Swimming with whale sharks


This 19 May 2020 BBC video says about itself:

Swim With The Biggest Fish In The Ocean | VR 360 | Seven Worlds, One Planet

Whale sharks share the fishermen’s catch in the seas of Indonesia. These gentle giants were once hunted and killed, but here their numbers are on the rise thanks to this special relationship. Stay in and explore their underwater world.

Why darter fish have stripes


This August 2016 video from the USA says about itself:

Darters are a small, wonderfully colored bottom-dwelling fish that looks more suited to live in a coral reef or tropical aquarium than the cold fast-flowing streams of the Smokies.

A diversity of darter species in the Park is indicative of the excellent water quality the Smokies has due to its protection from anthropogenic expansion.

The Great Smoky Mountains National Park has over a dozen species of these charismatic fish, and has one of the best-protected habitats for darters in the country.

From the University of Maryland Baltimore County in the USA:

How the darter got stripes: Expanding a sexual selection theory explains animal patterns

May 22, 2020

Summary: Scientists have shown for the first time that there is a strong correlation between the complex patterns on male darters and their highly-variable environments. The findings support and expand upon sensory drive theory, which states that the environment influences which sexual signals, like visual patterns, are selected for. Previous sensory drive research looked at simple signals (e.g. colors), but Hulse used Fourier analysis to greatly expand that work.

Samuel Hulse, a Ph.D. candidate at UMBC, spent a lot of time in waders over the last two years. He traipsed from stream to stream across the eastern U.S., carefully collecting live specimens of small, colorful freshwater fish known as darters and taking photos of their habitats. Then he brought them back to the lab to capture high-quality images of their coloration patterns.

Hulse developed a precise, quantitative analysis of those visual patterns, such as stripes, spots, and various mottled looks. His work shows, for the first time, a strong correlation between the complicated patterns on male fish and the fishes’ highly variable environments. The results were published today in Nature Communications.

These findings represent a major expansion of a theory in sexual selection known as “sensory drive”, which emphasizes how an animal’s environment can influence what sexual signals — like visual patterns — are selected for over time.

Driving progress

So far, sensory drive has successfully explained examples such as coloration in cichlids, a group of freshwater fish in Africa. Hulse was working to expand on this research.

Different species of cichlids live at different depths, and which colors the fish can easily see changes as you go deeper and there is less light. Why does this matter? The idea of sensory drive is that animals perceive visual signals, like colors, as more attractive when they are easier for their brains to process. And which signals are easier to process is dependent on the environment. When male fish are perceived as more attractive, they are more likely to reproduce, and their colors are more likely to be passed to the next generation of fish. So, if the theory of sensory drive is true, eventually, most male fish will have colors that are easy for mates to perceive in their particular environment.

In cichlid fish, “you see this depth-dependent change in the male colors as you go deeper,” Hulse says. With the new work, “we were able to expand on this theory to explain more complicated traits, such as visual patterns,” like stripes and spots.

Using math to understand biology

Hulse, who is also taking courses toward an M.S. in mathematics at UMBC, brought his quantitative skills to bear on this research. He used a measure called Fourier analysis to examine his fish images, looking at variations in color contrast.

For example, if you were to look at a photo of a grassy hill under a bright blue sky, the greatest contrast in brightness would be between the large areas above and below the horizon line. That contrast is on a larger scale than the differences in brightness between, say, tiny blades of grass. The differences between each blade are small, but occur frequently across the image.

Fourier analysis can translate the contrast patterns in an image into a representative set of mathematical sine and cosine waves. The low-frequency waves, which only swoop up and down once or twice across the entire image, represent large-scale differences, like above and below the horizon. High-frequency waves swoop up and down many times across an image and represent small-scale differences, like between blades of grass.

Researchers can look at the relationships between those waves — how much high-frequency versus low-frequency contrast there is in the image. Hulse’s work looked at that measure to examine the visual relationship between a habitat and the fish that lived in it. And sure enough, his calculations revealed a strong correlation, providing evidence of sensory drive in male darters.

Moving past “wishy-washy terminology”

One argument against the idea that these patterns are attractive to females is the idea of camouflage. Wouldn’t it make sense for animals to match the visual patterns of their environment to avoid getting eaten rather than to attract females? Darters are under strong predation pressure, so, Hulse says, it’s a valid point.

However, the fact that he found that only male fish match their environment is a strong argument in favor of sensory drive. Predators don’t discriminate between males and females, so you would expect females to also match their environment if camouflage was the reason.

“Quantitatively describing visual patterns is a big challenge, and there’s not one easy way to do that, so being able to use tools like Fourier analysis is wonderful,” Hulse says. “That actually lets us quantify some of these things that have historically been very hard to describe other than with wishy-washy terminology.”

Perfect timing

Tamra Mendelson, professor of biological sciences, is Hulse’s advisor and a co-author on the new paper. She had just begun formulating the ideas for this research with visual ecologist Julien Renoult, a colleague at Centre National de la Recherche Scientifique (CNRS) in Montpellier, France, and another co-author, when Hulse joined her laboratory in 2016.

“Julien had inspired me to take concepts from a field called human empirical aesthetics, which is the mathematical and biological basis of human appreciation of art, and apply them to animals’ appreciation of other animals,” Mendelson says. “I was super excited about it, but I didn’t have the mathematical chops to really take it as far as it could go.”

So, when Hulse arrived, “It was a perfect match. Sam is the ideal student to be doing this project.”

Hulse also spent several months in France working with Renoult to iron out some of the statistical challenges of the work — which were many. “The data analysis became a lot more complicated than we thought, and there were a lot of technical snags,” Hulse says. “So it was really great to be able to be there working directly with Julien, who has a lot of background with these sorts of methods.”

Bringing it all together

Hulse was drawn to this work by the unique blend of skills it requires. “I love the interdisciplinary nature of it. We’re bringing together field biology, sensory biology, a little bit of neurobiology, and image analysis,” he says. “That’s one of the most attractive things about this project for me — how much I get to learn and how much I get to take little pieces from so many different areas.”

Now, Hulse, Mendelson, and Renoult are excited to see where their new work leads. “There’s not a lot of theory in sexual selection that can be used to explain why you see one pattern evolve in one animal where you see a different one evolve in a closely related species,” Hulse says.

The new findings open the door to much more exploration with different species, including animals that live on land. In any group of animals that relies on vision, has visually distinct environments, and where the animals have distinct habitat preferences, Hulse argues, “this theory should hold.”

Japanese rice fish, male or female?


This 21 September 2019 video says about itself:

This is my new DIY Japanese Rice Fish mini pond. Japanese Rice Fish are called Medaka here in Japan and they are one of the most common pet fish here.

Rice fish look similar to guppies but they are actually pretty different in the way that these lay eggs whereas guppies are livebearers giving birth to live young. Adding to this, Japanese Rice Fish are super hardy. This doesn’t mean that you should neglect them but it means that they are very tolerant to fluctuating water conditions, they don’t require a filter, an air pump, or a heater.

From Nagoya University in Japan:

The ins and outs of sex change in medaka fish

May 21, 2020

Larval nutrition plays a role in determining the sexual characteristics of Japanese rice fish, also called medaka (Oryzias latipes), report a team of researchers led by Nagoya University. The findings, published in the journal Biology Open, could further understanding of a rare condition in humans and other vertebrates, where they genetically belong to one sex but also have characteristics of the other.

Decades ago, scientists found that medaka fish often undergo sex reversal in the wild. This involves genetically female larvae (meaning they have two X chromosomes) going on to develop male characteristics, or vice versa. This has made medaka fish a model organism for studying environmental sex development and other biological processes they have in common with vertebrates.

Now, Nagoya University reproductive biologist Minoru Tanaka and colleagues in Japan have gained further insight into the factors that affect medaka sex reversal, potentially providing direction for future research into similar conditions in other species.

Scientists had already discovered that environmental factors, such as temperature changes in the brackish and fresh waters where medaka fish live, are likely involved in their sex reversal. Tanaka and his team wanted to know if nutrition also played a role.

They starved medaka larvae for five days. This was enough time to affect their metabolism without killing them. Three to four months later, the team examined the fish and found that 20% of the genetically female medaka had developed testes and characteristically male fins. The same did not occur in larvae that were not starved.

Further tests showed that sex reversal in the fish was associated with reduced fatty acid synthesis and lipid levels. Specifically, starvation suppressed a metabolic pathway that synthesizes an enzyme called CoA, and disrupted a gene called fasn. These disruptions led to reductions in fatty acid synthesis. The scientists also found that a male gene, called dmrt1, was involved in the female-to-male reversal.

“Overall, our findings showed that the sex of medaka fish is affected by both the external environment and internal metabolism,” Tanaka says. “We believe lipids may represent a novel sex regulation system that responds to nutritional conditions.”

The team next plans on identifying other internal factors involved in medaka sex reversal. Future research should try to find the tissues or organs that sense changes in the internal environment and then produce key metabolites to regulate sex differentiation.

First fossil great white shark nursery discovered


This September 2014 video says about itself:

Scientists discover a great white shark pupping ground in the Sea of Cortez.

From the University of Vienna in Austria:

First fossil nursery of the great white shark discovered

Paleo-kindergarten ensured evolutionary success millions of years ago

May 22, 2020

Summary: An international research team discovered the first fossil nursery area of the great white shark, Carcharodon carcharias in Chile. This discovery provides a better understanding of the evolutionary success of the largest top predator in today’s oceans in the past and could contribute to the protection of these endangered animals.

The great white shark is one of the most charismatic, but also one of the most infamous sharks. Despite its importance as top predator in marine ecosystems, it is considered threatened with extinction; its very slow growth and late reproduction with only few offspring are — in addition to anthropogenic reasons — responsible for this.

Young white sharks are born in designated breeding areas, where they are protected from other predators until they are large enough not to fear competitors any more. Such nurseries are essential for maintaining stable and sustainable breeding population sizes, have a direct influence on the spatial distribution of populations and ensure the survival and evolutionary success of species. Researchers have therefore intensified the search for such nurseries in recent years in order to mitigate current population declines of sharks by suitable protection measures. “Our knowledge about current breeding grounds of the great white shark is still very limited, however, and palaeo-nurseries are completely unknown,” explains Jaime Villafaña from the University of Vienna.

He and his colleagues analysed statistically 5 to 2 million-year-old fossil teeth of this fascinating shark, which were found at several sites along the Pacific coast of Chile and Peru, to reconstruct body size distribution patterns of great white shark in the past. The results show that body sizes varied considerably along the South American paleo-Pacific coast. One of these localities in northern Chile, Coquimbo, revealed the highest percentage of young sharks, the lowest percentage of “teenagers.” Sexually mature animals were completely absent.

This first undoubted paleo-nursery of the Great White Shark is of enormous importance. It comes from a time when the climate was much warmer than today, so that this time can be considered analogous to the expected global warming trends in the future. “If we understand the past, it will enable us to take appropriate protective measures today to ensure the survival of this top predator, which is of utmost importance for ecosystems,” explains palaeobiologist Jürgen Kriwet: “Our results indicate that rising sea surface temperatures will change the distribution of fish in temperate zones and shift these important breeding grounds in the future.”

This would have a direct impact on population dynamics of the great white shark and would also affect its evolutionary success in the future. “Studies of past and present nursery grounds and their response to temperature and paleo-oceanographic changes are essential to protect such ecological key species,” concluded Jürgen Kriwet.

How sharks and rays evolve


This 21 May 2020 video says about itself:

Sharks and Rays by Annie Crawley

Sharks & Rays takes you on a journey to discover the wonders of sharks and rays from around the world. Join underwater photographer, filmmaker and ocean explorer, Annie Crawley to learn all about these amazing creatures. You learn the biology with complex information in easy to understand language.

Exclusive footage will have you diving with schooling hammerhead sharks, observing manta rays feeding, nurse sharks entering a state of tonic immobility, plus you will experience the first Shark Sanctuary in the world while diving in the blue waters of Palau. Whale sharks, hammerheads, great white sharks, electric rays, manta rays, reef sharks, mako sharks, dozens of species of sharks and rays from around our world’s ocean are explored in this program.

From Flinders University in Australia:

Ecosystem diversity drives the origin of new shark and ray species

May 19, 2020

Summary: Biologists how different oceanographic conditions in the Gulf of California and the Baja California Peninsula influenced formation of new species of sharks and rays.

What drives the evolution of new species of sharks and rays? Traditionally, scientists thought it required species to be separated by geographic or spatial barriers, however, a new study of elasmobranchs (the group of sharks and rays) has challenged this expectation — and found evolution is happening faster than many think.

Flinders University evolutionary biologists Dr Jonathan Sandoval-Castillo and Professor Luciano Beheregaray tested how different oceanographic conditions in the Gulf of California and the Baja California Peninsula (Mexico) influenced the formation of new species of guitarfish (genus Pseudobatos).

The team discovered four types, or ‘young species’, of guitarfish that have similar external appearance but are genetically different.

Each type of guitarfish appears to have adapted to one of the four separate regions of the Gulf of California. This promotes environmental tolerances which result in those guitarfish having improved odds for survival and reproduction in the region where they were born.

“We have shown that these four guitarfish species evolved quite quickly from the same common ancestor,” says Dr Jonathan Sandoval-Castillo.

“The process where several new species originate from one ancestor in a relatively short period of time is called adaptive radiation, and this is the first report of such a process in sharks and rays. Our results help changing the false popular belief that sharks and rays do not evolve, or only evolve very slowly,” says Prof Luciano Beheregaray.

These findings also have important implications for the management of exploited elasmobranch species, such as guitarfish in the Gulf of California which represents an important fishery for Mexico.

If these young species adapt and evolve to their local habitat conditions, they cannot be replaced by migrants from other habitats.

“If such species are incorrectly managed as a single stock, it can result in the over-exploitation and possibly extinction of the entire species.”

How giant prehistoric fish Titanichthys fed


This 30 December 2018 video says about itself:

Titanichthys is a genus of giant, aberrant marine placoderm from shallow seas of the Late Devonian of Morocco, Eastern North America, and possibly Europe. Many individuals of the species approached Dunkleosteus in size and build.

Unlike its relative, however, the various species of Titanichys had small, ineffective-looking mouth-plates that lacked a sharp cutting edge. It is assumed that Titanichthys was a filter feeder that used its capacious mouth to swallow or inhale schools of small, anchovy-like fish, or possibly krill-like zooplankton, and that the mouth-plates retained the prey while allowing the water to escape as it closed its mouth

From the University of Bristol in England:

Ancient giant armored fish fed in a similar way to basking sharks

May 19, 2020

Scientists from the University of Bristol and the University of Zurich have shown that the Titanichthys — a giant armoured fish that lived in the seas and oceans of the late Devonian period 380-million-years ago — fed in a similar manner to modern-day basking sharks.

Titanichthys has long been known as one of the largest animals of the Devonian — its exact size is difficult to determine, but it likely exceeded five metres in length; like in the basking shark, its lower jaw reached lengths exceeding one metre. However, unlike its similarly giant contemporary Dunkleosteus, there is no previous evidence of how Titanichthys fed.

Where the lower jaw of Dunkleosteus and many of its relatives had clear fangs and crushing plates, the lower jaw of Titanichthys is narrow and lacking any dentition or sharp edges suitable for cutting.

Consequently, Titanichthys has been presumed to have been a suspension-feeder, feeding on minute plankton by swimming slowly with the mouth opened widely through water to capture high concentrations of plankton — a technique called continuous ram feeding.

However, this has remained uncertain, as no fossilised evidence of suspension-feeding structures such as elongate projections that cover the gills in modern suspension-feeding fish has ever been found.

Instead, the team sought to investigate the question indirectly, using biomechanical analysis to compare the lower jaw of Titanichthys with those of other species. Their findings are reported today in the journal Royal Society Open Science.

Lead author Sam Coatham carried out the research while studying for his masters in palaeobiology at the University of Bristol’s School of Earth Sciences.

He said: “We have found that Titanichthys was very likely to have been a suspension-feeder, showing that its lower jaw was considerably less mechanically robust than those of other placoderm species that fed on large or hard-shelled prey.

“Consequently, those feeding strategies (common amongst its relatives) would probably have not been available for Titanichthys.”

The fossils of Titanichthys used in the study were found in the Moroccan part of the Sahara Desert by co-author Christian Klug, a researcher at the University of Zurich. He added: “When you do fieldwork in the Anti-Atlas, massive skull bones of placoderms can be found quite frequently.”

The team tested the resilience of the jaws by virtually applying forces to the jaws, using a technique called Finite Element Analysis (FEA) to assess how likely each jaw was to break or bend.

This revealed that the lower jaw of Titanichthys was much less resistant to stress and was more likely to break than those of the other placoderm species, such as the famous Dunkleosteus. Therefore, the jaw of Titanichthys probably would not have been able to withstand the higher stresses associated with their strategies of feeding on large prey, which thus exert more mechanical stress on the jaws.

This pattern was consistent in both sharks and whales, with the suspension-feeder proving less resistant to stress than the other species within the same lineage. Further analyses comparing the distribution of stress across the jaws showed similar patterns in Titanichthys and the basking shark, reinforcing this comparison.

It has been established that there were almost certainly giant suspension-feeding vertebrates living 380 million years ago, at least 150 million years before the suspension-feeding Pachycormidae (previously the earliest definitive example) and about 350 million years before the first baleen whales.

The research team believes that there are other extinct species that would have filled a similar ecological role, including other placoderms (armoured fish) and at least one species of plesiosaur.

Sam Coatham added: “Our methods could be extended to identify other such species in the fossil record and investigate whether there were common factors driving the evolution and extinction of these species.

“We suggest a link between oceanic productivity and the evolution of Titanichthys, but this should be investigated in detail in the future. An established link could have implications for our understanding of the conservation of modern suspension-feeders.”

Which fish eat crown-of-thorns starfish?


This August 2016 video says about itself:

Crown of thorns starfish are responsible for more than half of all coral loss on the Great Barrier Reef. Scientists are looking for ways to use their natural enemy, the giant triton, to disperse the starfish – including using the triton’s scent.

From the Australian Institute of Marine Science:

Fish feces reveals which species eat crown-of-thorns

Great Barrier Reef research finds the destructive starfish is eaten more often than thought

May 18, 2020

Crown-of-thorns starfish are on the menu for many more fish species than previously suspected, an investigation using fish poo and gut goo reveals.

The finding suggests that some fish, including popular eating and aquarium species, might have a role to play in keeping the destructive pest population under control.

The native starfish (Acanthaster solaris) is responsible for widespread damage to the Great Barrier Reef. Since 1962 its population has surged to plague proportions on three occasions, each time causing the loss of large amounts of hard coral. A fourth outbreak is currently underway.

Increasing the amount of predation on starfish has long been touted as a potential solution to preventing outbreaks. However, aside from a mollusc called the Giant Triton (Charonia tritonis), identifying what eats it has been a challenging task.

Now, a team of scientists led by Dr Frederieke Kroon from the Australian Institute of Marine Science in Townsville, Australia, has applied a genetic marker unique for crown-of-thorns, developed at AIMS, to detect the presence of starfish DNA in fish poo and gut contents.

Over three years, Dr Kroon’s team used it on samples taken from 678 fish from 101 species, comprising 21 families, gathered from reefs experiencing varying levels of starfish outbreak.

“Our results strongly indicate that direct fish predation on crown-of-thorns may well be more common than is currently appreciated,” said Dr Kroon.

The study, published in the journal Scientific Reports, confirms that at least 18 coral reef fish species — including Spangled Emperor (Lethrinus nebulosus), Redthroat Emperor (Lethrinus miniatus) and Blackspotted Puffer (Arothron nigropunctatus) — consume young or adult starfish on the reef.

Among the species were nine which had not been previously reported to feed on crown-of-thorns. These include the Neon Damsel (Pomacentrus coelistis), Redspot Emperor (Lethrinus lentjan), and the Blackspot Snapper (Lutjanus fulviflamma).

“Our findings might also solve a mystery — why reef areas that are closed to commercial and recreational fishing tend to have fewer starfish than areas where fishing is allowed,” said Dr Kroon.

She and colleagues from AIMS worked with researchers from CSIRO Land and Water and managers from the Great Barrier Reef Marine Park Authority to conduct the study.

“This innovative research sheds new light on the extent that coral reef fishes eat crown-of-thorns starfish,” said Mr Darren Cameron, co-author of the paper, and Director of the COTS Control Program at the Great Barrier Reef Marine Park Authority.

“A number of the fish species shown to feed on these starfish are caught by commercial and recreational fisheries, highlighting the importance of marine park zoning and effective fisheries management in controlling crown-of-thorns starfish across the Great Barrier Reef.”