A built-in pocket protector keeps sawfish from ‘sword fighting’ in the womb

Smalltooth sawfish develop their signature, long, tooth-lined snout while still in the womb. The needle-sharp teeth are encased in a specialized sheath that prevents the rays from cutting up their mother and siblings during gestation and birth. Now, scientists have gotten their first close-up look at this built-in pocket protector.

“It’s a cool thing Mother Nature figured out to protect mom from those calcified teeth and protect siblings from sword fighting in the uterus,” says fish biologist Gregg Poulakis of the Florida Fish and Wildlife Conservation Commission in Charlotte Harbor.

Observations of baby sawfish and laboratory analysis of tissue samples have revealed that the sheath is a tough, multilayered “second skin” that sheds within about four days after birth, Poulakis and colleagues report May 28 in Fishery Bulletin. The finding overturns a long-held assumption that the sheath was a fragile, gelatinous membrane.

“I think a lot of the descriptions historically are based on the fact that people have just seen them in pictures,” says Dean Grubbs, a fish ecologist at Florida State University’s Coastal and Marine Laboratory in St. Teresa who was not involved in the work. “It’s a significant structure … as you would expect it to be if it is going to essentially shield those really sharp points.”

The species, Pristis pectinata, is found primarily in waters off South Florida and the western Bahamas. The ray is so rare that it took Poulakis and colleagues 18 years of near-monthly research trips to collect a handful of sheath tissue samples.

The sheath feels like paraffin wax, Poulakis says: firm, but with a slight give. “You can’t peel it off.”

A combination of histology, scanning electron microscopy, micro-CT and elemental analysis of the samples show the sheath has two tissue layers that resemble an epidermis and a dermis, as well as proteins that look like keratin, reticulin and collagen. This suggests the sheath is a second skin, but the researchers emphasize more work is needed to confirm that’s what they’re seeing.

The research provides more insight into the life history of the smalltooth sawfish, which is critically endangered due to habitat loss and accidental entanglement in fishing nets (SN: 6/5/15). Scientists had begun to be cautiously optimistic that smalltooth sawfish were on the brink of a comeback, Poulakis says. But that burgeoning recovery is now threatened by an ongoing, mysterious die-off in Florida’s Lower Keys. Dozens of sawfish have died, and Poulakis and others are racing to determine why.

“It’s discouraging when something like this happens and makes us take a step or two back,” Poulakis says. “We’re taking a lot of samples that will help us learn about the species beyond this mortality event.”