Glittering seas: the science of ocean bioluminescence

11 November 2011

Fire and water don't normally mix but the ocean is full of living things that put on stunning firework displays. In a sparkling episode of Naked Oceans we celebrate Guy Fawkes night and Diwali as we go in search of some of the many marine animals that make their own light to hide, attack, escape, and woo. Chatting with ocean bioluminescence expert, Edie Widder, we find out about how and why so many ocean species emit light and how twinkling lights are being used to help track pollution through the seas. We also take our pick of the oceans' top 5 firework makers, including snails that glow like a green light bulb, squid that disappear before your eyes, and glowing seas that can be seen from space. And in Critter of the Month, underwater photographer Brian Skerry chooses a super-intelligent hunter.

In this episode

01:35 - Squid and octopus hide in the ocean midzone

A new study has shown that some squid and octopus species are able to shift their colouration between being transparent and coloured, in order to camouflage against the bioluminescent...

Squid and octopus hide in the ocean midzone

A new study has shown that some squid and octopus species are able to shift their colouration between being transparent and coloured, in order to camouflage against the bioluminescent searchlights of predator fish, and from producing a silhouette in downwelling light from above.In different depths of water, different camouflage strategies are most effective. In the mesopelagic - so mid-level waters - the best strategy is to be transparent, so most squid species here are not pigmented - so that they are harder for predators to spot, particularly from underneath - a dark silhouette against the sea surface is a bit of a giveaway.Down in the depths of the ocean however, there's very little light except from species with bioluminescent lures and searchlights. These would be reflected by a transparent body, so most species are coloured a dark red colour. But what if you're a species that lives in both the deep and mesopelagic zones? Adapting to one of the zones would make you vulnerable in the other.squid and octopusWell, Sarah Zylinski and Sönke Johnsen from Duke University in the United Statesreport that the two species that they studied - the octopus Japetella heathi and asquid, Onychoteuthis banksii - are able to shift between being transparent or coloured depending on depth.Both do this by expanding or contracting pigment containing cells calledchromatophores. When the two species were studied in a tank under ambient lighting, they both appeared transparent, but when a blue light was switched on and directed at them, to mimic the bioluminescent searchlights found in many deep sea species, they were seen to rapidly expand their chromatophores and take on a scattered deep red pattern.When reflectance of the surface of Japetella was measured, it was foundthat it reflected twice as much of the blue light when transparent as when thechromatophores were expanded. So when trying to hide from a predator with abioluminescent searchlight, it pays to be able to turn on your pigment cells, so theirsearchlight doesn't reflect back off you. But when they go back up into shallowerwaters, these species can shrink their chromatophores once more and take on the safer transparent form.

04:28 - What happens when endangered whales eat endangered fish

Conflict between killer whales and salmon highlight the importance of an ecosystems approach to conservation.

What happens when endangered whales eat endangered fish

What happens when the endangered marine mammals are busy scoffing endangered fish?

Southern resident killer whalesThat's exactly the conundrum that could soon be facing fisheries managers and conservationists in the Salish Sea in the northeast Pacific, the home of southern resident killer whales and their favourite food the Chinook salmon, both 'at-risk' species.

For the first time, scientists have worked out accurately how many salmon these resident killer whales eat, and hence how much conflict there is likely to be as plans go ahead to restore the depleted populations of both predators and prey.

Publishing in the journal PLos ONE, the big international research team, including from Oceans Initiative, set up a series of computer models linking age, length, weight, sex, and energy consumption of killer whales based on information from captured and captive animals - they didn't catch any killer whales themselves, but used existing information from whaling records.

These models were then used to calculate how many fish are eaten on an annual basis by the 87 killer whales that currently live in the Salish Sea.

And it turns out they already eat a lot of fish - it's estimated they eat 12-23% of Chinook salmonthe salmon from the Fraser River each year, compared to 10-40% natural mortality for the same fish. And the killer whale consumption could go up by 75% if efforts to boost killer whale populations are successful: the target is a 2% increase every year for the next 28 years.

Obviously, that means there will need to be a lot more salmon available for them to eat.

How to resolve the conflict between endangered predator and endangered prey?

Short term measures could include reducing the number of salmon caught by people in fisheries, while steps to boost the productivity of salmon take time to kick in - efforts like taking out dams that block the salmon migration routes, restoring their spawning habitats, and controlling diseases spreading from salmon farms.

One solution proposed is that the killer whales are officially allocated their share of the Chinook salmon under the Pacific Salmon Treaty.

This study highlights the importance of setting aside old-fashioned, single-species approach to managing fisheries and protecting species. Instead we need to look at the whole ecosystems, and consider the interactions between different species, in this case killer whales, salmon, and people.

Find out more:

Williams et al (2011) Competing conservation objectives for predators and prey: estimating killer whale prey requirements for chinook salmon.
PloS One.

07:29 - New way to detect fish poison ciguatera

A new way of detecting ciguatera, offers hope of combating this debilitating disease caught by eating infected fish.

New way to detect fish poison ciguatera

Imagine eating a piece of fish that looks, smells and tastes completely normal, but a few hours ater you're stuck in the toilet, vomiting and with tingling in your arms and legs. Well, this can happen if the fish you eat is contaminated with ciguatera toxin. This affects fish in tropical waters that eat a dinoflagellate species of algae that make the toxin. It affects between 20-60 thousand people every year and is the most common form of food poisoning caused by a natural toxin.BarracudaTesting fish for the toxin has up until now been a lengthy and time consumingprocess, involving injecting purified samples into mice, and watching to see ifthey become ill. Now Kentaro Yogi and his colleagues in Japan have developeda much faster and effective way of testing for the toxin, which has also allowedthem to identify 15 different forms of the toxin present in fish species caught in different areas, as well as identifying differences in the dinoflagellates responsible for producing the toxin.The team used liquid chromatography tandem mass spectrometry to analyse thetoxins, which were extracted and purified in a similar way to the mouse bioassaymethod. The peaks obtained from the analysis were compared with known referencesamples of the toxins and they accurately showed the separate toxins found. Theteam believe that with the production of more reference samples of the various toxins from extraction from natural sources and through synthetic chemistry, this method of detection could become widespread and would improve the speed and accuracy of testing for this unpleasant toxin.

09:42 - Fewer hitch hikers on Pacific turtles

Mini ecosystems clinging to sea turtle shells are more diverse in the Atlantic compared to the Pacific

Fewer hitch hikers on Pacific turtles

Mini ecosystems that cling to the shells of sea turtles and hitch hike across oceans are more diverse in the Atlantic compared to the Pacific, a new study from Mexico shows.

Green turtleA team of researchers led by Eric Lazo-Wasme had the tough job of going down to Teopa Beach in Jalisco, Mexico and examining the creatures that have hitched a ride on the female green turtles that arrive to lay their eggs in sandy nests.

These hitch-hikers are called epibionts and most of them have only been found living on turtles.

A total of 16 species of epibionts were found, including barnacles, crabs, seaweeds, a remora or shark sucker and even leaches. That compares with up to 90 species found attached to the shells of sea turtles in the Atlantic.

Exactly why there were fewer epibiont species attached to Pacific compared to Sea turtle epibiontsAtlantic turtles remains something of a mystery.

The study is published in the Bulletin of the Peabody Museum of Natural History and includes detailed pictures and descriptions of all the species found stuck onto the shells of green and olive ridley turtles, including instructions for future studies on how to collect these turtle hitch-hikers.

Find out more:

Lazo-Wasem et al (2011). Epibionts Associated with the Nesting Marine Turtles Lepidochelys olivacea and Chelonia mydas in Jalisco, Mexico: A Review and Field Guide.
Bulletin of the Peabody Museum of Natural History 52(2):221-240. 2011 doi: 10.3374/014.052.0203

11:21 - The science of ocean bioluminescence

Bioluminescence expert, Dr Edith Widder, introduces the spectacular light displays of the ocean.

The science of ocean bioluminescence
with Edith Widder, Ocean Conservation and Research Association

Edith - In fact in the open ocean environment most of the animals make light. YouAbralia drag a net out there and most of the animals you bring up are producing light and in some cases 80-90% of the animals. And it's fish, shrimp, squid, jellyfish, and they use it to help them survive. They use it to help them find food, to attract mates, and to defend against predators in all kinds of amazing and bizarre ways.

So for finding food you have things like the luminescent lures or built in flashlights to help them see in the dark. For finding mates they can flash certain species-specific patterns or have light organs that are shaped in a unique fashion that allows one species to identify another.

And then for defence there's just a huge range of different ways of using bioluminescence. For example, quite a few animals can release their bioluminescent chemicals into the water just the way a squid or an octopus release an ink cloud. These animals will release their light into the face of a predator, temporarily blinding the predator while they swim away into the darkness.

MelanocetusHelen - So there are all sorts things marine animals do with their homemade light- but how do they make it in the first place?

Essentially it comes down to a reaction between a substrate and an enzyme - generically referred to as luciferin and luciferaze - but, it's not as simple as that.

Edie - We think bioluminescence has evolved maybe 40 separate times, maybe as many as 50 separate times. It's a really unusual example of convergent evolution because there are very different chemistries in different animals. Some animals use the same chemistries and it's even across species and across phyla. But they're extremely different depending on different cofactors and different enzymes and different substrates. It's just very unusual in that respect.

For most of these animals they're making the chemicals that produce light out of the food that they eat. And usually some derivation of amino acids, enzymes or proteins. They  hold these chemicals usually in cells in their bodies and when they want to make light there's some activator usually a nerve action potential that triggers the release of something, maybe calcium or hydrogen, that's the missing element that then allows the light reaction to occur.

Some animals get their light form bioluminescent bacteria. So they have a symbiotic relationship with the bacteria. But the fish sometimes need to turn that light off and so they'll often have mechanical shutters that can close down around the light in some fish the light organ actually rotates back into the head just light the headlights on your Lamborghini.

Helen - Edie has spent a lot of time miles down beneath the waves, studying the glowing creatures of the depths, and she told me about the first time she went down in a submersible. Up until this point she'd only been conducting her research on bioluminescent organisms in the lab.

Edith WidderEdie - I went down to 880 feet, it was an evening dive, I turned out the lights and I was just blown away by the amount of light a saw. The bioluminescence light show was breathtakingly beautiful, but I also knew enough about it at this point to know how energetically costly it was to produce this light. And I thought, this has got to be one of the most important processes in the ocean, I couldn't understand why more people weren't studying it. And it completely changed the course of my career.

So, a lot of the reason I came to understand that it wasn't being studied more is there weren't tools available for studying it. Clearly you have to do a little bit more than go down and say, 'Oh wow everything glows!'. And so I started working with engineers to develop instruments that could better quantify bioluminescence in the ocean. I also doing more work from submersibles and eventually developed a technique that uses video image analysis to identify the animals by the type of flashes they produce. And it's turned out to be a very powerful tool for mapping the 3D distribution patterns of animals in the ocean.

Helen - Even with the advancing technologies for venturing into the depths in person, Edie began to think about what she was missing.

Edie - Of the 100s of dives I've made in submersible I've always felt like how muchMelanostomias is there out there that I'm not seeing because we're scaring it away with our bright lights and noisy thrusters.

And so I wanted to develop a camera that could be left quietly on the bottom of the ocean that was battery powered but unlike cameras that had been used in the past I wanted it to be completely unobtrusive. So I used red light, sort of the way people studying nocturnal animals use infra red light on land, trouble is you can't use infra red in the ocean because its absorbed so quickly in the water. So we used far red light and then a camera that was super sensitive to compensate for the fact that so much of that light is absorbed.

And so now we've started to study these animals in their natural habitat. I also developed an optical lure that imitates certain bioluminescent displays. That's proved very valuable in working out this language of light and what kind of displays different animals respond to.

Helen - As well as learning about the fascinating lives of glittering deep ocean species, Edie has also started applying her bioluminescence research to helping solve some of the problems facing the oceans.

Bioluminescent bamboo coralEdie - One of the big ones is tracking pollution. We have a lot of problem with non point-source pollution which is things like fertiliser and pesticides running off crop land, animals waste out of animal feed lots, toxic cocktails of hydrocarbons that run off our roads every time we have a rain storm. Just all kinds of things that are running off the land and having a very profound effect on coastal ecosystems.

And so I felt like what we really want to do here is make pollution visible. So we're actually using bioluminescence to do that. We use bacterial bioluminescence.

The reason bacteria glow all the time is that the bioluminescence is linked to the respiratory chain - so the breathing of the bacteria. And any pollutant that interferes with that, dims the light.

So the bioluminescent bacteria are akin to a canary in a coalmine. Miners used to take a canary down with them before they had sensors that could detect the poisonous gases that they had to be concerned about. They knew when the canary stopped singing or keeled over that they better get out of there very quickly. And that's what's known as a broad-spectrum bioassay. You're not measuring for a specific pollutant but a whole range of them. And that's what the bacteria do for us.

So we take sediment samples. When you're talking about water pollution most of Pachistomias microdonthe water pollution actually resides in sediments. So we take sediments and test the bacteria against these sediments and figure out where the toxins are accumulating in the environment.

And then we've also developed a water quality monitoring system called a Kilroy - because we hope these guys are going to be everywhere - that allows us to track the pollutant back to its source.

My dream is to get a pollution layer onto Google earth so that people can see the pollutants in their own back yards and in their favourite waterways and then work together to figure out what we can do to solve these problems.

Find out more:

ORCA (Ocean Research and Conservation Association)

Edith Widder
TED talk

21:53 - How glowing jellyfish revolutionsied science

The jellyfish that made rabbits glow green and revolutionised science features in our list of the top 5 ocean light-makers

How glowing jellyfish revolutionsied science

Aequorea victoria, the jellyfish that made rabbits glow green and revolutionised science, features in our list of the top ocean light-makers.

Aequorea victoria

Sarah - my first critter is a type of jellyfish called Aequorea victoria, or the 'crystal jelly', found off the West Coast of North America and out into the Pacific ocean.

It's a small jellyfish, growing up to around 10cm across, and is well named - it's almost completely colourless, but does show luminescence in a ring around the bottom edge of the main body.

The protein that causes the greenish glow of the crystal jelly's luminescence is known as GFP - the Green Fluorescent Protein. When the jelly is startled, its cells release calcium ions, that excite a blueish coloured protein called aequorin, which then stimulates the GFP to glow.

Osamu Shimomura first extracted GFP from the crystal jelly, and now it's used in many other areas of biology - and particularly to stain cells for fluorescent microscopy. The good thing about GFP is that unlike most stains that are used, it's not toxic to living cells, so it can be used to look at living tissues under the microscope. There's also several mutations of the GFP protein that have been developed - Blue FP, Yellow FP, Cyan and actually, Red fluorescent proteins have been extracted from corals. These can each be attached to a different protein or structure in a cell to see how they interact.

There's also been several transgenic animals produced by modifying their genes so they produce GFP - there've been mice, zebrafish (that you can buy in pet shops - that'd be quite cool to have glow in the dark fish in your tank at home), and even rabbits.

The crystal jellies have quite an interesting life cycle - they alternate between a sessile, or ground dwelling hydroid form, and the swimming medusa form. The medusae are either male or female and will release gametes into the water column, which meet and fertilise then settle onto the sea bed and grow into a hydroid colony. In the spring, the colonies then bud off more little medusae into the water column, that grown, start producing gametes, then die after about 6 months.

So they're a really good example of how the oceans are helping to advance other areas of research - a key point to think about when we're talking about conservation. This is a point made about the rainforests on land as well - the potential for new drugs etc in these untapped areas is a major reason to conserve species and ecosystems.

25:12 - Glowing seas spotted from space

A surreal nighttime phenomenon that's puzzled mariners for centuries was recently spotted from space.

Glowing seas spotted from space

Helen- Well, I nearly chose for one of my top firework makers the bioluminescent dinoflagelates that glow in the sea when you go diving at the right time in the right place and they just flash these fantastic lights when you move.

Sarah- I think those are in the beach, aren't they, in that film when they go swimming in that lagoon and you move your hands through water and it startles them and they produce all these teeny lights.

Helen- Yeah, it is absolutely brilliant. And it does happen like that, it really does. You can basically have a lot of fun pretending to be an underwater wizard because you can throw fireballs, it's fantastic. I have been swimming one time at night and come out and it stuck to my skin. So that if you actually then draw with your finger on your skin it glows again, it's like you are covered in this crazy paint. It is fabulous.

I am not choosing them, so I am cheating, slightly. Those dinoflagelates only flash when they are disturbed. But there is another phenomenon in the ocean that glows constantly and that is a thing called the milky seas effect.

Milky seas detailIt is a surreal night time phenomenon that mariners have puzzled over for hundreds of years. It's called milky because it looks white but is actually blue and but is such a low light that your eyes just use your rods which don't distinguish color, so it looks white.

It went unexplained for a long time, and it's still pretty mysterious; but, we think, the latest theories are, that it is caused by a type of bioluminescent bacteria, Vibrio harveyi. And these glowing seas have been spotted from space, this is fantastic.

This is all thanks to a meteorologist called Steve Miller, back in 2003 he was basically pondering the idea "could you see glowing seas from space", could we do this, is it possible. Everyone said no don't be stupid, it's far too brief, it's not bright enough, there is no way you could do it. But he was determined to see if he could figure this out.

So, like all good scientists he went straight to the internet and hunted around for descriptions of the milky seas effect. He found a report from the ship SS Lima, from Jan 1995 when it was in the Northern Indian Ocean and it went something like this:

"22:00 local time on a clear, moonless night a whitish glow was observed on the horizon and, after 15 min of steaming, the ship was completely surrounded by a sea of milky-white color with a fairly uniform luminescence. The bioluminescence appeared to cover the entire sea area, from horizon to horizon . . . and it appeared as though the ship was sailing over a field of snow or gliding over the clouds.."

Isn't that wonderful?

So, basically Steve got this report and then he wanted to figure out if that View of milky sea from spaceparticular milky sea could be spotted from space. So, he teamed up with some other researchers including bioluminescence expert Steve Haddock from the Monterey Bay Aquarium Research Institute.

They sifted through satellite images, and low and behold, there was the milky sea spotted from space in exactly the right place and the right time as SS Lima's description. It turned out to be an enormous patched of glowing sea, it measured over 15,000 square kilometers, the size of the county of Yorkshire if you are a Brit or the state of Connecticut if you are in the US, everyone else will just have to figure out how big that is.

A satellite image showed that it glowed on 3 consecutive nights, between the 25 and the 27th of January. Check this out, weirdly it is the same date, the 27th, although a hundred years earlier that the passengers on board the Nautilus, in Jules Verne's novel 20,000 Leagues Under the Sea, in a very similar part of the ocean sailed through a milky sea. How weird is that?

It only happens in the right conditions, and that is when there is a huge concentration of bacteria. It estimated that this particular milky sea had 40 billion trillion bacteria, and that is quite a lot. In fact, if you had a grain of sand for every one of those bacteria you'd cover the entire planet to a layer of 10cm thick in sand. It's huge.

Anyway, I had a quick chat with Steve Miller the other day and he said since that Milky seas paper figure2005 paper came out describing the satellite spotting of the milky sea in the Indian Ocean he has had lots of reports from sailors of milky seas but so far hasn't had satellite images to match it up. Because we're really talking extremely low light levels here, you've got to have very sensitive satellite images.

But the good news is, in October just past, NASA launched a new satellite called NPP and its carrying a new, improved low light sensor. So that does increase the chance of it spotting the milky sea from space and it perhaps gives researchers a chance to dash out on a ship and grab some of the sea water in the same place. And Steve told me if that happens he is going to put down his science hat and just frolic around in the glow.

Sarah- But, is that safe? Could you get infections from those kinds of bacteria or is it a safe thing to go swimming in?

Helen: That is a really good question. These are bacteria that are all the way through the oceans, just at very low concentrations compared to milky seas. But no, I think I might have to ask Steve that one, perhaps get him to go first and see what happens.

Find out more:

Milky Seas from Space - website with more info and links to Steve Miller and Steve Haddock's 2005 paper

30:07 - How bobtail squid vanish before your eyes

In our rundown of top ocean light-makers, the cute bobtail makes an appearance before disappearing before your very eyes.

How bobtail squid vanish before your eyes

In our rundown of top ocean light-makers, the adorable bobtail makes an appearance before disappearing before your very eyes.

Bobtail squid

This glowing critter uses bioluminescence to make itself invisible.

The problem with living the 3D environment of the ocean is that a potential predator could come at you from the sides, or above or below. Many species in the oceans show countershading - where the underside of the body is coloured lighter than the top of the body, to counteract the shading from being lit from above. But some species go further. Several species of squid use bioluminescence on the underside of their body to try and match the colour of the light coming down from above, so if a predator sees them from below, they don't see a dark shape against the light ocean surface, but the squid become almost invisible.

The idea of ventral bioluminescence as a form of countershading was supported by evidence from several species but it was only when an on-board study of squid caught in the open ocean in the 1970s and exposed to light from above showed that they fluoresced on their undersides to match the light level coming from above proved that it was a strategy being used. These results have been confirmed several times by other researchers.

Some of the most recent research has been into the Hawaiian bobtail squid, which I have to say is one of the cutest squid I've ever seen - it's like Disney have drawn it specifically to look round and cute - they're also known as dumpling or stubby squid! Well, these squid have a symbiosis with the bioluminescent bacteria Vibrio fischeri, which live in special crypts in two paired areas of the squid's body known as the light organs. The light organs have a reflective inner surface close to the body, so all the light gets directed away from the inside, and they also have a lens-like structure on the outside to help diffuse the light.

By using the muscles that are also used to control its ink sac, the squid can change the size and shape of the organ and so how much light is emitted, so it can match the light coming from above and disappear!

32:37 - Seashells that glow like lightbulbs

Another of our top ocean light-makers is the clusterwink snail that glows like a light bulb to scare off predators.

Seashells that glow like lightbulbs

Helen - In the mollusk group there are lots of glowing squid and octopus and so on. But in fact for gastropods, the seashells, there is really hardly any bioluminescence except for a little brilliant creature Hinea brasiliana, also known as the clusterwink, which has to be the best name for a creature.

Hinea brasilianaBasically they are called that because they group together in crevices at low tide, these live on the coast of Australia. Apparently once the tide comes in they wake up and scoot around busily, but when the tide comes in they all cluster together, hence the name cluster wink.

It's a family Planaxidae, when they are just sitting there in the day light they look like little yellow shells, fairly normal yellow shells. But put one in the dark and poke it and it glows bright green. It is basically a kind of burglar alarm, the idea is that is it shouting out for help and essentially saying "help, help I'm being attacked" to attract perhaps other predators of the thing that is attacking them and to basically make themselves look bigger.

A study came out earlier this year that looks into how this happens. They have two blobs of light emitting cells inside their bodies but it comes out like a whole glowing shell, a bit like a light bulb essentially.

Turns out this is caused by the crystalline structure of the shell that is specificallyHinea brasiliana filtering and diffusing the light to amplify it and it does it better than any commercially available material that we use to do similar things. It only works with green light, red and blue just gets stuck they don't get transmitted through the shell.

It is just really awesome, the researchers are now trying to bio-mimic this structure to try and make things that we can use for ourselves. And they are also looking at other members of the Planaxidae family to work out how this particular fireworks trick evolved.

Find out more:

Deheyn and Wilson (2011).Bioluminescent signals spatially amplified by wavelength-specific diffusion through the shell of a marine snail.
Proc. Roy. Soc B.

34:25 - Deep sea angler fish - queens of the glowing depths

With parasitic males and glowing lures to catch their dinner, female angler fish are the undisputed queens of the deep.

Deep sea angler fish - queens of the glowing depths

With parasitic males and glowing lures to catch their dinner, female angler fish are the undisputed queens of the deep.

Melanocetus

Sarah - The next bioluminescent critters I want to talk about are the anglerfish, particularly the family Ceratiidae, also known as the Sea Devils, which is a pretty good name.

These are those ugly deep sea fish with a long manoeuvrable lure on their heads that glows at the end. The lure is made up of a spine that has migrated through evolution from being attached to the front of the dorsal fin, to above the head. It's completely mobile and can be waved around in all directions, and it's used to lure prey, but may also be used to attract a mate. The very end of the lure is full of bioluminescent bacteria called Photobacteria.

So these fish don't bioluminesce in the same way as the crystal jellies, which produce their own light - they rely on a symbiotic relationship with these bacteria. The bacteria enter the bulbous end of the lure through pores and once inside can gain nutrients and protection from their host, while the fish gains the ability to use the bacteria's bioluminescence.

Now while the lure is used, just like a fishing lure - hence the name anglerfish - to lure prey, there is suggestion that they might also attract mates. Only the females have these lures - the sexual dimorphism, or the difference between males and females of these species of fish is extraordinary - females are reasonably large, ranging from a few centimetres in the smallest species, to over a meter in the case of Kroyer's Deep Sea Anglerfish. But the males are tiny - in Kroyer's Anglerfish, while the females can be over a meter long, males are generally around 14 centimetres!

And in smaller species the males are so tiny that originally researchers thought they were some kind of parasite living on the female specimens they caught. So possibly, along with their acute olfactory sense, the males can find the females by their lures. They then attach to the female permanently, actually FUSING with her blood stream and gradually atrophying until they are simply a pair of gonads that release sperm into the female in response to hormonal cues in her blood.

This bizarre method of reproduction is a good strategy if individuals of a species are scarce and widely spread out - having the male attached to her body means the female has a ready supply of sperm whenever she releases an egg.

37:54 - Critter of the month - killer whale

Underwater photographer Brian Skerry picks an intelligent ocean hunter for his critter of the month.

Critter of the month - killer whale
with Brian Skerry

Brian - Hello. My name is Brian Skerry and I'm a wildlife photojournalist specialising in Peeking orcamarine wildlife, underwater and ocean subjects. And if I were a marine creature I think I would like to be an orca, otherwise known as a killer whale.

I suppose that's somewhat of a shallow statement in the sense that I've chosen the most supreme creature in the sea, perhaps, an animal that I at least believe reins supreme pretty much over everything else. So I guess that tells you something about me.

I am just so impressed with orca. I think that they are on a level quite unlike anything else. I'd like to think that they're much smarter than human beings and that they have this ability and awareness that we can't even begin to understand.

I've only encountered orca a few times throughout my career in the ocean. The first time I had a real close encounter was back in 1994. I was in the Norwegian Arctic diving, free-diving, in very cold water in a place where they were gathering to feed on herring over the winter.

It was just magical, the cliffs and the fjords were immense and kind of spooky, it was this beautiful purple light in the sky. The sun never got very high at that time of year, this was in October. But on a few occasions I was able to get underwater with my dry suit, no scuba tank, just free-diving down, and see these animals in pods and in groups.

I had one day where a young orca, a juvenile, a baby, broke off from its mom and came around and circled me. And the whole time they're looking at you with this eye that you know has this incredible intelligence, this great awareness behind them.

I've had the privilege of being with many great underwater creatures from tiny ones to giant ones, and marine mammals are always very special because they are choosing to interact with you. They are making that decision that they want to know more about you and they're curious.

Orca is quite unlike anything else. They have this depth, this soul if you will, that goes back eons and if only we could tap into that I think we could learn so much.

The notion of coming back, if I could be reincarnated or if I could just snap my fingers and become an orca, I think would be pretty magical because I would learn so much about the ocean and have the ability to travel great distances and interact with everything that lives in the sea.

Ocean Soul by Brian SkerrySarah - That was Brian Skerry with those intelligent hunters the orcas, or killer whales - And did you know they are in fact big dolphins, a member of the Delphinidae family, the ocean dolphins.

This month Brian's fantastic new book, Ocean Soul, is published - it's a collection of some of his most stunning underwater photographs from his career taking pictures for National Geographic Magazine - so well worth checking out.

Find out more:

Brian Skerry

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