From so simple a beginning

Evolution and Biodiversity

Page 16 of 18

Trapped, encased, killed

Snails use their shells as a weapon against parasitic worms

a grove snail's shell can kill parasites

Parasitic roundworms that invade a snail’s shell may be trapped, encased and fixed permanently to the inner layer of that shell, as Robbie Rae shows.

Thanks to its shell, a snail is protected against damage, predators, heat and cold, drought and rain. But there is more, as Robbie Rae discovered. The snail also uses its shell as a defence system to eliminate parasitic roundworms (nematodes). These parasites attack snails since snails appeared on earth, about 400 million years ago. It is obvious that snails had to evolve a defence mechanism against these enemies, but until now, no defence mechanism was known.


In his lab, Rae exposed grove snails (Cepaea nemoralis) to the nematode Phasmarhabditis hermaphrodita for several weeks. This bottom dwelling animal, less than 2 millimetres long, is able to penetrate and kill many snail and slug species, but some snails are resistant, as for instance the grove snail. Rae studied the interaction between the grove snail and the worms to find out how the snail eliminates the parasites.

worms attached to the inner layer of the shell of a grove snailIt turned out that the cells on the inner layer of the shell do the job. They adhere to an invading worm, multiply, and swarm over the parasite’s body until it is entirely covered. Engulfed by the cells, it is fused to the inside of the shell and dies. By this procedure, grove snails not only encapsulate this lethal roundworm, but they use the immune reaction also to kill other, less dangerous nematodes, as experiments showed.

In nature, this is common practice. Rae collected grove snails and white-lipped snails (Cepaea hortenis) from the wild and observed that many snails had different species of roundworms attached to their inner shell surface, up to 100 worms in one shell. Also the garden snail (Cornu aspersum) – like the other two snail species an inhabitant of Western Europe – uses its shell to eliminate invading worms by encapsulation.

Old defence

Finally, he examined a large number of snails from museum collections, to conclude that many snails of many different species had nematodes attached to their shells. Trapped worms proved to be fixed permanently; they even can be found in snails that died a few hundred years before. As this defence mechanism is found to be widespread among the large and old clade of terrestrial snails and slugs, it must have evolved about 100 million years ago. Even some slug species eliminate parasitic roundworms by this mechanism. During evolutionary history, their shells have become reduced and internalised, but in many species they retained the ability to trap, encase and kill roundworms.

The vineyard snail (Cernuella virgata) is one of the species that is unable to get rid of the roundworm Phasmarhabditis hermaphrodita. Apparently, the parasite evades its immune reaction in one way or the other. As many slug species are also susceptible to this parasite, it is formulated into a biological control agent to be used against herbivirous slugs.

Willy van Strien

Large: grove snail, Cepaea nemoralis. Kristian Peters (Wikimedia Commons, Creative Commons CC BY-SA 3.0)
Small: nematodes fixed to the inner layer of a grove snail’s  shell. © Robbie Rae

Rae, R., 2017. The gastropod shell has been co-opted to kill parasitic nematodes. Scientific Reports 7: 4745. Doi: 10.1038/s41598-017-04695-5


Palm cockatoo drums with self-fashioned drumstick

Palm cockatoo makes a drumstick

With a female listening, palm cockatoo males may repeatedly strike a hollow branch or trunk with a stick. Robert Heinsohn and colleagues heard that the birds have good rhythm and that every male has his individual drumming style.

A palm cockatoo male from North Australia can produce different sounds while erecting its crest. That is impressive, but there is something that really stands out: it may start drumming.

Regular pulse

When a male is going to perform, it breaks off a twig, removes the leaves, trims it to approximately 20 centimetres, grasps it in one of both foots and starts beating repeatedly on a hollow branch or trunk. Instead of a stick, it may use a seed pod of a particular tree (Grevillea glauca, the bushman’s clothes peg) after adjusting the shape with its beak. It may continue drumming for a while, producing a sequence of up to 90 taps.

It is remarkable that the intervals between the taps don’t occur at random intervals; instead, the cockatoos produce a regular pulse, as Robert Heinsohn and colleagues assessed. They also noticed that each male has its individual, consistent style; some males have slow drumming rates, whereas others drum at a faster rate, or insert short sequences of faster drumming in the performance occasionally.


It is not known yet which function the performance might have. Palm cockatoos form monogamous pairs which occupy a large territory. The sound does not travel far enough to be heard by the neighbours, so a male cannot communicate with them by drumming; he always is playing solo. As most performances are attended by the female, the music probably is meant for her, and it may be a male’s way to inform its partner about its condition or age; the birds may live more than 50 years. We don’t know whether the females like the percussion and what rhythm they prefer.

Willy van Strien

Photo: Christoph Lorse (Via Flickr. Creative Commons CC BY-NC-SA 2.0)

The researchers explain their work on You Tube;
short fragment of a drumming cockatoo

Heinsohn, R., C.N. Zdenek, R.B. Cunningham, J.A. Endler & N.E. Langmore, 2017. Tool-assisted rhythmic drumming in palm cockatoos shares key elements of human instrumental music. Science Advances 3: e1602399. Doi: 10.1126/sciadv.1602399

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.

Sticky threads

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.

Modified glands

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

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

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

Slimy lips

Southern tubelip feeds on corals by kissing

Labropsis autralis feeding on corals

As one of only a few fish species, the tubelip wrasse Labropsis australis is able to feed on corals. Specialised lips protect the fish from being hurt, as Victor Huertas and David Bellwood show.

Lips of Labropsis australis bear lamellaeThe tubelip wrasse Labropsis australis, or Southern tubelip, looks like a normal fish. But it appears to have highly modified lips, as Victor Huertas and David Bellwood reveal after making a high-resolution image of the fish’s mouth. The lips form a protruding tube when the mouth is closed; they are thick and fleshy, bear lamellae much like a mushroom, and are covered with a thick layer of mucus, secreted by mucous glands.

Just like kissing

That’s noteworthy, as most wrasses, the group of fish species to which Labropsis australis belongs, have thin, smooth lips that are neither slimy nor protruding.

The remarkable lips facilitate un unusual diet, the researchers found out. Living on the Great Barrier Reef off the north coast of Australia, Labropsis australis feeds on hard corals – and that’s not easy, because the corals have a sharp skeleton covered by a layer of tissue with venomous stinging nematocysts, like jellyfish have. No wonder that most fishes don’t touch them. But Labropsis australis seems not to care.

The biologists recorded the fish’s behaviour with a high-speed camera to see how it managed to feed on corals. Analyzing the footage, they saw how the fish approaches its meal, closes its mouth, pushes its fleshy lips against the coral, sealing them over a small area, and rapidly sucks off some of the coral’s mucus and flesh. This sucking is accompanied by an audible ‘tuk’; it’s just like kissing.

Mucus is the key factor that enables these fish to feed on corals, the authors suppose. The thick mucus layer prevents the sharp edges and nematocysts of the coral from damaging the fish.

Willy van Strien

Photos: © Victor Huertas and David Bellwood
Large: Southern tubelip Labropsis australis
Small: Image of the lips of Labropsis australis

The kissing tubelip wrasse on a video made by Victor Huertas and David Bellwoo

Huertas, V. & D.R. Bellwood, 2017. Mucus-secreting lips offer protection to suction-feeding corallivorous fishes. Current Biology 27: R399–R407. Doi: 10.1016/j.cub.2017.04.056

Sweet snack

Wild bees can do without flowers – for a while

Andrena-bee visiting a flowerless shrub

When spring arrives in California, wild bees emerge before flowers appear that offer nectar, providing the animals with energy. To survive, they temporarily use sugary honeydew, as Joan Meiners and colleagues discovered.

It seems weird for bees to visit non-flowering shrubs, because they need flowers to find nectar, which contains sugars, and pollen, which contains protein; these nutrients are necessary for themselves and their larvae. Yet, in the Pinnacles National Park in California, Joan Meiners observed many wild bees of different species visiting shrubs on which no flower was to be found.


With a series of experiments, she and her colleagues found out what the bees were looking for at the non-flowering shrubs: the animals were accessing sugary honeydew, the sweet secretions of sap-feeding scale insects. It appeared that bees visit flowerless shrubs only early in the springtime, when they emerge while there are hardly any flowers blooming, and that all these bees belong to solitary species, not living in colonies where a stockpile of nectar is available. Apparently, in early spring honeydew is an alternative source of energy for these bees, a new discovery.

Now, the question is how the bees are able to find this alternative food source. They are specialists in detecting and distinguishing colours and scents. Flowers depend on bees for pollination, because as bees visit multiple flowers in succession, they transfer pollen from the stamens of one flower to the pistil of the next one, so that this second flower can grow seeds after fertilization. Because bees are indispensable, flowers attract them with showy scents, colours and shapes.

Still, bees manage to find the colourless, odourless honeydew as well.

Looking for food

Are they attracted by the black mold fungus that covers the honeydew? The researchers ruled out this possibility by painting a number of branches black: these branches were not visited by the bees. Do the scale insects form a clue to the honeydew? No, because if the sap-sucking insects were temporarily inactivated with a mild anti-insecticide, no bees were seen nearby; they only came when the scale insects were producing honeydew. But on the other hand, they did detect sticks on which the researchers had sprayed a sugar solution, and they did already within an hour.

The biologists propose that the bees are continuously looking for food, and if one bee locates some honeydew, other bees will notice and visit the food source as well.

Using honeydew as an extra source of energy, the bees can survive a period without nectar. But in the end they do need flowers, because the larvae cannot develop on a diet of sugars alone, but have to ingest a high amount of proteins, and therefore they need pollen. So, every female has to gather pollen for her offspring.

Once plants start flowering, bees lose their interest in honeydew-bearing shrubs and visit flowers instead. The mutual relationship between bees and flowers – where pollination is exchanged for food – is not jeopardized.

Willy van Strien

Photo: ©Paul G. Johnson

Meiners, J.M, T.L. Griswold, D.J. Harris & S.K.M. Ernest, 2017. Bees without flowers: before peak bloom, diverse native bees find insect-produced honeydew sugars. The American Naturalist, online May 30. Doi: 10.1086/692437

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

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

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

In need of a ride

Tadpoles are in a great hurry to get away

A male splash-back poison frog transports each tadpole to a pool to grow up

Things go bad if sibling larvae of the splash-back poison frog Ranitomeya variabilis grow up together: only one of them will survive. So, as soon as an adult frog approaches, tadpoles try to climb on its back and to get a ride to a safe place, Lisa Schulte and Michael Mayer write.

Mating pairs of the splash-back poison frog Ranitomeya variabilis, that occurs in Peru, lay two to six eggs at the surface of small water bodies in plants, for instance Bromelia species. In such ‘phytotelmata’ the risk for the eggs to be found by a predator is small. Later, the male returns to retrieve each larva upon hatching and transports it on its back to an unoccupied phytotelm that he already selected. He then returns to fish the next larva out of the water, until all the young are singly housed in different phytotelmata.


It is necessary for the larvae to get separated from each other, as the tadpoles are cannibalistic. If they stay together, only one of them will survive and grow up.

In some cases, however, the male doesn’t return to retrieve the hatching larvae. In such case, the abandoned tadpoles actively seek transport, as Lisa Schulte and Michael Mayer show. They collected clutches of eggs, took them to the lab and kept them in small plastic cups. After the tadpoles hatched, they were kept together and the researchers introduced an adult frog. That frog was either a conspecific male or a conspecific female, or a male of a different species.

In all cases, the tadpoles approached the adult frog, and many of them tried to climb onto its back quickly. Some succeeded. They actually jumped on the frog’s back, the researchers report; it looked like an attack.

High need

The tadpoles have a good reason to be so desperate. In a natural situation, a frog that shows up most likely is the male parent frog that revisits the phytotelm to save its young from cannibalism. The first tadpole to approach will be assisted to mount; the male will bend its back or push it up with its legs. After the male left with this lucky tadpole, there is no guarantee that he will return to get the other ones. If he can’t find an unoccupied phytotelm anymore, he will stay away. Hence the haste of the tadpoles.

But in the experiments, the visiting frog could also be a female, or a male of another species. In such cases, the tadpoles did not get any help. Yet they tried to get transport by mounting this frog on their own.

Obviously, the need to be saved is so high that the tadpoles don’t make any difference between their father and any other frog that happens to appear. And they had better not, because even a frog with no intention to bring tadpoles to a safe place may visit an unoccupied phytotelm, and rescue the hitchhiker.

However, when the researchers offered a plastic frog model, the tadpoles did not respond. They probably recognize a true frog by chemical cues.

Willy van Strien

Photo: John Clare (via Flickr. Creative Commons CC BY-NC-ND 2.0)

Schulte, L.M. & M. Mayer, 2017. Poison frog tadpoles seek parental transportation to escape their cannibalistic siblings. Journal of zoology, 5 mei online. Doi: 10.1111/jzo.12472

Rescue heroes

Birds free entangled group members from sticky seeds

Seychelles warbler on the ground runs a risk of becoming entangled in seeds

Seeds of the Pisonia tree can be dangerous for a little songbird like the Seychelles warbler: when these sticky seeds attach to its feathers, such a bird is not able to fly. Fortunately, the risk of entanglement is low, and in case of bad luck, help often arrives, as Martijn Hammers and Lyanne Brouwer observed.

Seychelles warblers lead a low-risk life on the tropical island of Cousin, belonging to the Republic of Seychelles: there are no natural enemies around that prey on the adult birds. High in the trees they safely glean insects from the leaves.

Seychelles warbler entangled in a cluster of seedsStill, they can be in trouble, Martijn Hammers and Lyanne Brouwer report. The most common tree, the Pisonia tree, produces seeds that become very sticky when they are ripe and fall to the ground. For foraging birds the risk of entanglement is low, but when they are on the ground to collect nest material – work performed by females – or to defend their territory, these seeds easily attach to their feathers; a bird may even get stuck in a cluster of seeds. That is bad luck. Just one of a few of these seeds can prevent a bird from flying, and cause it to die.


But if the victim is lucky, help will arrive. The biologists, who observed the behaviour of the Seychelles warblers during several years, sometimes saw a bird with sticky seeds attached. And in a few cases, another bird came to remove the seeds form the victim after having heard its alarm call. The helper picked and pulled the seeds off with his beak, rescuing the unfortunate animal.

Such rescue behaviour is rare and demanding. A helpful bird must be able to perceive what is going on, know what to do to help the victim and be willing to do it, putting itself at risk of becoming entangled as well. The helper is not just a random conspecific: in each case observed, it belonged to the victim’s family group. Often one or more grown-up young stay with their parents; also, a mother or grandmothermay join a breeding couple. In such cases, a family group lives in a territory, and relatives may help to rear the young. A rescue operation means that the family group remains intact and no help is lost.

Sea birds

The sticky Pisonia seeds do have a function. If they stick to a small songbird like the Seychelles warbler, the tree gains nothing. But more often, the seeds become attached to sea birds visiting the island. They have no difficulty flying – and take the seeds to another island. The tree has its seeds dispersed.

Willy van Strien

Photos: © Martijn Hammers

Hammers, M. & L. Brouwer, 2017. Rescue behaviour in a social bird: removal of sticky ‘bird-catcher tree’ seeds by group members. Behaviour 154: 403-411. Doi:10.1163/1568539X-00003428

Biting prey

Fish with venomous fangs have many imitators

Fangblenny Petroscirtes breviceps mimics a venomous species

Meiacanthus fish species are armed with venomous fangs that deter predators. Many nonvenomous fish species protect themselves from being attacked by mimicking the aposematic colours and the behaviour of Meiacanthus species. A large research team unravelled the evolution of the venomous fish.

A predator fish expecting to easily ingest a small Meiacanthus fish will prove to be wrong. This prey is armed with sharp teeth that inject venom into its enemy. Disoriented, the predator will release its victim – and will not go after the same fish anymore.

Meiacanthus species are the only fish with venomous fangs. They belong to the group of the saber-toothed blennies or fangblennies (Nemophini), which all have a pair of enlarged, hollow canines in the lower jaw. Nicholas Casewell, together with a large research team, has shown that these fangs must have originated in the common ancestor of these blennies. But only species of the genus Meiacanthus developed venomous fangs. They possess venom glands at the base of the fangs and grooves on the fangs to deliver the venom into the wound.

According to the researchers, the venom does not cause pain upon injection, but it reduces the blood pressure in the predator, which becomes weakened and disoriented so that the prey can escape unharmed from its mouth. Blood pressure reduction appears to be such a bad experience that the predator fish will never try to ingest a Meiacanthus again. The venom was found to contain three compounds that had never been found in fish before.


Some non-venomous fangblennies, as well as many fish species from other groups, profit from the aversion that predators have to Meiacanthus species by looking the same and behaving the same. While not mounting a defence against predators themselves, they are still protected from attacks thanks to this mimicry.

What do non-venomous fangblennies use their fangs for? To eat, probably. This holds at least for all Plagiotremus species, which feed on dermal tissue, scales, mucus, and fins of larger fish. If they look like Meiacanthus species, they can easily approach their victims, which are reluctant to attack.

Willy van Strien

Petroscirtes breviceps, with nonvenomous fangs in the lower jaw. ©Alex Ribeiro
CT-scan of the venomous species Meiacanthus grammistes. ©Anthony Romilio (University of Queensland, Australia)

Casewell, N.R., J.C. Visser, K. Baumann, J. Dobson, H. Han, S. Kuruppu, M. Morgan, A. Romilio, V. Weisbecker, S.A. Ali, J. Debono, I. Koludarov, I.Que, G.C. Bird, G.M. Cooke, A. Nouwens, W.C. Hodgson, S.C. Wagstaff, K.L. Cheney, I. Vetter, L. van der Weerd, M.K. Richardson & B.G. Fry, 2017. The evolution of fangs, venom, and mimicry systems in blenny fishes. Current Biology, March 30 online. Doi: 10.1016/j.cub.2017.02.067

Micro army

Cloud of semiautonomous pincers protects sea urchin

Collector sea urchin released a cloud of pincers

When a hungry fish approaches, the collector sea urchin releases a cloud of small biting, venomous pincers that deter its enemy before it has attacked. This peculiar defensive strategy is revealed by Hannah Sheppard-Brennand and colleagues.

In addition to their prominent spines, sea urchins and sea stars possess numerous smaller appendages as well: little pincer-like heads on a movable stalk, called pedicellariae. The pincers have different functions, such as catching food or removing debris – and tormenting predators. For in spite of the unattractive appearance that sea urchins and sea stars have, predators such as fish will pick at their tube feet and other soft tissue. Some species have pedicellariae with teethed jaws and a venom sac to deal with such predators, and anyone who once stepped on such a sea urchin will remember the painful experience.


One species, the collector sea urchin Tripneustes gratilla, deploys these venomous pedicellariae in an unique way, as Hannah Sheppard-Brennand and colleagues show: when harassed, this sea urchin will release a cloud of them in the surrounding water. Upon release, the pincer-like heads behave semi autonomously; they are mobile, have sensory structures and will bite and deliver their venom when they touch a supposed enemy.


Fish are deterred by such a swarm, as lab experiments revealed, and they will leave before they have bitten the sea urchin. Protected by his unique defensive army, the collector sea urchin is able to forage safely on grass and eelgrass during the day, when other sea urchin species have to take shelter, as well as during the night.

Willy van Strien

Collector sea urchin Tripneustes gratilla with a cloud of released pedicellaria heads. © Hannah Sheppard Brennand

Sheppard-Brennand, H., A.G.B. Poore & S.A. Dworjanyn, 2017. A waterborne pursuit-deterrent signal deployed by a sea urchin. The American Naturalist 189, online March 27. Doi: 10.1086/691437

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