If someone dropped you off several kilometres from your house, how would you know where you are and how would you find your way home? Chances are you would use visual clues – streets, buildings, mountains, rivers or perhaps the coastline.
Humans are visual creatures – we rely heavily on seeing things to navigate our world – but what if you were under water? Could you find your way home?
Sea creatures, such as crabs, use a range of different senses to find their way around their environment. One sense that researchers are finding important is sound. Instead of navigating by what they see, crabs can navigate by what they hear.
Crab larvae are able to tell a good habitat by what it sounds like. An active and living reef where crabs settle is also home to shrimp, kina and fish. All these animals make sounds that crabs use to navigate towards a reef and a potential habitat.
Why do crabs leave home in the first place?
Crab larvae develop in open water, not near the reef where they started their life. Fertilised crab eggs are released into the water and float with the currents while they develop into the larval form (zoea stage then megalopae stage).
This means that the crab larvae can be dispersed away from their ‘home’ habitat, which allows the crabs to settle new habitats and increase diversity at each reef location. However, it means that the larval crabs can end up drifting on the currents several kilometres offshore. Knowing how these tiny larvae (only millimetres in length) can find their way back to a suitable habitat from such a great distance has been the subject of research at the Leigh Marine Laboratory of The University of Auckland.
Making yourself at home
Discovering that crabs settle and metamorphose (change into an adult form) in response to reef noise was an unexpected result. Dr Jenni Stanley from the Leigh Marine Laboratory was actually studying something completely different when, one night, she left an experiment running.
Jenni had set up a tank in the laboratory that contained crab larvae. She had been playing reef noises through a speaker to see if it would disrupt the normal daily patterns of movement of the crabs, but nothing had been happening. However, one night when she had just about decided to give up on these experiments, Jenni forgot to turn the speaker off, and when she returned a couple of days later, she noticed something odd.
In the tank next to the speaker, Jenni noticed that the crabs had started to settle and moult into their adult form. It seemed like the noise had encouraged the crabs to change from their larval form and start developing into adults.
Normally, crabs will remain in the larval stages until they find a suitable habitat. When they find this place, they will settle and start to develop into adults. From these observations, Jenni decided to set up another set of experiments to see if the crab larvae will respond to the sound of the reef alone and if they use it as a sign of a good place to live.
From these experiments, Jenni noticed that, when the crabs heard the sound of the reef, they started to settle and develop into adults. This was particularly interesting because there were no other clues, no light changes, no smells and no changes in wave patterns. Crabs probably use a sum of all these clues in real life, but Jenni’s experiments showed for the first time that sound is also a very important clue for crabs.
Laboratory results versus real life
Sound in a tank doesn’t act the same way as it does in an open ocean – sound in a tank will bounce around off the walls and top of the water, creating echoes and reflections. It was important to determine if crabs responded to sound in the open environment, so Jenni designed an experiment that could test whether crab larvae in the ocean were able to detect and respond to sound cues.
Jenni caught many crab larvae and set them up in individual containers. She placed these containers at different locations around the coast. Some were close to a rocky reef, some were close to a sandy beach, and some were close to an estuary. Jenni left the containers in these places and then came back at regular intervals and measured how many crabs had settled and metamorphosed. She found that the containers by the reef had many more settled crabs than the containers in the other environments. The crabs had responded to the sound of the reef, recognising it as an appropriate place to set up home, and had begun to develop into adults.
Jenni is now working with other scientists to figure out how crab larvae hear these cues and how far the sound of the reef can travel. Starting from some basic research on the noises recorded under water, we now know that these sounds are very important to maintaining a healthy reef environment, signalling to crabs – and possibly other species – that a cosy reef is nearby waiting to welcome them home.
Nature of science
Scientific research quite often involves serendipity, where an unexpected result leads the research in a whole new direction. This was the case for Jenni Stanley who decided to work on crab settlement after accidently leaving a speaker on overnight in her laboratory and seeing unexpected settlement next morning.
Not all crabs use sound cues to find home. Read about how the New Zealand pea crab, a parasite of green-lipped mussels, locates mussel beds to settle in.
How small is that? – fill a matchbox with tiny items to model the tiny size of a crab and the huge distances they navigate to find their way back to their reef.