Segment regeneration in the vestimentiferan tubeworm, Lamellibrachia satsuma.

The ability to regenerate missing body parts varies among species. To elucidate the evolution of regenerative capability, an understanding of the regeneration mechanisms of diverse organisms is required. We focus on vestimentiferan tubeworms, which have a body plan that is unique among annelids. We found that the vestimentiferan Lamellibrachia satsuma is able to regenerate its posterior body parts, but not its anterior body parts. Based on observations of live specimens, we defined five stages in the process of posterior regeneration. The morphogenesis was observed in detail by a series of sections and scanning electron microscopy. The most posterior domain of the opisthosome differentiated from the blastema, while the anterior domain of the opisthosome regenerated from the remaining trunk region. We also examined the expression pattern of the engrailed gene during regeneration, and found that engrailed was expressed in the mesodermal cells of each segment.

Neuroanatomy of the vestimentiferan tubeworm Lamellibrachia satsuma provides insights into the evolution of the polychaete nervous system.

Vestimentiferan tubeworms are marine invertebrates that inhabit chemosynthetic environments, and although recent molecular phylogenetic analyses have suggested that vestimentiferan tubeworms are derived from polychaete annelids, they show some morphological features that are different from other polychaetes. For example, vestimentiferans lack a digestive tract and have less body segments and comparative neuroanatomy can provide essential insight into the vestimentiferan body plan and its evolution. In the present study, we investigated the adult nervous system in the vestimentiferan Lamellibrachia satsuma using antibodies against synapsin, serotonin, FMRMamide and acetylated α-tubulin. We also examined the expressions of neural marker genes, elav and synaptotagmin to reveal the distribution of neuronal cell bodies. Brain anatomy shows simple organization in Lamellibrachia compared to other polychaetes. This simplification is probably due to the loss of the digestive tract, passing through the body between the brain and the subesophageal ganglion. In contrast, the ventral nerve cord shows a repeated organizational structure as in the other polychaetes, despite the absence of the multiple segmentation of the trunk. These results suggest that the brain anatomy is variable depending on the function and the condition of surrounding tissues, and that the formation of the rope ladder-like nervous system of the ventral nerve cord is independent from segmentation in polychaetes.