Muscular system of Peltogasterella gracilis – A rhizocephalan with the modular type organization of interna

Rhizocephala is a group of highly modified parasitic crustaceans. The body of an adult female is formed de novo, and lacks most traits of free-living crustaceans. Despite the large number of studies dedicated to different aspects of rhizocephalan biology, the structure of the interna of these parasites is still poorly understood. The muscular systems specifically were described only for representatives of three families. The aim of our study is to visualize and describe the muscular system in the interna of Peltogasterella gracilis from the family Peltogasterellidae. P. gracilis has a colonial type of organization, with a common part of the interna and numerous externa-bearing modules. Interestingly, the muscular system has dissimilar organization in different parts of the interna. Muscular system in modules resembles that of Peltogaster paguri, while in the common part of the interna it's more similar to the muscular system of sacculinids and polyascids. We propose that the muscular system of rhizocephalans is a new structure in terms of evolution, and as such can be easily modified in respect to the functional characteristics of the different interna's parts.