A new angle on diving in whale sharks

Recently I featured a piece about how turtle hatchlings change their movement strategy several times in just the first few hours of life in order to suit their changing needs as they move across different types of sand.  Well, to go from the sublime to the ridiculous (or rather, just from the really small to the truly gargantuan) there’s a new paper out that shows that whale sharks, too, adjust the way they move according to their needs.  This new work follows nicely after Phil Motta’s paper earlier this year, also discussed here, which took a comprehensive look at how whale sharks feed.  Between them they make big strides in the autecologyof whale sharks.  The new paper, by Adrian Gleiss and Rory Wilson from Swansea University and Brad Norman from ECOCEAN, describes work they did at Ningaloo reef in Western Australia, perhaps the world’s best studied aggregation area for whale sharks.  They took a new type of accelerometer tag developed in Rory’s lab called a “daily diary” and deployed them on wild whale sharks to measure not only where they are (like traditional wildlife satellite tags) but also details about what the animals were doing: the beating of their tails and the rientation of their bodies in 3 dimensions.  From this information they could basically reconstruct the animal’s actions with computer game-like accuracy (indeed, the software looks a lot like something for the X-Box!).  The findings show Whale shark with accelerometer tag. Photo: Steve Lindfieldan animal with a surprising diversity of movement modes and a sophisticated approach to minimising the amount of energy they spend moving through the ocean.  They also help explain one of the most enduring mysteries of whale shark biology - a curious pattern of super-deep dives over the abyssal depths.

The first big observation is that whale sharks have 4 different types of dives and that these probably serve different purposes.  It was well known from traditional tagging studies (sat tags record depth data as well as location) that whale sharks dive quite a lot throughout the day, but the new daily diary tags showed that not all dives are the same and, in fact, they could be easily discriminated as one of four main types based on what the depth profile looks like: yo-yo (probably searching), V (horizontal movement), U (feeding at depth), and bottom bouncing (searching at depth).

Four dive types in whale sharks. (a) yo-yo, (b) V-shaped, (c) bottom bounce, (d) U-shaped

The second and a really key finding is that all the dive types feature a gliding descent and an active ascent.  In other words, they don’t beat their tails on the way down, but they do on the way up. Gliding converts their negative buoyancy (a sort of potential energy) into horizontal and vertical movement (kinetic energy).  The fact that their dive has both vertical (depth) and horizontal (forward motion) components, without active use of the tail, shows that their bodies are adapted to convert sinking to swimming.  Most of that effect comes from their pectoral fins, which serve as wings for gliding, but there is probably a big contribution from that incredibly broad head, which serves as a sort of lifting canard, a flat plane that creates lift at the front end. Gliding is an extremely efficient way to move; not only are they not spending energy operating their musculature to beat the tail through the water to create thrust, but the drag coefficient of water across their skin is a third as much when they glide as when they are actively beating their tails.

The third finding, which Gleiss and friends really go into in some, ahem, depth, is that angle of descent and ascent is consistently different for each type of dive and that they are optimal for whatever the purpose is.  For example, a V dive is meant to cover a great horizontal distance, so the gliding descent is at a shallow angle and the ascent angle is the one that minimizes the amount of energy spent to gain horizontal distance. In contrast, the angles of a yo-yo dive minimize the amount of energy spent to gain vertical distance.  The important point is that for any given purpose there is an optimal angle - one that uses the least energy for the most benefit - for both descent and ascent components.  Whale sharks adapt the geometry of their dives to stay in that optimum zone and minimize the amount of energy they spend on whatever they are doing.  Clever right?  Well, its probably not a conscious decision, but rather a state of tremendous efficiency towards which they have evolved: natural selection is a powerful tutor.

Diving geomtry in whale sharks. The angles of descent and ascent (the two thetas) are optimised for minimum energy expenditureIn the course of their study, the researchers solved one of the great mysteries of whale shark biology: the extraordinary deep dives whale sharks do when they are over abyssal depths (see the Brunnschweiler reference).  These dives tend to happen around dawn and dusk and may exceed 1600m or more in depth; in fact, we don’t know just how far down they go, because most tags have a self-preservation device that cuts them free of the animal at 1600m, lest the tag be crushed by the immense pressure of the overlying water.  We’re talking depths enough to turn a Styrofoam coffee cup into a shrinky-dink thimble, as well as changing enzyme kinetics and making the urea in their blood greatly more toxic, so the motivation to dive so deep must be compelling. There had been suggestions that they go down to rid themselves of parasites (as a parasitologist, I never bought that: parasites would easily co-evolve to tolerate such a strategy), or to clean their filter plates (at those depths particulate organic matter redissolves!) or even to “sleep”, although there is no evidence that sharks do so.    Well, it turns out that they are dives of the V type, optimised to spend the least amount of energy while achieveing maximum horizontal movement (HD in the figure above).  In other words, to travel far, they glide deep, and then gently beat their tails and ascend at a very shallow angle (steeper angles costing more energy and achieving less horizontal distance).  This is a strategy best used during migratory phases when travelling, not feeding, is your number one priority. 

To recap then, whale sharks turn out to have at least 4 main types of dives, each serving a different purpose from feeding to horizontal travel, and the geometry of each type of dive is optimised to achieve the goal while minimising the energy cost.  Overall, it paints a picture of an animal that is a paragon of efficiency, which is understandable given that they dwell in the nutrient-poor surface waters of the tropics, which are typically much less productive than the rich cold temperate and Arctic seas frequented by their fellow filter feeders: basking sharks and baleen whales.  I suspect that future studies will show that whale sharks deploy these movement strategies to travel efficiently between hotspots of tropical productivity, be they fish spawning events or patches of seasonal tropical upwelling, and that they are therefore extremely strategic masters of the feeding/travelling trade-off.

Gleiss, A., Norman, B., & Wilson, R. (2010). Moved by that sinking feeling: variable diving geometry underlies movement strategies in whale sharks Functional EcologyDOI: 10.1111/j.1365-2435.2010.01801.x

Brunnschweiler, J., Baensch, H., Pierce, S., & Sims, D. (2009). Deep-diving behaviour of a whale shark during long-distance movement in the western Indian OceanJournal of Fish Biology, 74(3), 706-714 DOI: 10.1111/j.1095-8649.2008.02155.x

A whaling conundrum

With tip of the cap to jfang at The Great Beyond

A recent proposal to limit whaling has been rejected by Japan and Australia, for opposite reasons. Japan, which takes almost a thousand whales a year, mostly Minke, objects to the 400 annual quota, which steps down after 5 years to 200 for another 5 years. Australia, which has a long history of opposing whaling, says the proposal doesn't go far enough; they're basically looking for a zero tolerance whaling policy.

Honestly, much as I hate the idea of even a single whale dying in the name of the imaginary research that Japan uses to defend commercial whaling, I think the Aussies might be being a little hard nosed in this case. Lets say the proposal is rejected, then the Japanese continue to take a thousand whales a year - how is that better? The art of negotiation is compromise, and in my view its always better to accept steps in the right direction, even if you don't get everything you want. Its like selling a car: you advertise for 10 grand, hope for 9, expect 8 and accept 7. If you hold out for 10, you're going to be disappointed most of the time.  Obstinacy doesn't help the cause.

In his vision for whaling, Peter Garret (Australia's environment minister) states that the right solution is to restructure the International Whaling Commission.  That may be so, but in the 2 years that it might take to do that, you could have saved 1,200 whales if you accept the current proposal first, and then go after the recalcitrant nations through a restructured IWC with more teeth.

There's a key line in the Great Beyond post linked above, from IWC chair Cristian Maquieira: “I don't think anybody will be happy with the numbers."  I often recognise that as the sign of a successful negotitation: a good outcome is not when everyone is happy, but when everyone is equally unhappy.

While we're talking genomes...

...here's something distinctly more marine. 

A little while ago I drew attention to Andrea Marshall's paper showing that there's not one but possibly three species of manta ray (see Whats A Manta Do?).  In the preamble for that post, I drew analogy between mantas and killer whales as monotypic species; that is, the only members of their genus, a taxonomic one-of-a-kind.  Well blow me down if some new genomics work with killer whales doesn't suggest that there's more than one species of those, too!  Morin and colleagues used a different approach than Marshall, whose work was mostly based on colors and patterns and tooth shape.  Instead, they used "massively parallel pyrosequencing" (try saying that with a mouth full of marbles) to show genetic differences in the mitochondrial genome.  So what the heck does that mean?  Well, lets just say its sequencing a whole bunch of DNA at once, using DNA not from the nucleus of the cell, but from its engine room: the mitochondrion.  The technology is actually a really, fantastic example of miniaturisation; perhaps I'll write about it one day.  But, I digress...  Morin and friends recommend three species of Orcinus orca, with two more subspecies as well.  Subspecies are not required by the taxonomic code, but they are eligible for separate protections under the Endangered Species Act, so its a meaningful result for conservation biologists too; they'll now have to make assessments of each species and subspecies to see which, if any, require additional protections.

To the experts, its not a total surprise that there are multiple species in either of these groups.  You can bet your bum that they set out to confirm a hunch that there are more than one, leaving the surprise for the rest of us less familiar with these beasties and who never saw the subtle differences.  That's OK, I like surprises, especially when they involve new and unexplored diversity, right under our noses.  Maybe we should take a harder look at a few more monotypics, for the inevitable species flocks hiding in the details or the DNA.  Whale sharks, basking sharks, Mola, anyone?

Morin, P., Archer, F., Foote, A., Vilstrup, J., Allen, E., Wade, P., Durban, J., Parsons, K., Pitman, R., Li, L., Bouffard, P., Abel Nielsen, S., Rasmussen, M., Willerslev, E., Gilbert, M., & Harkins, T. (2010). Complete mitochondrial genome phylogeographic analysis of killer whales (Orcinus orca) indicates multiple species Genome Research DOI: 10.1101/gr.102954.109

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...

Its a boy!

Congrats to the folks at Shedd Aquarium on the birth of a baby beluga whale - one of my personal favourite species (you will never meet a sweeter disposition in all the oceans).   Tell me this isnt the cutest little grey slug on the planet.

The little fella was born head first, which makes great sense for humans - where taking that first breath of fresh air is just a birth canal away - but not so great for whales, where you have to hold your breath until the rest of you comes out, before rushing to the surface.  In whales, such a head-first birth is considered a breech birth.  Nonetheless, he's apparently doing just fine.

He'll be grey for the first couple of years of life and then gradually get ever whiter, until he earns the common name for the species: "beluga" is Russian for white (and is actually pronounced more like b'loo-HA, with the H way in the back of the throat)