Fish that evolved legs has another surprise in store

A family of fishes that evolved “legs” to walk along the seafloor have another use for their strange appendages – tasting prey buried beneath the sand.

“This is a fish that grew legs using the same genes that contribute to the development of our limbs, and then repurposed these legs to find prey using the same genes our tongues use to taste food – pretty wild,” says Nicholas Bellono of Harvard University, USA.

Triglidae, commonly known as sea robins or gurnards, are a family of bottom-feeding ray-finned fish found in temperate and tropical seas worldwide.

Bellono and collaborators decided to study sea robins after learning that other fish follow them around to poach the unseen prey they can detect in the sand.

How do they manage it? The researchers found that the leg-like appendages of some types of sea robins are covered in sensory papillae – small structures similar to papillae found on tongues and along the gastrointestinal tract of many animals.

These sensory papillae contain touch-sensitive neurons able to perceive tactile sensations, as well as taste receptors for detecting chemical stimuli.

Sea robin using its leg-like appendages to move and probe for prey in the sand. Credit: Anik Grearson

“We were originally struck by the legs that are shared by all sea robins [but] make them different from most other fish,” says David Kingsley of Stanford University, co-author of a study describing the sensory papillae in in the journal Current Biology.

“We were surprised to see how much sea robins differ from each other in sensory structures found on the legs.”

The researchers buried capsules containing crude or filtered mussel extract, and single amino acids, in the sand of a fish tank containing a species of sea robin (Prionotus carolinus). The fish, which appeared to ‘‘scratch’’ at the sand with their legs while walking along the bottom of the tank, regularly found all buried prey-related items.

However, a second species of sea robin, Prionotus evolans, which lack sensory papillae, was not able to find the buried food. Instead, it used its legs only for getting around and for probing at visible prey.

The findings suggest that the ability to detect chemical stimuli is required for the digging behaviour seen in some sea robins.

Diver footage of a sea robin hunting on the sea floor. Credit: Anik Grearson

The researchers used genome sequencing, transcriptional profiling, and hybrid species to understand just how these sensory legs came to be. The findings are described in a second study published in Current Biology.

Their experiments identified a transcription factor, Tbx3a, plays a major role in the sea robins’ sensory leg development. Transcription factors are proteins that regulate – turn on and off – the expression of genes.

According to the researchers, Tbx3 has a role in posterior limb development in many vertebrates, including humans, mice, chickens, and other fish species that do not form walking legs.

The gene that encodes Tbx3a also plays a critical role in the formation of sea robins’ sensory papillae and their digging behaviour.

“Although many traits look new, they are usually built from genes and modules that have existed for a long time,” Kingsley says.

“That’s how evolution works: by tinkering with old pieces to build new things.”