Enigmatic marine invertebrates known as penis worms are shedding new light on the evolution of digestion, while posing a curious embrionic question: which develops first, the mouth or the anus?
To produce the first description of the entire fetal development of penis worms, or priapulids, Swedish and Norwegian researchers looked at which genes are expressed during the development of the gut, the mouth and the anus in three-day-old worm embryos. In particular, they investigated embryos belonging to Priapus caudatus worms.
Living in shallow waters and measuring up eight inches long, these worms are "living fossils" that have changed very little since their priapulid ancestors thrived on the ocean floor in the Cambrian period some 500 million years ago.
Detailing their findings in the journal Current Biology, the researchers noticed that priapulids form their guts like humans, fish, frogs, starfish and sea urchins. Furthermore, exactly the same genes are involved in the process.
"It does not mean that these penis worms are now closely related to humans," lead researcher Andreas Hejnol, at the Sars International Centre for Marine Molecular Biology in Norway, said.
"The fact that different animals share a common way of forming the gut suggests that the embryological origins of the human intestine and how it develops are much older than previously thought - most likely over 500 million years," Hejnol said.
Priapulids are thought to be among the first bilaterally symmetric animals (those with a left and right body side) which today make up 99 percent of all living animals.
Historically, bilaterally symmetric animals have been divided into two groups based on the difference in how the gut develops in the fetus. At an early stage of fetal development, some cells move into a region called the blastopore.
"The important point is that in some animals this region becomes the mouth, while in others it becomes the anus," Hejnol said.
The two groups - protostomes (from the Greek for "mouth first") and deuterostomes ("mouth second") - have defined separate branches of the evolutionary tree since 1908.
Priapulids and most other invertebrates fell into the protostome group, in which the mouth formed first, and the anus second.
Vertebrates and a few spineless animals were placed into the deuterostome category. (Yes, our anuses develop before our mouths).
Now several evolutionary biologists believe that the classification will have to be reconsidered.
"Here is an animal that is the poster child for early protostomes, and it develops just like a deuterostome," Mark Martindale, a developmental biologist at the University of Hawaii in Honolulu, told Nature.
"We've been using the name protostome for 100 years, and now it's clear that it doesn't mean anything," he said.
Apart from shaking the branches of the animal tree, the research shows how important it is to study the vast diversity of animals found in the oceans.
"Priapulids still hide a lot of secrets to unravel, which will have a great influence on our understanding of the origin of other major organs, such as the brain, blood or legs," Hejnol said.
Penis worms reproduce in winter time, so Hejnol and colleagues have to travel regularly to the west coast of Sweden during the ice-cold season to get them.
"We sail the fjords dredging in areas where they are abundant, collecting animals and later getting embryos from them in the lab," first author José M. Martín-Durán said.
"Although thrilling, sometimes the collection trips turn into real adventures, with low temperatures, snow or even frozen waters," he added.
Photo: Priapulus caudatus. Credit: Dmitry Aristov /Wikimedia Commons.