Morphology and development of worm snails

Worm snails (or worm shells) differ from other snails and slugs in that they form calcareous tubes in which their worm-like, roundish body resides. This worm-shaped body superficially resembles that of other marine worms, with to they are not closely related. Their tubes are permanently attached to the substrate, resulting in the animals being immobile because their body is physically connected to the inner side of their tube by a prominent retractor muscle. Although highly derived in their adult morphology, these animals show a classic, snail-like morphology during early development, including a coiled shell and a creeping foot, with which they glide over the substrate. These young snails develop in brood sacs inside the mother animal where they develop via a larval stage that is typical for other indirect developing marine snails and slugs. Once the snail-like morphology is established, the juveniles leave the brood sac and crawl out of the mothers tube. It has been proposed that these young snails settle in close vicinity to the mother animal, attach themselves permanently to the substrate, and commence their metamorphosis into the worm-shaped adult animal. The processes that lead to the snail-like morphology of the juvenile and those that govern its metamorphosis into the adult worm are hitherto unknown. Accordingly, the present project will investigate these developmental mechanisms both on the morphological and the molecular level. Specific focus will be on the development and transformation of the musculature, the nervous system, the calcareous shell and tube, as well as on the activity of selected genes that are known to be involved in establishing important morphological structures and that are responsible for shaping the body axes and symmetry planes in other animals. The data obtained will be compared from relevant data in published works in order to understand to what degree worm snails use similar or different mechanisms during their development as other animals, especially their closely related snail allies. This will lead to insights into the evolutionary and developmental processes that have resulted in a worm-like, sessile, tube-dwelling organism from a free-living, crawling miniature snail.