A study published in Science on the evolution of molluscan forms and genomes offers a fascinating glimpse into the complex and diverse biology of mollusks, shedding light on how they have evolved over hundreds of millions of years. Mollusks, one of the largest and most diverse animal phyla, are a key subject of study for understanding evolutionary processes, particularly with regard to their body plans, mineralized skeletons, and genomes.
Mollusks are known for their unique ability to repurpose ancient gene families to create novel structures, particularly when it comes to biomineralization, the process by which organisms produce hard mineralized structures such as shells. The origins of mollusks can be traced back to early small shelly fossils, some of which exhibit features that resemble modern mollusks, though it remains unclear exactly when they first appeared in the fossil record. These early fossils, like Odontogriphus and Shisania, suggest that the first mollusks may have lacked solid shells, yet the common ancestor of two major molluscan groups, Aculifera and Conchifera, is thought to have possessed a shell.
The study also reveals that the fossil record is rich with character mosaics, forms that combine traits not found in any living mollusk group. One of the key examples is Pelagiella exigua, an early Cambrian species that had a coiled shell but also exhibited features of flexible chaetae, which were interpreted as a stem gastropod.
A major theme in the evolution of mollusks is the extent of convergent evolution and character shifts in shorter time scales. For example, Juliidae, a group of gastropods with bivalved shells, was once thought to be a transitional form between gastropods and bivalves. However, the study shows that Juliidae represents a relatively recent adaptation to a specialized ecological niche, further emphasizing the plasticity of molluscan body plans. Such diversity highlights the ability of mollusks to adapt to various environments and ecological niches.
In conclusion, the study emphasizes the remarkable flexibility of the molluscan genome, which has allowed these organisms to evolve a wide array of body plans and adaptations over the course of their evolutionary history.
Sources: Science