An evolutionary enigma
In the muddy bottom sediments of the Gullmarsfjord, Sweden's only true fjord, lives one of the most perplexing creatures in the animal kingdom: Xenoturbella bocki. This marine worm, typically growing to about 1-4 centimeters long, has an extraordinarily simple body plan. It lacks a brain, an anus, a coelom (the main body cavity in most animals), and any organized reproductive organs. Its nervous system is a simple nerve net embedded in its skin, and it has a single opening on its belly that is both a mouth for eating and for excreting waste.
First collected in 1915 by Swedish zoologist Sixten Bock, the animal was not formally described until 1949 by Einar Westblad. Initially, it was classified as a type of flatworm. Its placement on the evolutionary tree of life has been a subject of scientific debate ever since. For a time, DNA analysis suggested it was a simplified mollusc, a conclusion that confounded zoologists. This was later proven to be an error caused by contamination from the bivalve molluscs that Xenoturbella eats. Genetic barcoding has identified DNA from bivalves like Abra nitida and Ennucula tenuis inside the worm, suggesting it feeds on their eggs or larvae. Direct feeding, however, has never been observed.
A record of the past
The central question surrounding Xenoturbella is whether its simplicity is primitive or a result of secondary simplification from a more complex ancestor. Recent genetic studies place it in a phylum called Xenacoelomorpha. The debate continues whether this group is an early offshoot of all bilaterally symmetrical animals (the Nephrozoa) or a sister group to the Ambulacraria (which includes starfish and sea urchins) within the deuterostomes. If it is an early branch, Xenoturbella is a model for what the common ancestor of most animals, including humans, looked like over 500 million years ago.
Its body is simple, its genome is not. Analysis of the Xenoturbella bocki genome shows it has a gene count and complexity typical of other bilaterian animals. This finding suggests that its simple form might be a derived trait, meaning it lost complex features over time. The animal reproduces by releasing eggs and sperm through temporary ruptures in its body wall. It develops directly from an embryo to a small version of the adult, without a distinct larval feeding stage.