Researchers of a new study found that fish larvae can differentiate the odor of the reef they live in from the surrounding reefs and use this ability to sniff their way back home when they drift away.
Researchers have always been perplexed about how fish larvae find their way back home after drifting several kilometers away from the reef they reside in. Previous studies established that these tiny larvae were able to differentiate the odor of the reef they live in from the odor of other reefs. Now in a new study, researchers from several universities collaborated to discover that the larvae can identify their reef's odor from several kilometers away and use this odor to find their way back home.
"In this collaborative study we expanded our work to demonstrate that the odor responses can also be detected under the field conditions," said Dr. Jelle Atema, Boston University Professor of Biology in a press statement. "This establishes for the first time that reef fish larvae discriminate odor in situ."
For the new study, researchers used an overflow plume in One Tree Island to observe how fish larvae responded in an open surrounding. They first caught larvae from cardinalfish [Apogonidae] and damselfish [Pomacentridae] which were in the settlement-stage. An O-DISC (ocean Drifting In Situ Chamber) was placed in the north and south of the island. This light-weight device is a transparent instrument that is translucent to light, sound and miniature turbulence. The fish were placed in the central chamber of the instrument and let adrift in the water. Researchers then observed the swimming activity and bearing of the larva using an underwater motion sensing and imaging system. These movements were tracked using odor cues.
Researchers found that the larvae of the two fish family reacted differently to the olfactory stimulus. The cardinalfish larvae speeded up their swimming and moved in a zig-zag pattern within the DISC chamber, suggesting they were using infotaxis, or sporadic odor cues, in their attempt to orient. However, they didn't seem oriented towards the reef. Contrarily, damselfish larvae slowed down their movements and swam towards the reef as if the odor cues acted like a compass for them.
"Ocean currents do not appear to influence the orientation of fish larvae," said Dr. Claire Paris, Professor at the University of Miami (UM) Rosenstiel School of Marine & Atmospheric Science. "They do not provide a frame of reference since larvae are transported within. Instead, we find that fish larvae navigate by detecting turbulent odor signals transported kilometers away from the reef. Subsequently they switch to a directional cue, perhaps magnetic or acoustic, which allows them to find the reef."
Though previous studies have established that fish like mature sharks and freshwater juvenile salmon navigate using olfactory signals, this is the first time researchers have found that fish larvae use similar odor cues.
According to the authors of the study, their findings emphasize on the importance of keeping a check on human activities that may alter the smells within the ocean. If these smells are altered, fish may not get their "wake-up" cue and remain in the open for longer, making them easy prey to bigger marine creatures.
This is not the first time a study has found that fish use their primary senses to find their way back home. A 2012 study conducted by researchers from the University of Bristol found that small coral fish use their sense of smell, sound and vision to navigate to their nursing ground.
Human activities are causing serious damage to ocean waters, causing fish to lose their sense of smell, according to a Canadian research and a NOAA report predicts the problem is "likely to be widespread in many freshwater aquatic habitats."
