Explorer of the Month: Erica Staaterman

Erica Staaterman is a PhD student at the University of Miami, where she studies acoustic navigation in larval fish, in the lab of Dr. Claire Paris. She’s also been doing interviews for the Marinexplore blog. Her previous work focused on acoustic communication in crustaceans, including mantis shrimp and lobsters. She is fascinated by underwater sounds, particularly those made by the “small critters.” Today she spoke to us about the importance of acoustic communication in marine organisms.

Erica (left) with a whale shark near Mexico

Kalle: You’re working on communication of sea animals. How many sea-animals communicate with acoustic signals? Are these just mammals or fish too?

Erica: Most of my work has actually been with crustaceans, like shrimp and lobsters, and sound production in those animals, which is very understudied. It’s actually one of the most wide open fields in terms of bioacoustics. You can probably say 90% of the research that’s done in communication in the ocean is done on the whales and dolphins. Of the remaining 10% around 8% of the research focuses on soniferous fish, and the rest is on crustaceans.

Kalle: Why such numbers?

Erica: We’re just starting to discover these things. Marine mammals are less than 1% of life in the ocean. But there has been a disproportionate effort put into studying the sounds of whales and dolphins, probably because it’s really interesting, they have big brain and they are beautiful. But the truth is that most animals in the ocean do make sounds, but we are just beginning to understand how, why and what kind of sounds they make. So it’s a really new field, which makes it very exciting. This makes that a very good opportunity for me, because most of the work that I do is new. As a scientist, you always want to pick something what no one else is doing.

Kalle: But how many sea animals communicate with sound?

Erica: Maybe 90% of the animals in the ocean make sound. No one really knows, but the thing that you have to remember about the ocean is, that most of it is dark and it’s generally not very practical to communicate visually. And sound travels five times faster in the water than it does in the air. This means that the ocean is really an ideal place to communicate acoustically. So it would be very surprising to find a marine animal that’s deaf. It just doesn’t make sense from the evolutionary perspective. Now, how many make sounds, that’s another question.

For example, some of my work is on detection of sound in larval fish and whether they may use the sounds of a coral reef to navigate towards the reef. At the end of their pelagic phase, which is the time they spend traveling out in the open ocean, before the settle back on the reef they have to find their way to home somehow, they have to navigate there. There is some evidence that they may be listening for the reefs and navigating towards them using these cues. So, that’s an example of an animal that can probably detect sound, but we don’t know yet whether they can produce sounds.

Some ambient reef sounds with some mysterious “growls” that Erica has not identified yet. The crackle sound you hear is the sound of snapping shrimp. Property of the Paris Lab, University of Miami.

Kalle: So basically most of sea animals have some sort of sound detection mechanism to monitor the sounds in the ocean and reflections from objects like reefs?

Erica: Yeah. And there are other signals like the sound of waves, wind, rain or even the movement of other animals, which is really just a vibration through the water. There are a lot of animals that have a little sensory hairs on their bodies that can feel these vibrations. And that is truly a form of sound. Any kind of vibration is sound. It may be a little bit weird for us as terrestrial animals, because we think of sound as just the pressure wave that hits our eardrum, but for animals in the sea, sound can mean a lot of things and one of those things is just the movement of water nearby. And that’s information for them. If they feel a giant predator go by and move the water, they are going to respond to that, probably try to swim away.

Kalle: How complicated are the communication signals? Is it just “danger!” and “let’s date” or something more?

Erica: Definitely the most common and well-studied type of signal is the “let’s date” type – a mating call. But there are also a lot of anti-predator signals, which is a response from the prey if a predator is nearby. Let’s say you’re a prey item and you sense that there’s a predator near you. You might make a sound to tell to predator “Hey, I see you”. This means in other words, for the predator it may not be worth it to pursue the prey because they’ve been detected and they’re no longer sneaky.

You also might make sound to warn your conspecifics. Lobsters for example are living in a den. If they make a sound they might be warning their family that “Hey, there’s a predator nearby, everybody take shelter!”

There are also some species that make sound towards the predator to tell them that they don’t taste good, that they’re spiny or toxic. It’s called acoustic aposemitism.

Then there’s a startle signal. We actually discovered that this is something that lobsters do. When a predator physically makes contact with them, like take a bite out of their antennae, they make a little squeak sound in order to try to startle the predator. The predator might hesitate for a moment and so the prey might have a moment to escape.

Finally, there’s also maintenance of territory through sounds. Animals, that have some sort burrow, nest or something similar that can be called a home, make sounds to say that this is their territory. These types of calls can often be the same as the mating call. It is usually not so straightforward that there is only one function per sound.

Staaterman, E., Clark, C. W., Gallagher, A., Claverie, T., deVries, M., and Patek, S. N. 2011. Rumbling in the benthos: acoustic ecology of the California mantis shrimp (Hemisquilla californiensis). Aquatic Biology 13: 97-105 (Featured article, cover story).

Kalle: How different is the signal pattern of different sea animals? And how complex is the pattern?

Erica: There’s a lot of variability depending on which animal you’re focusing on. Most animals make very simple acoustic patterns, but again whales and dolphins have different notes, syllables and songs. But when we talk about the basic grunting sound from the fish or squeaking sounds from lobsters, it’s pretty simple.

When talking about smaller brained animals, sometimes you see variability in the temporal pattern, like the rhythm. For example, I did a study on mantis shrimp. They live in the burrows in the mud. There make “rumble” sounds, and we found pretty regular rhythm to the sound. We also heard that off in the distance there was often a second individual rumbling with a slightly different rhythm. And that’s pretty much as complex as it gets if you’re a shrimp. But that rhythm can actually have information in it. We proposed that the sound is a combination of a mating call and territory defense signal. It’s possible that females are listening to the males calling back and forth, and they may have some sort of preference. They might prefer certain rhythm or pitch of the sound. So there is information in all of these patterns, but of course nothing as complex as for example bird song.

Hemisquilla californiensis, the California mantis shrimp

Kalle: So on the land we could compare it to the communication of insects?

Erica: Yes, the sounds of crustaceans are analogous to the sounds of insects.  And the fish sounds – I’m not an expert in that – but from what I’ve heard, they’re also pretty simple. Basic grunts, thumps and that sort of thing. There’s actually a great website called  Macaulay Library and they have a ton of different sounds out there. Most of the fish sounds, you’ll notice, are pretty simple.

Kalle: If there’s so much noise in the sea sea, made by sea life, which in some way describes the conditions down there – do you think it would be possible to start interpreting these sounds so that it tells us something about the physical variables of the environment?

Erica: From the bioacoustics perspective, there’s an idea that we might be able to monitor the life in the oceans by listening to it. There are few ways to go with this.

If you have a type of fish that makes sound when it’s spawning (or mating) and you have hydrophones in different locations in the water, you can find the location of the sound source, and therefor you can discover where are they mating. That’s important because in this way we can learn where the mating grounds of endangered species may be, and we can protect them from fishing.

The other type of acoustic bio-monitoring could happen in the future, maybe 10 or 20 years from now. We could use the sounds in the environment to give us an indication of the health or the diversity of the life there. So if we could just drop a hydrophone off the back of the boat and record sounds for a few minutes, we could have a sense how diverse and how healthy that habitat is. But the problem is, we don’t know yet what so many of these sounds are. I’ve recorded now about 18 months of data from coral reefs here in Florida and on my recordings I can only identify a small fraction of the sound that I hear. There are few fish that are identifiable, and the snapping shrimp. But a lot of it is a big mystery. I have a whole database of weird sounds I’ve recorded, and I don’t know which animal made which sound. So you can’t really monitor a habitat this way unless you have a well-developed catalogue of fish sounds. And it will take a lot of work before there’s enough data to make any conclusions.

Right now to monitor the health of a reef, a team of divers count the number of the fish they see in a minute, or they measure the percent coral cover per one square meter, and they do it twenty times and calculate averages. But it takes a lot of time and effort. It could be much easier to do this in the future, if we could use acoustic monitoring techniques.

Kalle: Are there any developments in this area already?

Erica: I don’t know of anybody who’s going around and cataloguing sea animal sounds on a mass scale. Actually my dream is to have a coupled video-audio system that I can use with the closed circuit rebreather, which is the silent scuba diving method. This is the best way for documenting the sounds of different animals. However, it will take many teams of people documenting ocean sounds for many years before we will even know a fraction of the sounds out there.

Erica checks on a passive acoustic recorder in the Florida Keys. Erica is a student in the lab of Dr. Claire Paris at University of Miami.

Kalle: How much interference is anthropogenic noise creating for sea life?

Erica: There are different types of human-derived, or “anthropogenic” sounds in the ocean. First, there are very loud pulses of sound made by sonar and seismic exploration devices. They’re not everywhere all the time. But the animals nearby can actually become deaf. There’s physical damage that can happen to fish.

Kalle: So even the fish can be damaged by sounds that are too loud, not just sea mammals?

Erica: Yes, even the fish. There’s been some work demonstrating that the fish can have hearing loss associated with those really loud and sudden sounds.

On the other hand, the shipping noise is a more pervasive problem, because there is just so much background noise all over the planet now. The noise that ships make elevates the background level. So if you’re an animal, trying to communicate to your neighbor and there’s constantly this loud background sound, you’re going to have to scream to your neighbor, or you just give up. It’s called acoustic masking. It’s the same concept as if you’re standing on the side of the road and trying to talk to somebody while heavy trucks pass by.

That really is a problem for animals that depend on communicating certain sounds for basic life functions like finding a mate or listening for the prey or defending their territory. If the animals can’t hear each other, there are consequences.

The California mantis shrimp making sound and then a boat goes overhead, suppressing all the shrimp voices.

Kalle: How serious do you think this background noise problem is?

One thing we have to keep in mind is the notion of cumulative stressors. Oceans are warmer, there’s ocean acidification, less prey is available because of overfishing, and of course there’s marine pollution. And then on top of that, you add the fact that there are these anthropogenic noises in the ocean – altogether that’s a lot of stress on an animal. The accumulation of all these things acting together is really what’s starting to concern scientists now. If animals are stressed, they generally don’t do very well. They may not die right away, but they may not reproduce successfully.

I personally don’t think that anthropogenic sound in the ocean is the one biggest thing that is going to kill off the fish. I think our biggest problems with the fish are humans eating too much of them, and a lot of toxins in the environment. But I do think that it’s this accumulation of stressors that is the ultimate challenge for us now – how can we preserve the life in the oceans when we are stressing the animals in multiple ways?

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