Sea Hares Plug Predators' Nose with Sticky Opaline

A sea hare has many ways of defending itself from predators; notable of these defenses is its ability to secrete purple ink and a sticky substance (opaline) to fend off predators. A new study has found that opaline impairs the predator's ability to smell.

"Sea hares have many potential predators, each with feeding habits [and] sensory systems... So, some chemicals may work on some predators and not on others," Dr. Charles Derby, one of the study authors, told BBC news. "A combination of mechanisms acting simultaneously may be more effective than any one alone," he said, explaining why the animal has so many defenses.

For the study, researchers from Georgia State University, USA, first obtained the water-soluble fraction of opaline, which lacks some amino acids, but is as sticky as opaline. They then applied this substance to the "nose" or antennas of some lobsters. To track the lobsters' sense of smell, researchers gave them "shrimp juice" and measured both chemosensory input and motor functions.

Study results showed that lobsters that had no opaline on their antennas were enticed with the shrimp treat. However, lobsters with the sticky opaline failed to detect the treat.

Further, researchers tested whether amino acids in opaline were inhibiting the smell of lobsters. For this, they first mixed all five key amino acids found in opaline and applied it to the antennas of the lobsters. Interestingly, the amino acid mixture increased the lobsters' appetite for shrimp juice.

Next, they tested the ability of carboxymethylcellulose - the sticky substance, to decrease the smelling prowess of lobsters. Study results showed that carboxymethylcellulose, alone or in mixture with amino acids, lowered the lobsters' ability to smell shrimp juice.

The study is published in The Journal of Experimental Biology.