Terrestrial hermit crabs love peanut snacks, but can only smell them when it's wet. Illustration Irene Goede. Used with permission.

The Caribbean hermit crabs in Anna-Sara Kr?ng’s laboratory are no picky eaters. They are eager to gobble down any fruit, nuts, fish or coconut flakes that comes their way. But above all else, these culinary connoisseurs prefer peanut flips. These snacks are always the first to disappear down their gullets when feeding time comes around.

The hermit crab’s refined taste is matched by its sense of smell, which is renowned amongst the biologists that study them. Robber crabs, close relatives of terrestrial hermit crabs, can locate coconuts from great distance by smell alone for example. Some hermit crab species can even smell death. When one crabs dies, others will track down its cadaverous scent in a macabre race to claim the shell it left behind.

Hermit crabs don’t have a single nose, like us. They have hundreds. Their inner antenna are covered by scores of thin and short hairs (called aesthetascs) with which they smell. The crabs flick these hairy antenna back and forth to sniff the air, or tap them on the ground to sample the dirt. A large part of the hermit crab’s brain is dedicated to processing the scents and tastes that their antenna pick up.

Consider Kr?ng’s surprise when she discovered, through a simple experiment, that her crabs could not smell the peanut snacks they seem to savour so much. This is what Kr?ng did: she placed a hungry hermit crab in a plastic box wit an upturned flower pot for shelter and two pitfalls on either side. A tasty snack awaited in one pitfall, whereas the other remained empty. Kr?ng would leave the crabs to their own devices for the night (hermit crabs are nocturnal creatures) and noted in which pitfall the creatures had scuttled by the next morning. If hermit crabs could pick up the scent, most of the crabs should end up in the pitfall that contained the food.

But the crabs failed the sniffing test. Hard. Sure, half of the hermit crabs were lucky to find a tasty piece of salmon or some peanut crisps, but the other half went hungry for a night. The hermit crabs stumbled into the pitfalls at random, oblivious of the odours that could have led them in the right direction. Only the freshly diced bananas and apples could coax the crabs: between 80 and 90 percent of the crabs managed to find these treats.

In parallel to these choice experiments, Kr?ng and her colleagues measured whether the crab’s antenna could detect a battery of different odours directly. She removed the antenna from euthanized crabs and placed them between electrical wires. She then applied puffs of different odours to the amputated antenna, to see whether they would elicit an electrical response. In total, Kr?ng tested 140 different compounds this way.

By fixing antenna between electrical wires, it is possible to make an 'antennogram': a recording of the antenna's electrical response to certain odours. Image Joby Joseph

Kr?ng found that the hermit crab’s odour palette most resembles that of their aquatic relatives. It certainly wasn’t as rich as that of insects, or robber crabs for that matter. “They did not respond to esters, lactones and ketones, which we know are typical terrestrial odourants. The limited set of compounds they did respond to were soluble in water, such as acids and amines”, Kr?ng writes in an e-mail.

Then it hit Kr?ng. What if the crabs depend on water to smell the world around them? This would explain why the crabs were able to find moist fruits, whereas the dry peanut crisps escaped detection by their probing antenna. To test this hypothesis, Kr?ng repeated the pitfall experiments, with a cup of water right next to the dry peanuts. As if by magic, the crabs now homed in on their beloved snacks. Around 75 percent of the creatures chose the correct pitfall. It must have been the peanut odours that lured them: the crabs were not attracted to a cup of water by itself.

Kr?ng also repeated the antenna experiments, but this time she made sure the air was moist before she released the odorous puffs. Bingo. The electrical response to certain odourants was three to ten times higher in humid air than it was in normal conditions. Kr?ng does not know for certain how water vapour affects the transmission of odours, but she thinks that the water helps them to diffuse through the thin walls of the aesthetascs.

It seems the hermit crabs are still struggling with their aquatic heritage. The ancestors of terrestrial hermit crabs crawled out of the water not too long ago. The oldest fossils of land dwelling hermit crabs are about 20 million years old. A blink of an eye, or a flick of an antenna, in the grand scale of things. The terrestrial track record of insects stretches as far back as 400 million years ago.

Caribbean hermit crabs are also called soldier crabs.

Trading water for land takes more than lungs and legs. The world smells different up here. Under water, molecules that are soluble in water carry farthest, whereas gaseous molecules are the most stable and reliable signals in the air. Kr?ng’s research shows that have not quite made the switch. The scents they perceive occupy the interface between water and land. Yes, they can smell food from great distance, but only when the air is moist.

Credit where credit is due: the hermit crabs cope perfectly well with their limited sense of smell. They live near coasts and on tropical islands, where the sea is always near. But how will they proceed from here? Will they continue to adapt to life on land? And if so, will their aesthetascs ever be tickled by the spicy scent of pine? Will they ever savour a whiff of sweet cinnamon, or enjoy the smells of freshly roasted peanuts in the morning?

Perhaps. The insects have done it before. Their ancestor evolved an entire new suite of genes that is dedicated to sensing terrestrial odourants, like esters and ketones. A new gene family for new smells. That sounds like a sensible solution, but it’s impossible to say whether hermit crabs will follow the same path.

For now, the peanut-loving hermit crabs have plenty of mysteries to divulge, says Kr?ng:

“Why do hermit crabs have such enormous olfactory lobes, while they can only sense such a limited number of odours? We still don’t know the answer to this. Also, their close relative, the robber crab (Birgus latro) seems to detect and also be attracted to insoluble odourants. We don’t yet know why this difference exists.”

Caribbean Hermit Crab crawling towards peanuts by Irene Goede. Website.
Antennogram by Joby Joseph
Kr?ng AS, Knaden M, Steck K, & Hansson BS (2012). Transition from sea to land: olfactory function and constraints in the terrestrial hermit crab Coenobita clypeatus. Proceedings. Biological sciences / The Royal Society PMID: 22673356