Sensing the underwater world

11 May 2011

How do marine animals sense the world around them? This month on Naked Oceans we explore the many ways aquatic critters see, sniff, and hear in their watery realm. We discover how tiny fish and lobsters find their way back home, we chat with a scientist who sends out underwater robots to listen for whales - even in the middle of a raging storm. And in Critter of the Month a legendary ichthyologist tells us a fantastic story about a shark on a plane.

In this episode

07:38 - How marine larvae find their way home

How do tiny fish and lobster larvae find their way home?

How marine larvae find their way home
with Jelle Atema, Boston University

Helen - This month on Naked Oceans we're looking into the sensory world of marine animals - obviously it's a very different realm down there compared our dry lives on land, but just how do ocean creatures find their way around and why is it important for conservation efforts that we understand what they get up to?

To get things going we chatted with Jelle Atema (yeller arti-mer) from Boston University to find out how-and-why he studies the ways marine animals build a picture of the world around them.

Jelle - My whole career is based on this intrigue with what other animals can see in American lobster the world. I decided that probably the most interesting place was underwater because it is so unfamiliar to us. So, if we want to enrich our understanding of sensing the information that is actually floating around in the environment it pays off to go to animals that live underwater and ask them some questions.

So, I joined a team of Australians in the Great Barrier Reef who were very much fish ecologists. They had until then pretty much considered larval fishes to be floating, passive particles who could not possibly swim against ocean currents and therefore were at the mercy of ocean currents.

I however came from a very different point of view. I look at little fishes and I look at their brain and I see highly sophisticated small versions of larger animals. And so I thought it cannot be, you cannot develop a big brain and all these senses for nothing and just float around willy-nilly in the ocean. 

So my intend was to go and mine the knowledge we had gathered from other animals to see if larval reef fishes actually can influence their trajectory in the wild, big ocean.

The answer, in short, is yes. We have discovered for instance that they use their olfactory sense to discriminate between the odours of different reefs. Even thought the reefs are very close together.

That means that they probably imprint on that at birth because at birth that's basically the only information they can gather. They are sitting there, often brooded in some species and so the local environment in which they hatch is the first experience they have with the world. And that knowledge can help them later on if they want to come back to the reef, because many of these reef fishes and also invertebrates are dispersing as larvae out in the open ocean and have to come back somewhere to a reef. 

So what better knowledge than where you were born. You know for sure that's a reef and if it doesn't work, go to another reef that is similar.

We also have just discovered that they have a sun compass. So just like birds and insects they can use the sun for general orientation. Other people have worked on sound and have discovered that they orient towards sound. And we expect that they will have a magnetic sense as well.

So here are little floating particles who have real knowledge of the ocean and can influence their return to the reef and thereby the structure of the population as a whole. And that has conservation implications.

Helen - We'll come back to those conservation implications shortly, but first, as well as reef fish, another marine animal that Jelle studies is the lobster.

Along the New England coast, where he works, there are some lobster fisheries that are doing fine, and others that are in serious trouble with lobster numbers dwindling. Jelle and his team set out to investigate what's going on, in particular in a place called Naragansett Bay on the north side of Rhode Island, and they made an interesting discovery which got them thinking about what it is that makes tiny lobster larvae head for home when they have the big wide ocean to choose from.

Jelle - What we discovered is that lobsters are different by their morphology. Lobstermen have always said that. They've said 'I know where this lobster comes from, we know this lobster is different from that lobster.'

So our surprising discovery was that within Naragansett Bay and only 30m distant we find two populations that are different. SO we looked carefully at the fine structure of who they're built: how long this leg is, how wide this leg is, the tail fan, anything we can measure. And based on that we compared these populations and we see that they are different.

Then we take a little piece of tissue, and we find that genetically they are different. And finally, we ask females from one and the other population to see where they would like to spend more time: with the side of the tank where a male from their own or from the other population is.

And it turns out that significantly, by a small margin, they prefer their own males.

Now these three things together could indicate that in fact we are dealing with a locally recruiting population because in order to get this kind of difference they have to repeatedly go back to the same place and reproduce in the same place.

That is hard to believe when you know how the lobster lives. Lobsters like the reef fishes disperse in the ocean, so they can travel a long distance that they are at sea. So how come they can still maintain a local population at 30km when they can move 100km?

In addition, once they settle, they are mobile adults not like reef fishes who sit where they are. But lobsters have been documented with tagging studies to be able to travel over 100s of miles

Helen - How are you going about answering that big question mark of how the lobsters don't just all being mixed up in one big population? What sort of studies are you doing?

Jelle - There are two directions of answer here. One is we want to first establish that what I just told you is really significant. I still don't want to walk on that thin ice because the implications are so significant that I haven't even published it yet.

The proposal that is right now in the hopper, is to go to a place where we saw this structure, in Naragansett Bay, and now sample 5 sites in Naragansett Bay and 5 sites outside Naragansett Bay, and sample it 3 times per year for 2 years.

So from that study, 2 years from now, hopefully if it gets funded, we would know how persistent and how different and at what scale these subpopulations are different. So that's the first step.

In the mean time we're working, because obviously that is really my interest, is How do they do it? We have a number of ideas about this. Once again it comes to larval imprinting that these animals may have an olfactory notion of where they were born and may have a preference for ocean water that smells like home.

In the beginning of their larval stages they cannot swim very much but in their 2nd part of their larval stage, they look like little lobsters and can swim incredibly well, faster than humans, even Olympic champions.

And so they can swim for days on end without feeding and so they can cover huge distances.

If on top of that it turns out they have navigational capabilities, let's say with a magnetic or a sun compass, they could steer in an innate direction. So if they were born automatically with the sense that they should be swimming north west - the way to go to the continent, right, if you're at sea - then that would already help them settle on the coast.

The olfactory capability is another thing. We have started that and we know that they have various preferences for instance for larval lobsters themselves so they can settle with each other. But we don't know yet if they prefer their local stock verses other stocks.

Helen - If we understand more about how local populations are seeding themselves, how they're working, we can' necessarily consider the lobsters as being this one, interconnected population. Maybe it's good news if there were local populations because they a local fishery could really look after it's own piece of sea and see benefits from what they do?

Jelle - I think you hit right on it. It is probably very good to have local management. Let's go back to the reef, because in the reefs you have these very distinct entities that are called reefs. Reef fishes can only live on reefs, in between there is an ocean desert where they can only live as plankton in their early larval life.

So, if you have a reef that you set aside as a management sanctuary and you are assuming that these animals will automatically seed all the nearby reefs, then you can expose the other reefs to fisheries and say well, it's okay because we have one reef that supplies all the larvae for everybody else. And this was one notion that people started with.

If it now turns out that most of these larvae are going back to their home reef, and not going to the other reefs except occasionally with a storm, then the recovery of the plundered reefs will take much, much longer. I'm not saying it will not happen, but it doesn't happen next year, it might happen in 10 or 30 years. So that has enormous management implications. 

If you now jump to the lobster territory which is not built of separate reefs but it is continuous coast, it seems less likely that lobsters can do this same population structure.

However if we actually manage to establish that they do regardless how they do it, we can use the same management implications for the reef as we can do for the lobsters.

Find out more

Jelle Atema's lab

17:32 - Fishy vision in the deep sea

How different is the vision of fishes living on kaleidoscopic coral reefs compared to their deep-sea-dwelling cousins?

Fishy vision in the deep sea
with Shozo Yokoyama, Emory University

The Coelacanth, Latimeria chalumnae model in the Oxford University Museum of Natural HistoryAs different species of fish evolved over huindreds of millions of years, their vision became adapted to where they live in the ocean. Coelacanths for example, live about 200m below the surface, and have ditched the pigments in their eyes that would allow them to see red and green light, as they no longer need them - they live in a monochrome world. But how can we track how vision has evolved over time? Well, by using a combination of the fossil record, and genetic techniques, and Shozo Yokoyama explains...

Shozo - When you look at the retina, we have one type of pigment which is expressed in rods which we call rodopsin, and then four kinds of pigments are expressed in cones. For example one type of cone pigment detects either UV or violet, another type of pigment detects red or green, that's other extremes. The other two types absorb light between blue and green. So essentially four kinds of cone pigment we have, in fish.

Sarah - And are all four of those pigments seen in all fish? Or are some of them lost within some groups?

Shozo - Oh yes, their compositions can be very different, depending on where you live.

Sarah - When we look at living species of fish, when we compare the visual pigmets of different groups today, what sort of differences do we see between them?

Shozo - I think different groups of fish do not make much difference, but we can see more similarities among them. But the biggest difference comes from where they live in this case the depth of the ocean. And that effects them a lot.

Sarah - So, if we compare something like a shallow water, a fish that perhaps lives on a coral reef compared to something that lives a very long way down where there's not a lot of light at all?

Shozo - As you go down in the ocean, as you go deeper, then the amount of light will be reduced a lot and the specific light can do down can penetrate, that is because the water absorbs red colours and UV colours. So, depending on where they live, their vision can be significantly effected.

I give you one example is the coelacanth, which live 200m under the ocean. They live close to the Comoros Archipelago in the Indian Ocean and they get very special light. So coelacanth kept only rod pigments, rodopsins in rods, and one kind of pigment in cones, that's all and other genes have been eliminated.

We can study that at the molecular level and the function levels, so we know what they did to live in that environment.

But if you come up in the shallow waters, fish can see practically everywhere from UV to far red. So whereas this example with coelacanth can detect only very narrow strip of light around 480nm.

Sarah - So is it a case that with something like a coelacanth because they live at a depth of the ocean where only a certain amount of the light spectrum gets through to them there's no point in having visual pigments that would allow them to see the red end of the spectrum or the UV end?

Shozo - Yes, that's the way it works. Actually they did that very perfectly so, the coelacanth's ancestor, palaeontological data says they used to live near the shallow water. Then maybe 200 million years ago or so they went down, they started going down to 200m. Their ancestor could see all sorts of light, from UV to red, but today's coelacanth can see only around 480nm, that's around blue. And at the same time, what they did was that they eliminated the gene which detected UV and red, so their gene structure of their visual pigment is very, very simple.

Sarah - And if we look even deeper, so right down into the deep, deep sea, I suppose a lot of the fish species that live down there don't have any need for cones that will see red light, because red light attenuates so much in water that there basically isn't any left down there, so they have no need to see it. But there are some species that produce their own red light, to be able to see prey species. Do we see a specialization of cones in those species to be able to see red light?

Shozo - Actually that's what we'd like to know. There's one fish called the loose jaw, which creates two kinds of light. Many bioluminescence produce 480 but this particular fish produces both 480 and around 700nm, that's red, right.

And the genetics of this one, we really want to clone these genes which we are doing. But a function has not been determined yet from a molecular genetics point of view. So, that's why I cannot tell you too much about it.

Sarah - So I guess it's a case of, if you produce the light you want to make sure that you can see it?

Shozo - Right, I think that's what they would do. Of course another purpose of the bioluminescence is for camouflage, right. So, if you go down in the ocean and look up, then the sky is kind of light, so fish want to make their belly whiter, that's the reason they use bioluminescence.

But if some of the red light, as you mentioned, is to be used for hunting or communication then they have to be able to see it. So to establish it we have to really clone the gene but we're not really there yet.

Find out more

Shozo Yokoyama's lab

23:54 - Underwater robots listen for whales

Underwater robots listen out for whales even in the middle of a raging storm

Underwater robots listen for whales
with Mark Baumgartner, Woods Hole Oceanographic Institution

Mark - Well one of the reasons we wanted to look into autonomous vehicles, or you can think of them as robots, to go to sea to do work for us is that it's very difficult to access marine mammals often.

It's expensive to go out on ships, and there's lots of times when we can't actually observes marine mammals. So for instance during difficult weather and when animals go far off shore it becomes difficult to get ships out to access them.

Ocean gliderA friend of mine, who works here at the Woods Hole Oceanographic Institution in the physical oceanography department, he has built a career on using these autonomous underwater vehicles for his physical oceanographic research.

And so we got together with the idea of equipping these vehicles with passive acoustic recorders. So not only can they be collecting a lot of oceanographic information but they can be listening for marine mammals as well.

And so in 2005 we developed a digital acoustic recorder and we put it on a glider and tried it for 5 days out off the coast of Massachusetts here.

We didn't realize we were going to have such a spectacular demonstration of this technology. When we put these things in the we put these things in the water a Noreaster, which is a powerful storm here on the east coast of the US, blew through the area when the gliders where in the water.

We were actually at sea at the time, trying to do research on North Atlantic right whales and we had to leave the area and come home, the storm was so intense. There were gale force winds, seas were up I think over 15 feet. These are conditions that are just not possible for us to work. But the gliders - no problem at all!

So they proved why we wanted to use them, right off the bat.

And they did a great job. They stayed at sea, they collected a lot of information, and the recordings were just wonderful that we got back.

Helen - What do these AUVs, these autonomous underwater vehicles, or ocean gliders, what do they look like? And how do you control them?

Mark - So, the ocean gliders the have long endurance because there's no motors associated with them. So the way they work, is they go up and down in the water column using what's called a buoyancy pump. They just suck in some seawater to become heavier than the surrounding water, and they sink. And when they get to the bottom of their dive they spit that water back out and they become buoyant and they float.

They have little stubby wings attached so that when they're going up and down, those wings provide a little lift, exactly the way and airplane glider works. 

The gliders themselves are sort of 4-feet long, maybe less than a foot in diameter. And so they're not very big. We paint them bright yellow so that we can see them. The way we operate them, is that they go to see, and we usually give them something to do. So, go to point A please, or stay in this one location.

And so they'll dive regularly to the bottom or to some specified depth then come back up to the surface. And every once in a while when they come back to the surface they sort of stick their but out of the water, and they have a GPS a Global Positioning System receiver on board, so they know where they are. And they also have a satellite phone, so they can phone home and tell us where they are and what they've been doing and what they've measured. We can see in real time what the water is like.

At that point we can communicate with the glider, it's a 2-way communication. You can say ok you've arrived at point A, please now go to point B.

And so you can sort of drive them but you're not sitting in the drivers seat necessarily, you're just telling the vehicle what to do, and it works out which direction it needs to go, how fast it needs to go.

Helen - Fantastic. And you said that a couple of years ago, you've tried one out and it seemed to work all the way through this great big storm and was wonderful for a few days.

Mark - Yup.

Helen - Have you now started using them  for a longer time? And what sort of things are you finding out?

Mark - So, we were involved in a pilot project to take gliders to an area that's fairly inaccessible to us and ships, in the Gulf of Maine here off the NE coast of the United States. There have been aerial surveys, they've been surveying the Gulf of Maine for many years and they found this pocket of right whales in the middle of the Gulf of Maine that seemed to go there every single year, kind of late fall/early winter. That's a very difficult time for us to operate on ships because that's getting into the beginning of the stormy season.

And so you can maybe stay out a day or two before seas get really rough and you have to come home again.

So we were thinking how can we go out there are figure out what it is about this habitat that's so important to the right whales. And to do that we typically take a ship and we go there and we take all kinds of measurements about what ocean conditions are like, what the food for the whales is like there, how much food is there, where's the food distributed. All of those things are impossible to observe from a plane. So we sent one glider, and two other autonomous underwater vehicles called profiling floats, which are very much like gliders only they don't fly they just up and down in the water column, and they stayed out there for a month.

So they were able to go to the area where the whales were and collect a lot of information about the environment that the whales had gone to exploit, we had acoustic recorders on all of those vehicles so we could tell where the animals were and where they weren't.

And so again it was  a good demonstration of the capabilities. They could stay there for a month and the weather, the sighting conditions didn't matter. It could be foggy, it could be night. They could still do their work.

Helen - What plans do you have next for these ocean gliders? Do you have things in the pipeline that you're going to be working on?

Mark - We're working on a system now where there can be not only are you recording the information on the glider, but you're also running software to detect marine mammals sounds and to classify them and say, Oo, that's a right whale sound or ah that's a humpback whale sound. Collect that information onboard the glider and then using satellite communication, basically phone that information home.

So we could potentially have these vehicles out in the ocean basically telling us what they're hearing, almost in real time. So you could say to the glider, come to the surface every 2 or 3 hours and tell me what you heard. And as scientists we'd like that information because we could send the gliders out to sort of do surveys for us. And find out where the animals are and then we can just go to those areas with planes or ships and study the animals, saving us an enormous amount of time.

For management purposes there's a lot of interest in this to mitigate chronic problem interactions between people and whales.

Right whales for instance are really prone to getting hit by ships and entangled in fishing gear. And so if you have of these vehicles out doing these surveys for you, you can cover a broad area for a long period of time to tell you where the animals are. And then you can build in safety features by saying well, we're not going to deploy fishing gear in this particular area because we know there are a lot of right whales there. 

Or over time, if you decided, well this particular area really seems to be important to right whales and this happens to be right where shipping lanes are, maybe we should move the shipping lanes.

And that actually has happened. A shipping lanes that went directly through a right whale habitat in the Bay of Fundy up in Canada, and those shipping lanes were eventually moved.

So it's just another surveillance technology that we can use to find out where the whales are so that we can better manage our activities around the whales.

Find out more

Mark Baumgartner's lab

Automomous Systems Laboratory

32:06 - Critters of the month - Lemon & nurse sharks

We ask legendary marine biologist Genie Clark aka 'The Shark Lady' to pick her favourite species

Critters of the month - Lemon & nurse sharks
with Eugenie Clark, Mote Marine Laboratory

We ask legendary marine biologist Genie Clark aka 'The Shark Lady' if she was a marine critter, which one she'd be, and why.

Genie - Never thought of being a marine animal!

Helen - Well, I guess the other question is which if your favourite?

Genie - Yeah, that's tough too because I have so many favourite. I supposed the lemon shark is my favourite shark because we got to know individuals and train them to press a target and ring a bell for their food. And that opened up a whole new field and interest.

Nurse sharkWe had nurse sharks very commonly here. The big nurse sharks didn't seem to catch on at all to the target but that's because they're almost blind. So they can't choose the correct target however we put it in the water. But the lemon sharks have keen eyesight.

But we later found out that nurse shark babies have keen eyesight and they loose it as they get older. And they feel their environment with their barbels.

So I remember taking a baby nurse shark to the crown prince [of Japan] who is now the emperor. I struck up a good friendship with a trained baby nurse shark that never made a mistake.

The airline gave me an extra seat for the shark. Most people didn't know, he wasNurse shark such a tiny little thing, he was less than 2 feet long. But he never made a mistake. 

And my assistant at that time, a high school kid Freddy Aronson, Freddy said, when I said, I have to take a present to the Emperor but they have everything. And he says, well, why don't you take our trained shark, nobody else in the world has a shark that never makes a mistake. So I took that.

Find out more

Mote Marine Laboratory

Eugenie Clark's website

Lemon sharks on Florida Museum of Natural History

Nurse sharks at Florida Museum of Natural History 

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