Pregnant male pipefish not so great at giving embryos oxygen

When a pipefish dad gets pregnant, his brood pouch delivers a surprisingly meager amount of oxygen to the embryos developing inside.

Broad-nosed pipefish (Syngnathus typhle) swimming in 100 percent oxygenated water in lab tanks delivered on average only about 50 percent of the oxygen to embryos in their brood pouches, says Inês Braga Gonçalves of the University of Zurich. Males stuck with worse water that was only 40 percent oxygenated kept the fluid in their brood pouches at only about 35 percent oxygenation. 

Yet oxygen matters for embryo growth. After two weeks, the youngsters in the lowest-oxygen environment were shorter and weighed less than the ones in airier pouches, Braga Gonçalves and her colleagues report June 3 in the Journal of Experimental Biology.

“We were surprised,” Braga Gonçalves says. She knows of no other direct test of how well pipefish dads manage embryos’ oxygen, but other work has hinted that males should manage well. Rich networks of capillaries line the inside of the males’ brood pouches, and direct measurements show that dads supply nutrients to the growing embryos and remove waste.

Broad-nosed pipefish dads provide fairly lush accommodations for their offspring. As breeding season starts, males grow two flaps that seal together to form a long, narrow incubation chamber for embryos. Females insert eggs into the pouch, sometimes adding to other females’ youngsters already inside. Then males seal the pouch and spend two or more weeks, depending on temperature, sustaining the next generation. Babies hatch out of their individual eggs days before they actually leave the pouch. When the time comes, males contort and spasm, unsealing the pouch and releasing miniature but fully formed pipefish.

Testing the oxygen situation inside male pouches wasn’t easy, Braga Gonçalves says. She searched for some tiny data collector she could fit inside the pouch along with the eggs and identified a company that produced related equipment. “They sent me an e-mail saying that if I gave them five years and 5 million Swedish kronor, they would do it for me.”

Moving to plan B, she devised a way to insert slim oxygen probes into the pouch every six days for the first 24 days of a pregnancy. The system worked well even though the probe was attached to a laptop. “We were in a wet room with tanks and pipes and seawater everywhere,” Braga Gonçalves says.

Oxygen levels in seawater where the pipefish breed in the wild fluctuate over the course of the day, but the concentrations in lab tests lie within natural ranges. Using males of various sizes, Braga Gonçalves and her colleagues discovered that the larger males were more likely to deliver oxygen more successfully than the little guys. Other research has shown that females of the species prefer larger males but failed to identify any particular benefit for size. Better oxygenated babies might finally explain some of the preference, Braga Gonçalves says.

This close look at male efforts to supply oxygen set Amanda Vincent of the University of British Columbia in Vancouver rethinking old assumptions about the topic. She has studied this species of pipefish as well as its relatives. The 52 genera of the pipefish and seahorse group encompass a range of daddy care, from tolerating glued-on eggs to providing a full pouch with one tiny opening. There are rich opportunities, she points out, for comparing these species and tracing the evolution of care.