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Rare worm spends part of life as a ‘swimming head’

A rare marine worm goes through a prolonged phase of development as little more than a head, researchers have found.

Paul Gonzalez, a graduate student at Stanford University’s Hopkins Marine Station, recently became a hunter, breeder, and farmer of the worms. He knew that some animals go through a long larval stage, a developmental strategy known as indirect development, and this rare worm was his chance to better understand that process.

The work, published in the journal Current Biology, suggests that many animals in the ocean likely share this trunk-less stage, and it may even shed light on the biological development of early animals.

Schizocardium californicum stages
Schizocardium californicum as a larva, juvenile, and adult. In the larval stage, S. californicum is little more than a swimming head. (Credit: Paul Gonzalez and Chris Patton/Hopkins Marine Station)

“Indirect development is the most prevalent developmental strategy of marine invertebrates and life evolved in the ocean,” says Chris Lowe, senior author of the paper and an associate professor of biology. “This means the earliest animals probably used these kinds of strategies to develop into adults.”

Most research animals commonly found in labs, such as mice, zebrafish, and the worm C. elegans, are direct developers, species that don’t go through a distinct larval stage. To understand how indirect developers differ from these, Gonzalez needed to study an indirect developer that was very closely related to a well-studied direct developer.

Acorn worms can even regenerate their heads

His best bet was a group of marine invertebrates called Hemichordata because there is already a wealth of molecular developmental work done on direct developers in this group. A flaw in this plan was that the indirect developers in this phylum were uncommon in areas near the station. Undeterred, Gonzalez poured through marine faunal surveys until a 1994 study gave him his big break: Schizocardium californicum, a species of acorn worm and indirect developer in the Hemichordata phylum, was once in Morro Bay, only two hours away.

Through contacting the researchers from that decades-old paper, Gonzalez obtained the exact coordinates of the worms. Once there, he pulled on a wet suit, readied his shovel, and began his hunt for the odd-looking ocean-dwellers.

Direct developers are more often used in research largely for reasons of practicality.

“Terrestrial, direct developing species develop fast, their life cycle is simple, and they are easy to rear in the lab,” says Gonzalez, who is lead author of the paper.

By comparison, indirect developers develop slowly, have a long larval stage, and their larvae are difficult to feed and maintain in captivity. The reproductive adults are also challenging to keep in the lab and, as Gonzalez has shown, collecting them can be an arduous process. However, the relative ease of studying direct developers has made for a lack of diversity in what scientists know about evolution and development, Gonzalez says.

“By selecting convenient species, we select a non-random sample of animal diversity, running the risk of missing interesting things,” he says.

Delayed trunks

After spending months perfecting the rearing and breeding techniques needed to study these worms, the researchers were eventually able to sequence the RNA from various stages of the worm’s development. They did this in order to see where specific genes are turned on or off in an embryo.

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They found that in the worms, activity of certain genes that would lead to the development of a trunk are delayed. So, during the larval stage, the worms are basically swimming heads.

“When you look at a larva, it’s like you’re looking at an acorn worm that decided to delay development of its trunk, inflate its body to be balloon-shaped, and float around in the plankton to feed on delicious algae,” says Gonzalez. “Delayed trunk development is probably very important to evolve a body shape that is different from that of a worm, and more suitable for life in the water column.”

As they continue to grow, the acorn worms eventually undergo a metamorphosis to their adult body plan. At this point, the genes that regulate the development of the trunk activate and the worms begin to develop the long body found in adults, which eventually grows to about 40 cm (15.8 inches) over the span of several years.

Why study these worms?

This research is only the beginning of the Lowe lab’s examination of indirect developers. These worms will never tell us about human diseases, unlike work with stem cells or mice, but they could reveal the intricacies of how life works for many organisms beyond the model species that we’ve studied so heavily. They may also show us how life in general came to be what it is today.

“Given how pervasive larvae are in the animal world, we understand very little about this critical phase in animal development,” says Lowe. “These are not the kind of species you want to pick if you want deep, mechanistic insights into developmental biology. But, if your goal is to understand how animals have evolved, then you cannot avoid using these species.”

Next, the researchers want to figure out how the acorn worm body development delay happens. They also have begun to sequence the genome of S. californicum.

Funding for this work came from the Natural Sciences and Engineering Research Council of Canada, the Dr. Earl H. Myers and Ethel M. Myers Oceanographic and Marine Biology Trust of Pebble Beach, NASAExobiology, and the National Science Foundation.

Source: Stanford University

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