Have Scientists Found A Manta Ray Maternity Ward?

Researchers may have just found the devil’s maternity ward… devil rays, that is. While taking a marine conservation field course in the Gulf of California as an undergraduate at Duke back in April 2014, Leo Chan Gaskins saw gillnets full of dead giant devil rays being hauled out of the ocean.

Based out of a small-scale fishing community in northern Sonora, the now doctoral student in marine science and conservation at Duke University’s Nicholas School of the Environment watched as the fishers were unable to return the animals back to the watery domain due to just how big they were.

A large species, giant devil rays (Mobula mobular) can grow to 17 feet (5.18 m) long and are famous for beauty and graceful swimming that looks like they are flying underwater. They can reach up 253.5 lbs (115 kg) in weight and are found in tropical and subtropical waters. The rays are also known for leaping high into the air, though the exact reason for these acrobatic feats is still not fully understood. They are filter feeders, unfurling the cephalic horns out front and funneling water towards their mouth to extract the tiny animals they eat. Mobula rays are assessed as Endangered by the International Union for Conservation of Nature (IUCN). This is because they are often targeted by fishers for their gill plates, used in a pseudo-medicinal soup called Peng Yu Sai. It is made of boiled manta gills, seahorses, and pipefish and is ‘prescribed’ to treat fevers, chickenpox, and help breastfeeding mothers. However, large numbers of devil rays die accidentally when caught as bycatch each year when they become entangled in fishing nets while foraging for food.

Gaskins watched as the female rays were cut open, and each one was pregnant with a pup (baby ray). “I determined that the pups were full‐term, as I revived one birthed into a net with the permission of the fisherman and it successfully swam away,” he wrote in the article that appears in the journal Ecology. Measuring the wingspans of two of the dead adult females caught and one of the dead pups confirmed the pups were full-term. Researchers believe sexually mature female giant devil rays average wing-to-wing width of 2.18 meters (7.2 ft); the two Gaskins measured were 1.97 meters (6.5 ft) and 2.17 meters (7.1 ft). The average measurement of a newborn pup is 1 meter (3.28 ft) and his measured .91 meters (2.98 ft).

“These measurements, along with the fact that each boat landed multiple pregnant rays, suggested that the fishers had unknowingly set their nets in a pupping zone,” Gaskins explained to Science Daily. This isn’t the fault of the fishers– in fact, nobody knew there was a birthing zone here! There have been past studies of giant devil rays that show how they use the Gulf of California as a mating and feeding zone in spring and early summer, but there is still much we are learning about these animals. But their favorite food, krill (Nyctiphanes simplex), is plentiful here around the time the expectant mothers are coming in so it makes sense a pupping zone would be here.

Further research is needed to confirm the possibility of this ‘maternity ward,’ but Gaskins thinks that the next step should be authorities and local fishers coming together to find a way to minimize the risk to these pregnant mothers out there each year in and around April. “These animals reach sexual maturity very slowly and they have only one pup per litter, so losing even a small number of pregnant females to by-catch can have dire effects on a local population’s birth potential and long-term survival,” Gaskins says, “Aside from the obvious humane reasons the fishers have for not wanting to harm endangered animals, entangled rays can damage their nets, drive away the desired catch, and greatly decrease their earnings. Ray meat is not valuable in the Gulf of California and only brings in about three pesos per kilo.” There isn’t a market in the Gulf of California for ray gill plates but Gaskins believes putting the protection in place can help the species in the long run.

Back in 2014, the undergraduate had no idea that what he was seeing was related to arguably the most important life stage for these animals. Yet scientists are still uncovering new information for animals, even when they are as iconic as a devil ray. “We still know very little about their global population size, annual movement patterns, and reproductive zones, so the discovery and protection of a previously unknown pupping ground would be good news for conservation,” says Gaskins, hopeful that protection is on its way for these devils.