How do you know what’s happening if you can’t see? Humans rely on their sense of sight and seeing to make sense of their world. We mainly use our eyes to work out where we are, who we are with and if there is danger present, but what would happen if you lived in the dark or wanted to feed at night? What other system could you use?
Professor John Montgomery from the Leigh Marine Laboratory is an expert in non-visual sensory systems – any system for sensing an environment that doesn’t rely on light and on using eyes. Humans have four main non-sensory systems – hearing, touch, smell and sound – but animals have many, many more.
The world of fish and other marine creatures is quite different from ours. Water doesn’t carry sensory information in the same way that air does, sound travels further, light refracts differently and scatters more, changes in pressure are more noticeable and it is often too dark to see. Therefore, marine creatures often rely less on vision and more on sensory organs adapted to pressure, vibration and electromagnetic fields.
Electromagnetic fields
One interesting non-visual sensory system is the electromagnetic field sensors used by sharks to detect their prey.
This finely tuned sense can detect a fish lying under sand or behind a rock. The question that faced researchers such as Professor Montgomery was how sharks filter out all the noise generated by such a sensitive receptor. Why doesn’t the shark detect its own electromagnetic field and get overwhelmed with sensory information?
As always, the animals are able to adapt. Prof Montgomery was able to show that the nerves that detect the electromagnetism are able to detect and cancel out ‘normal’ activity. This means that the shark doesn’t notice its own electric field and doesn’t react every time it moves a muscle, but it can still sense new things.
To achieve this remarkable feat, the nerves produce a negative pulse for every positive background pulse. This cancels the background activity and allows the shark to focus solely on external signals. This same idea is now used in noise-cancelling headphones that block out background noise, allowing you to listen to music in peace (even on a noisy plane journey).
Lateral line
Another important sense for marine creatures is the lateral line sense. This is a sort of combination sense, detecting both pressure and vibration.
It’s a little like a combination of touch and balance. Indeed, it is thought that the organs that humans use to detect balance are distantly related to the lateral line organs of fish. The lateral line refers to the placement of the sensory organs. They usually run in a line down the sides of the fish, from head to tail (laterally, in anatomical terminology). These sense organs allow the fish to detect where it is (like knowing where your hand is), how deep it is, what movement is happening around it and which way is up. There are many similarities between the lateral line and the human inner ear (balance organ). From studying the fish lateral line sense, we have also learned a lot about humans and how they work.
Vision and smell
As well as these quite different senses, fish and other marine animals also rely on senses that are much more familiar to us.
Many fish have eyes and can see under water, but they are often less dependent on their sense of vision as compared to humans.
Fish can smell. In the case of sharks, it is believed that they have an exceptionally good sense of smell and use this sense to help find prey.
Hearing
New research shows that fish can hear. The article Hearing explores both human hearing that of fish and shrimps. New Zealand reef noise and Studying sound under water look some more at what we know about sound under water and how sea creatures respond to the noises around them.
Nature of science
Science knowledge from one field can be very useful for work in another field. When Professor John Montgomery started to study how sharks detect movement in water, he didn’t imagine that the ideas that he proposed would end up being used in acoustics to help engineers design devices such as noise-cancelling headphones.
