Swimming Heads

Much of life begins not as a miniaturized version of what it will grow to become, but in a larval stage. But while we know much about the life cycles of “direct developers” (the Mini-mes), “indirect developers” are less studied and more mysterious. Which makes them very tempting for scientists to research. Scientists like Stanford grad student Paul Gonzalez.

“Terrestrial, direct developing species develop fast, their life cycle is simple, and they are easy to rear in the lab,” says Gonzalez. But indirect developing species are a bit more tricky. They develop slowly, have long larval stages, 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 found, collecting them can be an arduous process, too.

First he had to find them. His best bet was a group of marine invertebrates called Hemichordata because there is already a wealth of molecular developmental work done on many of the direct developers in this group, but there weren’t any near Hopkins Marine Station, where Gonzalez is based. So he went to the literature and found a 1994 study that gave him a big break. Schizocardium californicum, a species of acorn worm and one of the indirect developers in the Hemichordata phylum, was once in Morro Bay, only two hours away.

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.

But collecting the worms was only half the battle. Gonzalez then spent months perfecting the rearing and breeding techniques needed to study these worms. Eventually, he and his colleagues were able to sequence the RNA from various stages of the worm's development to see when specific genes are turned on or off in an embryo.

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 explains. “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.

Their findings were published this month in Current Biology and also may shed light on the very first animals’ development. “Indirect development is the most prevalent developmental strategy of marine invertebrates and life evolved in the ocean,” says study senior author Chris Lowe. “This means the earliest animals probably used these kinds of strategies to develop into adults.”