What happens if you cross a fish that has white spots on a black body with another fish that has black spots on a white body? You might think that you’d get a fish with a single uniform colour, or one with both types of spots. But the hybrid’s skins are very different and far more beautiful. It does not inherit its parents’ palettes, overlaid on top of each other; instead, it gets a mesmeric swirl of black and white that looks like a maze on its skin.
To understand where these hybrid patterns come from, you need to look at how fish decorate their skins in the first place. These patterns can be very complicated, as even the briefest swim through a coral reef will tell you, but they also vary from individual to individual – one trout will have a slightly different array of spots to another. These differences tell us that intricate patterns aren’t stamped onto a fish’s skin according to a genetically encoded blueprint. They’re living patterns, generated through a lively dance between a handful of molecules.
This article is reposted from the old WordPress incarnation of Not Exactly Rocket Science.
Getting excited when fish produce sperm would usually get you strange looks. But for Tomoyuki Okutsu and colleagues at the Tokyo University of Marine Science and Technology, it’s all part of a day’s work. They are trying to use one species of fish as surrogate parents for another, a technique that could help to preserve species that are headed for extinction.
Okutsu works on salmonids, a group of fish that includes salmon and trout. Many members of this tasty clan have suffered greatly from over-fishing in the last few decades, and their populations are dwindling their way to extinction.
If stocks fall below a critical level, they may need a jump-start. One strategy is to freeze some eggs to be fertilised artificially, in the way that many human eggs are in fertility clinics. But it’s much harder for fish eggs – they are large and have lots of fat, which makes them difficult to freeze effectively.
Okutsu’s group have hit on a more effective solution. They use transplanted sexual stem cells to turn another species of fish into surrogate parents for the endangered ones.
Underwater, fish make very difficult prey. When they sense sudden disturbances in the water around them, they respond within five thousandths of a second with a defensive reflex called the C-start. Their body contorts into a C-shape and with a flick of the tail, they rapidly zoom away from the potential threat. But one predator has a way of turning the fish’s defence against it, persuading the fish to swim towards danger. It’s the tentacled snake.
The tentacled snake (Erpeton tentaculatum) is a bizarre species, easily recognised by the pair of short “tentacles” on the front of its head. The snake is a master fisherman and it hunts in the waterways of South-East Asia. It relies on ambush, anchoring its tail and twisted the front of its body into a distinctive J-shape. Thus contorted, it waits motionlessly for a fish to swim past. When it strikes, it does so explosively, covering the distance to its prey in 15-20 milliseconds.
So the battle between the tentacled snake and its prey is a contest between two extremely fast movements – the strike versus the C-start. But the snake has a way of tipping the odds in its favour – it feints. As the fish approaches, it ripples its body towards it, sending the hapless prey darting in the opposite direction, straight towards the snake’s angled head. The snake anticipates this and executes a predictive strike, aimed at the position where the fish will end up. Sometimes, the fish swim directly into the snake’s mouth.
For the fish, there is no turning back. Its C-start is driven by two giant nerve cells called Mauthner neurons. Disturbances in the water excite the nearest of this pair, which in turn excites a large network of motor neurons on one side of the fish’s body and blocks the equivalent network on the other side. That triggers the C-start and sends the fish in the opposite direction. This whole process takes mere milliseconds and once the direction is set, it can’t be reversed. The snake’s feint corrupts an otherwise adaptive behaviour, turning it from a defence into a death march.
On the 3rd of October, 2006, Nicolas Makris watched a quarter of a billion fish gather in the same place. They were Atlantic herring, one of the most abundant fishes in the ocean and one prone to gathering in massive schools. This was the first time that anyone had watched the full scope of the event, much less capture it on video.
The first signs of the amassing herring appeared around 5pm and by sunset, the gathering had begun in earnest. Once a critical level of fish was reached, the shoal expanded at a breakneck pace, suddenly growing to cover tens of kilometres within the hour. By midnight, the shoal contained about 250,000,000 individuals – 50,000 tonnes of fish gathered in one place.
The ability of fish to congregate in gigantic schools may be familiar but until now, we’ve known remarkably little about the things that set off these gatherings. Without Facebook as a coordinator, what causes small groups of herring to take sociability to an extreme? Scientists have tried to follow gathering fish aboard research vessels but these can usually only see a small fraction of the massive schools are any one time.
Makris wasn’t so hampered. He used a new technique called Ocean Acoustic Waveguide Remote Sensing (OAWRS) that can visualise fish populations over vast distances in real-time. It needs two ships, one to send out sound waves in all directions and a second to pick up their echoes as they bounce off fish and floor alike.
In an instant, it can scan an area of ocean 100km in diameter, and it can update its images every 75 seconds, providing an unprecedented view of the genesis of herring shoals. The location was Georges Bank off the coast of Maine, where herring migrate to spawn in early autumn. Makris pointed his instruments at an area where herring historically gather, and waited.
You’re looking at the face of a new species of fish and judging by the two fearsome fangs, you’ll probably understand how it got its scientific name – Danionella dracula. The teeth do look terrifying but fortunately, their owner is a tiny animal just 15 millimetres long. Ralf Britz from London’s Natural History Museum discovered the fanged fish in a small stream in northern Burma, just two years ago. The more he studied them, the more he realised that they are physically extraordinary in many ways.
For a start, those are no ordinary teeth – they are actually just part of the fish’s jawbone. True teeth are separate from the jaws that house them and are made of several tissues including enamel and dentine. Those of D.dracula are protrusions of the jaw itself and are made of solid bone. The fish has rows of them in both its upper and lower jaw that look very convincingly like actual teeth. Even though it comes from a long line of fish that have lost their teeth, D.dracula has managed to re-evolve them through a completely unique route.
Secondly, D.dracula seems to be missing several bones, with 44 fewer than close relatives like the zebrafish, Danio rario. They haven’t disappeared – they never formed in the first place. Compared to other related fish, D.dracula stops developing at a much earlier point and retains the abridged skeleton of a larva throughout its adult life. It’s the Peter Pan of the carp family.
Earlier this year, I wrote about how the human obsession with size is reshaping the bodies of other species at an incredible pace. Unlike natural predators that cull the sick, weak and unfit, human fishermen prize the biggest catches and throw the smallest ones back in.
As a result, fish and other species harvested by humans are shrinking, often within a few generations, and are becoming sexually mature at an earlier stage. These changes are bad news for populations as a whole, for smaller individuals often have lower odds of survival and produce fewer offspring.
But David Conover from Stony Brook University has found a silver lining in this tale – selectively harvesting fish can lead to dramatic changes in body size, but these changes are reversible. Release them from the pressure of constant hunting, and some of the animals start to rebound to their previous state.
Conover spent ten years raising a commonly harvested species called the Atlantic silverside in six captive populations, each containing about 100 individuals. Every year, the fish produced a new generation and for five years, Conover would remove 90% of the fish, either by taking the largest ones, the smallest ones or randomly selected individuals. In every other way, the fish were all reared under exactly the same conditions. This constant upbringing ensured that any changes to their bodies would be the result of genetic influences rather than environmental ones.
Some of us have enough trouble finding the food we want among the ordered aisles of a supermarket. Now imagine that the supermarket itself is in the middle of a vast, featureless wasteland and is constantly on the move, and you begin to appreciate the challenges faced by animals in the open ocean.
Thriving habitats like coral reefs may present the photogenic face of the sea, but most of the world’s oceans are wide expanses of emptiness. In these aquatic deserts, all life faces the same challenge: how to find enough food. Now, a couple of interesting studies have shed new light on the tactics used by predators as large as sharks and as small as bacteria.
At a large scale, predators like sharks and tuna rely on chemical cues to give away the location of their prey. Sharks are particularly expert trackers, but powerful though their super-senses are, they can only come into play within a certain range. Over the large distances of the open ocean, they are more like blind hunters, hoping to stumble across some telltale sign of food.
David Sims from the UK’s Marine Biological Association found that many large marine predators use a search strategy called a ‘Levy walk’, although in this case it’s more of a swim. The strategy is formally described by a mathematical equation, but in simple terms, it means that an animal makes several short moves in its search for food, interspersed with a few long ones. The longer the ‘step’, the more infrequent they are.