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Expensive defence

18 June 2020 / wvanstrien

Ladybird cannot deal with all enemies at once

When a ladybird has to defer predators regularly, it is less able to resist pathogens and parasites, Michal Knapp and colleagues write.

When threatened, ladybird beetles try to avoid being eaten by excreting a yellow, smelly and bitter-tasting liquid from their legs. This reduces the appetite of hungry insects, lizards, birds or small mammals. The liquid is haemolymph, the insect variant of blood. You can see the phenomenon by provoking a ladybird.

But you shouldn’t do that, because ‘reflex bleeding’ decreases the ability to fight pathogens and parasites, as Michal Knapp and colleagues report.

They conducted experiments with the harlequin ladybird, Harmonia axyridis. The species originally lived in East Asia, was introduced in Europe and North America and nowadays also occurs in South America and Africa.

Precious blood

Haemolymph is an expensive means to scare away enemies. It contains nutrients, as well as blood cells, proteins and other compounds that ladybirds need to eliminate pathogens and parasites. The harlequin ladybird uses, among other compounds, the substance harmonine, which has a strong antimicrobial effect. Each bleeding causes a loss of these valuable components.

To measure the effect of this loss, Knapp triggered reflex bleeding in ladybirds twice a week, during three weeks. Contrary to his expectations, the treatment did not affect the survival of the beetles, and they did not lose weight.

He also, during a month, triggered newly hatched females daily to bleed, and found that their reproductive capacity was unaffected. In their first month of life, they produced as many eggs as females that were untreated. They started laying eggs a few days later, though, especially after losing a high volume of haemolymph. That may be of little importance, however, as the beetles live for months.

Costs

But bleeding, the defence mechanism against predators, comes at the expense of the resistance to other enemies, as it turned out. The concentration of blood cells and proteins in haemolymph had decreased. The concentration of harmonine and similar compounds has not been measured, but other research indicates that it also will have decreased.

Indeed, haemolymph of ladybirds that bled was found to inhibit bacteria less strongly. Probably, these ladybirds are less resistant to parasites as well, as blood cells take part in defence, but this has not been investigated.

Ladybirds successfully deploy constituents of haemolymph against all types of enemies – but they cannot fight them all at once at full power. If they have to deal with hungry predators frequently, their resistance to pathogens and parasites is reduced.

Willy van Strien

Photo: Harlequin ladybird, Harmonia axyridis. Timku (via Flickr, Creative Commons CC BY-NC-SA 2.0)

Source:
Knapp. M., M. Řeřicha & D. Židlická, 2020. Physiological costs of chemical defence: repeated reflex bleeding weakens the immune system and postpones reproduction in a ladybird beetle. Scientific Reports 10: 9266. Doi: 10.1038/s41598-020-66157-9

Game of patience

4 June 2020 / wvanstrien

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

Scattering sand

26 May 2020 / wvanstrien

Sea turtle creates decoy nests on the beach

Sea turtle eggs, buried in a sandy beach, are an attractive meal for some animals. Turtle mothers confuse these enemies with a series of decoy nests, Thomas Burns and colleagues think.

For a sea turtle female, it is physically demanding to lay her eggs. She crawls from the sea onto a sandy beach, selects a suitable place, digs a hole, lays dozens of eggs in it and refills the nest cavity. You would expect her to return to the sea as soon as possible, where she can move more easily and is safer.

But she doesn’t, as Thomas Burns and colleagues report. She first scatters sand around the refilled egg chamber. And then she starts to travel a convoluted path over a large area, further and further away from the nest, stopping periodically to scatter sand again. Only after having done this at several scattering stations, she will leave the beach. What is this extra effort good for?

Tasty

Sea turtles lay their eggs on tropical and subtropical beaches worldwide; afterwards, they don’t look after their clutches any more. The eggs develop in the warm sand, and the young turtles dig through the sand and crawl to the sea. All a mother can do for her offspring is to make sure not to betray the location of the nest, where she has dug in the sand, to predators. The tasty eggs are liked by various animals, including gulls, foxes, raccoons and wild pigs.

Biologists presumed that sea turtles scatter sand around their nests to disguise or camouflage them, so they won’t be noticed. But that can’t be the reason, Burns and colleagues argue after thoroughly studying the behaviour of leatherback sea turtle (Dermochelys coriacea) and hawksbill sea turtle (Eretmochelys imbricata). Because why, in that case, would sea turtles scatter sand also in places that are at considerable distance from the nest?

Big effort

The researchers, who worked on the islands of Trinidad and Tobago, point out that female sea turtles behave according to a fixed pattern until they have finished the nest. Thereafter, their movements become unpredictable. They take a random route on the beach, changing direction at each station.

Research also shows that scattering sand is a time-consuming and exhausting activity. The hawksbill sea turtle puts in as much energy as it does in excavating a nest hole and refilling it, and for the leatherback it is even the most strenuous activity. The hawksbill often scatters sand at more than ten stations, the leatherback may stop more than twenty times. Despite the great effort, the turtles persist: at the last stop they are as active as at the first.

The researchers’ conclusion: the sea turtles create a series of decoy nests. An natural enemy looking for eggs will mostly dig in vain and lose a lot of time. As a consequence, real nests are less easily found and therefore safer.

Willy van Strien

Photo: Eretmochelys imbricata. Gerwin Sturm (Wikimedia Commons, Creative Commons CC BY-SA 2.0)

Sources:
Burns, T.J., R.R. Thomson, R.A. McLaren, J. Rawlinson, E. McMillan, H. Davidson & M.W. Kennedy, 2020. Buried treasure—marine turtles do not ‘disguise’ or ‘camouflage’ their nests but avoid them and create a decoy trail. Royal Society Open Science 7: 200327. Doi: 10.1098/rsos.200327
Burns, T.J., H. Davidson & M.W. Kennedy, 2016. Large-scale investment in the excavation and ‘camouflaging’ phases by nesting leatherback turtles (Dermochelys coriacea). Canadian Journal of Zoology. Doi: 10.1139/cjz-2015-0240

Synchronous calling for safety reasons

15 May 2020 / wvanstrien

Pug-nosed tree frog male refrains from calling first

As soon as a pug-nosed tree frog male starts calling, other males in the neighbourhood follow suit. After a short time of noise, it is quiet again for a long period. Henry Legett and colleagues found an explanation for this pattern.

In pug-nosed tree frog, aka Panama cross-banded tree frog (Smilisca sila), males face a difficult dilemma. The frog lives in Central America. In order to reproduce, males have to attract a female by calling, which they do in the evening from a location along or above a water stream. But their calls reveal their presence not only to females, but also to their natural enemies, the fringe-lipped bat (Trachops cirrhosus) and midges. The enemies use sound to localise their victims.

According to Henry Legett and colleagues, the frog males reduce the risk by creating an auditory illusion in their enemies.

That illusion arises by the way in which animals, including humans, process sound. If, with a short interval (milliseconds), two or more identical sounds are produced by sources that are close to each other, we will perceive this as one sound, which originated at the source that uttered it first. So, we ignore reflections that occur in a furnished room or a forest, hearing sounds clearly as a consequence. The priority given to the first sound is called the precedence effect.

Followers

Because of this effect, pug-nosed tree frog males that call nearly synchronously with another male, can hide from their enemies’ ears. And, according to playback experiments by the researchers, this works out pretty well. They used two speakers that almost simultaneously produced the call of a male; alternately, one or the other speaker was leading. The response of bats, midges and female frogs was observed.

As results suggest, a male that closely follows another male calling runs a smaller risk of being captured by a bat and attracts less mosquitoes than the predecessor.

So following pays off – at least as far as safety is concerned. But what about reproduction? If females also have more difficulty finding followers, males won’t benefit from auditory hiding.

But as it turns out, chances are not too bad. The precedence effect is strong in other frog species, such as the túngara frog (Engystomops pustulosus), which inhabits the same region and whose males also call at night, but not synchronously. Compared to túngara frog females, the effect is weak in pug-nosed tree frog females. They chose following males less often than predecessors, but the difference is small. Also followers are approached by females.

Silence

The question remains why any tree frog male is the first to start the synchronous calling. After all, being the predecessor, its attractive power to females is only a bit stronger, while it is more likely to be eaten and bitten.

On the other hand, someone has to do it. If all males would remain silent, nothing happens. But the restraint of males to be the first explains the long periods of silence, interspersed with short, sporadic bouts of calls.

Willy van Strien

Photos:
Large: Pug-nosed tree frog Smilisca sila. Brian Gratwicke (Wikimedia Commons, Creative Commons CC BY 2.0)
Samll: fringe-lipped bat. Karin Schneeberger alias Felineora (Wikimedia Commons, Creative Commons CC BY 3.0)

Sources:
Legett, H.D., C.T. Hemingway & X.E. Bernal, 2020. Prey exploits the auditory illusions of eavesdropping predators. The American Naturalist 195: 927-933. Doi: 10.1086/707719
Tuttle, M.D. & M.J. Ryan, 1982. The role of synchronized calling, ambient light, and ambient noise, in anti-bat-predator behavior of a treefrog. Behavioral Ecology and Sociobiology 11: 125-131. Doi: 10.1007/BF00300101

Hoverflies entrapped

8 May 2020 / wvanstrien

Orchid deceives pollinators, but still offers a reward

The orchid Cypripedium subtropicum lures hoverflies by mimicking an aphid colony covered with honeydew. The hoverflies fall into a trap and while struggling out, they pollinate the flower, as Hong Jiang and colleagues write.

Pollination normally follows the principle ‘give a little, take a little’. Pollinators, such as bees, butterflies and flies, drink nectar from flowers, and through their visits they transfer pollen from one flower to another. A delicacy in exchange for pollen transport.

However, not all plants play a fair game. Some orchids, for instance, mimic a female wasp to attract male wasps. The males fruitlessly attempt to mate and while moving, they pick up or deposit pollen. Such deceptive flowers lure insects with false promises and make use of their services without paying a reward. On the contrary: a deceived male is just wasting its time.

Now, Hong Jiang and colleagues describe another form of deceit in Cypripedium subtropicum, an orchid species that grows in mountain forests in southwest China, Tibet and north Vietnam and that is pollinated by hoverflies. It promises its visitors not a mate, but food. Peculiarly, cheated insects do receive a reward from the plant – albeit an unusual one.

Aphid colony

The flowers of Cypripedium subtropicum are dark brown and have an enlarged labellum that has the shape of a pouch and is speckled with white hair tufts. In the eyes of hoverflies, the researchers think, the whole looks like an aphid colony covered with honeydew. And that’s what hoverflies are fond of. Honeydew is a sweet and sticky substance secreted by aphids because the sap they suck from plants contains an excess of sugars. Experiments showed that hoverflies don’t land on orchids that have their white hair tufts removed.

But the imitation goes further. The flowers also smell like an aphid colony: they emit an odour similar to that of alarm pheromones that aphids use to warn each other when danger is imminent.
And to finish it off, the white hair tufts are nutritious and contain a high amount of sugar – just like honeydew. Cypripedium subtropicum mimics the colour, smell and taste of an aphid colony.

Narrow way out

But when hoverflies enjoy the sweet food, it becomes clear that the orchid uses a trick to be pollinated. The labellum has an opening between the white hair tufts. At some moment while eating, a hoverfly will fall into the hole. Crawling back through the opening is impossible because of the curved margin. The animal is trapped.

The only way out is a narrow cleft at the top of the back of the pouch, through which the hoverfly can scramble out. It will first pass the flower’s pistil and then the stamens. When it squeezes along the stamens, a smear of pollen will attach on its back. And if it is trapped again during the next flower visit and tries to escape, it will deposit that pollen on the pistil. Subsequently, it picks up a new dose of pollen.

Cypripedium subtropicum forces hoverflies to pollinate it by trapping them, but at least they get an edible reward in return. The promise is not entirely false in this case.

Willy van Strien

Photo: ©Hong Jiang

Source:
Jiang, H., J-J. Kong, H-C. Chen, Z-Y. Xiang, W-P. Zhang, Z-D. Han, P-C. Liao & Y-i Lee, 2020. Cypripedium subtropicum (Orchidaceae) employs aphid colony mimicry to attract hoverfly (Syrphidae) pollinators. New Phytologist, online April 26. Doi: 10.1111/nph.16623

Backlit messages

24 April 2020 / wvanstrien

Humboldt squid is like an e-reader

It is difficult to communicate visually in the dark of the ocean, but Humboldt squid can do it, as Benjamin Burford and Bruce Robison show. It makes pigmentation patterns on its body visible by switching on backlight.

Humboldt squid or jumbo squid, Dosidicus gigas, is a social animal: individuals form groups to hunt prey, for instance lantern fish. Joint hunt requires good coordination, so that the whole group will swim in the same direction and decelerate synchronously to catch prey. And the squid manages to do so, without the animals bumping against each other or attacking each other, Ben Burford and Bruce Robison observed. Apparently, the animals communicate effectively.

Dark

This is remarkable, as the squid mainly lives in the dark. It spends the day hundreds of meters below surface and ascends to the surface only at night. So how do the animals communicate, the researchers wondered.

Squid species that live in light conditions are known to exchange messages with colour patterns on their body. Their skin contains chromatophores, small elastic bags filled with pigment that can be expanded. It was already known that Humboldt squid has chromatophores in one colour, reddish-brown. This enables it to display white-red patterns. But how can the animals show these patterns to each other in the dark?

By turning on backlight, as it turns out.

Glowing body

Burford and Robison studied the behaviour of the animals by filming during daytime at great depth with a camera mounted on a remotely operated vehicle and analyzing the footage.

In addition to chromatophores, Humboldt squid has so-called light organs, with cells that can produce light; this is called bioluminescence. Many deep-sea inhabitants have light organs in the skin, usually located at certain places, and convey messages by changing light intensity. For example, they show a pattern of spots indicating what species they are, they give a light show when courting, they flash to scare off an enemy or they lure prey with a lantern.

Humboldt squid uses bioluminescence in a different way. Its light organs are not embedded in, but located underneath the skin. And they are not located in certain places, but spread all over the body. By making its entire body glow yellow-green, Burford and Robison assume, Humboldt squid creates a backlight that reveals the white-and-red pattern of the chromatophores in the skin. It functions like an e-reader.

Deciphering the Humboldt squid

The squid has a whole repertoire of pigmentation patterns, as was already known. It can flash and flicker. It can make its caudal fins contrast with mantle, head and arms, or make the edge of the fins stand out; it can show stripes along the side of the mantle or on the arms, or create a stain between the eyes. Certain patterns are displayed only when the squid is hunting in a group, and some patterns appear in a fixed order. So, the system seems to enable complex, advanced communication.

The next challenge is to decipher that language. The camera used was not light-sensitive enough to read the patterns in detail, and it is still unknown how the animals respond to each other’s messages.

Willy van Strien

Researchers telling about their work on YouTube

Learn also about Humboldt squid mating behaviour

Sources:
Burford, B.P. & B.H. Robison, 2020. Bioluminescent backlighting illuminates the complex visual signals of a social squid in the deep sea. Proceedings of the National Academy of Sciences 117: 8524-8531. Doi: 10.1073/pnas.1920875117
Trueblood, L.A., S. Zylinski, B.H. Robison & B.A. Seibel, 2015. An ethogram of the Humboldt squid Dosidicus gigas Orbigny (1835) as observed from remotely operated vehicles. Behaviour 152: 1911-1932. Doi: 10.1163/1568539X-00003324
Rosen, H., W. Gilly, L. Bell, K. Abernathy & G. Marshall, 2015. Chromogenic behaviors of the Humboldt squid (Dosidicus gigas) studied in situ with an animal-borne video package. The Journal of Experimental Biology 218: 265-275. Doi:10.1242/jeb.114157
Benoit-Bird, K.J. & W.F. Gilly, 2012. Coordinated nocturnal behavior of foraging jumbo squid Dosidicus gigas. Marine Ecology Progress Series 455: 211-228. Doi: 10.3354/meps09664

Joint forces against brood parasite

7 April 2020 / wvanstrien

When yellow warbler is warning, red-winged blackbird will attack

The yellow warbler utters a specific alarm call when a brood parasite is nearby. The red-winged blackbird picks up the signal and attacks, as Shelby Lawson and colleagues write. Together, the birds protect their nests.

A bird’s nest with eggs or young is vulnerable. One of the dangers is that a heterospecific bird will lay an egg in it and charge the parents with the care of a foster young, like the cuckoo does. The red-winged blackbird, which breeds in wet areas in North and Central America, runs such risk. Here the brown-headed cowbird is the ‘cuckoo’, the brood parasite.

Although a young cowbird, unlike a cuckoo chick, does not eject its foster brothers and sisters out of the nest, its presence is to their detriment. The foreign chick demands so much attention that the legitimate young will suffer and starve or fledge in a bad condition.

So, the red-winged blackbird must keep the cowbird out of its nest. It takes advantage of the vigilance of the yellow warbler, another passerine bird that is visited by the cowbird, Shelby Lawson and colleagues show. The yellow warbler, in turn, takes advantage of the aggression of the redwing.

Defense

When yellow warblers detect a brown-headed cowbird, they utter a specific alarm signal, a ‘seet’ call. Upon hearing that call, all females respond appropriately: they immediately return to their nest (if they were not already there), repeat the seet and sit tightly on their clutch. As a consequence, a cowbird has no access.

Yellow warblers utter the seet call only in response to the brood parasite and only during the breeding period. To warn of predators, they have a different signal, and upon hearing that call, females will change perches and remain alert, but they won’t return to the nest. The combination of the specific alarm signal for brood parasites and the appropriate response of females is unique.

The researchers wondered whether red-winged blackbirds eavesdrop on that specific signal and take advantage of it. They play backed different sounds nearby redwings’ nests and observed their responses.

Both redwing males and females became aggressive upon hearing the seet of yellow warblers and attacked the speaker. They reacted as heated as in response to the chatter of brown-headed cowbirds. Also the call of a blue jay, a nest predator, aroused their aggression. Apparently, the response to the seet call is a general defence against various dangers that threaten a nest. The birds neglected the song of an innocent songbird.

Chatter of other redwings elicited the strongest defence response; the birds seem to consider conspecifics that invade their territory to be the greatest risk.

Together

The yellow warblers’ signal to warn of brood parasites is picked up by red-winged blackbirds, which respond by approaching the danger. This is to the benefit of yellow warblers: previous research had shown that their nests suffer less from parasitism by cowbirds if they breed in the neighbourhood of red-winged blackbirds. Redwings and yellow warblers often nest in loose aggregations; together they are able to resist the brood parasite.

So far, the red-winged blackbird appears to be the only bird species that understands and responds to yellow warblers’ warning of brood parasites.

Willy van Strien

Photos:
Large: Red-winged blackbird. Brian Gratwicke. (Wikimedia Commons, Creative Commons CC BY 2.0)
Small, upper: Female brown-headed cowbird. Ryan Hodnett (Wikimedia Commons, Creative Commons CC BY-SA 4.0)
Small, lower: Male yellow warbler. Mykola Swarnyk (Wikimedia Commons, Creative Commons CC BY-SA 3.0)
https://commons.wikimedia.org/wiki/File:Yellow_Warbler_Setophaga_aestiva_m_Toronto1.jpg

Researchers tell about their work on YouTube

Sources:
Lawson, S.L., J.K. Enos, N.C. Mendes, S.A. Gill & M.E. Hauber, 2020. Heterospecific eavesdropping on an anti-parasitic referential alarm call. Communications Biology 3: 143 . Doi: 10.1038/s42003-020-0875-7
Gill, S.A. & S.G. Sealy, 2004. Functional reference in an alarm signal given during nest defence: seet calls of yellow warblers denote brood-parasitic brown-headed cowbirds. Behavioral Ecology and Sociobiology 5671-80. Doi: 10.1007/s00265-003-0736-7
Clark, K.L. & R.J Robertson, 1979. Spatial and temporal multi-species nesting aggregations in birds as anti-parasite and anti-predator defenses. Behavioral Ecology and Sociobiology 5: 359-371. Doi: 10.1007/BF00292524

Pollen on wings

30 March 2020 / wvanstrien

Brush-like fireball lily is specialized for butterfly visits

On butterfly wings, the pollen of fireball lily, Scadoxus multiflorus, is transferred from one plant to another, as Hannah Butler and Steve Johnson show.

Like bees, butterflies are pollinators. They visit flowers to drink nectar, pick up pollen and deposit pollen grains on the pistil when visiting the next flower. Flowers pollinated by butterflies are often red or orange, because that colours are attractive to butterflies. In addition, the structure of the flowers is specialized for butterfly pollination, write Hannah Butler and Steve Johnson.

The ‘salver form’, in which petals form a platform on which a butterfly can settle while inserting its proboscis into a flower, was already known. In this model, pollen grains stick to the proboscis and the head of the butterfly. Now, Butler and Johnson describe another flower form with a different pollen transfer mechanism: the ‘brush model’.

Fluttering

A brush model plant is fireball lily, Scadoxus multiflorus, of Africa, also known as indoor plant. The stamens and pistils extend beyond the petals, and because the flowers are placed close together in an umbel, stamens and pistils of different flowers overlap. The plant cannot fertilize itself; pollen has to be brought from another plant for seeds to develop.

Large butterflies that visit the plant do the job. A frequent visitor is the mocker swallowtail Papilio dardanus, principally males. How does it transfer pollen from one plant to another?

The butterfly flutters along the inflorescence to inspect it, touching many stamens and pistils with the wings. When drinking nectar, it continues fluttering. The flat pollen grains from the stamens it touches stick between the scales at the ventral surface of the wings, as macro photos show. And part of the grains that a butterfly carries with it will fall on the pistils; the butterfly can pollinate several flowers during a single visit, even when it doesn’t drink nectar.

So, the brush-shaped umbel is a specialisation for butterfly-wing pollination. The fireball lily belongs to the amaryllis family. As it turns out, other red flowers of that plant family also have a brush model and deposit their pollen on butterfly wings.

Willy van Strien

Source:
Butler, H.C. & S.D. Johnson, 2020. Butterfly-wing pollination in Scadoxus and other South African Amaryllidaceae. Botanical Journal of the Linnean Society, online March 12. Doi: 10.1093/botlinnean/boaa016

Upside-down jellyfish stings at a distance

10 March 2020 / wvanstrien

Mucus contains numerous stinging-cell structures

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

Saving room for delicacy

20 February 2020 / wvanstrien

Cuttlefish won’t eat crab when there will be shrimp at night

Common cuttlefish exhibits a clear food preference: shrimp. If shrimp will be available at night, it refrains from eating crab during daytime, as Pauline Billard and colleagues show.

The European common cuttlefish Sepia officinalis, which occurs in Mediterranean Sea, North Sea and Baltic Sea, consumes different types of prey, but not with the same eagerness. Shrimp are its favourite food. It likes them so much, that it will skip a crab meal if it expects shrimp to be available later on, as research of Pauline Billard and colleagues revealed.

In the lab, the researchers conducted two experiments. First, they offered cuttlefish, kept in separate tanks, a crab during daytime. Some cuttlefish received an additional shrimp every night, the others were given shrimp only on some nights, in an unpredictable way.
The cuttlefish that received shrimp every night started to lower consumption of crab during daytime, saving room for their favourite food. The animals that were not sure about getting the delicacy at night maintained the consumption of crab during the day, eating enough in any case.
When the routine was reversed between groups – so, cuttlefish that had received shrimp every night changed to an unpredictable regimen and the other way round – the animals modified their behaviour accordingly.

Hunger

Then a second, more complex experiment was done. The animals again were offered crab during daytime, but now all of them got shrimp every other night. It took some time for them to get used to the regimen, but then they adjusted their behaviour. If they had received shrimp the night before, so when there would be no shrimp the following night, they consumed crab during daytime. Conversely, if there hadn’t been any shrimp the night before, they didn’t eat much crab, expecting to get shrimp the following night.

The animals’ behaviour can’t be explained by their feeding state, because in that case they would have eaten crab when they didn’t eat shrimp the night before, being more hungry.

Willy van Strien

Photo: Common cuttlefish. Amada44 (Wikimedia Commons, Creative Commons CC BY-SA 3.0)

Source:
Billard, P., A.K. Schnell, N.S. Clayton & C. Jozet-Alves, 2020. Cuttlefish show flexible and future-dependent foraging cognition. Biology Letters 16: 20190743. Doi: 10.1098/rsbl.2019.0743

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