Evolution and Biodiversity

Category: defence (Page 3 of 4)

Suicidal repair team

Young aphids die when closing a hole in their nest

Soldier nymphs in Nipponaphis monzeni repair their nest with their body fluid

Japanese aphids, Nipponaphis monzeni, inhabit galls on hazel. A hole in the gall wall would mean the end of the colony living there, were it not for aphid soldiers that give their lives to close it. Mayako Kutsukake and colleagues show how.

The Japanese aphid Nipponaphis monzeni is a social species, living in colonies. Juveniles, called nymphs, serve as soldiers for a period before they become adults and reproduce. It is their task to defend the nest, which is located in galls on the branches of evergreen witch hazel (Distylium racemosum), and to repair it in case of damage.

To close a hole, they show a spectacular and unique behaviour. In a self-destructive action, they discharge their body fluid to plug the gap. The liquid solidifies, forming a scab. Mayako Kutsukake and colleagues were curious about the mechanism.

Vulnerable nest

Colonies of Nipponaphis monzeni are founded by females that reproduce parthenogenetically. A colony of sisters is formed that are genetically identical and produce identical daughters.

gall on hazel in which Nipponaphis monzeni livesThe aphids induce the hazel on which they live to form a closed, hollow tumour, a gall. The animals inhabit this gall, sucking plant sap from the inner wall; in this phase, they are wingless. The gall remains small for a long time, but after three to five years it begins to grow rapidly during spring months and the following summer, it is fully grown – up to eight centimetres long – and home to thousands of aphids.

Winged aphids then appear in autumn. They make an opening in the wall and fly away to a second host tree, an oak, where they mate and produce a new generation of colony foundresses.

A full-grown gall has a lignified, hard wall, offering safety. But during growth, the wall consists of soft plant tissue and the nest is vulnerable. Moth caterpillars consuming hazel tree leaves easily tunnel into such gall, ingesting aphids as well. The soldiers will not tolerate this and attack the enemy: they climb onto it and sting it to death with their mouth parts.

But the hole that the caterpillar gnawed in the gall wall still remains. It has to be closed, otherwise enemies or pathogens may invade, or the nest may desiccate.

Skilful plastering

Japanese researchers had already shown how the soldier nymphs repair the hole with a self-sacrificing behaviour. Dozens or hundreds of them gather around the hole and eject large amounts of white body fluid (hemolymph, which is comparable to our blood) through two tubes on the abdomen. They mix the secretion with their legs and skilfully plaster it over the hole. Some soldiers are buried, others are locked out in the process. And all shrivel after losing their body fluid and will die.

Any way, the hole is fixed; the plug hardens and turns black. As a result, the colony is likely to survive the damage. After the sealing, the gall wall is healed, as the soldiers trigger the tree to cover the plasterwork on the inside by regenerating plant tissue.

Coagulation

Now, Kutsukake investigated the substances with which the soldiers repair a hole. The body fluid, she shows, contains many peculiar large cells of a hitherto unknown type that are packed with fat droplets and the enzyme phenoloxidase; the fluid contains long proteins and tyrosine, an amino acid.

When the soldiers discharge their body fluid, the cells rupture and the fat globules are released; the soldiers plug the gap immediately with a soft lipidic clot. At the same time, the other components come into contact with each other, and a coagulation process starts in which the proteins are linked to form a network that reinforces the lipid plug so that it becomes a scab.

The researchers assume that the process is derived from the process by which wounds heal. But in soldier nymphs’ hemolymph, the components are accumulated in extremely large quantities, far beyond what is necessary for wound healing.

With their unique repair behaviour, the soldier nymphs of Nipponaphis monzeni exhibit extreme altruism to defend the colony: they give their lives. Thanks to this sacrifice, a large part of their family survives. Otherwise the entire colony would have been lost.

Willy van Strien

Photos : ©Mayako Kutsukake
Large: Nipponaphis monzeni soldier nymphs plastering their hemolymphe over a hole
Small: gall in which Nipponaphis monzeni lives

On YouTube, the researchers show how soldiers fix a hole in the gall wall

Sources:
Kutsukake, M., M. Moriyama, S. Shigenobu, X-Y. Meng, N. Nikoh, C. Noda, S. Kobayashi & T. Fukatsu, 2019. Exaggeration and cooption of innate immunity for social defense. PNAS, 15 april online. Doi: 10.1073/pnas.1900917116
Kutsukake, M., H. Shibao, K. Uematsu & T. Fukatsu, 2009. Scab formation and wound healing of plant tissue by soldier aphid. Proceedings of the Royal Society B 276: 1555-1563. Doi: 10.1098/rspb.2008.1628
Kurosu, U., S. Aoki & T. Fukatsu, 2003. Self-sacrificing gall repair by aphid nymphs. Proceedings of the Royal Society London B (Suppl.) 270: S12-S14. Doi: 10.1098/rsbl.2003.0026

Young rebels

Ant larvae help to resist hostile take-over

The ant Formica fusca can resist parasites

When the nest of the ant Formica fusca is taken over by a parasitic queen of another species, the colony is lost. But the larvae help to limit the damage, according to Unni Pulliainen and colleagues.

An ants’ nest contains a large workforce serving the queen, which has the exclusive task to reproduce. Worker ants feed the queen and take care of her offspring, keep the nest clean and defend it. Their diligence attracts the attention of queens of other ant species that have not yet workers and for that reason could use some help. The black ant Formica fusca often suffers from such queens, which may invade a nest to exploit the workforce – thereby destroying the colony. But the host may resist, Unni Pulliainen and colleagues report.

Parasite

If a hostile queen tries to enter a nest of Formica fusca, which lives in clear-cut forest areas and along forest edges in Europe and parts of southern Asia and Africa, the workers may detect her and kill her. But that doesn’t always happen; sometimes, they accept her.

Once she’s inside, she can go on. She kills the resident queen or queens – in Formica fusca, a few queens usually live together in one colony – and she will start laying eggs. The workers have to raise her offspring as if they were the offspring of their own queen. The foreign queen, which outlives the workers, gradually acquires her own workers, while the original workers die. By temporarily parasitizing the Formica fusca colony, she founds her own.

Sabotage

But the enslaved workers can limit the damage by sabotaging. The workers can remove the foreign eggs. And the orphan ant larvae seem to help.

Ant larvae sometimes eat ant eggs, and Pulliainen wanted to know if Formica fusca larvae might be keen to consume the eggs of a foreign queen. In experiments, she offered larvae one egg each, either of their own queen or of a foreign queen, which belonged either to a parasitic species or to an innocent species that never invades other ants’ nests.

The larvae never consumed an egg of their own queen. But when they were given an egg from a parasitic queen, they consumed it in one in ten cases; eggs of an foreign innocent queen were consumed less often.

Future

The feeding behaviour of the larvae, albeit not very spectacular, may help to limit the damage. The eggs are nutritious and their consumption may increase the orphan larvae’s chance of survival. Male larvae can leave to reproduce as adults. And some of the female larvae will be future queens, which may found a new colony elsewhere. Female larvae destined to become workers can be successful too. They are not able to mate, but they can produce some sons, as sons develop from unfertilized eggs. The colony may be lost, but some larvae still have a future.

Willy van Strien

Photo: Formica fusca. Mathias Krumbholz (Wikimedia Commons, Creative Commons CC BY-SA 3.0)

Sources:
Pulliainen, U., H. Helanterä, L. Sundström & E. Schultner, 2019. The possible role of ant larvae in the defence against social parasites. Proceedings of the Royal Society B 286: 20182867. Doi: 10.1098/rspb.2018.2867
Chernenko, A., M. Vidal-Garcia, H. Helanterä & L. Sundström, 2013. Colony take-over and brood survival in temporary social parasite of the ant genus Formica. Behavioral Ecology and Sociobiology 67: 727-735. Doi: 10.1007/s00265-013-1496-7
Chernenko, A., H. Helanterä & L. Sundström, 2011. Egg Recognition and Social Parasitism in Formica Ants. Ethology 117: 1081-1092. Doi: 10.1111/j.1439-0310.2011.01972.x

Frightening days

Crayfish avoid light when renewing their armour

red swamp crayfish is anxious when moulting

Normally, the red swamp crayfish is rather fearless. But if it has to replace its carapace with a new one, its bravery disappears, as Julien Bacqué-Cazenave and colleagues report.

Crustaceans do not have a skeleton inside their body, like we do. Instead, they have a carapace, an external skeleton. This sturdy box in which they are packed protects them from physical harm. But there is a drawback: the carapace limits body growth. That is why the animals must, from time to time, replace their carapace with a larger one. The old one is shed, a new one is formed.

That is no trifle, as Julien Bacqué-Cazenave and colleagues show.

Process takes a month

The researchers wanted to know how the red swamp crayfish, Procambarus clarkii, is doing during a moult. The species originally occurs in Mexico and the south of the United States and has been introduced in many other places; it has settled as an exotic species in Europe.

Its moult is a lengthy and complex process. The chitin, of which the carapace consists, is secreted by the epidermis and the carapace is attached to it. So, it is must be separated from the epidermis, which has to form a new one. The attachments of the muscles that are anchored to the armour have to be transferred.

As soon as the old carapace is shed, the new one is exposed. This leaves the crayfish unprotected and vulnerable, as the newly formed carapace is thin and fragile in the beginning. It has to thicken and harden before it can protect the animal. The entire process of moulting takes about a month: two weeks before the old armour is shed and two more weeks until the new armour has hardened.

Anxious

The red swamp crayfish normally is courageous, but during the month of moulting, especially during the third week, it is not at ease, as experiments conducted by Bacqué-Cazenave show. He tested the animals every two or three days in a plus-maze with two illuminated and two dark arms. Crayfish that did not experience any stress spent 40 percent of their time in the illuminated part of the plus-maze. But when they were about to shed their carapace, they began to avoid the light a few days in advance, and the first week after moulting they stayed in the dark areas almost continuously. From earlier work, the researchers knew that the animals behave like this when they are anxious.

The aversion to light was indeed associated with moulting, according to tests in which the animals were given a hormone that initiates the moulting process, a so-called ecdysteroid. But when the animals were also given a tranquilizer, they did not avoid the illuminated areas. From this, the researchers conclude that the light aversion is an anxiety reaction.

Obviously, the period of moult is hard. But when it is over, the crayfish is safe in its armour for the next two to six months.

Willy van Strien

Photo: Andrew C (Wikimedia Commons, Creative Commons CC BY 2.0)

Source:
Bacqué-Cazenave, J., M. Berthomieu, D. Cattaert, P. Fossat & J.P. Delbecque, 2019. Do arthropods feel anxious during molts? Journal of Experimental Biology 222: jeb186999. Doi: 10.1242/jeb.186999

Romantic sea

Fairytale light shows of Cypridinid ostracods

ostracod produces light to escape from predator

With an amazing show of light pulses, male cypridinid ostracods try to attract a mate. Each species has its own specific show program, with either very short lasting flashes or bulbs that glow for several seconds. Nicholai Hensley and colleagues examined the chemistry behind.

It looks like a fairytale scene: dozens of blue lights dancing in the dark waters of the Caribbean Sea. The spectacle is visible to those who dive or snorkel at the beginning of the night. The light artists are ostracods of the Cypridinidae family, tiny crustaceans (less than two millimeters long) with a carapax consisting of two valves, like a clam shell.

They are also known as sea fireflies. Nicholai Hensley and colleagues study their behaviour and the chemistry behind their light.

Slimeballs

Ostracods produce light by expelling mucus containing a reactant, vargulin, and the enzyme c-luciferase, which react with oxygen in seawater emitting blue light. The ostracods use their light mainly to avoid predation. If a fish picks up an ostracod, the prey will produce a cloud of blue mucus that is pumped into the water via the gills of the fish. It makes the fish visible to its own predators. Startled, it will spit out the bite.

In ostracods of the family Cypridinidae that live in the Caribbean Sea, males use the same light reaction in a much more subtle way with a completely different purpose: they place luminescent slimeballs in the water in order to seduce a female into a mating. This courtship behaviour produces the fairytale scenes.

Train of lights

The light artist best known is Photeros annecohenae, one of the most abundant species off the coast of Belize. In the first dark hour of the night, when the sun is down and the moon is not shining, groups of males display above seagrass beds. They have to perform well, because competition is high. While there are as many females as males, most are unavailable. This is because they incubate fertilized eggs in a brood pouch, and during this period, they will not mate.

American biologists examined male courtship behaviour in the lab, using infrared light. A displaying male will first swim in a looping pattern just above the tips of the seagrass blades and place about three bright flashes of light, probably to draw attention. Then, while spirally swimming upward, it places weaker light pulses at regular intervals. It swims at high speed, slowing down when it releases a luminescent slime ball.

By doing so, it creates a train of about twelve consecutively flashing lights that can be 60 centimetres long. When finished, it descends to start a new series. Often other males join and start displaying in synchrony.

Interception

To choose a mate, females assess the light pulses that the males produce. If a female is attracted to a particular male, she will swim to him without producing any light herself. Thanks to his regular flashing pattern, she manages to meet him just above his last light pulse. Mission accomplished.

Sometimes males try to obtain a mate without producing light themselves. Instead, they intercept a female that is on her way to a performing male.

Starting a show, following another male’s show or sneaking to get a female are different tactics to acquire a mate and a male can easily switch among them.

Species-specific shows

In the Caribbean Sea, many other species of Cypridinidae also occur, and about ten species commonly live at the same place. Because they all have their own characteristic light show, a female has no difficulty finding a conspecific partner. The shows vary in the trajectory a courting male swims, the number of light pulses, the brightness of the light, the interpulse distance and time interval and the time that a pulse remains visible.

Romantic

Hensley investigated the cause of the variation in light pulse length. For although all species perform the same chemical reaction to make light pulses, the duration of the pulses varies greatly: some species, such as Photeros annecohenae, show flashes that last only a fraction of a second, others make light bulbs that continue to glow for 15 seconds.

The structure of the enzyme c-luciferase appears to vary between species, resulting in the light reaction to proceed faster in one species than in another. This determines how soon the light extinguishes. In addition, the reaction rate depends on the amount of vargulin compared to the amount of enzyme: the more vargulin, the longer it takes before it is all converted and the light disappears.

Courting males produce far less light than an animal that avoids predation. Romantic lights don’t have to be that big and bright.

Willy van Strien

Photo: Luminous cloud around a fish that intended to consume an ostracod. It will spit it out. © Trevor Rivers & Nicholai Hensley

Fifteen-scaled worm emits light to defend itself in another way

Sources:
Hensley, N.M., E.A. Ellis, G.A. Gerrish, E. Torres, J.P. Frawley, T.H. Oakley & T.J. Rivers, 2019. Phenotypic evolution shaped by current enzyme function in the bioluminescent courtship signals of sea fireflies. Proceedings of the Royal Society B 286: 20182621. Doi: 10.1098/rspb.2018.2621
Rivers, T.J. & J.G. Morin, 2013. Female ostracods respond to and intercept artificial conspecific male luminescent courtship displays. Behavioral Ecology 24: 877–887. Doi: 10.1093/beheco/art022
Rivers, T.J. & J.G. Morin, 2012. The relative cost of using luminescence for sex and defense: light budgets in cypridinid ostracods. The Journal of Experimental Biology 215, 2860-2868. Doi: 10.1242/jeb.072017
Morin, J.G. & A.C. Cohen, 2010. It’s all about sex: bioluminescent courtship displays, morphological variation and sexual selection in two new genera of Caribbean ostracodes. Journal of Crustacean Biology 30: 56-67. Doi: 10.1651/09-3170.1
Rivers, T.J. & J.G. Morin, 2009. Plasticity of male mating behaviour in a marine bioluminescent ostracod in both time and space. Animal Behaviour 78: 723-734. Doi: 10.1016/j.anbehav.2009.06.020
Rivers, T.J. & J.G. Morin, 2008. Complex sexual courtship displays by luminescent male marine ostracods. The Journal of Experimental Biology 211: 2252-2262. Doi: 10.1242/jeb.011130

Airmobile brigade

Hovering guards of bee colony position themselves orderly

Hovering guards defend the nest of Tetragonisca angustula

It is difficult for a robber bee to stealthy approach a colony of the stingless bee Tetragonisca angustula, because hovering guards will detect it. These guards arrange themselves in an organized manner, Kyle Shackleton and colleagues show.

Workers of the stingless bee Tetragonisca angustula defend their colony extraordinary well against their enemies. Some workers are dedicated guards; they are heavier than other workers and have longer legs. Such specialised soldier caste is not known in other bee species. And while during the day always guards are standing in or near the nest entrance, there are often also some hovering in front of it to keep an eye on the access route, especially in the afternoon. Such an airmobile brigade is unique too.

Robbery

The most important enemy is the robber bee Lestrimelitta limao. Robber bee workers do not collect nectar and pollen from flowers themselves, but get it from colonies of other species. They also steal food that is prepared for the larvae and nest constructing material. Tetragonisca angustula, with its large colonies, is vulnerable. No wonder, then, that there are guards that keep an eye on what is near the nest. It is important to deal with an approaching single robber bee at once, because it is a scout. It will recruit hundreds of others for a raid that will last for hours or days.

As more hovering guards are active, such a flying intruder is detected earlier and intercepted at a greater distance from the nest. The guards recognize the robber bee from its odour and colour; it is black and smells like lemon. The guards wrestle it to the ground by clamping to an antenna or wing. They are not able to kill it, because it is three times heavier than a they are. But they may stop it.

Maximal field of view

Often only a few hovering guards are hanging in front of the nest. Kyle Shackleton and colleagues now show that these guards do not choose their position randomly, but in a coordinated way. If two guards  are hovering, there will usually be one on the left and one of the right side of the access route to the nest. In case of three guards, it rarely occurs that all of them hover at the same side. And four guards mostly are distributed evenly; sometimes sometimes three guards hover at the one side and one at the other side, and it hardly happens that all four guards are at the same side. Because of this coordinated distribution, the hovering guards have a maximal field of view and they will discover an approaching flying enemy as fast as possible.

In case of immediate danger, more guards will be hovering in front of the nest. An even distribution between left and right is less important in that case, because together they will have a good view anyway. There is no surveillance at night, for in the evening the bees close the nest entrance with wax.

Willy van Strien

Photo:
A Tetragonisca angustula hovering guard bee next to a nest-entrance. Bibafu (Wikimedia Commons, Creative Commons CC BY-SA 3.0)

Sources:
Shackleton, K., D.A. Alves & F.L.W. Ratnieks, 2018. Organization enhances collective vigilance in the hovering guards of Tetragonisca angustula bees. Behavioral Ecology 29: 1105-1112. Doi: 10.1093/beheco/ary086
Grüter, C., C. Menezes, V.L. Imperatriz-Fonseca & F.L.W. Ratnieks, 2012. A morphologically specialized soldier caste improves colony defense in a neotropical eusocial bee. PNAS 109: 1182-1186. Doi: 10.1073/pnas.1113398109
Grüter, C., M.H. Kärcher & F.L.W. Ratnieks, 2011. The natural history of nest defence in a stinngless bee, Tetragonisca angustula (Latreille) (Hymenoptera: Apidae), with two distinct types of entrance guards. Neotropical Entomology 40: 55-61. Doi: 10.1590/S1519-566X2011000100008
Van Zweden, J.S., C. Grüter, S.M. Jones & F.L.W. Ratnieks, 2011. Hovering guards of the stingless bee Tetragonisca angustula increase colony defensive perimeter as shown by intra- and inter-specific comparisons. Behavioral Ecology and Sociobiology 65: 1277-1282. Doi: 10.1007/s00265-011-1141-2

Swirling lights

Scale worm deceives its enemy by detaching glowing scales

Scale worm emits light to escape from predator

If the fifteen-scaled worm is attacked by a lobster, green lights appear in the water. They are scales and tail segments that the worm released to escape, as Julia Livermore and colleagues show.

It is a cheerful sight: animals that emit light, such as fireflies. In marine habitats, this sight is much more common than on land, because most light artists live in dark water. They use their light to defend themselves against predators or to lure prey, or they perform a light show to seduce a partner.

The fifteen-scaled worm, Harmothoe imbricata, uses its light for defence, Julia Livermore and colleagues write. The worm, three to six centimetres in length, occurs everywhere on the northern hemisphere, from the inter-tidal zone to a depth of a few kilometres. On its back it carries fifteen pairs of disc-shaped scales for protection, which flash green light when the worm is irritated. It can also detach the scales, upon which they float through the water as little lights for some time. If it is in great danger, it also releases its posterior body segments, which then also emit light.

Deceived

The researchers show how this behaviour enables the worm to actually escape from its predators, crabs and lobsters. They brought worms into the lab and conducted experiments in which they exposed a worm to the American lobster, Homarus americanus, or the green shore crab, Carcinus maenas; a refuge for the worm to hide was provided.

If an enemy approaches, a worm sometimes tries to slowly swim away unseen. If that fails, its scales will flash and / or the worm detaches scales and sometimes also segments of its tail, which then emit light. Because of the movements of the animals, the parts will swirl in the water. The predator is deceived: it goes after the lights and grabs them – and it will eat the dropped scales and tail segments.

In the meantime, the worm gets a chance to safely escape, as it turns out. Especially when it drops tail segments, its chance of survival is high. The swirling lights therefore function as an effective defence, but also an expensive one: the worm sacrifices protective scales and sometimes also a piece of its tail. That will regenerate, but it takes a while: a few days for the scales, a few weeks for the tail. But the sacrifice may save its life.

Willy van Strien

Photo: Harmothoe impar, a scale worm that is closely related to Harmorhoe imbricata. Saxifraga-Eric Gibcus

The researchers filmed experiments with a crab predator

A flashlight fish uses light to lure prey

Sources:
Livermore, J., T. Perreault & T. Rivers, 2018. Luminescent defensive behaviors or polynoid polychaete worms to natural predators. Marine Biology 165: 149. Doi: 10.1007 / s00227-018-3403-2
Verdes, A. & D.F. Gruber, 2017. Glowing worms: biological, chemical, functional diversity or bioluminescent Annelids. Integrative and Comparative Biology 57: 18-32. Doi: 10.1093 / icb / icx017
Plyuscheva, M. & D. Martin, 2009. On the morphology of elytra as luminescent organs in scale worms (Polychaeta, Polynoidae). Zoosymposia 2: 379-389. Doi: 10.11646 / zoosymposia.2.1.26

Startling

When under attack, skink exposes its entire blue tongue

Blue-tongued skink deters attack by protruding its blue tongue

As their name indicates, blue-tongued skinks possess a blue tongue. The lizards sometimes protrude it to show the striking colour. Arnaud Badiane and colleagues explain why.

The blue-tongued skinks from Australia, Indonesia and Papua New Guinea have a cryptic colour which protects them from hunting predators. But sometimes, they suddenly expose their large tongue, which catches the eye because of a striking blue colour. This behaviour seems odd, as it reveals the animals’ presence after all.

Now, Arnaud Badiane and colleagues argue that also this tongue display offers protection from predators, just because the blue colour stands out against the background. A blue-tongued skink uses its tongue as a defensive strategy at the last moment, they say, when a predator is about to strike. The sudden appearance of the blue tongue startles or overawes the enemy – offering the skink a chance to escape.

Reflexive recoil

The researchers substantiate their arguments with experiments in which individuals of the northern blue-tongued skink, Tiliqua scindoides intermedia, were approached by models of predators: a snake, a monitor lizard, a bird, a fox or, as a control, a piece of wood.

The tested skinks behaved normally until such a predatory enemy came very close. When attack was imminent, they suddenly opened their mouth widely and showed the entire tongue by sticking it out as far as possible. To a piece of wood the threatened skinks responded less strongly than to a predator model. To a bird and a fox they protruded their tongue most often, and to a fox or a snake they exposed the largest area of their tongue. In order to increase the shock effect, they inflated their body and hissed.

The back of the tongue has the most intense colouration, and the tested skunks exposed this part when an enemy was in close proximity. The blue colour is detectable to the visual system of the natural enemies.

Predators cannot learn to ignore such suddenly exposed blue flag, the biologists assume: a recoil reflex is inevitable. They still have to investigate how predators respond in reality. If they really are startled, blue-tongued skins in distress would rightly rely on their tongue as the last defence.

Willy van Strien

Photo: northern blue-tongued skink, Tiliqua scindoides intermedia. ©Shane Black

Source:
Badiane, A., P. Carazo, S.J. Price-Rees, M. Ferrando-Bernal & M.J. Whiting, 2018. Why blue tongue? A potential UV-based deimatic display in a lizard. Behavioral Ecology and Sociobiology 72: 104. Doi: 10.1007/s00265-018-2512-8

Camouflage suit

Covered with sponges, a crab is poorly visible

Spider decorator crab Camposcia retusa covered with sponges

The spider decorator crab Camposcia retusa adorns its legs and carapace exuberantly with sponges, probably to mislead predators, Rohan Brooker and colleagues write. The crab accumulates more decorations when it has no access to shelter.

Equipped with a lot of sponges, complemented by some algae and dead organic matter, the spider decorator crab Camposcia retusa moves around: a weird appearance. The crab is associated with tropical coral reefs in the Indian Ocean and the western Pacific Ocean. Why would this little animal, with a carapace that is a few centimetres wide, carry so much stuff that probably hampers its mobility?

According to Rohan Brooker and colleagues, a highly decorated crab is less visible to its predators. In addition, many sponges are noxious or toxic, and they may deter predators that perceive such a crab in spite of its camouflage.

The researchers wanted to learn more about the crab’s decorating behaviour. From reefs, they caught a number of crabs to study their decoration patterns. They then conducted a manipulative behavioural experiment on crabs in tanks to which they added red polyester pompoms of different sizes to see how the crabs would use them.

Velcro

They found that the animals covered their carapace and the third and fourth sets of walking legs most (they have four pairs of walking legs). In the experiments, they placed the largest and heaviest pompoms only on the hind legs, which are the strongest. The chelipeds – which the crabs use for feeding and communication – and the first set of legs were hardly decorated. The parts of the body on which items are distributed are equipped with hooked seta like those of Velcro, to which pieces of sponge and other material are easily attached.

Defence

In another experiment, the crabs either got a shelter in the form of a PVC elbow in their tank or no shelter. The crabs that had no access to shelter decorated more than the crabs that had shelter, hence the conclusion that the decoration is primarily an antipredator defence. Because the animals accumulate and retain a wide range of materials, camouflage most likely is the main effect of decoration. And because they prefer to attach sponges, it may also serve as a deterrent. It would be great if the researchers now would go on to show that predators have more difficulty perceiving a prey in camouflage suit, or that they are deterred by the sponges.

Decoration occurs in many animal species, most frequently in aquatic species. The spider decorator crab Camposcia retusa is a beautiful example of this behaviour.

Willy van Strien

Photo: Patrick Randall (via Flickr, Creative Commons CC BY-NC-SA 2.0)

Three examples of decorated crabs on YouTube: 1, 2, 3

Source:
Brooker, R.M., E.C. Muñoz Ruiz, T.L. Sih & D.L. Dixson, 2017. Shelter availability mediates decorating in the majoid crab, Camposcia retusa. Behavioral Ecology, online Oct. 17. Doi: 10.1093/beheco/arx119

Cryptic leaf colour

Camouflage protects alpine plants from herbivory

Corydalis hemidicentra has stone coloured leaves

In the high mountains of China, Corydalis plants can be found with leaves that are coloured like stone. That is no coincidence: plants without a stone colour are easily detected by butterflies and devoured by caterpillars, show Yang Niu and colleagues.

Apollo butterfly oviposits near Corydalis plantsThe leaves of the alpine plant Corydalis hemidicentra don’t have a fresh green colour; instead, they have the colour of stones: they are either dark grey, reddish brown or greyish green. That is unusual, but it is for a good reason. The plants grow on bare and open stony ground in the very high mountains of southwest China. A normal green leaf colour would attract plant-eating insects, while a cryptic colouration protects the plants from herbivores.

Butterflies’ eyes

The main enemies of the mountain plants are Apollo butterflies, such as Parnassius cephalus. Butterfly females search for a Corydalis plant, which they locate visually, and lay their eggs on the rocks next to it. After emergence, the caterpillars find their meal ready to eat and they consume the plant almost completely.

leaves of Corydalis hemidicentra match against their backgroundThe colour of the leaves of Corydalis hemidicentra almost always match against the background: where the rock is grey, the leaves are grey too; reddish brown plants grow on reddish brown scree; and greyish green plants are found among greyish green stones. Yang Niu and colleagues show that the colour of the plants is similar to the background colour not only to our eyes, but also to butterflies’ eyes. The cryptic colouration arises because the leaves not only contain green pigment (chlorophyll), as normal, but also red pigment (anthocyanin) and air-filled spaces that are white, and the leaf colour is genetically determined.

Pollinators

Previously, Niu had studied another alpine plant, Corydalis benecincta, of which a green and a grey morph exist. He had found that Apollo butterflies detect the green plants much more easily, and as a the consequence, most green plants are damaged by caterpillars, while grey plants often escape. When plants escape from the enemy, their colour is unimportant: greyish green plants perform as well as green plants. Also in Corydalis hemidicentra non-camouflaged individuals will disappear by herbivory, while camouflaged plants survive. That is why the leaf colour of the plants matches against the background.

While camouflage makes the plants invisible for butterflies, they need to be found by pollinators. Thanks to the strikingly coloured flowers – light blue in Corydalis hemidicentra, purplish pink in Corydalis benecincta – they are easy to find to them. But those flowers don’t appear until the plants are no longer at risk, that is: after the period when butterflies are laying their eggs.

So, not only many animals are camouflaged against their surroundings, but there are also plants with background matching leaves, especially in bare mountain areas. In a well-grown area, plants that are attractive to herbivores are camouflaged best by a normal green colour.

Willy van Strien

Photos: ©Yang Niu

Sources:
Niu, Y., Z. Chen, M. Stevens & H. Sun, 2017. Divergence in cryptic leaf colour provides local camouflage in an alpine plant. Proceedings of the Royal Society B 284: 20171654. Doi: 10.1098/rspb.2017.1654
Niu, Y., G. Chen, D-L. Peng, B. Song, Y. Yang, Z-M. Li & H. Sun, 2014. Grey leaves in an alpine plant: a cryptic colouration to avoid attack? New Phytologist 203: 953-963. Doi: 10.1111/nph.12834

Useful cigarette butts

House finch has to accept harmful side effects

House finches add cigarette butts to their nests to repel parasites

Smoked-trough cigarette filters are noxious, still some bird species add them to their nest lining, where the nicotine will repel blood-sucking parasites. They do so only when they need to, as Monserrat Suárez-Rodríguez and Constantino Macías Garcia show.

Spent cigarette filters are popular among some bird species, for instance the house finch. The birds weave cellulose fibres from discarded butts into the lining of their nests, together with more conventional soft materials such as feathers, fur or cotton. Monserrat Suárez-Rodríguez en Constantino Macías Garcia wondered whether the birds collect cellulose from butts accidently, or whether they do it to protect their young against blood-sucking parasites: lice and ticks. From earlier research, they knew that ectoparasites are repelled by nicotine, and the more smoked-through cigarette butts could be found in a nest, the smaller the amount of parasites was. Weight gain and fledging success of young increased with the proportion of cellulose from butts in the nest lining.

But they also knew that the butts are harmful to adult birds and their offspring. Next to nicotine, the butts contain more than 400 different substances such as heavy metals and insecticides, many of which are toxic. The substances may enter the birds’ bodies through the skin or the lungs.

Damage

The research team had analysed blood samples of parents and young and found nuclear abnormalities in many red blood cells (in contrast to human red blood cells, those of birds contain a nucleus with dna). The larger the proportion of butts in the nest lining, the more genotoxic damage was seen. Red blood cells live for only two to four weeks, so the damage may have no serious consequences. But other cells types likely are damaged too. The question is whether the benefits of adding cigarette butts to the nest lining – less parasites, resulting in better growth – are large enough to outweigh these costs.

The answer will depend on how much the butts are needed to fight off parasites.

Ticks

Now, experiments reveal that house finches act accordingly: they bring more smoked-through cellulose fibres from cigarette butts to their nests if parasites are present than if they’re not. The researchers removed the nest lining from a number of nests shortly after the young hatched, and added a piece of felt instead; by doing so, they removed the bulk of the tick population from the nest as well. They measured the amount of butts in the original lining. They added living ticks to some of the artificial felt nest linings, dead ticks to other linings and nothing to the remaining linings. After the young fledged, they collected the artificial linings to investigate how much butts the parents had added.

It appeared that the birds collected more butts if the researchers had added living ticks to their nest, so when it was useful to bring butts. Also birds that had brought a large amount of butts into their original nest lining, collected many butts now as well; apparently, they had experienced a high parasitic load during incubation.

The birds don’t collect cigarette butts randomly, the conclusion is, but in response to the presence of ectoparasites; so, it is a form of self-medication.

Willy van Strien

Photo: house finch male feeding young. Susan Rachlin (Wikimedia Commons, Creative Commons CC BY 2.0)

Sources:
Suárez-Rodríguez, M. & C. Macías Garcia, 2017. An experimental demonstration that house finches add cigarette butts in response to ectoparasites. Journal of Avian Biology, online September 1. Doi: 10.1111/jav.01324
Suárez-Rodríguez, M., R.D. Montero-Montoya & C. Macías Garcia, 2017. Anthropogenic nest materials may increase breeding costs for urban birds. Frontiers in Ecology and Evolution 5: 4. Doi: 10.3389/fevo.2017.00004
Suárez-Rodríguez, M. & C. Macías Garcia, 2014. There is no such a thing as a free cigarette; lining nests with discarded butts brings short-term benefits, but causes toxic damage. Journal of Evolutionary Biology 27: 2719–2726. Doi: 10.1111/jeb.12531
Suárez-Rodríguez, M., I. López-Rull & C. Macías Garcia, 2013. Incorporation of cigarette butts into nests reduces nest ectoparasite load in urban birds: new ingredients for an old recipe? Biology Letters 9: 20120931. Doi: 10.1098/rsbl.2012.0931

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