Category: predation (Page 1 of 2)

Theridiosoma gemmosum catapults its web to passing prey

A Theridiosoma gemmosum female does not wait for a mosquito to fly into her web. If she hears one approaching, she strikes her web at it, Sarah Han and colleagues write. The mosquito cannot escape.

Most female spiders that construct a web, feed on the insects that have flown into it. But Theridiosoma gemmosum takes a different approach: this spider catapults her web at a passing prey, usually a mosquito, to capture it.

If she did not, such a mosquito would escape the web. A mosquito flies with its front legs extended forward, and as soon as the legs touch a spider’s web, the mosquito reverses its flight and the spider misses her prey. But Theridiosoma gemmosum is ahead of this avoidance strategy by taking action when she hears a mosquito coming close, Sarah Han and colleagues show.

Theridiosoma gemmosum belongs to the ray or slingshot spiders (Theriodiosomatidae), small spiders that use their web as a catapult. Theridiosoma gemmosum is only a few millimetres in size. The web is also small, and it is difficult to find one. The species is widely distributed in wet environments such as river banks and swamps in the northern hemisphere.

Ready to strike

A slingshot spider makes a planar orb web, spins a thread from the centre and attaches the end to a twig. Then she sits in the middle of the web, grabs the centre with her four back legs and the anchor thread with her four front legs. By letting her front legs run over the thread, she stretches the web a few centimetres; the web becomes cone-shaped and is now ready to strike. The spider, sitting on top of the cone, holds the loose piece of thread between front and back legs with her pedipalps (the ‘boxing gloves of spiders).

And then she waits until a flying insect passes the base of the cone. If this happens, she releases the thread; the web snaps back, the spider whizzing backwards with it. The more the web was stretched, the more powerfully it shoots back. If it hits the unfortunate passer-by, the spider has captured her prey; the insect sticks to the threads and cannot escape. Otherwise, she immediately picks up the thread to tighten the catapult again.

Sound

The researchers wondered what exactly made the spider release her web. They conducted experiments in which they tethered a mosquito to a paper strip, in such a way that it could make normal flying movements. They moved it towards a web of Theridiosoma gemmosum. High-speed camera footage shows that the spider shoots its web at lightning speed at a mosquito when it is within reach, but before it touches it with its front legs and realizes the danger.

How does a slingshot spider perceive that a mosquito is within reach? Not with her eyes: the spider does not see sharply and, moreover, she is facing away from the mosquito. But she has special long hairs on the hind legs that sense the airborne vibrations caused by the wing beats. Moreover, the vibrations propagate over the threads of the web, and she detects that too. From the combination of this information, she probably infers where a mosquito is.

There is another piece of evidence that slingshot spiders respond to sound: they shoot their webs also at the snap of a finger or at the sound of a tuning fork. This does not produce any result, but apparently, slingshot spiders take every chance.

Willy van Strien

Photo: Theridiosoma gemmosum. ©Portioid (via iNaturalist, Creative Commons CC BY-SA)

The catapult in slow motion on YouTube

Sources:
Han, S.I. & T.A. Blackledge, 2024. Directional web strikes are performed by ray spiders in response to airborne prey vibrations. Journal of Experimental Biology 227: jeb249237. Doi: 10.1242/jeb.249237
Alexander, S.L.M. & M.S. Bhamla, 2020. Ultrafast launch of slingshot spiders using conical silk webs. Current Biology 30: R928-R929. Doi: 10.1016/j.cub.2020.06.076

Hunting trumpetfish swims closely alongside other fish to remain undetected

By swimming next to another fish, the trumpetfish can approach its prey closer without them noticing. It works, Samuel Matchette and colleagues observed.

In order to ‘surprise’ its prey when attacking, a trumpetfish keeps itself invisible. The elongate, extremely slender fish, typically just over half a meter long, often waits in vertical position between corals and sponges for prey to come close enough to strike; its prey are smaller fish and shrimp. As the predator fish takes on the background colour while waiting, it does not stand out.

But that strategy is useless in water with little cover. In such scenery, the predator applies another camouflage trick: it shadows a harmless fish by swimming very closely alongside it. And that works well, as Samuel Matchette and colleagues show: prey do not see their enemy approaching.

Clear response

The trumpetfish, Aulostomus maculatus, is related to seahorses and pipefish; it lives in the western part of the Atlantic Ocean. It was already known that it often swims aligned with other fish. It arches for instance over the back of a large fish, taking on that fish’s colour. The idea already existed that in doing so, it conceals its characteristic outline to approach its prey undetected. Fish that it hides behind are usually harmless herbivores, such as parrotfish, from which prey animals do not flee. 

But the question still was: does it really work? Does it enable the predator to approach its prey more closely undetected?

On coral riffs near Curaçao, the researchers conducted tests that answered these questions in the affirmative.

They studied the defensive behaviour of bicolour damselfish (Stegastes partitus). These fish live in colonies, are on the menu of trumpetfish and react clearly when seeing a predatory fish: a few individuals inspect the predator and then all quickly seek shelter.

Models

Matchette and colleagues exposed damselfish colonies to a three-dimensional printed and painted model of a trumpetfish alone, a parrotfish alone, or a parrotfish with a trumpetfish attached. They moved those models over a colony, one at a time, and videotaped the colony’s reaction.

As might be expected, damselfish were not much startled by a parrotfish passing on its own. In contrast, they responded strongly to a trumpetfish; they inspected it intensively and then quickly retreated.

And the combination of parrotfish and trumpetfish? This did not elicit a stronger reaction than a parrotfish alone. Conclusion: bicoloured damselfish do not notice a trumpetfish that shadows a parrotfish. The camouflage trick – hiding behind moving fish – works well.

That is: as long as the parrotfish tolerates the company. It often chases the trumpetfish away.

Willy van Strien

Photo: Trumpetfish. Becky A. Dayhuff (Public Domain)

Shadowing behaviour on YouTube

Sources:
Matchette, S.R., C. Drerup, I.K. Davidson, S.D. Simpson, A.N. Radford & J.E. Herbert-Read, 2023. Predatory trumpetfish conceal themselves from their prey by swimming alongside other fish. Current Biology 33: R781-R802. Doi: 10.1016/j.cub.2023.05.075
Aronson, R., 1983. Foraging behavior of the west Atlantic trumpetfish, Aulostomus maculatus: use of large, herbivorous reef fishes as camouflage. Bulletin of Marine Science 33: 166-171.

Gorareduvius assassin bug is covered in resin

A Gorareduvius assassin bug uses resin as a tool to overpower prey. That works fine, as Fernando Soley and Marie Herberstein show.

Assassin bugs prey on insects; they grasp them with their forelegs, stab them with their proboscis and suck them out. Insects that are trapped try to escape, of course, but some species of assassin bugs impede that. They coat their body with sticky resin so that prey items cannot escape so quickly.

One of these resin users is an otherwise poorly known Gorareduvius species. It is successful with its sticky strategy, as Fernando Soley and Marie Herberstein show.

Meticulously applied

Gorareduvius lives in Western Australia, where it resides in hummocks of curly spinifex grass, which produces resin. The assassin bug scrapes resin off the grass leaves and applies it meticulously to its body, particularly onto its forelegs. Every individual does it, a Gorareduvius assassin bug is always sticky.

Soley and Herberstein conducted experiments in which they staged interactions between this assassin bug and two types of fast-moving prey: ants and flies. In some trials the assassin bug was allowed to keep its resin equipment, in other cases the researchers gently removed it.

Equipped with resin, Gorareduvius more successfully captures ants and flies, as it turned out. The prey can still escape, but the resin stops them for a while, giving the assassin bug more chance to stab them. As a result, capture attempts of resin-equipped assassin bugs are more often successful than those of clean assassin bugs.

Tool

The assassin bug family (Reduviidae) contains about 7000 species. Part of these species cover themselves in resin to enhance prey capture, which the researchers consider as tool use. The habit has arisen at least three times independently.

Willy van Strien

Photo: Gorareduvius, an assassin bug; on the forelegs, above the nod, you see small lumps of resin and where the antennae branch you see the large proboscis. ©Fernando Soley

Sources:
Soley, F.G. & M.E. Herberstein, 2023. Assassin bugs enhance prey capture with a sticky resin. Biology Letters 19: 20220608. Doi: 10.1098/rsbl.2022.0608
Zhang, J., C. Weirauch, G. Zhang & D. Forero, 2016. Molecular phylogeny of Harpactorinae and Bactrodinae uncovers complex evolution of sticky trap predation in assassin bugs (Heteroptera: Reduviidae). Cladistics 32: 538-554. Doi: 10.1111/cla.12140

Snake and frog wait for each other to initiate action

When snake and frog meet, an endurance game begins. The one that moves first takes a risk, as Nozomi Nishiumi and Akira Mori show. If the snake attacks, it will see its prey escape. If the frog jumps, it will be captured.

As the snake slowly slides closer, the frog remains motionless. Doesn’t that frog perceive the danger? Or can’t it flee, because it is frozen with fear? Neither, Nozomi Nishiumi and Akira Mori write. The best strategy is to remain motionless as long as possible.
In Japan, the biologists investigated interactions between the Japanese striped snake Elaphe quadrivirgata and one of its prey species, the black-spotted pond frog Pelophylax nigromaculatus. Tension is mounting, as staged encounter experiments showed, because neither animal will take action. And with good reason.

Intercepted

Of course, the frog could initiate flight by jumping away if the snake approaches. But then it is at a disadvantage. Because after take-off kicking, it can’t change his speed and direction. The snake will respond immediately and try to intercept the frog in mid-air, with a good chance of success. So, it is best for the frog to remain motionless.

Also, the approaching snake should refrain from initiating strike behaviour at the frog. Once it has started projecting its head, it can no longer adjust the direction. The frog can evade the strike by jumping away, and chances are high that it will succeed. The snake can make another attempt to capture the frog, but it looses some time because it has to assume the correct posture.

So, the opponents wait for the other to initiate action. The one that gives up first, takes a risk. Sometimes it is the frog that takes pre-emptive action and jumps – with a high chance of being caught. Other times, the snake launches into a strike – and the frog is likely to escape.

No chance

But if both predator and prey persevere, something must happen in the end. At some point they have to switch from waiting to taking action. When the snake has approached the frog to a distance of about six centimetres, the prey has no chance to escape anymore; the snake can successfully grab it. The frog should not wait that long: just before the snake is dangerously close and about to strike, it must jump. That can go wrong, but at least, escape is not yet excluded.

It is a game of patience, but also a game of life and death. In that sense, the tests in which snake and frog are forced to face each other are somewhat cruel, as some frogs were eaten. In nature however, as the researchers state in an ethical note, this is daily practice.

Willy van Strien

Photos:
Large: Japanese striped snake, Elaphe quadrivirgata. Ʃ64 (Wikimedia Commons, Creative Commons CC BY 3.0)
Small: black-spotted pond frog, Pelophylax nigromaculatus. Alpsdake (Wikimedia Commons, Creative Commons, CC BY-SA 3.0; flipped)

Source:
Nishiumi, N. & A. Mori, 2020. A game of patience between predator and prey: waiting for opponent’s action determines successful capture or escape. Canadian Journal of Zoology 98: 351-357. Doi: 10.1139/cjz-2019-0164

Shepherd’s purse seedling benefits from animal proteins

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

A 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.