From so simple a beginning

Evolution and Biodiversity

Month: January 2018

Idea of lenses abandoned

30 January 2018 /

Brittle star senses light with network of photosensitive cells

Ophiocoma wendtii possesses network of light-sensitive cells

A network of thousands of photosensitive cells allows brittle stars to detect dark places where they can hide from predators, Lauren Sumner-Rooney and colleagues write. No lenses are involved, as has been hypothesized.

The brittle star Ophiocoma wendtii, which lives on coral reefs in the Caribbean, has a strong aversion to light and during the day it retreats into dark crevices, where it is safe from predators. So, it perceives a difference between dark and light places, and this is possible thanks to an impressive network of thousands of light-sensitive cells across the entire body surface, Lauren Sumner-Rooney and colleagues discovered.

Microlenses

At the same time, they reject the existing idea that the dorsal side of the arms is covered with microlenses, as described by for instance Joanna Aizenberg and colleagues. These lenses were thought to focus incident light onto light-sensitive cells beneath; these cells would then transmit a signal to nerve fibres and from these signals neural centres would construct an image of the environment. In fact, the whole animal would act as one compound eye.

Those lenses don’t appear to exist.

Where did the idea come from? Brittle stars have an internal skeleton consisting of a spongy, porous form of calcite (calcium carbonate). The calcite plates of the arms extend into many bumps at the surface, which are hemispherical and transparent. They look just like tiny lenses – and so they were assumed to be tiny lenses.

But now, Sumner-Rooney succeeded in locating cells with light-sensitive pigments. She found many such cells, but not beneath the proposed microlenses, where the focal points should be. Instead, the light-sensitive cells occur at the surface in between the putative lenses, embedded in the skin; they are regularly arranged across the entire body. She also found bundles of nerve fibres that project towards these cells, and no nerve fibres that terminate beneath the ‘lenses’.

Safe place

In conclusion: the brittle star Ophiocoma wendtii possesses thousands of light-sensitive cells at the surface, but the transparent crystal bumps (the putative lenses) are not associated with them. The bumps are completely covered with skin, which is also in contradiction with an optical role. Also, no neural centres are found that could process the signals. With the extensive network of photosensitive cells the animals can distinguish light from dark very coarsely and find a safe place.

Willy van Strien

Photo: Ophiocoma wendtii. © Lauren Sumner-Rooney

Sources:
Sumner-Rooney, L., I.A. Rahman, J.D. Sigwart & E. Ullrich-Lüter, 2018. Whole-body photoreceptor networks are independent of ‘lenses’ in brittle stars. Proceedings of the Royal Society B 285: 20172590. Doi: 10.1098/rspb.2017.2590
Aizenberg, J., A. Tkachenko, S. Weiner, L. Addadi & G. Hendler, 2001. Calcitic microlenses as part of the photoreceptor system in brittlestars. Nature 412: 819-822. Doi: 10.1038/35090573

Blacker than black

18 January 2018 /

Almost no light escapes from of bird of paradise feathers

many birds of paradise have velvety super black feathers

Many birds of paradise have beautiful colours, the brightness of which partly is an illusion, created by dark feathers that surround coloured patches. These feathers are not normal black, but velvety super black, as Dakota McCoy and colleagues show.

Birds of paradise, which mainly occur in New Guinea, deserve their name. The bird family includes many species in which the males have brilliant colours, wear exuberant plumage ornaments and perform exciting dances. With their spectacular appearance, they try to seduce females.

Black feathers play an important role in their courtship, Dakota McCoy and colleagues write. The black feathers that these birds display are not normal black, but super black: they absorb almost all light – more than 99.5 percent – that falls on it. Against this velvety super black background, blue and yellow colours seem brighter than they really are; it looks as if the colours were luminescent. Such super black material is extremely rare in nature.

Ragged, curled edges

The researchers show that the deep black appearance is brought about by the special surface structure of the smallest components of the feathers. A feather consists of a shaft on which barbs are implanted, and the barbs are densely packed with barbules. Normally, these barbules are smooth and just bear hooks that interlock to make the feather stiff. The black feathers of crows and ravens have such normal barbules, as do the black feathers of birds of paradise that play no role in their show, such as back feathers.

But the barbules of super black feathers are highly modified. They have very ragged, curled edges with which deep, curved cavities in between, and this structure retains almost all light that falls on it. A normal black surface absorbs 95 to 97 percent of the incident light and reflects the remaining 3 to 5 percent. But in the micro jungle of spikes and cavities of super-black feathers, the light hits obstacles that scatter it again and again, and each time part of the light is transmitted into the material, where it is absorbed. Ultimately, less than half a percent of the incident light is reflected, so the feathers look super black for someone who faces the male – for instance a choosy female.

Photo: Victoria’s riflebid, Ptiloris victoriae, courting male. Francesco Veronesi (Wikimedia Commons, Creative Commons CC BY-SA 2.0)

Watch paradise birds in a video of BBC Earth, and another one of BBC Earth, and one of Cornell University featuring the magnificent riflebird.

Source:
McCoy, D.E., T. Feo, T.A. Harvey & R.O. Prum, 2018. Structural absorption by barbule microstructures of super black bird of paradise feathers. Nature Communications 9:1. Doi: 10.1038/s41467-017-02088-w

Fireproof

10 January 2018 /

‘Heart of Flame’ bromelia protects spider against flames

Heart of flame bromelia shelters spiders from fire

In case of a fire event on the Brazilian cerrado, many animals are killed. But the spider Psecas chapoda, which lives on a bromeliad plant, has a chance to survive, Paula de Omena and colleagues write.

Female Psecas chapoda on Bromelia balansaeThe terrestrial bromeliad plant Bromelia balansae and the spider Psecas chapoda are strongly associated with each other. The spider lives almost exclusively on this prickly plant, in the centre of which it is protected against its predators. On the leaves, adult spiders hunt, court and mate, females lay their eggs, and spiderlings grow up. Up to twenty spiders may inhabit one plant. Conversely, although the plant can do without Psecas chapoda, it benefits when it is inhabited by spiders, because it extracts nutrients from their faeces; the predatory spiders also protect the plant against herbivorous critters.

Fires

Now, Paula de Omena and colleagues discovered that this nice mutualism offers an additional benefit to the spider: it can survive a fire on the plant, because the leaves provide shelter and protection from the heat of the flames. Strikingly, the plant is known as ‘Heart of Flame’, as the centre turns bright red when it is about to bloom.

The bromeliads and spiders live in South America, including the Brazilian cerrado: a savanna-like area with trees and shrubs. In the dry period, which lasts about half a year, natural fires frequently rage. The researchers assumed that in the centre of the plants the spiders are sheltered from the heat of flames, and to find out whether they were right, they counted plants and spiders in a small and isolated cerrado fragment before and after a natural fire event.

Recovery

The day after the fire, the number of spiders had strongly decreased, and also the percentage of bromeliads occupied by spiders was low. But in the centre of a number of plants with intact leaf structures, the researchers found spiders that had survived the fire, and thanks to their survival, the spider population recovered within five months.

Without this possibility to hide, far fewer spiders would survive a fire event and it would take much longer for the population to return to pre-fire levels. A new fire would probably break out before the population fully recovered – so that Psecas chapoda would run a risk to disappear completely. Thanks to the plants, this does not happen.

Willy van Strien

Photos
Large: ‘Heart of Flame’, Bromelia balansae. João Medeiros (Wikimedia Commons, Creative Commons CC BY 2.0)
Small: jumping spider Psecas chapoda, female, on Bromelia balansae. ©Gustavo Q. Romero

Sources:
De Omena, P.M., M.F. Kersch-Beckr, P.A.P. Antiquera, T.N. Bernabé, S. Benavides-Gordillo, F. C. Recalde, C. Vieira, G.H. Migliorini & G.Q. Romero, 2017. Bromeliads provide shelter against fire to mutualistic spiders in a fire-prone landscape. Ecological Entomology, online December 20. Doi: 10.1111/een.12497
Romero, G.Q., P. Mazzafera, J. Vasconcellos-Neto & P.C.O. Trivelin, 2006. Bromeliad-living spiders improve host plant nutrition and growth. Ecology 87: 803-808. Doi: 10.1890/0012-9658(2006)87[803:BSIHPN]2.0.CO;2

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