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Colour meanings

3 August 2022 / wvanstrien

Aegean wall lizard with white throat is more brave

An Aegean wall lizard with striking throat colour will run off fast when a predator looms, Kinsey Brock and Indiana Madden write.

In Aegean or Erhard’s wall lizard, Podarcis erhardii, different colour morphs exist: the animals have either a white, yellow or orange throat. The lizards can be found on walls in South-eastern Europe, in a dry landscape with tough shrubs. They have several predators: snakes, birds, and mammals.

When a predator appears, a lizard will flee. But that implies that it must stop what it was doing: sunbathing or foraging for food. For that reason, it will not leave until necessary. Kinsey Brock and Indiana Madden wanted to know whether the three colour morphs have a similar flight initiation distance. They checked the distance they could approach a lizard before it ran away.

Careful

The throat colour of the Aegean wall lizard is genetically determined. Most animals, males and females alike, have a white throat; yellow and orange are less common. There are also individuals with mosaic throat colours, but they are rare. Brock and Madden investigated lizards with plain throat colour on the Greek island of Naxos.

You can get most closely to the white-throated wall lizards, they found; lizards with an orange throat run off earliest; yellow-throated animals are in between.

So, animals with an orange throat are the most careful. They also stay closest to a refuge: a crevice in a wall or dense vegetation. And once they fled, they are slower to reappear than animals with yellow or white throats.

It is in line with lab research showing that white-throated males are the most aggressive, bold, and brave.

Striking colour

An orange-throated Aegean wall lizard probably is more wary because it is more detectable. The grey-brown blotchy body has a camouflage colour, but a yellow, and especially an orange throat stands out against the background. This makes it easier for a predator to discover a lizard with an orange throat, so, in turn, it must flee earlier to escape from the enemy.

Willy van Strien

Photo: Male Podarcis erhardii with white throat. Gailhampshire (Wikimedia Commons, Creative Commons CC BY 2.0)

Source:
Brock, K.M. & I.E. Madden, 2022. Morph‑specific differences in escape behavior in a color polymorphic lizard. Behavioral Ecology and Sociobiology 76: 104. Doi: 10.1007/s00265-022-03211-8

Partnership

21 July 2022 / wvanstrien

Young spotted bowerbird joins older male

In company of a subordinate, a spotted bowerbird male stands stronger: his bower is safe, and more females are impressed, according to observations by Giovanni Spezie and Leonida Fusani.

Bowerbird males keep themselves to themselves. To seduce females, they each build their own bower with courtship platforms. They keep a far distance from each other; in the spotted bowerbird, the average distance is no less than 1 kilometre. Yet the owner of a bower often has company of a subordinate male. Giovanni Spezie and Leonida Fusani wondered what such male is doing there. Is he a younger male learning skills from an older one? Or does he actively participate in the activities, is it a form of collaboration?

The spotted bowerbird (Ptilonorhynchus maculatus or Chlamydera maculata) is one of 21 species of bowerbirds that exist, and it lives in eastern Australia. It has an erectile lilac crest on the nape.

A male builds a lane of grass and twigs with a platform on both sides of mainly greyish objects, such as bones and stones. He decorates the place with berries, leaves and pieces of glass. Females will visit and enter the bower to watch the male calling and dancing next to his bower. The performance can last an hour. With his elaborate bower and energetic courtship display, he shows his quality. If she likes it, she will mate.

Males can devote all their time to show off, because taking care of the young is a females’ task. Some males attract several females, but all the effort of many others are in vain.

Adequate reaction

To find out what subordinate males are doing at bowers, the researchers made motion-activated video recordings. They analysed the footage to see if such male just watches, or also participates in bower maintenance and courtship. And if he helps, is he, like the bower owner, able to adapt his behaviour to the reaction of a visiting female, for example if she threatens to leave? Does the bower owner benefit from the help? And the helper himself?

Although subordinate males are less active than bower owners, they behave similar and respond to female behaviour in the same way (unless the researchers missed subtle differences). So, the relationship between an owner and subordinate seems unlike that of teacher and apprenticeship, the researchers suggest.

Both participants benefit

Rather, the subordinate seems to be a helper. In his presence, the bower is less likely to be plundered by competing males. Males often destroy each other’s bower or steal precious ornaments to embellish their own place. In the spotted bowerbird, marauding is less common than in other species, but the presence of an extra male even reduces the risk. That is why a bower owner may tolerate the presence of another male.

In addition, an owner with a helper has more courtship success.

The owner thus benefits from the company of a subordinate. In turn, the auxiliary male also benefits; sometimes he has an opportunity to mate with a visiting female. In addition, there is a chance that he will gain ownership of the bower. A partnership between males may last for years.

Related?

The collaboration would be most useful if the males were related, for example brothers, so that the subordinate indirectly has some reproductive success via the bower owner. But researchers have not yet investigated whether that is the case.

It is questionable. Other research had shown that males pay little attention to family relationships. They don’t necessarily place their bower near relatives, but they don’t avoid them either. And if they maraud a bower, it is the neighbour’s bower, regardless of whether the birds are relatives.

Willy van Strien

Photos:
Large: spotted bowerbird. Greg Miles (Wikimedia Commons, Creative Commons CC BY-SA 2.0)
Small: bower of spotted bowerbird. Davidgregsmith (Wikimedia Commons, Creative Commons CC BY-SA 4.0)

Sources:
Spezie, G. & L. Fusani, 2022. Male–male associations in spotted bowerbirds (Ptilonorhynchus maculatus) exhibit attributes of courtship coalitions. Behavioral Ecology and Sociobiology 76: 97. Doi: 10.1007/s00265-022-03200-x
Madden, J.R., T.J. Lowe, H.V. Fuller, R.L. Coe, K.K. Dasmahapatra, W. Amos & F. Jury, 2004. Neighbouring male spotted bowerbirds are not related, but do maraud each other. Animal Behaviour, 68: 751-758. Doi: 10.1016/j.anbehav.2003.12.006

Tripedal gait

16 June 2022 / wvanstrien

Lovebird parrot climbs on three legs

When climbing vertically, a lovebird parrot has an extra leg at its disposal: its beak, according to research by Melody Young and colleagues.

Woodpeckers, nuthatches, treecreepers, parrots, and parakeets: all these birds are able to move up against a tree trunk. Woodpeckers, nuthatches, and treecreepers do so by hopping forward, both legs briefly releasing from the ground simultaneously. Parrots and parakeets do it differently. They clamber – using their beaks as a third leg, as Melody Young and colleagues show.

Everyone has observed parrots and parakeets using their beaks when climbing up. But do they really use their beak as a leg, or just for support and balance, in the same way as birds often use their tail? To find out, Young investigated the climbing skills of the rosy-faced lovebird, Agapornis roseicollis, a parrot from Southwest Africa.

Novel function

She brought six animals into the lab and let them walk across a runway at different inclinations. She filmed their gait with a high-speed camera and measured the force that legs, beak, and tail exerted on the substrate.

The lovebirds often used their beak and tail when walking if the runway was set up steeper than 45° inclination. If it was positioned vertically, beak and tail were always necessary. In that case, the beak functioned as an extra leg, as it turned out. The animals put both legs and beak forward in turn: right leg, left leg, beak, right leg, left leg, beak. Measured forces also showed that the beak plays a similar role as the legs in propulsion.

The tail helps support and balance the bird.

Parrots have given their beak a second function as an extra leg to climb with. The neck muscles must also have been adapted to this new task.

Willy van Strien

Photo: Rosy-faced lovebird. User Nbansal4732 of the English Wikipedia (Wikimedia Commons, Creative Commons CC BY-SA 2.5)

Source:
Melody W. Young, M.W., E. Dickinson, N.D. Flaim & M.C. Granatosky, 2022. Overcoming a ‘forbidden phenotype’: the parrot’s head supports, propels and powers tripedal locomotion. Proceedings of the Royal Society B 289: 20220245. Doi: 10.1098/rspb.2022.0245

Detering owls by buzzing

30 May 2022 / wvanstrien

Greater mouse-eared bat mimics the sound of bees and wasps

Owls avoid the buzzes of angry bees and wasps. The greater mouse-eared bat takes advantage of that fear by mimicking the sound, Leonardo Ancillotto and colleagues show.

A greater mouse-eared bat in stress behaves weird: it buzzes like a startled group of bees or wasps. Leonardo Ancillotto and colleagues noticed this when they handled the animals during their research. They wondered whether the bats mimic the sound of alarmed bees and wasps when they feel threatened by a potential predator to deter it. It was worth a study.

The greater mouse-eared bat, Myotis myotis, occurs in most European countries. Its enemies are owls, which are nocturnal like the bats.

Larynx

To find out, the researchers first analysed sound recordings of buzzing bats and compared that to the buzzing sounds that several species of bees and wasps produce when they are harassed and defend their nests. Among those species were honeybee (Apis mellifera) and hornet (Vespa crabro). And yes: the buzzing sounds were similar, especially to the ears of an owl.

The similarity is remarkable because the sound is created in different ways. Bees and wasps buzz by beating their wings, while bats produce the sound with the larynx.

Next, the researchers conducted playback experiments in which they broadcasted the buzzing sounds of honeybee, hornet or greater mouse-eared bat to a number of barn owls and tawny owls. The buzzing of the bat was most similar to that of honeybee and hornet. In addition, these insects live in tree cavities, in which owls are interested. As control, they broadcasted the communication calls of another bat species, the European free-tailed bat (Tadarida teniotis).

Experience

The owls moved away from loudspeakers that emitted buzzes, whether these were produced by honeybee, hornet, or greater mouse-eared bat. Bat communication calls, in contrast, attracted them. Wild owls, which may have encountered angry bees or wasps and suffered painful stings, were even more averse to buzzing sounds than owls that had been raised in captivity.

Does it make sense that owls, which are nocturnal animals, are afraid of bees and wasps, which are active during the day? Yes, that fear is conceivable. Honeybees fly until late evening in summer and hornets may fly at night, under moonlight or artificial light. Barn owls appear already at dusk, and when they have hungry young to feed, tawny owls sometimes even hunt during the day.

Apparently, the owls are afraid of bees and wasps and the bats delude them. Buzzing like bees or wasps, acoustic mimicry, may be all they can do to escape from their predator.

Willy van Strien

Photo: Greater mouse-eared bat. Kovács Richárd (Wikimedia Commons, Creative Commons CC BY-SA 3.0)

Source:
Ancillotto, L., D. Pafundi, F. Cappa, G. Chaverri, M. Gamba, R. Cervo & D. Russo, 2022. Bats mimic hymenopteran insect sounds to deter predators. Current Biology 32: R408-R409. Doi: 10.1016/j.cub.2022.03.052

Food supply according to demand

23 May 2022 / wvanstrien

Leafcutter ant prevents fungus garden from starving

The fungus-breeding ant Acromyrmex ambiguus checks its crop and intervenes if it threatens to starve, Daniela Römer and colleagues show.

Leafcutter ants grow fungus on plant material in their underground nests with chambers. The ant workers cultivate their crop with care, according to research by Daniela Römer and colleagues on Acromyrmex ambiguus: if the fungus garden in one of the nest chambers deteriorates because of lack of food, they will bring it more pieces of fresh green leaves.

Ants are unable to digest plant leaves, but, thanks to their crop, fungus growing ants still are herbivorous. The fungus breaks down plant material and converts it into digestible, nutritious globules. The larvae are completely dependent on that food, the workers also eat it.

It was already known that leafcutter ants, which live in North, Central and South America, take excellent care of their crops. They have to, of course, because if the fungus dies, all their effort to grow it is wasted and the larvae have no food.

Even distribution

Acromyrmex ambiguus is such leaf cutter species; it lives in nests holding thousands of chambers in which fungus grows and brood is raised. Römer wanted to know how the workers distribute the leaf material they collect – i.e., food for the fungus – equally among those chambers.

The experiments she conducted once again show how skilled the small farmers are. The researchers have a colony of Acromyrmex ambiguus in the lab. It is housed in an artificial nest with a number of nest chambers that are filled with fungus, about thousand workers, and ant brood. For the experiments, they placed three of those nest chambers serially, each with its own access tube connected to a main corridor. Chambers with fresh leaf material and with water and honey for the workers were connected to one end of the main corridor. At the other end was a box to which the ants could bring waste. Video cameras recorded the workers’ behaviour.

In experiments in which the fungus in every nest chamber was in good condition, the workers distributed the pieces of leaf evenly among those three chambers: they delivered a similar amount to each one.

Undernourished

But in several trials, the researchers had starved the fungus in the center chamber by disconnecting it from chambers with pieces of leaf during two days before the experiment. The workers in that chamber had been unable to provide food for the fungus. As a result, the normal greyish-green top layer of the fungal mass had vanished.

In these experiments, the ants brought much more pieces of leaf to the center chamber than to the other two. They probably noticed that the fungus crop in this chamber was in bad condition and demanded more food because of the smell it emitted. So, they tried to save the dying fungus garden with extra care.

Workers of Acromyrmex ambiguus, and probably also those of other leafcutter ants, are farmers that monitor how their crop is doing. If its condition deteriorates, they react appropriately.

Willy van Strien

Photos: two different fungus growing Acromyrmex species
Large: Acromyrmex octospinosus. Deadstar0 (Wikimedia Commons, Creative Commons CC BY-SA 3.0)
Small: Acromyrmex balzani. Alex Wild (Wikimedia Commons, Creative Commons, Public Domain)

Source:
Römer, D., G.P. Aguilar, A. Meyer & F. Roces, 2022. Symbiont demand guides resource supply: leaf‑cutting ants preferentially deliver their harvested fragments to undernourished fungus gardens. The Science of Nature 109: 25. Doi: 10.1007/s00114-022-01797-7

Emergency leap after mating

11 May 2022 / wvanstrien

Spider male escapes from cannibalism

With a catapult mechanism, a male of the spider Philoponella prominens manages to escape his hungry partner after copulation. Shichang Zhang and colleagues recorded it on video.

For many male spiders, mating is life-threatening. Because to a female, a male not only is a supplier of sperm that she can use to fertilize her eggs, but also a tasty snack. And when he has given his sperm, he is just a meal. Dying without siring offspring is no option. So, he has to proceed with caution, and leave immediately after finishing copulation.

A Philoponella prominens male, a spider species from woods of central China, is very accomplished. After mating, he swiftly leaps away, out of her reach, Shichang Zhang and colleagues show. They recorded mating and leaping with a high-speed camera.

High pressure

During mating, which lasts half a minute, he folds his two front legs against her, the researchers observed. By suddenly stretching them afterwards, he pushes off and shoots away. He had already secured himself before with a safety line of silk, which he had tied to the edge of her web. After leaping, he crawls back via that line to mate with her again. He is able to repeat the action up to six times.

Spiders move their legs not only with muscles, but also use hydraulics. They bend the legs by contracting flexor muscles but lack extensor muscles. Instead, they fill the joints with body fluid at high pressure, so that the legs stretch by released hydraulic power as the flexor muscles are relaxed. In this way, a male Philoponella prominens jumps from his partner. He reaches a speed of about seventy centimeters per second, spinning around at high speed. A female is unable to grasp him.

The leap is lifesaving, as the researchers showed. If they prevented a male from leaping with a fine brush, he was grabbed by his partner and eaten. As if he were just prey.

Willy van Strien

Photo: Philoponella prominens, mala above female. © Shichang Zhang

Emergency leap on video

Another spider male that has to be careful: Maevia inclemens

Source:
Zhang, S., Y. Liu, Y. Ma, H. Wang, Y. Zhao, M. Kuntner & D. Li, 2022. Male spiders avoid sexual cannibalism with a catapult mechanism. Current Biology 32: R341-R359. Doi: 10.1016/j.cub.2022.03.051

First migration trip in Caspian tern

28 April 2022 / wvanstrien

Father teaches young bird how to travel

Young Caspian terns learn from their father how to migrate to the wintering grounds. When, in following years, they make that autumn trip independently, they remember their fathers’ lesson, Patrik Byholm and colleagues show.

A young Caspian tern that is born at the end of May along the coast of Finland or Sweden, will migrate to West Africa at the end of summer to hibernate there. Its father’s job is to guide it on that first journey, Patrik Byholm and colleagues noted.

The researchers wanted to know how information about autumn migration – route and stopover sites – is passed on from one generation to the next. To find out, they equipped birds with GPS tracking devices.

The Caspian tern, Hydroprogne caspia, is found in many places in the world. In Europe, it also breeds along the Black Sea and the Caspian Sea, and in North America along ocean coasts and the great lakes. Some birds from Finland and Sweden make a stop in the Netherlands during their migration to Africa. They travel singly or in small family groups, which are single-parent families.

Reduced tempo

The collected travel data shows that couples that started a nest with two or three eggs in spring and raised their young together, leave each other after the breeding season. They travel separately, sometimes weeks apart, to the wintering area.

Young birds travel with one of the parents, and mostly, that is the father. They cannot travel safely on their own: young terns that for one reason or another lose contact with parents, are captured by birds of prey. So, they stay close to their father during the trip. He teaches them the route and knows good stopover sites, where the birds can roost and forage during the migration. The lesson is learned: the young birds will follow the same route southwards in the years that follow, using the same stopover sites.

Fathers that accompany one or a few young, will adjust their tempo a bit. They progress less quickly than adult birds traveling alone. This is mainly because young birds take more time to rest.

After arrival, the bond between father and young loosens and parental care finishes. They gradually spend less time without each other, and after a month or two they stop seeing each other at all. Sometimes, a young travels a little further south, in the company of another congener.

Willy van Strien

Photo: Colony of Caspian tern. Dmitry Mikhirev (Wikimedia Commons, Creative Commons CC BY-SA 3.0)

Source:
Byholm, P., M. Beal, N. Isaksson, U. Lötberg & S. Åkesson, 2022. Paternal transmission of migration knowledge in a long-distance bird migrant. Nature Communications 13: 1566. Doi: 10.1038/s41467-022-29300-w

Increasing efficiency in brood parasite

12 April 2022 / wvanstrien

Cuckoo catfish improves its timing

It is not easy for a cuckoo catfish to get its eggs adopted by intended host parents, because these are wary. But it learns the trick by experience, as Holger Zimmermann and colleagues show.

Cuckoo catfish dump their eggs at host parents to let them take care of their offspring: they are brood parasites. That seems easy and, in a way, it is, because the eggs can develop safely without the real parents having to worry. But they do have to bring them to the host parents, and that is not so easy. In that sense, cuckoo catfish spend more effort for their offspring than most fish, which simply lay eggs and leave them behind.

They have to practice the art of parasitism, Holger Zimmermann and colleagues write. The cuckoo catfish (Synodontis multipunctatus) is, as far as we know, the only fish species that, like a cuckoo, relegates the raising of its offspring to others. It lives in Lake Tanganyika in Africa.

Abuse

It takes advantage of species of cichlids that have the most extensive form of parental care, the so-called mouthbrooders. In these species, mothers take the fertilized eggs in the mouth and keep them there until they hatch, after a few weeks.

During spawning, such mouthbrooding cichlids circle around each other and release eggs and sperm; in between acts, they defend the spawning site against intruders.

But a group of cuckoo catfish may intrude. They consume some cichlid eggs before the mother has been able to collect them and drop a few eggs themselves and fertilize them. The cichlid mother panics and collects her eggs as fast as she can; in her haste, she also takes up catfish eggs.

The catfish must interfere at exactly the right time, when the female cichlid is busy laying eggs; it’s a matter of seconds. By experience, they learn to improve the timing of egg laying and fertilization, Zimmermann shows with experiments in tanks, in which he exposed cichlids (4 males and 12 females) to three cuckoo catfish pairs.

Sharper timing

The researchers searched for host parents that have no resistance against the underwater cuckoo. With resilient host parents, the learning process of the parasite would not show up. They selected the mouthbrooder Astatotilapia burtoni, which lives in Lake Tanganyika and is known to the catfish. But they took a population from a neighbouring river, where the cuckoo catfish does not occur. The chosen host parents have no innate defences against cuckoo catfish, nor do they learn to avoid it, but they do behave aggressively towards any fish that disturb the spawning to predate on eggs.

Unexperienced cuckoo catfish almost never managed to get their eggs taken up by these host parents. Only 3 percent of their attempts succeeded. But after some time – in the experiments after four months, about 30 attempts – things got much better: more than 25 percent of the attempts now was successful. That success rate did not increase further. Experienced catfish also managed to consume more eggs of the host parents in the brief time available.

The improvement was possible because the parasites learn to lay and fertilize their eggs at precisely the right time, as behavioural observations showed. In addition, groups of catfish improve the coordination of their intrusive act.

Host parent is loser

Most attempts fail, though, even in experienced cuckoo catfish, because the vigilant cichlids outsmart their enemy. But that does not matter, because the profits for the parasite are large if the action does succeed. A host mother then carries on average five parasite eggs. The catfish will hatch sooner than the cichlids, and the young catfish devour some cichlid embryos.

The host mother is the loser. She is abused and produces fewer young of her own.

Willy van Strien

Photo: Cuckoo catfish. Calwhiz. (Via Flickr, CC BY-NC-ND 2.0)

Cichlids from Lake Tanganyika have learned to coexist with cuckoo catfish

Source:
Zimmermann, H., R. Blažek, M. Polačik & M. Reichard, 2022. Individual experience as a key to success for the cuckoo catfish brood parasitism. Nature Communications 13: 1723. Doi: 10.1038/s41467-022-29417-y

Flower opener

22 February 2022 / wvanstrien

Without flying fox no fruits on Dillenia tree

The flowers of the tree Dillenia biflora cannot open on their own. That means that the pollinator, a flying fox, has an extra job to do, Sophie Petit and colleagues write.

The relationship between a plant and its pollinators can be special, and the tree Dillenia biflora may have one of the most remarkable. Its peculiar flowers cannot open. Their petals are fused and form a globose corolla, a lid that covers the anthers and stigma.

Dozens of Dillenia species exist that are pollinated by bees that come to collect pollen; these species do not offer them nectar. So, it was a mystery what happens in Dillenia biflora with its permanently closed flowers and inaccessible anthers. There must be pollinators, because the tree produces fruits with seeds and the flowers cannot self-pollinate. Sophie Petit and colleagues wondered: who are the pollinators, and how do they do it?

Long teeth

The flowers produce scent, are pale-coloured, large and stout and last only one night. People had noticed large bats, or flying foxes, near the trees at night, and pollen was found in flying fox droppings. These findings suggested that these animals play a role in pollination. On the floor, corollas can be found with four tiny holes.

To find out what happens, the researchers placed video cameras near trees at night, when flying foxes are active. They conducted their research on Fiji’s two largest islands, Vanua Levu and Viti Levu, where Dillenia biflora grows in rainforests.

The footage was clear. Trees are visited at night by the Fijian blossom bat Notopteris macdonaldi, which roosts during the day in large groups in caves. The animals grasp the lid of a flower with their four long canines, pull it away and drop it. Hundreds of anthers and a stigma then are accessible.

Mutual dependence

A flying fox has good reason to do the job: unlike those of related species, the flowers of Dillenia biflora turn out to contain a copious amount of nectar. While the animal is drinking, its snout gets covered with pollen, part of which ends up on the stigma of the next flower it visits. That flower is then pollinated and will form seeds.

It is beneficial for a flower to remain closed. The contents then are safe from rain, from insects that feed on them, and from moths and geckos that sip nectar without pollinating the flowers.

The researchers suspect that more Dillenia species have a similar exclusive relationship with flying foxes, because more species exist with flowers that do not open. They also think there are more bat species that open the flowers and enjoy the hidden food source, such as the Pacific flying fox, Pteropus tonganus.

These trees are completely reliant on nectar-feeding bats that remove the corollas: without their visit, the flowers are aborted and don’t reproduce. Conversely, nectar is the main food source for these flying foxes. That has implications for nature conservation. Many Dillenia species are threatened, and to conserve them, it is necessary that the bats do well. In turn, the flying fox Notopteris macdonaldi is a vulnerable species, and its conservation requires that the trees do not disappear.

Willy van Strien

Photos:
Large: The Pacific flying fox Pteropus tonganus also possibly opens Dillenia flowers; it feeds on fruits, pollen and nectar. Paul Asman, Jill Lenoble (Wikimedia Commons, Creative Commons, CC BY 2.0)
Small: Labeled flower of Dillenia biflora. © Sophie Petit

Source:
Sophie Petit, S., A.T. Scanlon, A. Naikatini, T. Pukala & R. Schumann, 2022. A novel bat pollination system involving obligate flower corolla removal has implications for global Dillenia conservation. PLoS ONE 17: e0262985. Doi: 10.1371/journal.pone.0262985

Valves closed

10 February 2022 / wvanstrien

Blue mussels learn to avoid parasites

Blue mussels adapt their behaviour when parasitic larvae are nearby, according to research by Christian Selbach and colleagues.

During food intake, blue mussels, Mytilus edulis, run a risk. The bivalve molluscs feed by filtering water. It enters through an inlet and flows over gills, which not only take oxygen from the water, but also food particles, mainly plankton. These particles get stuck on a mucous layer and are transported to the stomach. The water exits through an outflow opening.

With the inflow of water, mussels may ingest larvae of a harmful parasite.

Mussels that encountered the parasite before, have learned to be more careful. If they notice the presence of parasites in the water, they close their valves and stop filtering to avoid further infection, Christian Selbach and colleagues show.

Intermediate host

The parasite, the fluke (or trematode) Himasthla elongata, has a complicated life cycle in which mussels are indispensable. The cycle starts in a bird that lives near or at sea, such as an oystercatcher, common eider, or scoter; in these animals, adult parasites thrive. They mate and produce eggs that end up in the water with the bird’s faeces. The eggs hatch and the larvae, so-called miracidia, are eaten by common periwinkles; the small snails are the first intermediate host.

In the snails, the parasites develop into the next larval stage, the cercariae, which also end up in the seawater. These are the larvae that infect filtering mussels, which are the second intermediate host. Mussels live in the tidal zone, near the coast, where they can form large shell reefs.

After ingestion by mussels, the parasitic larvae form cysts, a resting stage. Infected mussels grow poorly and are vulnerable to predation by oystercatcher, eider or scoter. And that completes the circle: those birds are the primary host. Once a bird has eaten infected mussels, the parasites mature, and the story starts all over again.

Shut off

If infective larvae are present in the water, mussels cannot help ingesting them when filtering. The only thing they can do to avoid infection is to stop taking in water. But that has a price, because it also means that they cannot take in oxygen and food.

Yet they stop, according to Selbach’s experiments in which he exposed mussels to infective larvae. But they have to learn it.

Mussels that have no previous experience with the parasites go on filtering when they are exposed to larvae. But mussels that met the parasite before and got infected, now shut themselves off. They reduce filtration activity and close the valves with the adductor muscles, which costs energy. But apparently, it would be worse to ingest another dose of parasitic larvae.

Now, it would be interesting to find out how the mussels notice that there are infective larvae around; that is still unclear.

Willy van Strien

Photo: blue mussel. Inductiveload (Wikimedia Commons, public domain)

Source:
Selbach, C., L. Marchant & K.N. Mouritsen, 2022. Mussel memory: can bivalves learn to fear parasites? Royal Society Open Science 9: 211774. Doi: 10.1098/rsos.211774

Aegean wall lizard with white throat is more brave

Careful

Striking colour

Young spotted bowerbird joins older male

Adequate reaction

Both participants benefit

Related?

Lovebird parrot climbs on three legs

Novel function

Greater mouse-eared bat mimics the sound of bees and wasps

Larynx

Experience

Leafcutter ant prevents fungus garden from starving

Even distribution

Undernourished

Spider male escapes from cannibalism

High pressure

Father teaches young bird how to travel

Reduced tempo

Cuckoo catfish improves its timing

Abuse

Sharper timing

Host parent is loser

Without flying fox no fruits on Dillenia tree

Long teeth

Mutual dependence

Blue mussels learn to avoid parasites

Intermediate host

Shut off

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