In the darkness of the deep ocean, some animals create their own light. Among these is the Hawaiian bobtail squid Euprymna scolopes, which forms a partnership with the luminous bacterium Vibrio fischeri. The squid houses colonies of these bacteria in special light organs, and it can control the brightness and direction of their illuminations. But these organs do much more than produce light – they detect it too.
Deyan Tong from the University of Wisconsin has discovered that the organs generate nervous signals when they sense light and they’re loaded with proteins responsible for detecting it. The light organs are effectively an extra set of primitive eyes, each equipped with its own “iris” and “lens”. The squid comes equipped with a pair of living, ‘seeing’ flashlights.
Scientists have studied the light organ of E.scolopes for over 20 years and its similarity to an actual eye hasn’t gone unnoticed. The core of the organ where the bacteria live is surrounded by a reflective layer of tissue and part of the squid’s ink sac. These can expand and contract like an iris to control how much light escapes the core. The entire package is covered by a thick, transparent tissue – a “lens” – which diffuses the light produced by the bacteria.
The giant cephalopods (squids and octopuses) of the deep sea have captured the imagination for centuries. But despite our fascination with these creatures, they are still enigmas, their behaviour illuminated only by the occasional lucky video or the presence of scars on animals they fight with. For many species, including the famous giant squid, we still know relatively little about what they eat and what position they occupy in their ecosystems.
Yves Cherel from the Centre d’Etudes Biologiques de Chize has some new answers about the behaviour of deep-sea cephalopods and they came from a most unorthodox technique– he studied remains recovered from the stomachs of dead sperm whales.
It’s clear that sperm whales feed on squid and octopuses. Sucker-shaped scars along the backs of some individuals have led people to picture titanic battles between the whales and their giant prey. Once eaten, the cephalopods’ soft bodies are easily digested, but they also have hard, parrot-like beaks that aren’t easily broken down.
By looking in the stomachs of three sperm whales stranded in the Bay of Biscay, Cherel recovered hundreds of beaks from 19 separate species – 17 squids including the giant squid, the seven-arm octopus (the largest in the world) and the bizarre vampire squid. Together, these species represent a decent spread of the full diversity of deep-sea cephalopods.
The Japanese pinecone fish searches for food with living headlights. This hand-sized fish harbours colonies of light-producing bacteria in two organs on its lower jaw. The beams from these organs shine forward, and when night falls and the fish goes searching for food, its jaw-lamps light the way.
Elsewhere in the Pacific Ocean, the Hawaiian bobtail squid also uses luminous bacteria, but theirs act as a cloaking device. They produce a dim glow that matches the strength of moonlight from above, hiding the squid’s silhouette from hungry fish below. It’s a mutual relationship; the squid gets protection and it pays its residents with sugars and amino acids.
The glowing bacteria of these two animals may have different uses, but they are actually the same species – Vibrio fischeri, a free-swimming bacterium found in almost all of the world’s oceans. V.fischeri isn’t inherited; instead, it colonises the light organs of both fish and squid when they are young. Its challenge is to recognise the right partners among the myriad of species in the ocean, and not end up in the wrong body.
But its potential hosts, the bobtail squid and the pinecone fish, are incredibly different animals, separated by over 550 million years of evolution. How does one bacterium manage to form tight alliances with such disparate hosts?.
Incredibly enough, it does so with a single gene. Mark Mandel from the University of Wisconsin found that the strains of V.fischeri in the squid contain a gene called RscS that is missing or very different in those found in fish. RscS was the genetic innovation that allowed a fishy bacterium to set up shop in the body of a squid.
Imagine that you hand is made of jelly and you have to carve a roast using a knife that has no handle. The bare metal blade would rip through your hypothetical hand as easily as it would through the meat. It’s clearly no easy task and yet, squid have to cope with a very similar challenge every time they eat a meal.
The bodies of squid, like those of their relatives the cuttlefish and octopus, are mainly soft and pliant, with one major exception. In the centre of their web of tentacles lies a hard, sharp and murderous beak that resembles that of a parrot. The beak is a tool for killing and dismembering prey and the large Humboldt squid (Dosidicus gigas) is known to use its beak to sever the spinal cord of fishy prey, paralysing them for easy dining.
The Humboldt squid’s beak is two inches long and incredibly hard (difficult to dent or scratch), stiff (difficult to bend out of shape) and tough (resistant to fractures). This combination of properties makes the beak harder to deform than virtually all known metals and polymers. That’s all the more remarkable because unlike most animal teeth or jaws, it contains no minerals or metals. It’s made up solely of organic chemicals and manages to be twice as hard and stiff as the most competitive manmade equivalents.
By comparison, the mass of muscle that surrounds and connects to the beak is incredibly soft, the equivalent of a jelly hand gripping a bare metal blade. With such mismatched tissues, how does the squid manage to use its killer mouth without tearing the surrounding muscle to shreds?