A new study has added to the growing body of research that proves not all animals resemble their adult forms during their developmental stages. A rare species of marine worm was found to be nothing more than a swimming head without a body.
According to researchers, Schizocardium californicum, a type of acorn worm, is basically a mere head swimming around collecting plankton during its larval stage. The work, which sheds light on what is called indirect development, was published in the Dec. 8 issue of Current Biology.
Direct And Indirect Development
Direct development is a kind of development in which a young organism becomes an adult without going through a distinct larval stage. Animals that are commonly used in research labs, such as mice, are direct developers. In contrast, indirect developers only turn into adults after going through a larval stage and undergo metamorphosis.
To have a deeper understanding of how indirect developers differ from direct developers, biologists Paul Gonzalez and Chris Lowe from Stanford's Hopkins Marine Station studied S. californicum, a worm with protracted larval stage. S. californicum is an indirect developer that can be closely compared with a direct developer that has been studied at length.
Lowe, associate professor of biology and the paper's senior author, said that indirect development is the most prevalent developmental strategy of marine invertebrates and that is how life evolved in the ocean.
Stages Of Development
The researchers sequenced the worm's RNA from different phases of its life cycle to determine how and where genes delay development. They concluded that some of the S. californicum's genes are dormant and cause delayed development into a full-grown body.
They found certain genes leading to the development of a trunk are delayed, resulting in the worm having a long larval stage during which it is basically a swimming head.
"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," said Gonzalez.
As the worm larvae grow, they will start sprouting a trunk. The genes that regulate the development of this body part turn on, and the worms begin to transform, slowly getting the elongated body of an adult. The worms grow to an average of 16 inches over several years.
"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," added Gonzalez.
The researchers believe S. californicum may not be the only creature having this strange growth strategy.
