark crevices, where it is safe from predators. So, it perceives a difference between dark and light places, and this is possible thanks to an impressive network of thousands of light-sensitive cells across the entire body surface, Lauren Sumner-Rooney and colleagues discovered.

At the same time, they reject the existing idea that the dorsal side of the arms is covered with microlenses, as described by for instance Joanna Aizenberg and colleagues. These lenses were thought to focus incident light onto light-sensitive cells beneath; these cells would then transmit a signal to nerve fibres and from these signals neural centres would construct an image of the environment. In fact, the whole animal would act as one compound eye.

Those lenses don’t appear to exist.

Where did the idea come from? Brittle stars have an internal skeleton consisting of a spongy, porous form of calcite (calcium carbonate). The calcite plates of the arms extend into many bumps at the surface, which are hemispherical and transparent. They look just like tiny lenses – and so they were assumed to be tiny lenses.

But now, Sumner-Rooney succeeded in locating cells with light-sensitive pigments. She found many such cells, but not beneath the proposed microlenses, where the focal points should be. Instead, the light-sensitive cells occur at the surface in between the putative lenses, embedded in the skin; they are regularly arranged across the entire body. She also found bundles of nerve fibres that project towards these cells, and no nerve fibres that terminate beneath the ‘lenses’.

In conclusion: the brittle star Ophiocoma wendtii possesses thousands of light-sensitive cells at the surface, but the transparent crystal bumps (the putative lenses) are not associated with them. The bumps are completely covered with skin, which is also in contradiction with an optical role. Also, no neural centres are found that could process the signals. With the extensive network of photosensitive cells the animals can distinguish light from dark very coarsely and find a safe place.

Willy van Strien

Photo: Ophiocoma wendtii. © Lauren Sumner-Rooney

Sumner-Rooney, L., I.A. Rahman, J.D. Sigwart & E. Ullrich-Lüter, 2018. Whole-body photoreceptor networks are independent of ‘lenses’ in brittle stars. Proceedings of the Royal Society B 285: 20172590. Doi: 10.1098/rspb.2017.2590
Aizenberg, J., A. Tkachenko, S. Weiner, L. Addadi & G. Hendler, 2001. Calcitic microlenses as part of the photoreceptor system in brittlestars. Nature 412: 819-822. Doi: 10.1038/35090573

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Blacker than black

Almost no light escapes from of bird of paradise feathers

many birds of paradise have velvety super black feathers

Many birds of paradise have beautiful colours, the brightness of which partly is an illusion, created by dark feathers that surround coloured patches. These feathers are not normal black, but velvety super black, as Dakota McCoy and colleagues show.

Birds of paradise, which mainly occur in New Guinea, deserve their name. The bird family includes many species in which the males have brilliant colours, wear exuberant plumage ornaments and perform exciting dances. With their spectacular appearance, they try to seduce females.

Black feathers play an important role in their courtship, Dakota McCoy and colleagues write. The black feathers that these birds display are not normal black, but super black: they absorb almost all light – more than 99.5 percent – that falls on it. Against this velvety super black background, blue and yellow colours seem brighter than they really are; it looks as if the colours were luminescent. Such super black material is extremely rare in nature.

The researchers show that the deep black appearance is brought about by the special surface structure of the smallest components of the feathers. A feather consists of a shaft on which barbs are implanted, and the barbs are densely packed with barbules. Normally, these barbules are smooth and just bear hooks that interlock to make the feather stiff. The black feathers of crows and ravens have such normal barbules, as do the black feathers of birds of paradise that play no role in their show, such as back feathers.

But the barbules of super black feathers are highly modified. They have very ragged, curled edges with which deep, curved cavities in between, and this structure retains almost all light that falls on it. A normal black surface absorbs 95 to 97 percent of the incident light and reflects the remaining 3 to 5 percent. But in the micro jungle of spikes and cavities of super-black feathers, the light hits obstacles that scatter it again and again, and each time part of the light is transmitted into the material, where it is absorbed. Ultimately, less than half a percent of the incident light is reflected, so the feathers look super black for someone who faces the male – for instance a choosy female.

Photo: Victoria’s riflebid, Ptiloris victoriae, courting male. Francesco Veronesi (Wikimedia Commons, Creative Commons CC BY-SA 2.0)

Watch paradise birds in a video of BBC Earth, and another one of BBC Earth, and one of Cornell University featuring the magnificent riflebird.

McCoy, D.E., T. Feo, T.A. Harvey & R.O. Prum, 2018. Structural absorption by barbule microstructures of super black bird of paradise feathers. Nature Communications 9:1. Doi: 10.1038/s41467-017-02088-w

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‘Heart of Flame’ bromelia protects spider against flames

Heart of flame bromelia shelters spiders from fire

In case of a fire event on the Brazilian cerrado, many animals are killed. But the spider Psecas chapoda, which lives on a bromeliad plant, has a chance to survive, Paula de Omena and colleagues write.

Female Psecas chapoda on Bromelia balansaeThe terrestrial bromeliad plant Bromelia balansae and the spider Psecas chapoda are strongly associated with each other. The spider lives almost exclusively on this prickly plant, in the centre of which it is protected against its predators. On the leaves, adult spiders hunt, court and mate, females lay their eggs, and spiderlings grow up. Up to twenty spiders may inhabit one plant. Conversely, although the plant can do without Psecas chapoda, it benefits when it is inhabited by spiders, because it extracts nutrients from their faeces; the predatory spiders also protect the plant against herbivorous critters.

Now, Paula de Omena and colleagues discovered that this nice mutualism offers an additional benefit to the spider: it can survive a fire on the plant, because the leaves provide shelter and protection from the heat of the flames. Strikingly, the plant is known as ‘Heart of Flame’, as the centre turns bright red when it is about to bloom.

The bromeliads and spiders live in South America, including the Brazilian cerrado: a savanna-like area with trees and shrubs. In the dry period, which lasts about half a year, natural fires frequently rage. The researchers assumed that in the centre of the plants the spiders are sheltered from the heat of flames, and to find out whether they were right, they counted plants and spiders in a small and isolated cerrado fragment before and after a natural fire event.

The day after the fire, the number of spiders had strongly decreased, and also the percentage of bromeliads occupied by spiders was low. But in the centre of a number of plants with intact leaf structures, the researchers found spiders that had survived the fire, and thanks to their survival, the spider population recovered within five months.

Without this possibility to hide, far fewer spiders would survive a fire event and it would take much longer for the population to return to pre-fire levels. A new fire would probably break out before the population fully recovered – so that Psecas chapoda would run a risk to disappear completely. Thanks to the plants, this does not happen.

Willy van Strien

Large: ‘Heart of Flame’, Bromelia balansae. João Medeiros (Wikimedia Commons, Creative Commons CC BY 2.0)
Small: jumping spider Psecas chapoda, female, on Bromelia balansae. ©Gustavo Q. Romero

De Omena, P.M., M.F. Kersch-Beckr, P.A.P. Antiquera, T.N. Bernabé, S. Benavides-Gordillo, F. C. Recalde, C. Vieira, G.H. Migliorini & G.Q. Romero, 2017. Bromeliads provide shelter against fire to mutualistic spiders in a fire-prone landscape. Ecological Entomology, online December 20. Doi: 10.1111/een.12497
Romero, G.Q., P. Mazzafera, J. Vasconcellos-Neto & P.C.O. Trivelin, 2006. Bromeliad-living spiders improve host plant nutrition and growth. Ecology 87: 803-808. Doi: 10.1890/0012-9658(2006)87[803:BSIHPN]2.0.CO;2

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Alternative imaging

Scallops see thanks to hundreds of tiny mirrors

scallop eyes are similar to reflecting telescopes

If a potential predator is in sight, a scallop moves away. It sees the danger with special eyes, which don’t use a lens for image formation, but a concave mirror instead. Benjamin Palmer and colleagues visualized the scallops’ eyes.

great scallop has many blue eyes

With many bright blue eyes along the edge of their mantles, scallops scan their environment continuously. The eyes are peculiar. They do not use a lens to form an image of the outside world, like almost all other eyes do (including ours), but a concave mirror; the eyes are similar to reflecting telescopes. As a second oddness, each eye contains not one retina, but two.

Using various microscopic imaging techniques, Benjamin Palmer and colleagues took a detailed look at the eyes of the great scallop, Pecten maximus, an inhabitant of the Atlantic Ocean which is appreciated in the kitchen. It has about two hundred eyes, each about one millimetre in size.

At the back, the eyes appear to be ’tiled’ with a mosaic of thin, square guanine plates, that are neatly placed next to each other. There are twenty to thirty layers of tiles, and the system reflects almost all incoming light: it is a highly reflective mirror. The fact that the crystals are thin square plates is prove that the scallops have strong control over the crystallisation process, because guanine crystals would take a different form when growing in the lab.

Guanine is also known as the nucleobase G, one of the four letters of the genetic material, the DNA; but it has quite a different application here.

The mirror is curved, it is concave, so that reflected light is focused in front of it. It has no regular shape, but is flattened in the middle. As a result, light that falls in obliquely, that is, from the periphery of the field of view, is focused slightly closer to the mirror than the light falling in perpendicular, from the centre of the field of view. Each eye has two retinas in front of the mirror which absorb the reflected and focused light: closest to the mirror a retina on which an image is formed of the peripheral field of view, in front of it a retina for the central field of view (incoming light has to pass through the retinas before it hits the mirror).

On the outside of the retinas, the eyes also have a lens, but this lens is weakly refracting and it hardly contributes to the imaging.

Thanks to the many eyes, a scallop can see if a predator is approaching. In case of danger, it makes sure to get away: scallops can move by opening and closing their valves quickly. Though it is not quite like swimming, they can escape if they have to.

Willy van Strien

Large: eyes of a scallop. Matthew Krummins (Wikimedia Commons, Creative Commons CC BY 2.0)
Small: great scallop. ©Ceri Jones (Haven Diving Services)

Watch a scallop moving in its habitat

Palmer, B.A., G.J. Taylor, V. Brumfeld, D. Gur, M. Shemesh, N. Elad, A. Osherov, D. Oron, S. Weiner & L. Addadi, 2017. The image-forming mirror in the eye of the scallop. Science 358: 1172-1175. Doi: 10.1126/science.aam9506

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Disarmed, but not impotent

Disabled cactus bug produces more sperm

male Narnia femorata that dropped a leg grows larger testes

With their enlarged hind legs, male cactus bugs fight with each other to defend a territory or to achieve access to a female. What will become of a male that lost one of those weapons, Paul Joseph and colleagues wondered.

The leaf-footed cactus bug Narnia femorata can drop (autotomize) a leg when this leg is grasped by a predator, entrapped or damaged. Thanks to such self-amputation the bug survives the incident, but from now on it has only five legs left to stand on and to walk on; a leg that is lost is not regenerated. For a male, it is extra annoying if it has to sacrifice one of its two hind legs, because it uses them to fight with other males for the possession of a territory or the access to a female. However, if it loses a hind leg before it is fully grown, it can compensate for it, write Paul Joseph and colleagues.

cactus bug narnia femorata preferably feeds on cactus fruitsIn the southwest of the United States, Mexico and parts of Central America, the bugs live on cacti, for instance on the prickly pear cactus Opuntia mesacantha. They feed on the plants, preferably on the ripe fruits, and females lay their eggs on them, selecting parts with ripe fruits.

Males try to defend a territory on a cactus. If an intruder shows up, both males position themselves rear to rear to display, kick and wrestle with their hind legs until one of them gives up. In the presence of a female – when there is a lot at stake – the fight is fiercer, and the male with the largest hind legs will be the winner. The hind legs of males are real weapons, they are enlarged and serrated.

A male that loses one of its hind legs is in problems. It cannot defeat an intact rival and the chance that it will mate a female has decreased considerably. But it may compensate for its disability, Joseph hypothesized, by growing larger testes. This would be possible if the leg is lost before the male is full-grown; bugs don’t go through a complete metamorphosis with a pupal stage, but they grow gradually.

In order to find out whether juvenile males grow larger testes after losing a hind leg, Joseph experimentally induced juvenile bugs to drop a leg by grasping the leg with a pair of forceps and tickling with a small paintbrush, mimicking what can happen in the wild. As expected, after such treatment the testes grew extra large, while everything else developed as it normally does.

And is it useful to have enlarged testes? The researchers paired disabled and untreated males each with a female for 24 hours. Afterwards, they counted how many eggs the females laid and how many of them hatched, meaning that they had been fertilized. They noticed that most females produced about twenty eggs, independent of whether or not they had mated. Clutches of females that had been paired with an untreated male were more likely to contain eggs that hatched than clutches of females with a disabled partner. Apparently, males that dropped a hind leg less often succeeded in mating.

But if disarmed males managed to mate, they fertilized a larger proportion of the eggs. Their enlarged testes produced more sperm, and so they sired more offspring than intact males.

In conclusion, males can compensate for the loss of a weapon by investing more in testes growth – but only if they lose it when still young. Otherwise, it is just bad luck.

Willy van Strien

Large: leaf-footed cactus bug Narnia femorata; male that dropped a hind leg. ©Christine Miller
Small: leaf-footed cactus bug male on cactus fruit. Cotinis (via Flickr; Creative Commons CC BY-NC-SA 2.0)

Joseph, P.N., Z. Emberts, D.A. Sasson & C.W. Miller, 2017. Males that drop a sexually selected weapon grow larger testes. Evolution, 20 november online. Doi: 10.1111/evo.13387
Procter, D.S., A.J. Moore & C.W. Miller, 2012. The form of sexual selection arising from male-male competition depends on the presence of females in the social environment. Journal of Evolutionary Biology 25: 803–812. Doi: 10.1111/j.1420-9101.2012.02485.x

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In its own bubble

Alkali fly manages to stay dry in very wet water

Alkali fly can enter the alkaline Mono Lake

Each insect would drown in Mono Lake, a saline soda lake in California. Each insect, except for the alkali fly, which has unique adaptations to survive the extreme environment, as Floris van Breugel and Micael Dickinson show.

Only bacteria, algae and brine shrimp tolerate the saline water of Mono Lake in California- and the alkali fly Ephydra hians, which flourishes here. It is an amazing critter. The larvae develop in the water, feeding on the algae. Adult flies, which occur in great numbers along the shores, regularly crawl into the water to feed on algae or to lay their eggs. They are the only adult insects that are able to dive into the briny water and emerge alive, Floris van Breugel and Michael Dickinson report.

Several insect species exist that survive submersion in the water of lakes or streams, thanks to a water-repellent layer of hydrocarbons (waxy substances) on their cuticle and tiny hairs that trap a layer of air. But they would be wetted and drown in Mono Lake. That is because the water of this lake contains a large amount of sodium carbonate, a salt known as baking soda, the presence of which renders the insects incapable of keeping the layer of air intact; such water is ‘wetter’ than pure water and penetrates into the layer of air. Sodium carbonate is one of the substances that make the water strongly alkaline.

But the alkali fly easily dives into the alkaline water and when it emerges, it is completely dry. The researchers show that this is possible because the diving fly possesses a very dense mat of hairs and a layer of superhydrophobic hydrocarbons which, combined, prevent wetting. Upon entering the water, a stable air bubble forms around the fly, which enables it to spend 15 minutes underwater; the bubble protects it from the hostile water and offers a supply of oxygen.

In the past 60,000 years, Mono Lake became more salt and more alkaline because it has no outlet, and as water is evaporating, the concentrations of mineral salts gradually rise. Limestone columns, named tufa, developed, along which the flies descend below water surface.

The alkali fly’s ancestors had to enter the lake to forage in a time when the lake was still fresh and algae were the only food available, the authors hypothesize. While the lake gradually became more alkaline, the fly adapted to the new conditions. Nowadays, it can safely graze underwater, as no fish predators occur. The flies are preyed upon by many birds that forage near the lake, like gulls.

When oil, from decaying organic matter or sunscreen and other cosmetics, is floating on the water, even the alkali flies are wetted and drown, in spite of their unique adaptations.

Willy van Strien

Photo: Alkali fly, pictured under water inside its protective air bubble. Floris van Breugel/Caltech

Van Breugel, F. & M.H. Dickinson, 2017. Superhydrophobic diving flies (Ephydra hians) and the hypersaline waters of Mono Lake. PNAS, online Nov. 20. Doi: 10.1073/pnas.1714874114

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Second hand meal

Sea slug consumes the food of its prey

the pilgrim hervia steals the prey of its prey

The diet of the pilgrim hervia Cratena peregrina, a sea slug, not only consists of the hydroids on which it lives; the animal also feeds on prey that was captured by the polyps, Trevor Willis and colleagues report.

The pilgrim hervia Cratena peregrina is a fairylike beautiful creature. Its white back bears tens of red protuberances with a luminescent blue tip, much like little candles. The sea slug occurs in the Mediterranean Sea and the eastern Atlantic Ocean on the branched colonies of hydroids such as Eudendrium ramosum and Eudendrium racemosum. The colonies provide shelter, the polyps are edible and possess defensive weapons that are useful. And according to Trevor Willis and colleagues, there is even more: the polyps capture food which the sea slug then may steal and consume.

Hydroids are cinidarians, just like jellyfish, and their mouth is surrounded by tentacles which grasp their prey, mainly zooplankton. They also possess stinging cells that are able to eject a harpoon (nematocyst) with a toxic content to paralyze prey or to deter predators from attacking.

But the pilgrim hervia is not deterred. It devours polyps without being bothered by stinging cells, as has already been known for a long time. In one way or another it is protected against injury by nematocysts that are fired while it is feeding, and many nematocysts that are ingested remain undischarged. Undischarged harpoons are not digested, but remain structurally intact while passing through the digestive system; some of them are discarded in the faeces, but others are sequestered and stored in cellular vesicles (cnidosacs) at the tips of the dorsal appendages.

So, the sea slug incorporates its prey’s weapons, and it can use them to frighten off hungry fish predators. Because of the bright aposematic coloration, predators quickly learn not to touch this beautiful but unpalatable morsel.

Now, Willis shows that Cratena peregrina not only obtains second hand weapons from the hydroids, but also takes the prey that the polyps have captured. Laboratory experiments revealed that the sea slugs preferentially feed on polyps that are handling prey, for instance brine shrimp. By doing so, the sea slug ingests two food types in one bite: its prey and its prey’s prey. The second hand food, zooplankton, appears to be a substantial part of its diet – and the sea slug doesn’t have to capture this mobile prey by himself.

Willy van Strien

Photo: Cratena peregrina on Eudendrium ramosum. Français (Wikimedia Commons, public domain)

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Willis, T.J., K.T.L. Berglöf, R.A.R. McGill, L. Musco, S. Piraino, C.M. Rumsey, T.V. Fernández & F. Badalamenti, 2017. Kleptopredation: a mechanism to facilitate planktivory in a benthic mollusc. Biology Letters 13: 20170447. Doi: 10.1098/rsbl.2017.0447
Greenwood, P.G., 2009. Acquisition and use of nematocysts by cnidarian predators. Toxicon 54: 1065-1070. Doi:10.1016/j.toxicon.2009.02.029
Aguado, F. & A. Marin, 2007. Warning coloration associated with nematocyst-based defences in aeolidiodean nudibranchs. Journal of Molluscan Studies 73: 23-28. Doi:10.1093/mollus/eyl026
Martin, R., 2003. Management of nematocysts in the alimentary tract and in cnidosacs of the aeolid nudibranch gastropod Cratena peregrina. Marine Biology 143: 533-541. Doi: 10.1007/s00227-003-1078-8

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