Shark relatives deliver surprises

Skates – a group of fish closely related to sharks and rays – have provided scientists with not one but two surprises.

One group of researchers has investigated a species that can walk – blowing away the idea that walking-style locomotion developed in fish only when they began to leave the water for land.

And a second group report an entirely different skate species that uses deep sea hydrothermal vents as incubators for its eggs – only the second time using volcanic heat as a nursery aid has been detected since the Cretaceous period.

Skates, together with sharks and rays, are considered to be very primitive vertebrates. Supported by a framework of cartilage rather than bone, they are thought to have remained unchanged for millions of years.

It came as a surprise, thus, in 2003 when a species known as the little skate (Leucoraja erinacea) was observed using its set of small pelvic fins to walk across the ocean floor.

Until then, it had been assumed that fin-walking was a behaviour that emerged only in fish species that inhabited environments, such as tidal zones, that required sporadic land-based locomotion. The little skate is a benthic, or bottom-dwelling, species and never goes anywhere near dry land.

Recently, a team led by neuroscientist Jeremy Dasen from the New York University School of Medicine in the US set out to investigate how the skate developed, and controls, its gait.

To do this he and his team used RNA sequencing to discover the genes that influence the fish’s motor neuron development. What they found was extremely interesting.

“They use a neural and genetic developmental program that is almost identical to the one used by higher vertebrates, including humans,” he reveals.

The researchers found that the types of neurons the skates used to bend and flex their fins were exactly the same as the ones mammals use to move their limbs.  What’s more, the cells that connect motor neurons and determine the sequence in which muscles are activated – called interneurons – were also identical.

In an evolutionary sense, therefore, the ability to walk was genetically inherent in the vertebrate lineage before it became a useful strategy for conquering new, dry, environments.

“These findings suggest [that] the genetic program that determines the ability of the nerves in the spinal cord to articulate muscles actually originated millions of years earlier than we have assumed they appeared,” Dasen says.

In a paper published in the journal Cell, the researchers suggest that the little skate could now become a useful model through which to better understand the nerves that control walking and how they develop.

Meanwhile, in a paper published in the journal Scientific Reports, another group of researchers reveals that a species known as the deep-sea skate (Bathyraja spinosissima) has evolved a novel way of incubating its eggs.

Rather than keeping its eggs warm by sitting on or burying them – strategies used by scores of other egg-laying vertebrates – they take advantage of the heat pumped out by underwater hydrothermal vents.

The discovery was made in the ocean near the Galapagos Islands by a team led by Pelayo Salinas-de-León of the Ecuadorian Charles Darwin Research Station, based on the islands.

Salinas-de-León and his colleagues were using a remote-operated underwater vehicle to explore the sea bed around the Galapagos, when they came across an active thermal vent. They observed 157 large egg cases, each of which boasted horn-like structures at each end which marked them as eggs from a cartilaginous species.

Retrieving four of them for DNA analysis, the scientists found that they belonged to the deep sea skate. Further research established that 58% of the eggs were left within 20 metres of the vent’s “black smoker” chimney, and almost all the rest sat in areas where the water was warmer than usual.

The species, as its name suggests, lives in extremely deep waters. It has one of the longest incubation periods known – with eggs taking months, sometimes years, to hatch.

Salinas-de-León’s team suggest that the Galapagos population has learned to lay eggs in the warmer waters near the vent because the heat speeds things up and decreases embryo development time.

The use of heat pumped from deep within the earth as a nursing aid is extremely rare. There is evidence that some land-based dinosaurs did so, and there are reports that a rare bird from the northern islands of Tonga, known as a megapode, takes advantage of volcanic heat when nesting.

Salinas-de-León and colleagues suggest that this is the only time the strategy has ever been noted in a marine environment.