Who needs a body? These larvae are basically swimming heads

When they’re young, most creatures are miniature versions of their adult selves. But not Schizocardium californicum – during its larval stage, the creature is basically just a swimming head.

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

When it comes to animal development, nature follows a few very distinct paths. Take mammals for example. You’re born with all your organs and sure – you drastically change as you grow up, but the main biological setup remains the same. Other creatures however, like some amphibians and insects, have it completely different. They first develop as larvae which are completely different from adult individuals and only after a while do they metamorphose into their final form.

This process has been studied in great detail by biologists but there is still a big knowledge gap surrounding it. Graduate student Paul Gonzalez at Stanford University’s Hopkins Marine Station wanted to bridge that gap – so he became a hunter, breeder, and farmer of a rare marine worm.

Most research animals commonly found in labs, such as mice, zebrafish, and the worm C. elegans, are direct developers — they don’t go through a larval stage.

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

By contrast, larval species develop slowly, have a long larval stage, and the larvae are difficult to maintain and feed in captivity – these being the main reasons why direct growers are preferred in the lab. But choosing convenient species has led to a lack of diversity in what scientists know about evolution and development, Gonzalez said, and he wanted something different.

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

Ultimately, he focused on Schizocardium californicum, a rather rare marine worm.

“By selecting convenient species, we select a non-random sample of animal diversity, running the risk of missing interesting things,” he said. “That’s what brought me to the Lowe lab. We specialize in asking cool evolutionary questions using developmental biology and molecular genetics, and we’re not afraid to start from scratch and work on animals that no one has worked with before.”

The two spent months and months honing their techniques for the study. They gathered RNA from various stages of the worm’s development to see what genes are turned on and off. What they found is that during their larval stage, the genes which determine the development of the trunk (the rest of the body) are almost completely switched off — so, in other words, the larvae are just 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,” said 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.”

This came as quite a surprise and shows just how much we still have to learn about these creatures. Sure, they won’t teach us much about the human body or advanced mammals, but they might shed some light on how life evolved to the diversity we see today as well as evolution itself.

“Given how pervasive larvae are in the animal world, we understand very little about this critical phase in animal development,” said 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.”

Journal Reference: Paul Gonzalez, Kevin R. Uhlinger, Christopher J. Lowe. The Adult Body Plan of Indirect Developing Hemichordates Develops by Adding a Hox-Patterned Trunk to an Anterior Larval Territory. DOI: http://dx.doi.org/10.1016/j.cub.2016.10.047

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