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By Jeff Kubina. Creative Commons. Some rights reserved.
Fish can hide in the open ocean by manipulating how light reflects off their skin, according to researchers at The University of Texas at Austin. The discovery could someday lead to the development of new camouflage materials for use in the ocean, and it overturns 40 years of conventional wisdom about fish camouflage.
The researchers found that lookdown fish camouflage themselves through a complex manipulation of polarized light after it strikes the fishes’ skin. In laboratory studies, they showed that this kind of camouflage outperforms by up to 80 percent the “mirror” strategy that was previously thought to be state-of-the-art in fish camouflage.
The study was published this month in the Proceedings of the National Academy of Sciences (PNAS). The research was funded by the U.S. Navy, which has an interest both in developing better ocean camouflage technologies and in being able to detect such strategies if developed by others.
“The open ocean represents a challenging environment for camouflage,” said Molly Cummings, associate professor of integrative biology in the College of Natural Sciences. “There are no objects to hide behind in three-dimensional space, so organisms have to find a way to blend in to the water itself.”
For the past few decades the assumption has been that the optimal camouflage strategy for open ocean fish is to reflect sunlight like a mirror. Many fish, including the lookdown, have reflective skin elements that can act like mirrors.
Such a strategy works well for certain aspects of light, such as color and intensity, which tend to be distributed homogenously in the region surrounding the fish. The mirror strategy is not optimal, however, when light is polarized, which occurs when individual waves of light align parallel to one another.
“In the polarized light field, there is a lot of structure in the open ocean,” said physicist Parrish Brady, a postdoctoral associate in Cummings’ lab. “Humans can’t see it, but more than 60 different species of fish have some degree of polarization sensitivity. They can perceive the structure in the light.”
The contours of the polarized light field in the open ocean environment are constantly changing except at noon, when the sun is directly overhead. A fish needs to do more than deploy the straight mirror strategy in order to stay camouflaged. Cummings hypothesized that perhaps nature had evolved a strategy to do just that.
She and Brady caught some lookdowns, which are known as good camouflagers. In tanks in the lab, they simulated the sun passing over the ocean during the course of a day, and they used a custom-built polarimeter to measure how the lookdowns reflected the polarized light.
They found that the lookdowns were able to manipulate their reflective properties in ways that were close to the theoretical optimum and far better than a standard mirror.
“The nifty thing is when we mimicked the light field when the sun is overhead, as it would be at noon, the fish just bounced back that light field,” said Cummings. “It acted like a mirror. Then we mimicked the light field when it’s more complex, and the lookdown altered the properties of the polarized light it was reflecting so that it would be a better blend into its specific background at different times of day.”
The lookdown’s “polaro-cryptic” mirror skin functions by selectively reducing the degree of polarization and transforming the angle of polarization of the reflected light depending on the conditions.
The researchers’ next task is to understand how the fish are accomplishing this feat.
Cummings said it might be an entirely passive process, with different elements of their skin automatically responding to the angle of the sunlight. Or, the lookdowns may aid in their camouflage by subtly altering their bodies’ orientation relative to the sun, or by neurologically ramping up certain processes.
What the researchers discover about these mechanisms will be of particular interest to the Navy, which at present isn’t as good as lookdowns at open ocean camouflage.
“From an evolutionary biologist viewpoint, I am always excited when evolution is one step ahead of humans,” said Cummings. “There is this problem out there — how to blend in to this environment, and though we haven’t quite solved it yet, an animal has. We can identify these basic biological strategies, and perhaps materials scientists can then translate them into useful products for society.”
Photograph by Eric Lemar/Shutterstock.
Scientists from the Hebrew University of Jerusalem and the Technion-Israel Institute of Technology have discovered why Heteroxenia corals pulsate. Their work, which resolves an old scientific mystery, appears in the current issue of PNAS.
One of the most fascinating and spectacular sights in the coral reef of Eilat is the perpetual motion of the tentacles of a coral called Heteroxenia (Heteroxeniafuscescens). Heteroxenia is a soft coral from the family Xeniidae, which looks like a small bunch of flowers, settled in the reef walls and on rocky areas on the bottom of the reef. Each “flower” is actually a living polyp, the basic unit which comprises a coral colony. Apparently, the motion of these polyps, resembling flowers that are elegantly spreading out and closing up their petals, is unique in the animal kingdom.
Except for the familiar swimming motion of jellyfish, no other bottom-attached aquatic animal is known to perform such motions. Pulsation is energetically costly, and hence there must be a reasonable benefit to justify this motion.
The perpetual motions of jellyfish serve them for swimming, predation and feeding. The natural explanation would be that that the Heteroxenia’s spectacular motions are used for predation and feeding, however several studies indicate that these corals do not predate on other animals at all. If predation is not the reason for pulsating, there must be another explanation to justify the substantial energetic expense by the Heteroxenia.
Maya Kremien found the answers to these questions, while working on her master’s research at the Interuniversity institute for Marine Sciences in Eilat under the supervision of Prof. Amatzia Genin from the Hebrew University and Prof. Uri Shavit from the Technion in a joint research funded by the National Science Foundation.
After watching several coral colonies with an underwater infrared-sensitive camera night and day, the researchers found their first surprising discovery: Heteroxenia corals cease to pulsate and take a half-hour break every single day in the afternoon hours. At this stage, the afternoon “siestas” remained unexplained.
The labs of Prof. Genin and Prof. Shavit conduct work on the interaction between biological processes of aquatic creatures and the water motions which surround them. Apparently aquatic animals affect the flow and at the same time are absolutely dependent on that flow. In order to solve the mystery of the Heteroxenia coral, the research team developed (as part of Ph.D. work by Tali Mass) an underwater measuring device called PIV (particle imaging velocimetry), which allows measurement of the flow field just around the coral very accurately. The system consists of two powerful lasers, an image capturing system and computation ability. A special set of lenses releases a sheet of light in short, powerful pulses so that the imaging system can capture pairs of snapshots of natural particles moving with the flow. The computational system then performs a mathematical analysis of the pairs of photos, producing a huge database of flow field maps, from which the flow speed, characteristics of solutes transport, and turbulent mixing intensity are calculated.
The measurements were performed at night with the support of divers who volunteered to assist the research team. It was found that if a diver lightly touched the coral, the polyps “close” and remain motionless for a few minutes, after which the coral returns to its normal pulsation activity. The researchers used this behavior in order to repeatedly measure the flow field around the Heteroxenia during pulsation and rest.
These measurements led to the research group’s next discovery. Analysis of the direction of water flow indicated that the motion of the polyps effectively sweeps water up and away from the coral tissues into the ambient water. Corals need carbon-dioxide during daytime and oxygen during nighttime, as well as nutrients (such as phosphate and nitrogen) during day and night. One of the challenges for coral colonies is to render their surrounding waters rich in essential commodities by efficiently mixing the water around them.
By using the sophisticated measuring system, the researchers calculated the mixing intensity of the water as a result of the coral’s pulsation. The unexpected discovery was that even though the polyps’ motions are uncoordinated (i.e. each polyp starts its period of motion at a different time), the accumulated effect of the polyps’ activity is a significant enhancement of the flow around the colony, particularly in the upward direction which sweeps water away from the coral, hence reducing the probability of re-filtration of the same water.
However, these findings still did not yet answer the question of why a coral would invest so much energy to move its tentacles. After receiving a permit from the Israel Nature and Parks Authority, the research team collected a few Heteroxenia colonies from the sea in order to run a series of laboratory experiments. All corals were returned back to their original location after the experiment terminated. The Hypothesis was that the pulsation motions enhance the coral’s photosynthesis rate.
Corals are among the most ancient creatures surviving on our planet. One of the “secrets” of their amazing survival abilities is that they “host” photosynthetic algae in their tissues. The symbiotic algae provides the coral with essential nutrients and lives off the waste of the coral.
In a previous study of the same research team (which the results of were also published in PNAS) it was found that the motion of water around corals is essential in order to enhance the efflux of oxygen from the coral tissues. Without water motion, the oxygen concentration in the coral tissues would rise and the photosynthesis rate would drop.
The answer to the question as to why the Heteroxenia pulsates was finally revealed through the lab experiments. First, the photosynthesis rate of a pulsating Heteroxenia was measured, and it was found to be on an order of magnitude higher than that of a non-pulsating colony. Next, in order to prove that the mechanism of pulsation is intended to sweep away oxygen, the researchers artificially increased the oxygen concentration in the measurement chamber so that even when the coral managed to mix water via pulsation, it was replacing oxygen-rich water with new water, which, unfortunately for the coral , was also rich in oxygen. And indeed it was found that the photosynthesis rate was low in this case, and even when the coral was constantly pulsating, the oxygen concentration remained high and photosynthesis remained low, as if the coral was at rest (i.e. not pulsating).
The elegant motion of Heteroxenia has been fascinating the scientific society and capturing the attention of researchers for nearly 200 years (Jean-Baptiste Lamarck, 1744-1829), yet it has not been explained. Now, in the study of Kremien, Genin and Shavit, it was found that the pulsation motions augment a significant enhancement in the binding of carbon dioxide to the photosynthetic enzyme RuBisCo, also leading to a decrease in photorespiration. This explanation justifies the investment of energy in pulsation — the benefit overcomes the cost. In fact, thanks to pulsation, the ratio between photosynthesis to respiration in Heteroxenia is the highest ever measured in stony and non-pulsating soft corals.
The findings of this study indicate that pulsation motions are a highly efficient means for sweeping away water from the pulsating body, and for an increased mixing of dissolved matter between the body and the surrounding medium. These two processes (expulsion of medium and mixing of solutes) may lead to future applications in engineering and medicine. Currently the research group is focusing on attempts to broaden the results of this study and on developing mathematical models which could serve various applicative purposes.
Source: Science Daily
Photograph by Aleks.K/Shutterstock.
Newcastle Researchers Leapfrog Ahead in World-First
University of Newcastle researchers have successfully developed a method to freeze frog embryonic cells in a world-first breakthrough that could slow the threat of extinction to hundreds of frog species.
The researchers have separated, isolated and frozen the embryonic cells of an Australian Ground Frog (the Striped Marsh Frog, Limnodynastes peronii), using cryopreservation techniques that will now allow for cloning.
This is the first time anyone in the world has successfully used slow-freezing techniques on amphibian cells, project leader at the University of Newcastle, Professor Michael Mahony, said.
“Almost 200 frog species have been lost in the past 30 years due to disease and a further 200 species face imminent threat – this is the worst rate of extinction of any vertebrate group,” he said.
“Amphibian eggs and early embryos, unlike human eggs and embryos, are large in size and have traditionally presented a challenge to researchers attempting to cryo-preserve and store frog genomes, as they would shatter during the freezing process.
“The new technique, developed by our University of Newcastle researchers, will act as an insurance policy to buy us time for species on the edge of extinction, as we search for answers to diseases and other threats.”
Professor Mahony said the development would have wider implications for other species facing extinction.
“Not only will it help us preserve the genetic diversity of frogs, but this discovery could also help in the conservation of other species with large embryonic cells, such as fish.”
The University of Newcastle is leading the world on research into amphibian protection. This latest discovery follows on from recent work with other universities on the Lazarus project, which generated live embryos using cells from an extinct Australian frog.
The technical work was led by Dr John Clulow and Professor Michael Mahony, alongside Ms Bianca Lawson and Mr Simon Clulow.
Courtesy of the University of Newcastle.
Two weeks ago, a group of sailors off the coast of New Zealand leaned over the side of their boat, dropped a contraption into the Pacific Ocean and watched it disappear. Using an app they’d downloaded to a smartphone, they logged a reading from the underwater device, along with their GPS location and the water temperature. In just a few minutes’ time, they had become the first participants in a new program launched by the UK’s Plymouth University Marine Institute which allows citizen scientists to help climatologists study the effects of climate change on the oceans.
Photo: Richard Kirby
Researchers writing in the Proceedings of the Royal Society A say they have developed a new robotic fish that has lateral line sensing capabilities.
The FILOSE team members have spent four years investigating fish lateral line sensing, which is a sensing organ found in aquatic vertebrates used to detect movement and vibration in the surrounding water. This organ essentially helps a fish’s orientation in the water. The team set out to understanding how a fish detects and exploits flow features in water, and then use their findings to develop efficient underwater robots based on biological principles.
Flow can be measured and characterized on many salient features that do not change. This “flowscape” is a flow landscape that helps fish and robots orient themselves, navigate, and control their movements in water.
Photo: Prof. Maarja Kruusmaa and FILOSE fish robot. Credit: Jelena Pijonkina