In Southwestern France, a group of fish have learned how to kill birds. As the River Tarn winds through the city of Albi, it contains a small gravel island where pigeons gather to clean and bathe. And patrolling the island are European catfish—1 to 1.5 metres long, and the largest freshwater fish on the continent. These particular catfish have taken to lunging out of the water, grabbing a pigeon, and then wriggling back into the water to swallow their prey. In the process, they temporarily strand themselves on land for a few seconds.
Other aquatic hunters strand themselves in a similar way, including bottlenose dolphins from South Carolina, which drive small fish onto beaches, and Argentinian killer whales, which swim onto beaches to snag resting sealions. The behaviour of the Tarn catfishes is so similar that Julien Cucherousset from Paul Sabatier University in Toulouse describes them as “freshwater killer whales”.
If you want to find an ocean animal that kills with speed, don’t look to sharks, swordfishes, or barracuda. Instead, try to find a mantis shrimp. These pugilistic relatives of crabs and lobsters attack other animals by rapidly unfurling a pair of arms held under their heads. One group of them—the smashers—have arms that end in heavily reinforced clubs, which can lash out with a top speed of 23 metres per second (50 miles per hour), and hit like a rifle bullet. These powerful hammers can shatter aquarium glass and crab shells alike.
Most research on mantis shrimps focuses on smashers, but these pugilists are in the minority. The majority are “spearers”, whose arms end in a row of fiendish spikes, rather than hard clubs. While the smashers actively search for prey to beat into submission, the spearers are ambush-hunters. They hide in burrows and wait to impale passing victims. They’re Loki to the smashers’ Thor.
Given their differing lifestyles, you might expect the spearers to be faster than the smashers. They rely on quick strikes to kill their prey, and they target fast victims like fish and shrimp rather than the tank-like, slow-moving crabs favoured by smashers. But surprisingly, Maya DeVries from the University of California, Berkeley, found that the fastest spearer strikes at just a quarter of the speed of the fastest smasher.
“It is an awe-inspiring experience to be faced with a 3-metre-long, 500 kilogram predator, the size of a racehorse, as it launches itself out of the water and slides on its belly for a couple of seconds, coming to a halt barely a metre away from where I stood, without any barrier between me and it.”
That was how Erich Fitzgerald met Sabine the leopard seal.
Leopard seals are like the lions of the Antarctic. They are huge, powerful predators, known for their brutal killing strategy. They bite penguins and seal pups with their big canines, and thrash them onto the surface of the water to flay and dismember their prey.
But Fitzgerald, David Hocking and Alistair Evans have shown that these predators can take smaller prey in a very different way. They suck krill and small fish into their mouths and sieve them in the manner of whales, by passing their mouthfuls of water through tightly interlocking teeth. It’s astonishing behaviour that allows them to dine from the top and bottom of the food chain. As Fitzgerald told me: “This is equivalent to a lion hunting down zebras, but also regularly feasting on ants or termites.
I’ve written about the story for Nature News. Head over there for the full details.
In 2010, an article in Rolling Stone likened the investment bank Goldman Sachs to “a great vampire squid wrapped around the face of humanity, relentlessly jamming its blood funnel into anything that smells like money.”The creature it was referring to does exist – it’s not a true squid, but one of their close relatives. But despite its terrifying name and appearance, it’s not a vampire. It doesn’t suck blood. It doesn’t have a “blood funnel”.
In fact, thanks to newly published observations, we now know that the vampire squid is a garbage-eater. It extends living fishing lines from its body to snag a rain of rubbish falling from the surface, getting fat on a menu of faeces and corpses.
The Goldman Sachs metaphor still works, doesn’t it?
The vampire squid belongs to the cephalopods, the group that includes squid, octopuses and cuttlefish. But it’s an evolutionary relict that appeared well before any of these more familiar animals. Its body is gelatinous and blood-coloured, as if the internal organ of a larger animal had broken free. It swims with two wing-like flaps, sees with opal-blue eyes, and lights up the surrounding water with flashing organs found all over its body, and especially at the tips of its arms.
Two of these arms have been modified into white thin filaments, which coil up into special pockets, and can extend to 8 times the animal’s length. The other eight arms are connected by a cloak-like web that can be inverted over the vampire squid’s body to reveal a muddy charcoal interior, lined with fleshy spines. You can see where the name comes from.
The vampire squid lives all over the world, but we know very little about what it does. That’s partly because it lives at incredible depths – 600 to 900 metres below the surface, in pitch blackness. This level is known as the oxygen minimum zone (OMZ) and unlike the vampire squid, it’s well-named. While a few animals thrive here, most are choked off by the lack of oxygen.
The vampire squid copes by having an incredibly slow metabolism, blood proteins that hug oxygen molecules with an unyielding grip, and a body that so closely matches the density of water that it neither floats nor sinks. It rarely wastes energy on unnecessary movements. It simply hangs in the darkness.
But even though it lives life in the slow lane, the vampire squid needs food, and that’s in short supply in the oxygen minimum zone. What does it eat? To find out, Hendrik Hoving and Bruce Robison from the Monterey Bay Aquarium Research Institute (MBARI) analysed footage of 170 vampire squids, taken over the last decade by the institute’s submersibles.
The videos, along with feeding experiments on captive vampire squids, revealed that they use their filaments like mobile spider webs. They extend these into the surrounding water to ensnare particles of food falling from above. The filaments are covered in tiny hairs, probably for catching these particles. They also have neurons that connect to a particularly large part of the creature’s brain, presumably so it can sense what’s stuck to its fishing lines.
When the time is right, it retracts the filaments, transfers the food to its other arms, and coats them in mucus secreted from its arm tips. It then conveys these delicious balls of mucus-bound detritus into its mouth, possibly with the help of the spines within its cloak.
This strategy is very different to that of other cephalopods, most of which are active hunters that attack and kill their food. Vampire squids are definitely not that, as Hoving and Robison confirmed by checking the stomachs of captured specimens. They found eggs, algae, pellets of faeces, bits of jelly, crustacean body parts—antennae, eyes, some shells, whole copeopods—and flesh from another deep-sea squid. In both kind and quantity, these remnants don’t reflect the diet of a hunter.
Instead, Hoving and Robison think that the vampire squid is mainly a ‘detritivore’ – a rubbish-eater. With few predators in the oxygen minimum zone, it can afford to sacrifice powerful swimming muscles or a high metabolism. Instead, it leads a relatively passive lifestyle, collecting the plentiful snowing debris with its two modified arms. With these adaptations, it can greatly extend the reach of its mouth, while its body—and its life—literally hangs in the balance.
Reference: Hoving & Robsion. 2012. Vampire squid: detritivores in the oxygen minimum zone. Proc Roy Soc B http://dx.doi.org/10.1098/rspb.2012.1357
All images from Hoving and Robison
More on cephalopods
Some folks just can’t help being loud in bed, but noisy liaisons can lead to a swift death… at least for a housefly. In a German cowshed, Natterer’s bats eavesdrop on mating flies, homing in on their distinctive sexual buzzes.
Based on some old papers, Stefan Greif form the Max Planck Institute for Ornithology knew that Natterer’s bats shelter in cowsheds and sometimes feed on the flies within. What he didn’t know was how the bats catch insects that they shouldn’t be able to find. They hunt with sonar, releasing high-pitched squeaks and visualising the world in the returning echoes. Normally, the echoes rebounding from the flies would be masked by those bouncing off the rough, textured surface of the shed’s ceiling. The flies should be invisible.
And they mostly are. Greif filmed thousands of flies walking on the shed’s ceiling, and not a single one of them was ever targeted by a bat. That changed as soon as they started having sex. Greif found that a quarter of mating flies are attacked by bats. Just over half of the attacks were successful and in almost all of these, the bat swallowed both partners.
Photo by Mariano Sironi, Instituto de Conservación de Ballenas, Argentina, via BBC
Like most seagulls, the kelp gull is an opportunist. It will catch fish and other small prey, but it’s not above scavenging at landfill sites. And off the southern coast of Argentina, some kelp gulls have developed a taste for whale.
Between June and December, Southern right whales gather to breed in the waters off Peninsula Valdes in Argentina, and every year, thousands of tourists go out to watch them. Kelp gulls are watching too. As the whales surface for air, the birds land and rip pieces of skin and blubber form their backs, inflicting gaping wounds up to 20 centimetres long. The whales violently arch their backs to submerge whatever they can below the water, before hurriedly swimming away.
The first of these attacks was documented in 1972, and they have been getting worse. In 1974, just 1 per cent of whales had gull-inflicted wounds. By 2008, 77 per cent of them carried such injuries.
Insects have been around for almost 400 million years. That’s plenty of time for evolution to fashion countless horrific deaths for them. Case in point: some insects die because a little worm vomits glowing bacteria inside their bodies.
The worm is Heterorhabditis bacteriophora, a microscopic creature used by gardeners the world over to control insect pests. Its accomplice-in-insecticide is a shiny bacterium called Photorhabdus luminescens, which only lives in the worm’s guts.
When the worm infiltrates an insect, it vomits out the bacteria. These reproduce madly and produce toxins that kill the insect, converting its fallen cells into nutrients that nourish the worm. The bacteria also make amino acids that the worm needs to reproduce, and antibiotics that kill other bacteria trying to colonise the insect. (In the US Civil war, soldiers were sometimes contaminated with P.luminescens, which gave their wounds a mysterious blue shine and protected them from blood poisoning – they called it the “angel’s glow”.)
Even though most spiders are harmless to us, many people suffer from a crippling fear of them. Imagine then, what a grasshopper must feel. The threat of venomous fangs isn’t something that the insects can shrug off. It’s a perpetual danger that chemically alters their bodies, triggering changes that ripple through an entire ecosystem.
Now, Dror Hawlena from Yale University has found just how far-reaching these changes can be. In an elegant experiment, he showed that the fear instilled by spiders can extend into the very soil, affecting how quickly leaf litter decays.
Hawlena raised red-legged grasshoppers in outdoor enclosures, half a metre wide. Half the enclosures contained a single nursery-web spider, whose mouthparts had been glued shut, so they couldn’t actually kill any of the hoppers. Their presence, however, was felt.
For engineers looking to create the next generation of armour, the ocean is the place to look. Animals from snails to crabs protect themselves with hard shells whose microscopic structures imbue them with exceptional durability, surpassing even those of most man-made materials. They are extreme defences.
The mantis shrimp smashes them apart with its fists.
That’s the animal that David Kisailus from the University of California, Riverside is studying. “People have been studying molluscs for decades because they’re thought to be very impact-resistant,” he says. “The mantis shrimp eats these guys for dinner.”
In a swarm of buzzing mosquitoes, every insect probably looks the same to you. You wouldn’t notice that some have swollen abdomens, engorged with red blood, while others are hungry and empty. You wouldn’t differentiate between the antennae of the males (fluffy) and the females (straight). But there is one animal that can spot all of these traits, using eyes that have lower resolution than yours and a nervous system that’s far simpler.
E.culicivora is an East African jumping spider that feeds on mammal blood. Don’t worry: it’s not going to bite you. This indirect vampire only attacks mosquitoes that have recently bitten mammals, and it’s an incredibly discerning diner.
Jumping spiders are famously fussy anyway. They sit and wait for just the right victim to come along, spotting them with large eyes and pouncing upon them with well-judged leaps. Some eat other spiders, but only eat certain species. E.culicivora stalks mosquitoes, but it only female malarial mosquitoes that have recently fed. It ignores: males; individuals that aren’t full of blood; and insects of the wrong species (including other mosquitoes).
“It is the pickiest predator that we know of,” says Ximena Nelson, who studies the spider at the University of Canterbury in New Zealand.
To be that choosy, the spider must have very keen senses. Smell clearly plays a role (the spider is drawn to the odour of both bloody mosquitoes and human feet, and they themselves smell sexier once they’ve drunk some blood). But vision is important too. Even if all scents are blocked, E.culicivora can still pounce on exactly the right kind of prey. Now, Nelson, together with Robert Jackson, has worked out the visual cues that it uses.
They confronted captive spiders with lures built from body parts of dead mosquitoes, which had been glued together in different combinations like miniature Frankenstein’s monsters. The spiders saw two lures at a time, and Nelson noted which they pounced upon. “They are easy-to-handle, patient spiders,” she says. “Being so picky, it means we can ask them questions and get answers regarding their preferences that makes it seem like they answered in English.”
Nelson and Jackson found that the spiders always went for mosquitoes with blood-filled abdomens, rather than empty or sugar-filled ones, no matter which head had been stuck on top. The head matters too, though. When given a choice between two lures with bloody abdomens, the spiders picked the one with a female head rather than the one with a male head.
To check that the spiders weren’t relying on the smell of the lures, Nelson also showed them virtual mosquitoes on a screen. Again, they were more likely to pounce on virtual prey with female antennae than identical ones with male antennae. Human eyes would find it hard to tell the difference. The spiders’ eyes (and it has four pairs) have no such problem.
Having worked out the cues it uses, Nelson and Jackson are working to build the spider’s “decision tree”: the mental steps it makes in order to decide whether to pounce or hold. For now, all we know is that these preferences are innate. No learning is required. The spider appears to be born with some mental template of the ideal mosquito.
This feat is all the more impressive because the spider’s eyes and brain are so simple. The front pair is the largest and most sensitive, but even they probably only have a thousand or so receptors. The young spiders, which are just as fussy as the adults, probably just have 300 receptors per eye.
It seems hard to believe that with so few receptors these spiders can achieve that level of visual detail,” says Nelson. She says that the spider’s receptors are packed tightly in the central part of its eye, so it might be possible for it to see in extreme detail for a small part of its visual field. It probably also processes the information from its eyes in sophisticated way, but no one yet knows how it, or other jumping spiders, do this.
Reference: Nelson & Jackson. 2012. The discerning predator: decision rules underlying prey classification by a mosquito-eating jumping spider. Journal of Experimental Biology http://dx.doi.org/10.1242/jeb.069609
Images all by Robert Jackson
More on amazing spiders: