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Tiny Fish Make “Eyes” at Their Killer

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Small prey fish can grow a bigger ‘eye’ on their rear fins as a way of distracting predators and dramatically boosting their chances of survival, new scientific research has found.

Researchers from Australia’s ARC Centre of Excellence for Coral Reef Studies (CoECRS) have made a world-first discovery that, when constantly threatened with being eaten, small damsel fish not only grow a larger false ‘eye spot’ near their tail – but also reduce the size of their real eyes.

The result is a fish that looks like it is heading in the opposite direction – potentially confusing predatory fish with plans to gobble them up, says Oona Lönnstedt, a graduate student at CoECRS and James Cook University.

For decades scientists have debated whether false eyespots, or dark circular marks on less vulnerable regions of the bodies of prey animals, played an important role in protecting them from predators – or were simply a fortuitous evolutionary accident.

The CoECRS team has found the first clear evidence that fish can change the size of both the misleading spot and their real eye to maximise their chances of survival when under threat.

“It’s an amazing feat of cunning for a tiny fish,” Ms Lonnstedt says. “Young damsel fish are pale yellow in colour and have this distinctive black circular ‘eye’ marking towards their tail, which fades as they mature. We figured it must serve an important purpose when they are young.”

“We found that when young damsel fish were placed in a specially built tank where they could see and smell predatory fish without being attacked, they automatically began to grow a bigger eye spot, and their real eye became relatively smaller, compared with damsels exposed only to herbivorous fish, or isolated ones.

“We believe this is the first study to document predator-induced changes in the size of eyes and eye-spots in prey animals.”

When the researchers investigated what happens in nature on a coral reef with lots of predators, they found that juvenile damsel fish with enlarged eye spots had an amazing five times the survival rate of fish with a normal-sized spot.

“This was dramatic proof that eyespots work – and give young fish a hugely increased chance of not being eaten.

“We think the eyespots not only cause the predator to attack the wrong end of the fish, enabling it to escape by accelerating in the opposite direction, but also reduce the risk of fatal injury to the head,” she explains.

 The team also noted that when placed in proximity to a predator the young damsel fish also adopted other protective behaviours and features, including  reducing activity levels, taking refuge more often and developing a chunkier body shape less easy for a predator to swallow.

 “It all goes to show that even a very young, tiny fish a few millimetres long have evolved quite a range of clever strategies for survival which they can deploy when a threatening situation demands,” Ms Lonnstedt says.

Their paper “Predator-induced changes in the growth of eyes and false eyespots” by Oona M. Lonnstedt, Mark I. McCormick and Douglas P. Chivers appears in the latest issue of the journal Scientific Reports.

Source: Australia’s ARC Centre of Excellence for Coral Reef Studies (CoECRS)

Posted August 20th, 2013.

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Clawing to Get the Girls

male guppy with genitalia marked

Photograph courtesy of the University of Toronto.

TORONTO, ON – Some males will go to great lengths to pursue a female and take extreme measures to hold on once they find one that interests them, even if that affection is unrequited. New research from evolutionary biologists at the University of Toronto shows that the male guppy grows claws on its genitals to make it more difficult for unreceptive females to get away during mating.

Genitalia differ greatly in animal groups, even among similar species – so much so that even closely related species may have very different genitalia. The reasons for these differences are unclear but sexual conflict between males and females may be a source. Sexual conflict occurs when the fitness interests of males and females differ, which is rooted in differences in egg and sperm sizes. Males invest less than females in reproduction because sperm is cheap to produce, and larger eggs are most costly to make. This difference results in a conflict in which males are interested in mating with as many females possible but females are more selective with their mates.

The researchers examined the role of a pair of claws at the tip of the gonopodium of the male guppy (Poecilia reticulata) – essentially the fish’s penis.

“Our results show that the claws are used to increase sperm transfer to females who are resisting matings,” says Lucia Kwan, PhD candidate in U of T’s Department of Ecology and Evolutionary Biology and lead author of a paper published this week in Biology Letters. “This suggests that it has evolved to benefit males at the expense of females, especially when their mating interests differ.”

The researchers tested two ideas for the function of the claws – one for their role in securing sperm in place at the tip of the gonopodium just before it is inserted into the female, the other for grasping unreceptive or resistant females during mating to aid in sperm transfer. For the latter, Kwan, former graduate student Yun Yun Cheng and faculty members Helen Rodd and Locke Rowe used a phenotypic engineering approach. They surgically removed the pair of claws from one set of males and compared the amount of sperm transferred by them with a group of males who hadn’t been declawed after they had all mated with receptive or unreceptive females.

“Clawed males transferred up to three times more sperm to unreceptive females compared to declawed males,” says Kwan. “The claw has evolved to benefit the males at the expense of females, and implicates sexual conflict between the sexes in the diversification of the genitalia in this family of fish. This provides support that this important selective force is behind an evolutionary pattern that evolutionary biologists have been trying to unravel for over a century.”

Source: EurekAlert 

Posted July 25th, 2013.

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How Fish Swim

How do fish swim? It is a simple question, but there is no simple answer.

Researchers at Northwestern University have revealed some of the mechanical properties that allow fish to perform their complex movements. Their findings, published on June 13 in the journal PLOS Computational Biology, could provide insights in evolutionary biology and lead to an understanding of the neural control of movement and development of bio-inspired underwater vehicles.

“If we could play God and create an undulatory swimmer, how stiff should its body be? At what wave frequency should its body undulate so it moves at its top speed? How does its brain control those movements?” said Neelesh Patankar, professor of mechanical engineering at Northwestern’s McCormick School of Engineering and Applied Science. “Millennia ago, undulatory swimmers like eels that had the right mechanical properties are the ones that would have survived.”

The researchers used computational methods to test assumptions about the preferred evolutionary characteristics. For example, species with low muscle activation frequency and high body stiffness are the most successful; the researchers found the optimal values for each property.

“The stiffness that we predict for good swimming characteristics is, in fact, the same as the experimentally determined stiffness of undulatory swimmers with a backbone,” said Amneet Bhalla, graduate student in mechanical engineering at McCormick and one of the paper’s authors.

“Thus, our results suggest that precursors of a backbone would have given rise to animals with the appropriate body stiffness,” added Patankar. “We hypothesize that this would have been mechanically beneficial to the evolutionary emergence of swimming vertebrates.”

In addition, species must be resilient to small changes in physical characteristics from one generation to the next. The researchers confirmed that the ability to swim, while dependent upon mechanical parameters, is not sensitive to minor generational changes; as long as the body stiffness is above a certain value, the ability to swim quickly is insensitive to the value of the stiffness, the researchers found.

Finally, making a connection to the neural control of movement, the researchers analyzed the curvature of its undulations to determine if it was the result of a single bending torque, or if precise bending torques were necessary at every point along its body. They learned that a simple movement pattern gives rise to the complicated-looking deformation.

“This suggests that the animal does not need precise control of its movements,” Patankar said.

To make these determinations, the researchers applied a common physics concept known as “spring mass damper” — a model, applied to everything from car suspension to Slinkies, that determines movement in systems that are losing energy — to the body of the fish.

This novel approach for the first time unified the concepts of active and passive swimming — swimming in which forcing comes from within the fish (active) or from the surrounding water (passive) — by calculating the conditions necessary for the fish to swim both actively and passively.

The paper, “A Forced Damped Oscillation Framework for Undulatory Swimming Provides New Insights into How Propulsion Arises in Active and Passive Swimming,” was authored by Patankar, Bhalla, and Boyce E. Griffith, assistant professor of medicine and mathematics at New York University.

The work was supported by the National Science Foundation (NSF).

Source: McCormick

Posted June 25th, 2013.

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New Blenny Discovered

Photograph by D. Ross Robertson and Carole C. Baldwin, CC-BY 3.0.

A new species of tiny blenniiform fish has been discovered in the biodiversity rich waters of the southern Caribbean. Haptoclinus dropi is only around 2 cm in length with a beautiful color pattern that includes iridescence on the fins. The proposed common name of the species is four-fin blenny, due to the division of the dorsal fin into four sections, which is a distinguishing feature of the genus and unique among blenniiform fishes. The study was published in the open access journal Zookeys.

This beautiful new species was discovered as a lucky bycatch during targeted specimen catching at 157-167 m depth off Curaçao as a part of the Smithsonian Institution’s Deep Reef Observation Project (DROP). The new species, Haptoclinus dropi, gets its name from the project’s abbreviation and is one of numerous new ray-finned fish species emerging from this project.

For DROP expeditions the Substation Curaçao‘s manned submersible Curasub was used to catch specimens. While generally used as tourist attraction because it travels at much greater depths than divers can reach, the Curasub is also used for scientific marine research. Targeted fish specimens are collected with the sub’s two flexible, hydraulic arms, but very often small non-targeted fish are also caught in the process.

“Below the depths accessible using scuba gear and above the depths typically targeted by deep-diving submersibles, tropical deep reefs are productive ocean ecosystems that science has largely missed. They are home to diverse assemblages of new and rare species that we are only just beginning to understand,” explains the lead author of the study Dr Carole Baldwin, Smithsonian Institution.

Source: Pensoft Publishers

Original Source: Baldwin CC, Robertson RD (2013) A new Haptoclinus blenny (Teleostei, Labrisomidae) from deep reefs off Curaзao, southern Caribbean, with comments on relationships of the genus.ZooKeys 306: 71–81, doi: 10.3897/zookeys.306.5198

Posted June 7th, 2013.

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New Way to Camouflage

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.”

Source: EurekAlert!

Posted June 6th, 2013.

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