When New Zealand marine crab larvae are played the sound of tidal turbines, they halt their metamorphosis into an adult form. That’s one of the findings of Rutherford Discovery Fellow Dr Craig Radford’s team, in the little-known, underwater world of crab hearing. The Senior Lecturer at the University of Auckland tells us about another new finding that crabs can make sounds, how far fish can hear, and why hydrophones are a good way of monitoring marine habitats.
Crabs don’t have ears, do they? How do they hear?
For over 50 years, the mechanism and even existence of crustacean hearing has been fiercely debated. The lack of convincing evidence for how they hear is especially embarrassing given the tremendous advancements made in our understanding of crustacean behavior that clearly shows they produce sounds and can respond to acoustic cues. They do have a structure analogous to that which fish use for hearing, so we’re using medical technologies such as CT scanning to study this. Then we’re developing physiological techniques to understand the neural mechanisms of how they respond to sound.
We’ve done work showing that crabs, especially larval crabs, do respond to sound. They have a range of sensory hair cells which could be responding but they also have an organ called a statocyst that could be doing it. I have a Masters student who is studying the hearing sensitivity of snapping shrimp, the purple rock crab, red rock crab, a swimming crab, and the burrowing mud crab.
A lot of our early work has been done on the larvae of animals that live on reefs. The larvae respond to reef sounds to direct their movement towards reef habitats. When they settle, the larvae go through metamorphosis. We’ve shown that if you play reef sounds to a reef crab they’ll metamorphosise faster. We’ve shown that for a range of different crab species in New Zealand and in the tropics.
Do crabs also make sounds?
Some recent work from Italy has shown that the paddle crab can produce sounds. That particular species has some ridges on its pinchers and it scrapes them along its body, making a sawing noise. We’ve done preliminary work with some of the crabs we’re holding here and they’re also making this noise. This is really new – I haven’t got any field recordings of it yet. What we don’t know is how that sound is used in a behavioural context.
The rasping sound of the New Zealand crab Ovalipes catharus.
Is man-made underwater noise a problem?
The evidence so far is suggesting that it’s going to be a big problem. Since the industrial revolution noise under water has increased and animals are starting to change behaviours. If, for example, a whole lot of fish go to an area and they use sound to synchronise their spawning, then if the noise is too high the communication signals of the fish will become masked by the noise. With our larval fish work, if noise levels are too high we could be disrupting the behaviours. We had a PhD student who did some work on tidal turbine sounds, showing that we can disrupt the metamorphic process in crustaceans with man-made sounds. The crabs will eventually metamorphose, but their condition and survivability will be compromised.
How can we tell the underwater environment is getting noisier?
New Zealand has a quieter environment than most of the world, which is one of the reasons why doing this work in this part of the world is good. We’ve got a major port in Auckland harbour, but it’s small compared with the port of Singapore. But we do have enough shipping activity to measure the effect.
What we’re also doing is trying to use ambient sound as a proxy for monitoring diversity. We did some recent work putting hydrophones in and out of the Leigh Marine Reserve and at these same sites we did traditional reef surveys, counting fish on scuba dives. We got quite a strong correlation to some new indices that a group in Europe recently developed. So that’s the first evidence that we can use acoustics as a tool to measure reef diversity. This is great because just dropping a hydrophone over the side is a lot cheaper than a boat full of divers, and it also offers the opportunity to get into places where you can’t dive.
How far can fish hear?
All fish have the same basic hearing structures. But some fish also have ancillary hearing structures which increases their sensitivity to sound, so you get a range of species that can hear really well. We did some physiology work on fish larvae and some modelling and the range that they can hear over is anywhere between 2-3 kilometres or up 10-50 kilometres. Goldfish are one species that hears very well, they’re kind of the lab rat for hearing studies in fish. It makes sense that fish that live in rivers and streams have to hear really well to be able to segregate different sounds.
The big mystery with fish is how they determine sound direction. We know they can, but it is a mystery how they do it. A shark fin cutting through the water’s surface is a striking image. We hypothesise the reason for this swimming behaviour is that it enables sharks to better localise sounds to direct them across ocean basins or towards prey. The exceptional auditory system of elasmobranch (sharks, rays and skates) has evolved over hundreds of millions of years, yet we still understand surprisingly little of how they determine sound direction. Using a range of techniques such as passive acoustics, magnetic resonant imaging, electrophysiology and behavioural tagging studies, we will figure out how sharks determine sound direction.
These interviews are supported by the Royal Society of New Zealand, which promotes, invests in and celebrates excellence in people and ideas, for the benefit of all New Zealanders.