A new study indicates that some marine worms go through a prolonged phase, probably for months, with little more than head.
The study about the trunk-less larval stage, a developmental strategy known as indirect development, known to be shared by many animals in the ocean, may shed light on the biological development of early animals.
"Indirect development is the most prevalent developmental strategy of marine invertebrates and life evolved in the ocean," said Chris Lowe, an associate professor of biology at Stanford University. "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.
The research targeted a group of marine invertebrates called Hemichordata, because there is already a wealth of molecular developmental work done on direct developers in this group, and finally focused on Schizocardium californicum, a species of acorn worm and indirect developer in the Hemichordata phylum.
At Stanford's Hopkins Marine Station, graduate student Paul Gonzalez, lead author of a paper published in the journal Current Biology, became a hunter, breeder and farmer of the rare marine worm.
"Terrestrial, direct developing species develop fast, their life cycle is simple and they are easy to rear in the lab," said Gonzalez. 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.
After spending months perfecting the rearing and breeding techniques needed to study these worms, the researchers were able to sequence the ribonucleic acid (RNA) from various stages of the worm's development. 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," Gonzalez was quoted as saying in a news release. "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."
The acorn worms continue to grow, 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, or 15.8 inches, over the span of several years.
The research is the beginning of the Lowe lab's examination of indirect developers. "Given how pervasive larvae are in the animal world, we understand very little about this critical phase in animal development," said Lowe, senior author of the paper. "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."
