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Entries in pelagic (11)


Shedding some light on lanternfish

Diaphus parri, a lanternfish from the Coral Sea. Img: Adrian Flynn

Adrian Flynn’s PhD is about myctophids, or lanternfishes.  These are not well known to most people, but they’re probably among the most abundant animals on the planet, because they live in the largest habitat there is: the deep mid-waters of all the oceans of the world, neither near the surface nor on the bottom, what scientists call the mesopelagic zone (meso = middle, pelagic = in the water column).  Lanternfish, as the name implies, produce their own light from organs arrayed on the skin of the head and body.  These organs, which generate light through the action of the enzyme luciferase, allow the fish to signal each other, to find food and to disguise their own outline against the gloom from above, when seen from below.  Its a neat trick, but not nearly unique in the deep pelagic zone.  Indeed, it seems that just about everything down there makes light in some form or another (if you want some google fodder for that, check out Edith Widder’s work, she’s got a great talk on too).  Lanternfishes can be hard to study because they’re hard to collect in tact; they’re sort of flabby and the skin comes off very easily. But not to be deterred, Adrian is undertaking an ambitious study of how different species of lanternfishes are distributed from the tropic of Capricorn to the waters of the Antarctic - their biogeography - and also how the numbers of any given species are affected by oceanographic factors like major currents and places where deep nutrient-rich water comes up to the surface (upwelling). 

A nice ventral view of D. parri, showing the light organs arrayed on the skin

So far his results are showing that the deep pelagic zone is not as homogenous as previously thought.  It seems that lanternfish distribute themselves into biogeographic zones somewhat according to latitude, but more so according the oceanographic features like major currents and landforms like islands.  He’s had a couple of real eye opening results too.  In one case, they observed - for the first time in Australia - a lanternfish spawning aggregation, off the coast of Cairns in far north Queensland.  That’s cool, but what was a real trip was that the laternfish in question (the Dana lanternfish) was a species known only from Tasmania, almost 2,000km away!  How did they get up there?  How will their young get back again?  On another expedition, they found lanternfish close to the surface at Macquarie Island, a remote rock in the Great Southern Ocean.  The island juts up into the prevailing currents, causing upwelling that brings nutrients and lanternfishes alike well within the foraging range of penguins and seals/sea lions that nest on the island.  It looks like lanternfish in this unique location are an important part of the diet of at least three penguin species as well as the pinnipeds (seals), and that’s a pretty novel discovery.


Whats happening all the time in 90% of the oceans, and half the time in the other 10%

To explain the title a little better - most of the oceans are in total darkness all the time, and even the sunlit zone is an inky realm every night when our star visits the other side of the planet.  Accepting that we can't easily visit the bathypelagic zone (the deepest bits) without submersibles or ROV's (remotely operated vehicles), then perhaps the best feel we can get for what's happening in the vast majority of the oceans is to don SCUBA gear and dive the surface of the open ocean, but in the dark.  In preparation for doing a bit of that later this year, I've been looking at "black water night diving" stuff on YouTube.  Honestly, the idea invokes in me a healthy amount of fear, but if these videos are anything to go on, then I hope that will soon be replaced by wonderment and fascination.

Pelagic plankton. I love the flatfish at 0:28. If you know what the spongy looking thing at 1:40 and 4:04 is, please let me know.

Humboldt squids - I especially liked the face-on attack at 2:00 and the strobing at 2:30

I guess this is the most obvious anxiety. The one at 1:20 just gives me the heebie-jeebies!

This video isn't so much pelagic as reef, but the spawning sea cucumbers and then the palolo worms about 5:40 in are just great, and I love the music, which (curiously) is from that abysmal Mel Gibson flick Passion of the Christ.


Implications of the first sighting of whale sharks in the gulf oil slick

I recently experienced a moment of genuine dread regarding the oil spill in the Gulf of Mexico, and it was neither a familiar nor comfortable feeling. What is it that invoked such a powerful feeling after a disaster that has been underway for the last 80-odd days, now? Something that struck a little close to home, of course: the first direct impact to whale sharks. You may have seen this story coming across the wires over the past two days about NOAA scientists who, while on an aerial survey of the impacted area, observed 3 whale sharks swimming among ribbons of surface oil, not 4 miles from the epicenter of the Deepwater Horizon spill. This observation has serious implications; let me explain.

Whale sharks are widely-ranging tropical migratory sharks that are unusual among their more toothy relatives in that they eat plankton. Two of the adaptations they use to pursue this lifestyle – surface filter feeding and an exquisite sense of smell – make them especially susceptible to the impacts of the oil spill. I had all but convinced myself (perhaps wishfully thinking) that whale sharks would be able to sense the altered chemistry of the affected water bodies and avoid the area. It now seems that this is not the case; the observation by the NOAA scientists suggest that either whale sharks cannot tell the difference between polluted and unpolluted water, or they can tell the difference but do not alter their behaviour in such a way as to avoid the ribbons and plumes. As USM researcher Eric Hoffmayer states in the article, this is the realization of the worst fears of whale shark scientists, and I count myself among those.

How can it be that whale sharks are unable to tell the difference if their sense of smell is so good? One simple explanation is that the olfactory abilities may be extremely selective. Scientists don’t know exactly what sort of chemicals whale sharks are homing in on when they seek out patches of food in the ocean – indeed, addressing this question is one of the goals of this year’s whale shark research program at Georgia Aquarium – but we have some good candidate molecules. If the whale shark sense of smell is highly tuned to these compounds and relatively insensitive to other families of chemicals, like hydrocarbons (oil and gas), then it’s certainly possible that whale sharks simply cannot detect the problem.

That’s when the second adaptation, surface filter feeding, becomes a liability for whale sharks trying to negotiate the deadly emulsions and surface slicks in the Gulf. To fully appreciate why this is such a problem, we need to look a little more closely at the filtration apparatus whale sharks use to feed.

Like most plankton-feeding fishes, whale sharks use filters in the mouth/gill cavity to sift food particles from the water (see the exellent illustration by Emily Damstra at right). And like most plankton-feeding fishes, these filters develop from structures associated with the gills and gill rakers (cartilaginous rods that come off the leading edge of the gills and protect the gills from fouling and shape the current of water across the breathing surface). Where whale sharks differ radically from other planktivores like, say, anchovies, is that they do not have feathery interlocking gill rakers that serve to filter the plankton but can be disengaged from each other to allow bulk water flow out through the gill opening. Rather, their filters are so derived and so heavily branched that they form a single continuous pad that occupies the space between gill arches; it looks a lot like a black scouring pad. The gill arches cannot be disengaged from each other; thus, anything that goes in the mouth must be small enough to pass through the filters (less than 2mm, or about 1/12th of an inch), or it must be swallowed, or be spat back out through the mouth (something they are surprisingly good at!). In a paper currently in the review process, comparative anatomist Phil Motta from USF is describing the full functional anatomy of these structures; he took the photo of the filter pad surface shown hereabouts based on material samples from Georgia Aquarium.

The implication here is that oil that finds its way into the mouth, if it is not to be swallowed or to foul the filters, must be continually spat back. OK, I hear you say, perhaps if the whale sharks avoid feeding, there won’t be a problem. If only it were that easy. Whale sharks do not only use their mouths for feeding, they use them for breathing. They need to be passing water continually across the filters and thence across the gills, in order to keep the body supplied with oxygen. For the whale shark swimming in oil-affected waters, therefore, the animal’s breathing needs and the susceptibility of their feeding filters to fouling are in complete opposition.

If whale sharks are swimming into oil-polluted waters and fouling their filters with oil, what does that mean? In my best estimation, it means that the oil spill represents an extremely serious threat to whale shark health. I am by no means the first person to suggest this. Nature identified whale sharks as one of the 5 species most likely to be affected by the oil spill, and other scientists like Bob Hueter from Mote Marine Laboratory have also highlighted the risks. The true toll that the spill exacts on the Gulf of Mexico whale shark population will not be known for some time, but the thought of dead or dying whale sharks sinking silently into the depths (dead sharks generally sink, not float) is yet more motivation to put an end to the spill and to undertake immediate and extensive research and conservation programs to assess the damage and plan a road to recovery for the whale sharks – and all the other affected wildlife – in the Gulf of Mexico.


Mountains of Pelagic Diversity

If you ever saw the dramatic seamount scene in Blue Planet (and if you haven’t, where ya been??), then you are probably familiar with the idea that submarine mountains can attract lots of animals; as Attenborough puts it, they “create oases where life can flourish in the comparatively empty expanses of the open ocean”.  In that spectacular BBC sequence, jacks and tuna swarm an Eastern Pacific seamount peppered with colourful schools of barberfish, Anthias and goatfish.  Then the sharks cruise in, including silkys and hammerheads, there for a clean from the faithful barberfish.
There’s a paper in the latest issue of PNAS that quantifies the richness of seamounts, so beautifully depicted by those geniuses at the BBC Documentary department.  The authors, led by Telmo Morato from the Secretariat of the Pacific Community in New Caledonia, analysed data gathered by longline fisheries in the western and central Pacific, close to and remote from seamounts .  In a sense, a longline is a standardized sort of sampling unit like a quadrat, so they can be analysed across locations to measure differences in diversity.  They accounted for differences between total catch per longline using the statistical process called rarefaction which is a practical application of one of my favourite fundamental biological patterns – the species accumulation curve - which I’ve discussed before (here and here).  It looks like a great dataset with great spatial resolution and pretty good coverage in the tropics, though the equatorial zones are less well-represented.
I don’t think anyone would be surprised by their result that, yes, seamounts are diverse places.  When they broke it down by species, about 2/5 (15 species) showed positive association with seamounts; this group included both sharks and fish.  Interestingly, 3 species (pelagic stingrays, albacore and shortbilled spearfish) showed negative associations with seamounts, while 19 showed no measurable association.  So, the net effect is positive, but there's clearly some structure in the data, depending on what species you look at.  Nor, I think , would most people be surprised by the distance effect they found, wherein sample diversity decreased with distance moved away from the peak of a seamount, and most sharply in the first 10 or so kilometers.  What was surprising, to me at least, was that both the absolute diversity and the distance effect they found were greater on seamounts (left) than they were for coastal zones (center). 
I would have thought that coastal zones, with their larger area, more complex topography and currents, coastal upwelling and inputs from the land, should have had higher diversity.  Indeed, it kind of goes against the island biogeography ideas, that as we go away from the largest habitat towards smaller more distant patches, diversity drops; if you think of seamounts as underwater islands and continental shelves as underwater mainlands, perhaps you’ll see what I mean.
There’s a couple of reasons I can think of to explain the observed difference.  Perhaps there is something intrinsic to seamounts, some feature of topography or productivity that makes them real magnets for diversity.  Under this scenario, they are true biodiversity hotspots.  Alternatively, perhaps coastal zones once were more diverse than seamounts but have been denuded by our actions, so that only the remote and submarine mountains remain as examples of what once was.  Perhaps it’s a bit of both, or some other concept (that you should propose in the comments).  Either way, Morato et al. show us that we may be successful at protecting widely roaming pelagic species by strategically preserving relatively tiny specks of submarine oases.  Since reading their paper, I have enjoyed thinking of schools of pelagics, hopping from mountaintop to mountaintop, skipping across vast plains of abyssal ocean, and as usual dreaming about diversity and all the fantastic forms of life in the 3D wonderland of the open ocean.  It just makes you want to down tools and grab the next slow boat bound for Cocos, doesn't it?

Morato, T., Hoyle, S., Allain, V., & Nicol, S. (2010). Seamounts are hotspots of pelagic biodiversity in the open ocean Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0910290107


Plastiki update

If you're not following the unusual Plastiki expidition, its a boat made of thousands of plastic drink bottles, sailing across the Pacific to raise awareness of plastic pollution in the oceans.  They've now travelled 1900 nautical miles and are about 1000 miles east of Hawaii.  Follow them here or on Twitter as @Plastiki


What's a Manta do?

Manta rays (Manta birostris) surely vie for the title most spectacular among the large animals in the ocean. Not only do they grow to enormous sizes, but they are placid, graceful, and generally unafraid of humans, which means we can get close to them in the water and really appreciate how incredible they are, up nice and personal. I always thought that mantas were a one-of-a-kind species - the only member of its genus - like humans, whale sharks, koala bears or killer whales. Luckily, Andrea Marshall is not like me. She and her colleagues took a closer look at the body features, colours and patterns on lots of mantas from all around the world and they concluded that there are at least two, and possibly even three, manta ray species. They’re not the first people to propose this, so technically what they have done is “resurrect” the name Manta alfredi, the Prince Alfred manta, which had been made a synonym of Manta birostris some time ago (read the paper for the full sordid taxonomic history of mantas). The differences between the two species are subtle and mostly to do with the colour of the lips, wings and shoulders, the spots on the belly and the presence or absence of a bony mass near the base of the tail, but nonetheless they probably reflect real differences between the animals and, under the current definition of “species”, they probably cannot successfully interbreed. The third potential species they call “Manta sp. cf. birostris” which is taxonomist shorthand for “as-yet undescribed manta species sort-of like M. birostris”.

If you have ever been to the Georgia Aquarium, you may have seen one or both of their mantas in the Ocean Voyager exhibit. If you look closely at these and compare them to the Marshall paper, you’ll see that one (called “Nandi”) is Manta alfredi and the other (“Tallulah”) is more like Manta sp. cf. birostris. Its slightly ironic that in light of this new paper, neither of them is the “actual” or original “manta ray”.  Of course, they are both still spectacular animals!

Who cares about all this anyway? What does it matter if there’s one or three or a dozen manta species? As it happens, it matters a great deal! Taxonomy underlies everything else in biology. What good is a population estimate, for example, if that estimate confuses two species? We would grossly overestimate both, potentially leading to overexploitation. More generally, how can we understand migration patterns, breeding grounds, diets, ecological roles or behaviour, if we are constantly confounded? These are, of course, somewhat self-centered concerns about the quality of our science or management decisions; a species count is about the most fundamental measure of nature that we have, and those sorts of diversity stats are predicated on a decent taxonomy. Consider this: how much of a ginormous “oops!” would it be if we were to protect a species in one area of ocean, only to learn that the animal in the area we didn’t protect was actually a different species?   Perhaps a more important reason it matters is for the mantas themselves and the rest of their ecosystem.  Each species has an intrinsic right to exist and a value to the ecosystem its part of. 

I just love the idea that even for familiar, charismatic mega-animals like mantas, if we look a little closer, nature shows us hidden diversity: surprising, unexpected, and exciting.

Marhsall, Andrea D., Compagno, Leonard J.V., & Bennett, Michael B. (2009). Redescription of the genus Manta with resurrection of Manta alfredi (Krefft, 1868) (Chondrichthyes; Myliobatoidei; Mobulidae) Zootaxa, 2301, 1-28


I'm surfing on the inside

I've always loved the idea of internal waves; the idea that gentle, rolling, and sometimes very large waves roll along, not on the surface of the sea, but deep below it.  How is that possible?  The best explanation is by analogy:  If you've ever swum in a lake in early summer, where your body was bathed in warm still surface waters but your legs were down in the icy deeper layer, then you've crossed the boundary that internal waves call home.  They travel along "density boundaries" and the most common of those is the bit where water goes from warm at the surface to cold at depths, called the thermocline; there are other types of density boundaries too, such as where fresh water overlies denser briny water.  Sometimes you can even see density boundaries near the surface; the change in density affects the way light passes through the water so you can sometime see a shimmery sort of distortion, even though the water is clear.  There's a good one shown here. The picture at right from the Institute of Hydromechanics at the University of Stuttgart shows an artificial internal wave produced as part of an experiment; they dyed the different densities of water to show it better.

The bigger the change from warm to cold in a thermocline, the bigger the density difference, with the colder water being more dense.  Really sharp thermoclines like this have some interesting properties, such as the ability to reflect some types of sonar.  In fact, Navy submarines have been known to "hide" below a good thermocline, and then be revealed for all to "see", by a passing internal wave.  The sub is below the thermocline one minute, then above it the next, exposed and vulnerable to the next sonar ping - oops. 

Internal waves can even "break", like a surf wave on a beach.  This image from Memorial University in Canada shows a model of how this happens, with the wave coming in from the left and breaking on the bottom as it gets shallower; all the while the water's surface is calm.  The internal wave doesn't look quite the same as a regular beach wave because the density difference isn't nearly as much as when you go from water to air and the internal wave pushes up against the heavy, viscous overlying water, but the principle is the same.

I don't know if its possible to somehow surf on an internal wave.  I doubt it, because the drag of the overlying water would be much more than you would experience in air, but its pretty cool to think about.  Its probably a good thing if you can't do it, because Al surfing is about the only thing scarier than Al dancing...


CITES epic fail?

David Helvarg has a scathing OpEd piece in the Huffington Post yesterday, and rightfully so.  CITES, the Council for International Trade in Endangered Species recently failed to give proposed protections to the northern bluefin tuna and several species of threatened sharks, apparently caving to the desires of Japan and other nations with similar pro-harvest agendas.  I dont know how much data you need to be convinced that these populations are threatened to the point of collapse, but even if if there were equivocation on the science (and there's not), why not err on the side of safety?  Just as line calls in baseball go to the batter, decisions regarding endangerment should always go to the organism.

The way I see it, bluefin are stuck in a positive feedback loop of ever increasing commodity value, feeding more intense searching/fishing efforts, further reducing the population and thereby driving the value yet higher.  Its a trajectory that only ends one way, and it ain't a good one.

Oh, and if what Helvarg says is true about the Japanese embassy serving bluefin sashimi at a reception for the CITES delegates, then wow. Just, wow.  I sincerely hope those were artificially reared and not ranched or wild-caught...


2 of the 10 worst jobs in science

Popular Science has just published its annual "Ten worst jobs in science" issue, and two of them are in marine science!  How is this possible?  Marine science is clearly the best job since, well, ever.  Hmmmm, lets take a closer look.

1. Oceanic Snot Diver.  The name sounds gross enough, but what does it mean?  Well, it turns out that they are talking about collecting "sea snot" true enough, but to call it nasty is a bit of a beat-up, IMHO.  Scientists call this stuff "aggregates", and its an incredibly important part of the nutrient cycle in the ocean.  Really, sea snot is just the secreted mucus and fecal casts of hordes of plankton.  Wait a sec, when you write it like that, it does sound gross!  Its biggest role is in "exporting" nutrients from the surface layers of the ocean, where the sun sponsors all that plankton growth, to the dark depths, where sunlight never penetrates, but life nonetheless thrives.  Not only do some animals down there eat the stuff (ew), but at those crushing depths, some of the snot also dissolves under the immense pressure of all the water above it, much like snow melting before it reaches the ground.  In this way, the snot plays a very important part in taking nutrients produced at the surface, and dissolving them in the water at great depths.  Maybe not the most attractive concept, but pretty important in the grand scheme of things.  Like Tom Cruise says in The Firm: "Its not sexy, but its got teeth". 

2. Whale slasher.  OK, I have to concede that one.  I've seen a few stranded whales being cut up on the beach (this is called a necropsy, not an autopsy, which is reserved for people only), and it pongs.  I'm not talking sweat sock pong, or even doggy-breath-after-eating-goose-poo pong, but serious, invasive, gets-into-your-hair, throw-your-clothes-away stink.  While the cause of death is always interesting, wading through week-old whale giblets that have been baking on the beach?  Not so fresh...


Bon Voyage, Plastiki

Some enterprising folks have built a tri-maran out of 12,000 two liter plastic softdrink bottles.  Dubbed the "Plastiki", she took to sea today, on her maiden voyage from San Francisco to Sydney.  Along the way, the Plastiki will serve as a floating demonstration platform for sustainable technologies, and will spend some quality time in the Great Pacific Garbage Patch, where vast amounts of floating plastic debris are becalmed in the heart of the enormous gyre current of the North Pacific.

Follow the Plastiki on Flickr here, and on their website here.  Best of luck and kind winds, folks.  If you begin to lose buoyancy, I guess you can always reach overboard and grab another bottle from among the flotsam, you know, to repair the hull with.  :-/


Whale sharks arrive early at Ningaloo Reef, Australia

I have never been to Ningaloo (great name, right?) - its WAY on the other side of Australia from where I grew up - but its a fascinating place.  Whale sharks gather there every year, and this year it looks like they turned up earlier than usual.  Whale shark aggregations are amazing events and the one at Ningaloo is one of the biggest.  Why gather? Why Ningaloo?  Why then?  How do they know where to go?  These are just some of the great mysteries of whale shark gatherings; its amazing that we know so little about the worlds largest fish.