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Some bilateral animals have highly specialized genomes, particularly model organisms like fruit flies, nematodes, zebrafish, and planarians. Evolutionary biologists have observed this specialization since their genomes were sequenced. Conversely, marine invertebrates such as sea urchins, amphioxus, scallops, and ribbon worms exhibit conserved genome structures that have remained unchanged for over 500 million years. Typically, a phylum's genome structure is either conserved or specialized. Studies on Annelida, however, reveal a rare exception.

Most annelids are marine, but some live on land and in freshwater. Assistant Research Fellow Yi-Jyun Luo's team from the Biodiversity Research Center analyzed the chromosomal-level genomes of 23 annelid species. They found that while most annelids have conserved genome structures, Clitellates— including leeches and earthworms— exhibit a marked difference. The team developed a rearrangement index, which showed that leeches and earthworms possess the most highly rearranged genomes among bilaterians. The research, published on August 14, 2024, in Molecular Biology and Evolution, was funded by the Academia Sinica Career Development Award.