Upside-down jellyfish stings at a distance

Mucus contains numerous stinging-cell structures

Upside-down jellyfish releases mucus containing stinging cell masses

The water around upside-down jellyfish is dangerous for small animals and itching for snorkelers. Mobile cell structures, released by the jellyfish, are responsible, as Cheryl Ames and colleagues show.

The upside-down jellyfish Cassiopea xamachana doesn’t swim like jellyfish normally do, but settles upside down on muddy soils of mangrove forests, seagrass beds or shallow bays, its eight oral arms with exuberantly branched flaps facing upward. These jellyfish occur in warm parts of the western Atlantic Ocean, the Caribbean Sea and the Gulf of Mexico, often in large groups.

The habit of lying on the bottom is not the only odd trait of this animal. It is also unusual in hosting unicellular organisms inside its body, the so-called zooxanthellae. Like plants, these organisms convert carbon dioxide and water into carbohydrates and oxygen, using energy from sunlight. They donate part of the carbohydrates to the jellyfish in exchange for their comfortable and safe accommodation.

And then there is a third peculiarity: the water surrounding a group of upside-down jellyfish ‘stings’, as snorkelers know. Cheryl Ames and colleagues discovered how the upside-down jellyfish is responsible.

Mobile cell structures

The carbohydrates that upside-down jellyfish receive from the resident microorganisms are the main source of energy. But the jellyfish also need proteins. That is why they supplement the diet with animal food.

To capture prey, jellyfish use stinging cells. These cells contain stinging capsules, ‘harpoons’, and are filled with a poison blend; the harpoons are able paralyze or kill small critters. Their stings also scare off enemies.

Upside-down jelly has stinging cells on its oral arms. The animal is pulsating, causing water movements that drive prey to the arms, where it is trapped. But, unlike other jellies, the upside-down jellyfish also is able to sting at a distance. How?

If prey is around or if the jellyfish is disturbed, it releases large amounts of mucus, which contain microscopic spherical bodies with an irregular surface, as the current research shows in detail. The bodies consist of an outer cell layer, with stinging cells and ciliated epithelial cells. The content is gelatinous like the jellyfish itself; often zooxanthellae are present, but whether they are active and provide carbohydrates is unknown.

Killing

The cell structures, which the researchers have termed cassiosomes, are produced in large quantities on the jellyfish’s arms. Whenever disturbed, the jelly starts emitting them after five minutes in a mucus cloud and continues for hours. Thanks to the cilia, the spherical bodies are motile. They swim around in the mucus for fifteen minutes and then sink down. They go on rotating and displacing for days, and gradually become smoother and smaller to eventually disintegrate after ten days.

The cassiosomes are capable of killing prey animals, laboratory tests show. Brine shrimp, for example, is often instantly killed upon contact with the cell structures.

While doing their work, the researchers experienced that the water in the test tanks was indeed stinging.

Of all peculiarities that upside-down jellyfish possess, this may well be the strangest: loose jellyfish pieces that remain alive for days independently of the main body, move around and help capture prey and scare enemies. The researchers now know that a few closely related jellyfish species release similar small ‘grenades’.

The cell masses in the mucus of upside-down jellyfish had been seen before, at the beginning of the twentieth century, but were thought to be parasites. Nobody could not fancy by that time that it was jellyfish tissue.

Willy van Strien

Photo: Bjoertvedt (Wikimedia Commons, Creative Commons CC BY-SA 4.0)

Source:
Ames, C.L., A.M.L. Klompen, K. Badhiwala, K. Muffett, A.J. Reft, M. Kumar, J.D. Janssen, J.N. Schultzhaus, L.D. Field, M.E. Muroski, N. Bezio, J.T. Robinson, D.H. Leary, P. Cartwright, A.G. Collins & G.J. Vora, 2020. Cassiosomes are stinging-cell structures in the mucus of the upside-down jellyfish Cassiopea xamachana. Communications Biology 3: 67. Doi: 10.1038/s42003-020-0777-8

Modest carnivore

Shepherd’s purse seedling benefits from animal proteins

Shepherd's purse is carnivorous during early development

Shepherd’s purse seeds derive nutrients from small animals by attracting, ensnaring, killing and digesting them, Hattie Roberts and colleagues discovered. The plant is carnivorous during early development.

Most carnivorous plants have a striking appearance: bladderworts, Venus flytrap, pitcher plants and sundews capture small critters with ingenious traps or sticky leaves. But who would suspect shepherd’s purse of having a carnivorous diet? The plant, which occurs almost everywhere, looks innocent. Still, it captures small invertebrates and uses their proteins when germinating, Hattie Roberts and colleagues write. Apparently, also less spectacular carnivores exist among plants.

Animal proteins

shepherd's purse seeds attract, ensnare, kill and digest small invertebratesA shepherd’s purse plant (Capsella bursa-pastoris) does not capture any bugs, but the seeds do. In moist soil, they are covered with a tough, sticky mucus layer. Years ago, John Barber already showed that germinating seeds excrete an attractive substance that lure small invertebrates, which subsequently are entrapped and killed by a toxin – Barber used mosquito larvae in his experiments. The seeds also release enzymes that digest animal proteins and take up the building blocks, amino acids.

The seeds seem to use animal proteins for growth, Barner stated. They also attract and kill nematodes (roundworms that live in soil), unicellular organisms and bacteria.

Food supply

Now, Hattie Roberts completes the story. She germinated seeds in presence or absence of nematodes and monitored germination and growth of the seedlings. The experiments showed that the seeds do better when animal nutrients are available. In soils in which nematodes were present, more seeds germinated successfully than in soils without nematodes, and the seedlings were bigger and heavier when measured ten days after germination. After three weeks, young plants that had germinated in soil with nematodes had longer roots and larger leaves.

Although seeds of shepherd’s purse are able germinate without animal nutrients, they do better with such nutrients. On soils with low nutrient levels, seeds derive more benefits from animal nutrients than on soils with high nutrient levels, as shown by the experiments.

Shepherd’s purse seeds are small and contain only a minimal food supply for the seedling. That is why the plants profit from extra nutrients during early development.

Willy van Strien

Photos:
Large: shepherd’s purse flowers. Harry Rose (Wikimedia Commons, Creative Commons CC BY 2.0)
Small: shepherd’s purse seeds. Kinori (Wikimedia Commons, public domain)

See also:
Carnivorous plants with spectacular traps: bladderwort and Venus flytrap

Sources:
Roberts, H.R., J.M. Warren & J. Provan, 2018. Evidence for facultative protocarnivory in Capsella bursa-pastoris seeds. Scientific Reports 8: 1012. Doi: 10.1038/s41598-018-28564-x
Barber, J.T., 1978. Capsella bursa-pastoris seeds. Are they “carnivorous”? Carnivorous Plant Newsletter 7: 39-42.

Small bites, huge gulps

Thousands of copepods are ingested each day by little auk

feeding in little auk is similar to that of whale

Little auks have a way to gather food that is unusual in birds, Manfred Enstipp and colleagues show. The birds feed ‘almost like a whale’, as the title of  their publication suggests.

The little auk (or dovekie), a seabird species of arctic regions which gathers its food while diving, does not have easy prey to catch. It feeds mainly on copepods (small crustaceans) and it needs an estimated 60,000 prey items per day. It is difficult to capture these tiny animals, as they respond to threats with fast movements. It’s difficult to grasp something as small like copepods under water anyway: the object is pushed away when you try. How does a little auk get all the copepods it needs?

Just by opening the beak while swimming and filtering the prey from the water, Manfred Enstipp and colleagues assumed. But when they filmed little auks with a high-speed camera in a test pool containing copepods and analysed the footage, their idea turned out to be wrong.

Whale

Little auks, as the researchers observed, search for copepods with their eyes. Upon prey detection, they go after it, stretching their neck. They extend their gular pouch so that under pressure arises in the oral cavity, and open their bill slightly. As a consequence, a big gulp of prey-laden water is sucked in. The bird then retains the prey while it expels the excess water through the nostrils and from the back of the bill. The whole procedure takes about half a second. By capturing prey in quick succession – and with some luck it sometimes has two items in one trial – a little auk gathers its food.

Many fish species use this ‘suction-feeding’ method to catch prey, but for a bird it is unusual. It is reminiscent of the way baleen whales feed: they take in large quantities of water and expel it through the baleens, retaining the plankton.

Willy van Strien

Photo: Allan Hopkins (via Flickr, Creative Commons CC BY-NC-ND 2.0)

Source:
Enstipp, M.R., S. Descamps, J. Fort & D. David Grémillet, 2018. Almost like a whale – First evidence of suction-feeding in a seabird. Journal of Experimental Biology, online May 29 mei. Doi: 10.1242/jeb.182170 

Collateral benefit

Bird disperses eggs of stick insects it swallowed

brown-eared bulbul disperses eggs of stick insects

Some stick insects are even more like plants than you might think at first glance. Just like plant seeds, the eggs can be dispersed by a bird, Kenji Suetsugu and colleagues show.

Stick insects are perfectly camouflaged: they do not stand out among the plants. Yet insect-eating birds are able to find them and will eat them. And that is the end of the story for such tiny animal.

Well, it may not be, Kenji Suetsugu and colleagues report. If an unfortunate female stick insect is carrying mature eggs, a few of these appear undamaged in the bird’s droppings, and some may even hatch.

Younsters

The researchers, working in Japan, point out that the eggs of stick insects resemble plant seeds: they have the same size and colour and feel the same thanks to a hard shell. Hence their suggestion that the eggs might survive passage through a bird’s digestive tract like plant seeds do. Many plant species produce fruits that are eaten by birds or other animals; the seeds remain intact, are excreted and germinate. Is something similar possible for the eggs of stick insects?

eggs of stick insect after passage through a bird's digestive tractTo find out, they mixed mature eggs of three stick insect species with an artificial diet and fed this to a brown-ear bulbul, one of the main predators of the insects. Afterwards, they examined the bird’s droppings under a stereomicroscope and discovered a small number of intact eggs, and from some of these eggs a young stick insect hatched later on.

Such scenario is also possible when a bird swallowed a gravid female, the authors think. The youngsters that hatch after passage through a bird’s guts would have to find an appropriate food plant to live on, but that is always the case. Normally, a female just drops her eggs to the ground and does not provide any care.

Parthenogenetic

young stick insect, hatched from egg that passed through a bird's gutsSo, sick insects not only look like plants, but they also exhibit a surprising plant-like trait: dispersal of offspring by birds, which is unique in insects.

Dispersal by an avian predator is only possible for species that reproduce parthenogenetically, for in that case females carry eggs that can develop without fertilization. A number of stick insect species exhibit parthenogenesis, including the species that were studied here.

Flyways

Dispersal of insect eggs via a bird’s digestive tract is not entirely comparable to dispersal of plant seeds. Plants produce fruits that have to be eaten to disperse their seeds. In contrast, a female stick insect has no intention to be captured by a bird to have her eggs transported – by being camouflaged, she tries to prevent just that. But if she is unlucky enough to become a bird’s meal, it is a collateral benefit if some eggs survive and young hatch, if only a few.

The hard eggs probably have not evolved to facilitate avian dispersal, the authors suggest, but to decrease the risk of attack by parasitoid wasps, which lay their eggs in other insects’ eggs.

Stick insects are immobile. Thanks to the birds they may reach new places to live. An interesting question is whether distribution patterns in the insects, to be unravelled by DNA research, overlap with birds’ flyways; that would strengthen the idea that the eggs are sometimes dispersed like plant seeds.

Willy van Strien

Photos
Large: Brown-eared bulbul (tongue visible). Alpsdake (Wikimedia Commons, Creative Commons BY-SA 4.0)
Small: stick insect (Ramulus irregulariterdentatus) eggs that passed through a bird’s digestive tract and a young stick insect that hatched from such egg. ©Kenji Suetsugu

Source:
Suetsugu, K., S. Funaki, A. Takahashi, K. Ito & T. Yokoyama, 2018. Potential role of bird predation in the dispersal of otherwise flightless stick insects. Ecology, online May 29. Doi: 10.002/ecy.2230

Deceit, abuse and benefits

Complex relationships between arum, blowflies and lizard

Dead-horse arum flower is attractive to lizard

With its smell of rotting carrion, the dead-horse arum Helicodiceros muscivorus is irresistible to blowflies and a lizard. The blowflies will be abused, the lizard benefits. Ana Pérez-Cembranos and colleagues unraveled these complex relationships.

On islands in the Mediterranean Sea, a plant occurs with a very bad smell, the dead-horse arum, Helicodiceros muscivorus. Its odour contains chemical components that are also emitted by a decomposing dead animal. It irresistible to a female blowfly searching for carcasses to lay her eggs on to make sure that the carnivorous larvae will have food. The dead-horse arum takes advantage of that behaviour.

The plants release their odour on the first day of blooming. Blowflies that perceive the smell cannot ignore it. Upon approaching the source, they find a pink or red curved bract, the spathe, with the hairy end of the spadix (inflorescence), which produces the smell. When they land, the spadix turns out to be warm. To blowflies, the imitation is perfect: this is rotting carrion. Guided by the heat, they crawl into the tube that is formed by the base of the spathe around the lower part of the spadix, which bears female and male florets.

Once inside, the blowflies don’t find what they need, which is decaying meat. But if they want to leave, they cannot. Spikes on the spadix keep the door closed. The blowflies are trapped.

Unintentionally, they provide a service to the arum during their imprisonment in the floral chamber. The female flowers at the bottom of the spadix are blooming this first day, and blowflies that had been misled by the arum before, now deliver the pollen that they picked up on that occasion. The plant has its female flowers pollinated.

The next day, the female flowers have faded and the male flowers are mature. The stench and the heat disappear, the spikes wilt and the blowflies escape, and while passing the male flowers, they are loaded with pollen. And here is the second benefit to the plant: the blowflies take the pollen with them to female flowers elsewhere – if at least they find another foul smelling arum on their way and are again misled into visiting it.

So, the blowflies are coerced to pollinate the dead-horse arum without receiving any reward such as nectar. On the contrary: they lose time that they should have spent on searching for genuine carcasses.

Now Ana Pérez-Cembranos and colleagues show that the Balearic lizard Podarcis lilfordi is also misled by the arum’s odour. The animal is omnivorous and sometimes forages on carcasses, which are also attractive as a heat source; lizards are cold-blooded and when the weather is cool, they may use a rotting carcass as a perching site for basking. In addition, they capture the blowflies that arrive at the cadaver in search for a site for oviposition.

The lizards respond to the smell of the dead-horse arum as they do to the smell of a carcass and will approach the source. If that turns out to be a dead-hors arum instead of a dead animal body, they will not find a meat meal, but they do find a basking place and blowflies, which they take from the spathe or grab from the tube. The lizards thus take away a number of pollinators, but, according to the researchers, enough are left to ensure pollination.

So, the lizard isn’t an enemy of the arum. And after the flowering period, when fruits are ripe, a mutualism even develops between both. The lizards consume the fruits and disperse the seeds in their faeces; passage through the lizard’s intestine increases the probability of germination. On Aire Island, a the small island off the southeastern coast of Menorca, where the research was done, the dead-horse arum is a newcomer. It is estimated to have grown there for only about fifty years. In that period, it spread rapidly over the island and nowadays it locally occurs in great densities. That is because of the lizard, which has learned to eat the fruits and now is the main disperser of the seeds, the researchers think.

Willy van Strien

Photo: Balearic lizard on the spathe of the dead-horse arum © Ana Pérez-Cembranos

Sources:
Pérez-Cembranos, A., V. Pérez-Mellado & W.E. Cooper, 2018. Balearic lizards use chemical cues from a complex deceptive mimicry to capture attracted pollinators. Ethology  124: 260-268. Doi: 10.1111/eth.12728
Angioy, A-M.,  M. C. Stensmyr, I. Urru, M. Puliafito, I. Collu & B. S. Hansson, 2004. Function of the heater: the dead horse arum revisited. Proceedings of the Royal Society London B 271: S13-S15. Doi: 10.1098/rsbl.2003.0111
Stensmyr, M.C., I. Urru, I. Collu. M. Celander. B.S. Hansson & A-M. Angioy, 2002. Rotting smell of dead-horse arum florets. Nature 420: 625-626. Doi: 10.1038/420625a

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)

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

Saving energy

Venus flytrap controls the trapping process in several ways

Venus flytrap has several mechanisms to save energy

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 of the Venus flytrap end in a two-lobed trapThe 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 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 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 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 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

Photos
Large: ©Andrej Pavlovič
Small: Olivier License (via Flickr, Creative Commons CC BY-NC-ND 2.0)

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

Floral dress

Pollinators are deceived by flower mimicking crab spider

Epidacus heterogaster sucessfully mimics a flower

The spider Epicadus heterogaster is coloured strikingly like a flower, and bees are lured by the colour to become prey, as Camila Vieira and colleagues show. The masquerade is completed by a conspicuous abdomen, mimicking a flower’s shape.

The crab spider Epicadus heterogaster, which lives in Brazil, always seems to be dressed in a carnival costume that makes it look like a flower: it has a white, yellow or purple body colour and conspicuous abdominal protuberances. By mimicking a flower, it attracts insects that use to visit flowers to collect nectar, meanwhile pollinating the flowers. All it has to do next, is extend its legs and grab the victims – and the pollinators become prey.

Now, Camila Vieira and colleagues present proof that the crab spider’s colour attracts pollinators.

Like many flowers, Epicadus heterogaster has an ultraviolet component in its body colour. We cannot see that colour, but insects do and some insects prefer it. The spider’s colour stands out clearly against the green leaves on which she awaits her visitors.

In order to demonstrate that the spiders’ colour indeed lures insects, the researchers treated anesthetized females with sunscreen that blocks ultraviolet light. When they applied the sunscreen on a female’s back, passing pollinators no longer saw an ultraviolet colour and didn’t approach the spider; in contrast, they avoided it. But when the sunscreen was applied not on the dorsal side, but on the ventral side of a spider, it remained attractive to pollinators. Its flowerlike appearance undoubtedly  contributes to the deceit.

Juvenile female spiders are coloured like adults, also mimicking a flower, but they exploit their disguise in another manner. They’re not sitting on a leaf to attract pollinators, as they are too small to be of any interest to them anyway, and by being conspicuous, they would attract predators. Instead, youngsters are observed mostly on flowers, where they are perfectly camouflaged.

Willy van Strien

Photo: Alex Popovkin (Wikimedia Commons, Creative Commons CC BY 2.0)

Epicadus heterogaster on YouTube

Source:
Vieira, C., E.N. Ramires, J. Vasconcellos-Neto, R.J. Poppi & G.Q. Romero,2017. Crab spider lures prey in flowerless neighborhoods. Scientific Reports 7: 9188. Doi: 10.1038/s41598-017-09456-y

Glue-coated silk

Ground spider immobilises prey with sticky threads

Ground spider captures prey with sticky threads

Many ground spiders can handle large or dangerous prey. They swathe their victims’ legs with threads that are coated with a tough glue, as Jonas Wolff and colleagues show.

Ground spiders (Gnaphosidae) do not capture their prey by building a web, but hunt for them on the ground. For some of them, this is a risky venture because they aim for large or dangerous prey, such as ants and other spiders. Jonas Wolff and colleagues found out how they managed to capture these prey.

The researchers recorded the behaviour of a number of ground spiders during an attack with a high speed camera and analyzed the footage. When a ground spider perceives a prey, they observed, it first tries to grab it with its front legs. If it fails because the victim is large or dangerous, it quickly switches to another tactic and applies sticky silk threads to the soil and to the opponent’s legs and mouth parts to immobilise it; sometimes, an entangled prey spider still tries to defend itself by biting – hunting is never without risk.

Other spider species don’t produce such sticky silk threads. Spider silk is made in glands, and comparing the silk glands of ground spiders that swathe the legs of their prey with those of other species, the researchers found clear differences.

Spiders possess different types of silk glands, each of which makes another type of silk. The glands produce a liquid mixture of silk proteins and have a tapering duct ending in a nozzle-like opening (the spigot) on a spinneret; spiders have one to four pairs of spinnerets on their abdomen and make threads by pulling the protein mixture from the spigots.

In most spider species, the largest pair of spinnerets bears the spigot of a single large ampullate gland. This gland produces the strong silk of which draglines are made: the structural threads of webs of web-building species and the drop lines that spiders make when they drop. In addition, there are the spigots of numerous tiny piriform glands; they produce short glue-coated fibres of which spiders spin attachment disks to link the threads of their web at the intersections, or to anchor a dragline to, for instance, a tree.

In ground spiders that use sticky threads to capture their prey, the silk glands are strongly modified. The ampullate glands are relatively small, while the piriform glands are enlarged and have wide spigots. Ground spiders entangle their prey with the silk threads pulled from these enlarged piriform glands. The threads are coated with a layer of ductile, tough glue, perfect for swathing and immobilising struggling prey.

So, ground spiders developed a novel use of piriform silk, and the morphology of the silk glands is adapted to this novel use. The consequence is that they are not able to make functional draglines and fully functional attachment disks. Like many other spider species, they build a silk shelter in which they hide (of yet another type of silk), but they can’t anchor that shelter to the substrate as well as other spider species do. That’s the price they have to pay for their exclusive hunting method.

Willy van Strien

Photo:
Mouse spider Scotophaeus blackwalli. Richard Pigott (via Flickr; Creative Commons CC BY-SA 2.0)

Source:
Wolff, J.O., M. Řezáč, T. Krejčí & S. Gorb, 2017. Hunting with sticky tape: functional shift in silk glands of araneophagous ground spiders (Gnaphosidae). Journal of Experimental Biology 220: 2250-2259. Doi: 10.1242/jeb.154682

Fast trapping device

Water flea has no chance against bladderwort bladders

Southern bladderwort possesses fast trapping device

There is no escape for a water flea that hits one of the submerged suction traps of bladderwort. Within a split second, the trapdoor opens and closes and the water flea has disappeared, as Simon Poppinga and colleagues show.

Some carnivorous plants, which feed on small animals, possess motile traps to capture insects or other prey. The fasted motile trapping device belongs to aquatic bladderwort species, according to Simon Poppinga and colleagues.

greater bladderwort, leaves with trapsThe stems with yellow flowers of these floating water plants are visible above water level, while the leaves, bearing bladder-like suction traps, are below surface. Using a high-speed camera, the researchers recorded the action of the suction process in Southern bladderwort bladders when trapping a water flea.

Each trap is filled with water, sometimes also with some air, and because water is continuously pumped out, there is a negative pressure within. The bladder entrance is closed with a flexible door which is fixed along the upper part, resting on a threshold and bulging outwards; it bears trigger hairs that are sensitive to touch.

As soon as a water flea touches a trigger hair, the door will invert its curvature, bulging inwards. In that position it can’t resist the water pressure and swings open. The water flea is sucked into the bladder with a velocity that increases to 4 meters per second. It is unable to interfere with the process in any way. As soon as it is in, the water flow decelerates. The strong acceleration and deceleration immobilize the animal and maybe even kill it. And if still alive, the water flea will die soon due to anoxia.

The door recloses and regains the convex curvature. The whole process took only 0.01 second, and within a couple of hours, the prey will be digested.

Willy van Strien

Photos:
Large: Southern bladderwort. Abalg (via Wikimedia Commons. Creative Commons CC BY 3.0)
Small: leaves with suction traps of greater bladderwort. H. Zell (via Wikimedia Commons. Creative Commons CC BY-SA 3.0

Source:
Poppinga, S., L.E. Daber, A.S. Westermeier, S. Kruppert, M. Horstmann, R. Tollrian & T. Speck, 2017. Biomechanical analysis of prey capture in the carnivorous Southern bladderwort (Utricularia australis). Scientific Reports 7: 1776. Doi:10.1038/s41598-017-01954-3