How marine larvae find their way home

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  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