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Family ties

6 February 2018 / wvanstrien

Caring treehopper mother recognizes her offspring

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.

They 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.

Sap feeders

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.

Trample

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.

Apparently, 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

Sources:
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

Idea of lenses abandoned

30 January 2018 / wvanstrien

Brittle star senses light with network of photosensitive 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.

Microlenses

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’.

Safe place

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

Sources:
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

Blacker than black

18 January 2018 / wvanstrien

Almost no light escapes from of bird of paradise 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.

Ragged, curled edges

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.

Source:
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

Fireproof

10 January 2018 / wvanstrien

‘Heart of Flame’ bromelia protects spider against flames

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.

The 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.

Fires

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.

Recovery

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

Photos
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

Sources:
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

Alternative imaging

13 December 2017 / wvanstrien

Scallops see thanks to hundreds of tiny mirrors

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.

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.

Tiles

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.

Danger

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

Photos:
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

Source:
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

Disarmed, but not impotent

6 December 2017 / wvanstrien

Disabled cactus bug produces more sperm

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.

In 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.

Fierce fight

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.

More sperm

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

Photos
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)

Sources:
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

In its own bubble

30 November 2017 / wvanstrien

Alkali fly manages to stay dry in very wet water

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.

Wax

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.

Limestone columns

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

Source:
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

Second hand meal

23 November 2017 / wvanstrien

Sea slug consumes the food of its prey

The diet of the pilgrim hervia Cratena peregrina, a sea slug, not only consists of the hydroids on which it lives; this 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.

Cnidosacs

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.

Prey of prey

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)

Watch the sea slug on YouTube

Sources:
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

Camouflage suit

6 November 2017 / wvanstrien

Covered with sponges, a crab is poorly visible

The spider decorator crab Camposcia retusa adorns its legs and carapace exuberantly with sponges, probably to mislead predators, Rohan Brooker and colleagues write. The crab accumulates more decorations when it has no access to shelter.

Equipped with a lot of sponges, complemented by some algae and dead organic matter, the spider decorator crab Camposcia retusa moves around: a weird appearance. The crab is associated with tropical coral reefs in the Indian Ocean and the western Pacific Ocean. Why would this little animal, with a carapace that is a few centimetres wide, carry so much stuff that probably hampers its mobility?

According to Rohan Brooker and colleagues, a highly decorated crab is less visible to its predators. In addition, many sponges are noxious or toxic, and they may deter predators that perceive such a crab in spite of its camouflage.

The researchers wanted to learn more about the crab’s decorating behaviour. From reefs, they caught a number of crabs to study their decoration patterns. They then conducted a manipulative behavioural experiment on crabs in tanks to which they added red polyester pompoms of different sizes to see how the crabs would use them.

Velcro

They found that the animals covered their carapace and the third and fourth sets of walking legs most (they have four pairs of walking legs). In the experiments, they placed the largest and heaviest pompoms only on the hind legs, which are the strongest. The chelipeds – which the crabs use for feeding and communication – and the first set of legs were hardly decorated. The parts of the body on which items are distributed are equipped with hooked seta like those of Velcro, to which pieces of sponge and other material are easily attached.

Defence

In another experiment, the crabs either got a shelter in the form of a PVC elbow in their tank or no shelter. The crabs that had no access to shelter decorated more than the crabs that had shelter, hence the conclusion that the decoration is primarily an antipredator defence. Because the animals accumulate and retain a wide range of materials, camouflage most likely is the main effect of decoration. And because they prefer to attach sponges, it may also serve as a deterrent. It would be great if the researchers now would go on to show that predators have more difficulty perceiving a prey in camouflage suit, or that they are deterred by the sponges.

Decoration occurs in many animal species, most frequently in aquatic species. The spider decorator crab Camposcia retusa is a beautiful example of this behaviour.

Willy van Strien

Photo: Patrick Randall (via Flickr, Creative Commons CC BY-NC-SA 2.0)

Three examples of decorated crabs on YouTube: 1, 2, 3

Source:
Brooker, R.M., E.C. Muñoz Ruiz, T.L. Sih & D.L. Dixson, 2017. Shelter availability mediates decorating in the majoid crab, Camposcia retusa. Behavioral Ecology, online Oct. 17. Doi: 10.1093/beheco/arx119

Saving energy

1 November 2017 / wvanstrien

Venus flytrap controls the trapping process in several ways

Carnivorous plants should control their energy budget, otherwise the benefits of capturing insects will not compensate for the costs. The Venus flytrap has several mechanisms to limit waste of energy, Andrej Pavlovič and colleagues discovered.

To us, it is almost impossible to catch a fly. But the Venus flytrap has no difficulty. The plant (Dionaea muscipula) occurs in North and South Carolina (United States), where it grows in sunny, wet areas on poor soil; it can grow there by ‘eating’ insects. The catch of a fly yields lots of nutrients, but the process also demands lots of energy, and the balance between yield and costs must be positive, otherwise the plant will not grow. So, it has evolved a number of control mechanisms to minimize waste of energy, as Andrej Pavlovič and colleagues point out.

The leaves are arranged in a rosette, and each leaf has a double-lobed trap at the top with a row of ten to twenty teeth at the edge of each lobe. Glands along the edges secrete a sugary substance that attracts insects. Each lobe has a few trigger hairs that respond when touched by an insect, causing the trap to snap shut rapidly. The central zone of the trap contains glands secreting enzymes that digest a trapped prey and proteins that enable the glands to absorb the nutrients that are released upon digestion.

The Venus flytrap has to invest a lot of energy to keep the traps operational and to produce lures, digestive enzymes and absorption proteins. How does the plant control these costs?

1: Two times

First of all, a trap will not snap shut until trigger hairs are touched at least twice within twenty seconds, when there is a fair chance that an insect has landed. So, a trap will not close when a for instance a wind-blown dust grain touches a trigger hair.

2: Panic

But not every animal that landed turns out to be a nice fat fly. Upon closure, small gaps between the marginal teeth allow little insects that are not worth the effort to digest them to escape. If the trap is empty, it will reopen again after a few hours. But if a large insect is encased, it will struggle in panic, and his movements induce the trap to seal hermetically. After the trigger hairs have been touched at least five times, the secretion of digestive enzymes and absorption proteins starts, and the more movements the prey makes, the more enzymes and proteins will be secreted.

3: Limited reaction

Still, a trap may snap shut, close tightly and secrete digestive enzymes and absorption proteins in vein. This happens when it is damaged. The cause of the error is to be found in the evolution of carnivorous plants, for the habit to capture insects probably evolved from defence mechanisms against herbivorous insects. In ordinary plants, herbivory generates an electrical signal, which in turn stimulates the accumulation of plant hormones, jasmonates. These will induce the plants to synthesize toxins that harm the insects, not only locally on the place of damage, but also elsewhere in the plant, as a precaution. In carnivorous plants, such as the Venus flytrap, things have a bit changed. In these plants, the presence of an insect triggers an electrical signal that induces the accumulation of jasmonates; these hormones stimulate the secretion of digestive enzymes and absorption proteins. The electrical signal also induces the trap to close.

Now, Pavlovič conducted an experiment in which he repeatedly wounded a trap of Venus flytraps by piercing it with a needle to mimic herbivory, and noticed that the trap showed the same response as when the trigger hairs wouldhave been touched by an insect: the trap closed and jasmonates accumulated; if he continued damaging every few minutes, the traps secreted digestive enzymes and absorption proteins – to no end. But the misplaced reaction was limited to the local trap that was damaged and did not occur elsewhere in the plant, in contrast to defence reactions agains herbivores.

4: Process stops

The secretion of digestive enzymes and absorption proteins does not run at full speed from the start. Only when certain substances from an enclosed prey are released, the rate of secretion increases to the highest speed. If there is no prey, the process will stop. So, the plant doesn’t waste much energy when it is misled.

After about ten days the fly is digested and the fall will reopen again.

Willy van Strien

Watch the trapping process

Sources:
Pavlovič, A., J. Jakšová & O. Novák, 2017. Triggering a false alarm: wounding mimics prey capture in the carnivorous Venus flytrap (Dionaea muscipula). New Phytologist 216: 927-938. Doi: 10.1111/nph.14747
Böhm, J., S. Scherzer, E. Krol, I. Kreuzer, K. von Meyer, C. Lorey, T.D. Mueller, L. Shabala, I. Monte, R. Solano, K.A.S. Al-Rasheid, H. Rennenberg, S. Shabala, E. Neher & R. Hedrich, 2016. The Venus flytrap Dionaea muscipula counts prey-induced action potentials to induce sodium uptake. Current Biology 26: 286-295. Doi: 10.1016/j.cub.2015.11.057
Libiaková, M., K. Floková, O. Novák, L. Slováková & A. Pavlovič, 2014. Abundance of cysteine endopeptidase dionain in digestive fluid of Venus flytrap (Dionaea muscipula Ellis) is regulated by different stimuli from prey through jasmonates. PLoS ONE 9: e104424. Doi:10.1371/journal.pone.0104424
Pavlovič, A., V. Demko & J. Hudák, 2010. Trap closure and prey retention in Venus flytrap (Dionaea muscipula) temporarily reduces photosynthesis and stimulates respiration. Annals of Botany 105: 37-44. Doi:10.1093/aob/mcp269

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Venus flytrap controls the trapping process in several ways

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