Photograph by Radim Blazek, Matej Polacik and Martin Reichard.
African annual fish take the adage ‘live fast, die young’ to a whole new level with the discovery that their short lifespan is accompanied by the most rapid sexual maturation of any vertebrate species. The find, reported in the open access journal EvoDevo as part of a series on extreme environments, adds to our knowledge of extremophile lifestyles.
Extreme environments can give rise to extreme adaptations. The tiny annual fish of Africa live in temporary puddles created by seasonal rainfall, and so must grow and reproduce quickly in order to lay their hardy eggs before the waters dry up.
African annual fish can grow up to 23% of their body length in a day, report Martin Reichard and colleagues, who studied wild-caught fish in captivity. One species, Nothobranchius kadleci started reproducing at 17 days old, at a size of just 31 mm, with a related species, N. furzeri maturing only one day later. The fish then produced eggs that developed to the hatching stage in as few as 15 days, making the time from one generation to the next as little as month – the most rapid sexual maturation time and minimum generation time of any known vertebrate species.
When the pools dry up, dormant embryos can survive in the dried mud for months, until the next rains come and the life cycle begins again. In the lab, half of embryos skipped dormancy when incubated on a peat substrate in a Petri dish. In the wild these individuals would populate secondary pools produced within a single rainy season after the primary pool desiccated. The findings suggest that rapid growth and maturation do not compromise subsequent fecundity.
Animals with a long life span can afford to take things slow. The tiny cave-dwelling salamander, olm (Proteus anguinus), which lives for over 100 years, takes 16 years to reach sexual maturity. But when the risk of mortality is high or lifespan shorter, animals reach sexual maturity earlier. The tiny goby, Schindleria, and females of house mouse lab strains (Mus musculus) become sexually mature at just 23 days old.
Earlier studies of a laboratory strain of an African annual fish suggested that it took the fish four weeks to mature, but this may have been an over-estimate. Previous reports of early maturation were based on anecdotal evidence, but this study is based on quantitative data and demonstrates that the rapid growth rate in the lab is still an underestimate compared to that in the wild.
Source: BioMed Central
Posted September 6th, 2013. Add a comment
Much as human siblings can have vastly different personalities despite their similar resemblance and genetics, two closely related species of electric fish from the Amazon produce very different electric signals. These species, new to science, are described in the open access journal ZooKeys by Drs. John Sullivan of Cornell University in Ithaca, New York, Jansen Zuanon of theNational Amazonian Research Institute in Manaus, Brazil and Cristina Cox Fernandes of the University of Massachusetts, Amherst.
The two new species are bluntnose knifefish, genus Brachyhypopomus, that live under rafts of unrooted grasses and water hyacinth along the margins of the Amazon River called “floating meadows.” These are weakly electric relatives of South America’s famous electric “eel” (not a true eel) that can produce strong electric discharges of hundreds of volts. By contrast, these weakly fishes produce pulses of only a few hundred millivolts from an organ under the body that extends out onto a filamentous tail. Nearby objects in the water create distortions to the electric field that are sensed by receptor cells on the fishes’ skin. In this way, they are able to “electrolocate” through their complex aquatic environment at night. Their short electric pulses, too weak to be sensed by touch, are also used to communicate the sender’s species identity and gender to other electric fishes.
“The most striking differences between these two similar species have to do with their electric organs and their electric organ discharges, or EODs,” says lead author John Sullivan, Curatorial Affiliate at theCornell University Museum of Vertebrates. “If it weren’t for these traits, we undoubtedly would have thought they were a single species. The one we are calling Brachyhypopomus bennetti has a huge electric organ, a short, fat tail, and produces a monophasic EOD; the other one that we’re calling Brachyhypopomus walteri has a more typical electric organ, a long thin tail, and a more typical biphasic EOD.”
It turns out the monophasic EOD of the new species Brachyhypopomus bennetti is highly unusual. Most species of this kind of knifefish produce EOD waveforms with both a positive and negative phase to them, as viewed on an oscilloscope: essentially alternating current. In this way, there is no net positive or negative current generated by the signal. “All of this fish’s relatives, including its newly described sister species, have biphasic EODs,” says Sullivan. “For that reason we know that this trait evolved in this species’ lineage. The interesting question is why.”
One widely accepted idea is that the biphasic EOD with its reduced amount of direct current (DC) is an adaptation to hide from predatory fish, like catfishes and electric eels, that are equipped with a type of electroreceptor that are sensitive to DC. So why would one species seemingly court danger by evolving a monophasic EOD?
The only other electric fish in the Amazon with a similar monophasic EOD is the fearsome electric eel. This fish has both a weak EOD used for electrolocation and communication as well as a much more powerful EOD used to stun prey and for defense. A theory proposed by Dr. Philip Stoddard of Florida International University contends that, in much the same way that the Viceroy butterfly—a species tasty to birds—evolved wing color patterns to mimic the distasteful Monarch butterfly, the harmless B. bennetti ‘s EOD waveform evolved to mimic that of the electric eel, a species electroreceptive predatory fishes may have learned to avoid.
In this paper, the authors suggest an additional possible benefit of of B. bennetti’s monophasic EOD. Unlike biphasic species, B. bennetti’s EOD waveform is largely unaffected after their tails are partially bitten off by predators, a common type of injury in this species. They suggest that this species’ preference for floating meadow habitat near river channels may put them at particularly high risk of predation and ‘tail grazing’ by other fishes.
The authors show that the EOD waveforms of Brachyhypopomus species with biphasic EODs are severely altered after such injuries, whereas those of B. bennetti are not. “Any change to the EOD waveform likely impairs electroreception and communication and the monophasic EOD waveform may have been favored by natural selection in a species that suffers a lot of tail injuries,” says Sullivan. “Selection for both EOD stability and mimicry of electric eels could be going on simultaneously…both hypotheses make predictions that should be tested,” said Sullivan.
Source: Pensoft Publishers
Posted September 4th, 2013. Add a comment
Photograph by Dr. Dwight Smith.
Scientists are taking the public with them to study the world’s coral reefs, thanks to 360 degree panoramas from Google’s underwater street-view format. Results from this pioneering project – which will allow ecologists to harness people power to discover how coral reefs are responding to climate change – will be presented at INTECOL, the world’s largest international ecology meeting, in London this week.
Professor Ove Hoegh-Guldberg of the University of Queensland leads the research associated with the Catlin Seaview Survey. The Survey uses image recognition technology to automatically assess creatures on the seabed; so far it has already taken hundreds of thousands of images on the Great Barrier Reef and in the Caribbean.
“This new technology allows us to rapidly understand the distribution and abundance of key organisms such as corals at large scales. Our expeditions in 2012 to the Great Barrier Reef recorded over 150 km of reef-scape using these methods,” he says.
The project is now being expanded by building citizen science into the research, which he hopes will raise awareness and provide more data. “We are planning to involve online citizens to help us count a wide range of organisms that appear in the high-definition images. Anyone with access to a computer will be able to help us log creatures such as stingrays, turtles, fish and Crown of Thorns starfish.”
“Only 1% of humanity has ever dived on a coral reef and by making the experience easily accessible the survey will help alert millions of people around the world to the plight of coral reefs,” he says.
Professor Hoegh-Guldberg will also report findings from ground-breaking research on the impact of climate change on the Great Barrier Reef. At Queensland’s Heron Island research station, he has been running the first-ever long-term climate simulation experiments using computer-controlled systems to manipulate carbon dioxide levels and temperature to simulate past, present and future climate conditions around coral reefs.
“Coral reefs have had a hard time adjusting even to the conditions we find ourselves in today with respect to high carbon dioxide levels and sea temperatures. Our work is showing some interesting observations, such as the lack of adaptation of reef communities to the changes that have occurred up until the present,” he explains.
“Worse still, our results show that even under the most moderate climate change projections from the Intergovernmental Panel on Climate Change, most corals will struggle to survive and reefs will rapidly decalcify.”
Exposing coral and their symbiotic microorganisms, known as dinoflagellates, to future ocean conditions is also revealing how these key organisms cope with changes in acidity and temperature.
Professor Hoegh-Guldberg’s experiments show that responses involve the whole organism, not only one or two features of its biology. “The idea that evolution is likely to operate rapidly within these systems is largely unfounded. The more complex the response, the greater the number of biological systems involved, and the greater the number of genes that will have to be changed in coordination to enable organisms to survive,” he says.
Source: Alpha Galileo
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. 1 comment
Zebra danio. Photograph by Oliver Lucanus.
Chronic stress can lead to depression and anxiety in humans. Scientists working with Herwig Baier, Director at the Max Planck Institute of Neurobiology in Martinsried, recently discovered a very similar link in fish. Normally, the stress hormone cortisol helps fish, as in humans, to regulate stress. Fish that lack the receptor for cortisol as a result of a genetic mutation exhibited a consistently high level of stress. They were unable to adapt to a new and unfamiliar situation. The fishes’ behaviour returned to normal when an antidepressant was added to the water. These findings demonstrate a direct causal link between chronic stress and behavioural changes which resemble depression. The findings could also open the door to an effective search for new drugs to treat psychiatric disorders.
In stressful situations, the body releases hormones in order to ready itself for a fight or flight reaction. But it is equally important for the hormone level to return to normal after a certain time. If that does not happen, chronic stress can result, a condition that is linked to depression and anxiety, among other things. Whether stress is a trigger or merely a side effect of such affective disorders remains unclear.
The indication of a causal relationship between stress and depression comes from totally unexpected quarters. An international team led by Herwig Baierfrom the Max Planck Institute of Neurobiology in Martinsried and the University of California in San Francisco observed that zebrafish suffering from chronic stress as a result of a genetic mutation showed signs of depression in behavioural tests. The zebrafish is a popular model organism for biological and medical research. So far, however, it has not been an obvious research object for the study of psychiatric disorders. This may be about to change.
“These mutant fish behaved very strangely when we moved them to a new aquarium,” reports Herwig Baier. All animals experience stress upon moving to an unfamiliar environment. Being separated from members of their own species places the fish under added pressure. Zebrafish initially act withdrawn in this situation and swim around very hesitantly in the first few minutes. But ultimately, curiosity prevails, and they begin to investigate their new tank. However, the fish with the mutation had a particularly strong reaction to the isolation: they sank to the bottom of the tank and stayed completely still. They took an exceptionally long time to get used to the new environment.
An analysis of these “lethargic” fish showed that they had an extremely elevated concentration of the stress hormones cortisol, CRH and ACTH. “We therefore postulated that these fish were suffering from chronic stress and were exhibiting certain aspects of depressive or perhaps hyper-anxious behaviour,” says Baier. To put this assumption to the test, the scientists added the antidepressant fluoxetine (marketed under the trade name Prozac, among others) to the water. Shortly afterwards, the fish’s behaviour returned to normal.
What was it that made these fish so different? The scientists uncovered a mutation in the glucocorticoid receptor, which is present in almost all of the body’s cells and which binds the hormone cortisol. Normally, when cortisol is bound to this receptor it restricts the release of the stress hormones CRH and ACTH. It is this regulating mechanism that enables humans and many animal species to cope with stress. In the type of fish the scientists examined, however, the glucocorticoid receptor was unable to function, and so the level of stress hormones remained high.
“Although there are a whole range of drugs available for depression, no one yet knows what the relationship is between their effect and the stress hormones,” explains Herwig Baier. “Our findings provide the first evidence of a possible connection.” Understanding the molecular and neurobiological relationships between stress regulation and affective disorders is important in the search for new treatments and drugs. The scientists’ discovery is therefore also of interest to the pharmaceutical industry, given that the zebrafish could turn out to be a good model organism for a large-scale screen for new drugs.
Source: Max Planck Institute of Neurobiology
Posted August 6th, 2013. 1 comment
Photograph courtesy of Penn State.
Raising fish in tanks that contain hiding places and other obstacles can make the fish both smarter and improve their chances of survival when they are released into the wild, according to an international team of researchers.
“It’s a key problem in that we are very good at rearing fish, but we’re really not very good at releasing those animals in the wild such that they survive,” said Victoria Braithwaite, professor of fisheries and biology, Penn State. “There’s a mismatch between the way we raise them and the real world.”
Juvenile Atlantic salmon raised in tanks that including pebble and rock hiding places and floating artificial plants were better able to navigate mazes and showed signs of improved brain function compared to the salmon reared in standard hatchery tanks, Braithwaite said. This may help conservation fish hatcheries raise and release fish that are better adapted to survive in the wild.
Conservation fish hatcheries raise cod, salmon, trout and other types of fish and release them in places where their species may be threatened, or where their populations are declining.
“The philosophy of most fish hatcheries is to rear a large number of fish and hope some survive,” said Braithwaite. “What this study is suggesting is that you could raise fewer, but smarter fish, and you will still have higher survivability once you release them.”
The researchers, who released their findings today (July 31) in the Proceedings of the Royal Society B, placed pebbles and rocks at the bottom of the tank and added plastic plants weighted down so they would float vertically in the water. Braithwaite said the objects created a more natural, three-dimensional ecosystem.
“In the hatchery the world is homogenous, life is boring and monotonous,” Braithwaite said. “The water flow is the same, you don’t have to find your food and you don’t have to avoid predators.”
The researchers also moved the objects around about once a week during the eight-week study, which took place in Norway.
When the researchers placed the salmon in a maze, the fish raised in the enriched tanks made fewer mistakes when trying to escape the maze, Braithwaite said. The performance of the salmon from the enriched tank continued to improve with each trial, and they learned to solve the maze much faster than fish reared the standard way.
The brains of the fish from the enriched tank were also different from the fish raised in the standard hatchery tanks, according to the researchers.
They noted increased expressions of a gene in a region of the fish’s brain that is associated with learning and memory, an indication of increased brain function and growth. The fish raised in standard tanks did not show this sign of increased brain development.
Interacting with the environment can influence gene expression in the brain, Braithwaite said.
“The brain is a very plastic organ, it’s a dynamic structure,” said Braithwaite, who worked with Ann Gro Vea Salvanes, professor of biology; Olav
Moberg, doctoral student; Tome Ole Nilsen, researcher in marine development biology; Knut Helge Jensen, senior engineer in evolutionary ecology, all at the University of Bergen, Norway; and Lars O.E. Ebbesson, group leader of integrative fish biology, Uni Research, Bergen. Braithwaite said the enriched tanks created significant improvement in the intelligence and adaptability of the fish, but were relatively inexpensive and easy to implement. Owners of fish hatcheries should be able to afford the creation of enhanced tanks.
Photograph courtesy of Zookeys.
The new species Echinophyllia tarae is described from the remote and poorly studied Gambier Islands, French Polynesia. Although the new species is common in the lagoon of Gambier Islands, its occurrence elsewhere is unknown. Echinophyllia tarae lives in protected reef habitats and was observed between 5 and 20 m depth. It is a zooxanthellate species which commonly grows on dead coral fragments, which are also covered by crustose coralline algae and fleshy macroalgae.
This species can grow on well illuminated surfaces but also encrusts shaded underhangs and contributes to the formation of coral reefs in the Gambier. It is characterized by large polyps and bright often mottled colourations and it is very plastic in morphology like most hard corals. Patterns of partial death and recovery of the species were often observed and could be due to competition with other benthic invertebrates like the soft-bodied corallimorpharians or zoanthids which can co-occur with this species.
Stony corals are currently under threat by the effects of global warming, ocean acidification and anthropogenic changes of reef structures. Although corals represent a relatively well studied group of charismatic marine invertebrates, much has still to be understood of their biology, evolution, diversity, and biogeography. The discovery of this new species in French Polynesia confirms that our knowledge of hard coral diversity is still incomplete and that the exploration efforts of recent scientific expeditions like Tara Oceans can lead to new insights in a remote and previously poorly studied locations.
This species is named after the Tara vessel which allowed the exploration of coral reefs in Gambier. Moreover, the name “tara” in the Polynesian language may refer to a spiny, pointed object, which applies well to the new species typically featuring pointed skeletal structures. In the same language, Tara is also the name of a sea goddess.
Source: Benzoni F (2013) Echinophyllia tarae sp. n. (Cnidaria, Anthozoa, Scleractinia), a new reef coral species from the Gambier Islands, French Polynesia. ZooKeys 318: 59, doi:10.3897/zookeys.318.5351
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.”
Posted July 25th, 2013. 1 comment
Photograph by Bluehand/Shutterstock
The mighty tuna is more closely related to the dainty seahorse than to a marlin or sailfish. That is one of the surprises from the first comprehensive family tree, or phylogeny, of the “spiny-rayed fish,” a group that includes about a third of all living vertebrate species. The work is published July 15 in the journal Proceedings of the National Academy of Sciences.
The spiny-rayed fish are an incredibly diverse group, including tuna and billfish, tiny gobies and seahorses, and oddities such as pufferfish and anglerfish. The fish occupy every aquatic environment from coral reefs and open oceans to lakes and ponds. It includes all the major commercially fished species — all of which are threatened. But until now, no one has had any idea exactly how more than 18,000 species in 650 families are related to each other, said Peter Wainwright, professor and chair of evolution and ecology at the University of California, Davis and senior author on the paper.
“There has been a ‘bush’ at the top of the family tree leading to the rest of the vertebrates,” Wainwright said. “Now we have this beautiful phylogeny of one-third of all vertebrates.”
The study also shows that after roaring along for their first 100 million years, the pace of evolution of the spiny-rayed fish downshifted about 50 million years ago.
Some groups of fish have gone along steadily for millennia; others have gone through bursts of rapid evolution. Overall, the researchers found that the rate at which new species formed was fairly constant across the group from their origin to about 50 million years ago, then dropped about five-fold and has remained at that level since.
That might mean that these fish have essentially filled the available spaces, Wainwright said.
“It’s not uncommon in evolution to see a rapid diversification followed by a slowdown, but it’s never been seen on such a scale before,” he said.
Wainwright’s laboratory worked with the lab of Tom Near, a former postdoctoral scholar at UC Davis now at Yale University, and colleagues at the University of Tennessee, The Field Museum in Chicago, Florida Atlantic University and CUNY Staten Island to construct the family tree. Matt Friedman, a paleontologist at the University of Oxford, England, added fossils that helped set dates for branches of the tree.
The researchers looked at 10 genes in more than 500 fish species representing most of the families of spiny-rayed fish. They used the genetic data to construct a tree, grouping related families together. They also looked at the pace of evolution — the rate at which new species formed — in different branches, and across the group as a whole.
The spiny-rayed fish originated about 150 million years ago, separating from more primitive fish, such as lampreys, sharks and sturgeon, and from the ancestors of salmon and trout. Since then, they have spread into every aquatic habitat on Earth.
The tree shows some interesting relationships. For example, tuna are more closely related to seahorses than to swordfish or barracuda. The oddly shaped pufferfishes are related to anglerfish, the only fishes whose bodies are wider than they are deep.
Cichlids, a family that includes about 2,000 species of freshwater fish known for brooding their young in their mouths and a favorite for studies of evolution, are related to the engineer gobies, an obscure family of just two species that live on coral reefs and raise their young in a nest.
Wainwright’s special interest is in the evolution of fish jaws. Fish have two sets of jawbones, an outer jaw and “pharyngeal jaws” in the throat that adapted to different functions. In some fish, the lower pharyngeal jaw is fused into a single solid bone that can be used to crush prey such as shellfish.
Biologists had assumed that this fused jaw had evolved once and then spread into different groups of fish. Instead, the new tree shows that this structure evolved at least six times in different groups of fish.
Source: Alpha Galileo Foundation
The starfish compound eye (red cups) is seen at the tip of the arm. Each red cup corresponds to a single optical unit, ommatidium, in an arthropod compound eye. Photograph courtesy of Dan-Erik Nilsson, Lund University.
A study has shown for the first time that starfish use primitive eyes at the tip of their arms to visually navigate their environment. Research headed by Dr. Anders Garm at the Marine Biological Section of the University of Copenhagen in Denmark, showed that starfish eyes are image-forming and could be an essential stage in eye evolution.
The researchers removed starfish with and without eyes from their food rich habitat, the coral reef, and placed them on the sand bottom one meter away, where they would starve. They monitored the starfishes’ behavior from the surface and found that while starfish with intact eyes head towards the direction of the reef, starfish without eyes walk randomly.
Dr Garm said: “The results show that the starfish nervous system must be able to process visual information, which points to a clear underestimation of the capacity found in the circular and somewhat dispersed central nervous system of echinoderms.”
Analyzing the morphology of the photoreceptors in the starfish eyes the researchers further confirmed that they constitute an intermediate state between the two large known groups of rhabdomeric and ciliary photoreceptors, in that they have both microvilli and a modified cilium.
Most known starfish species possess a compound eye at the tip of each arm, which, except for the lack of true optics, resembles arthropod compound eye. Despite being known for about two centuries, no visually guided behavior has ever been documented before.