Big-eyed conch snails use vision to jump away from predators

Snails with the largest eyes are keeping a lookout for predators, new research suggests.

While the basic eyes of most snails look like small black dots, conch snails known as strombids have eyes that are up to 10 times bigger than any of their relatives. These mobile, stalked eyes can peer in all directions and help these snails detect predators in their tropical shallow-water habitats.

A new study, led by Dr Alison Irwin during her PhD research at the Natural History Museum, sheds light on how these remarkable eyes support the snails’ unique escape strategies.

“Large, high-resolution eyes are normally a characteristic of predators that rely on vision to hunt prey,” Alison explains, who is now based at the University of Copenhagen. “Given their significant evolutionary cost, we only expect to find such eyes in animals where they are vital for survival.”

“That means it’s really strange to find conch snails with such good eyesight. They’re algae-grazers, not predators, so they must be using their excellent vision for something else.”

The findings of Alison and her co-authors suggest that the vision of strombids and their large-eyed relatives evolved in parallel with their ability to move quickly. While most snails crawl, conch snails possess an exceptionally large foot that allows them to make sudden leaps along the seabed.

Dr Suzanne Williams, who supervised Alison’s PhD, suggests that the combination of excellent vision and agility allows them to avoid becoming prey to slow-moving predators like cone snails.

“Cone snails are a famous marine predator with really toxic venom,” she says. “They fire venom in darts at their prey, which can include worms, fish and some conch snails.”

“Cone snails are, however, notoriously slow. This marriage of good vision and high speed allows these conch snails to dart out of danger and live to graze another day.”

The findings of the study were published in the journal Systematic Biology.

The benefits and drawbacks of vision

Vision is one of the most important senses, as it allows animals to detect changes in light, find food and navigate the world around them. However, vision comes at a cost. Large eyes require significant energy to develop and maintain.

This balance in cost and benefit means that only animals which really depend on eyesight have evolved it.

But even for the animals which do rely on vision, there is another trade-off they have to make between seeing in greater detail or seeing more contrast. For the conch snails, the researchers found those species which were more active during the day in shallower waters tended to favour resolution over sensitivity to spot predators.

There is a way to get around this trade-off by making the eyes bigger. However, this also increases the amount of energy that the eyes need, which brings the evolutionary dilemma full circle.

Keeping an eye out

Stromboidea sea snails provide some of the best examples of nature’s different approaches to eyesight. While some members of the group, like the strombids, have well-developed eyes at the end of long, mobile eyestalks, others have small ones on tiny eyestalks that are tucked under their oral tentacles.

As part of her PhD Alison wanted to investigate how the differences between the conch snails and their smaller-eyed relatives had come about. The work was co-supervised by Professor Nicholas Roberts at the University of Bristol and Professor Elizabeth Harper at the University of Cambridge.

“Unlike the big, colourful eyes of the conch snails, their smaller-eyed relatives have a reduced lens and disorganised retina while their eyes are at the base of the tentacles rather than at the tips – all of which limits vision,” Alison says.

“We wanted to find out when large eyes evolved in this wider group of snails, how many times it’s happened in their evolutionary history and what factors were linked to that.”

Using the DNA of snails loaned from museums around the world, such as the Muséum national d'Histoire naturelle in Paris, the researchers assembled a family tree of these snails.

They found that this tree had two main branches – one containing large-eyed snails, including the conch snails, and one representing smaller-eyed snails. This suggested that large eyes evolved only once, with only a single shift from being at the base of the tentacles to their tips.

“It seems that the group of large-eyed snails inherited their large eyes from a common ancestor,” Alison says. “These then got even bigger on two further occasions within the conch snail family.”

The evolutionary push for better eyesight likely coincided with increasing pressure from predators during the Mesozoic Era. At this point snail-eating predators were becoming more prominent in shallow, tropical marine habitats, so large eyes would have been useful to spy escape routes to leap away from danger.

This may have also coincided with the evolution of another interesting characteristic, a unique curve in the shell called a ‘stromboid notch’.

“Both eyes can peer out from underneath the notch, giving a greater view of their surroundings while still partially shielded by the shell,” Suzanne says. “This means they are able to quickly retract into their shell if they spot a predator that they can’t leap away from.”

The researchers hope to continue investigating these animals to get a deeper insight into how senses have helped to shape the animal kingdom.

“These are a fascinating group of animals, and there’s a lot more we’d like to investigate,” Suzanne says. “We’re currently working on a deep-sea group of snails that are losing their vision as they move into deeper water. We want to investigate which genes are lost and how their loss affects the morphology of the eye.”

“I’m looking forward to working with more students to carry on this research.”