The smooth dogfish, Mustelus canis, is a small shark that feasts on lobster, squid, and other small shellfish, finding its food by tracking odour plumes. It doesn't just follow its nose, but also relies on the movement of the water and vibrations, picked up by its lateral line system, to find its meal. Jayne Gardiner and Jelle Atema wanted to know how the dogfish used their different senses – smell, mechanoreception by the lateral line and vision – to track odour plumes in a large flow tank (p. 1925).
The sharks had two odour plumes to choose from: seawater or yummy squid juice. Each plume was squeezed out of a small nozzle to create a relatively smooth `oozing' odour source, which became turbulent as it flowed over a brick 15 cm downstream of the nozzle. When the lights were on, and all their senses were intact, the sharks preferred the turbulent part of the odour plume, indicating their choice by biting or nudging the brick. When the team knocked out the lateral line using streptomycin, the sharks had to search for longer, but couldn't distinguish between turbulent and oozing odour, suggesting that they couldn't tell the difference between smooth and turbulent flow without their lateral lines.
When the team plunged the sharks into darkness, the intact sharks had no problems choosing the turbulent odour, however those whose lateral lines weren't working rarely found the odour plumes. The few sharks that did find them couldn't tell the seawater or squid odours apart. This shows that sharks need both their lateral line and sense of smell to track odours. If their lateral line is not working, vision can help them to find the source of the smell, but if the lights go out, the fish have big problems, and might go hungry.
- © The Company of Biologists Limited 2007