New Data on the Musculature Organization of the Atypical Phoronid Phoronis embryolabi Allow Reconstruction of Its Distinctive Lifestyle Characteristics

Abstract

Phoronids are marine invertebrates with a global distribution and are often abundant in benthic communities. Their morphology, anatomy, and ultrastructure is rather uniform, including the organization of their musculature. However, Phoronis embryolabi, which is characterized by an unusual body regionalization, exhibits a distinct morphology in its trunk musculature. This study uses histology, electron microscopy, computer microtomography, histochemistry, and confocal laser scanning microscopy to characterize the musculature in various trunk regions of P. embryolabi, a species that resides commensally within the burrows of burrowing shrimps. This phoronid species is considered the closest relative of Phoronis pallida, which has a unique syncytial musculature. The musculature of P. embryolabi comprises transverse and longitudinal muscles, organized in a single layer, with the absence of diagonal musculature. The longitudinal muscles are organized into bundles, each comprising cross-striated cells in the central part and smooth cells in two marginal parts. Phoronis embryolabi features several sphincters located between the head region and the rest of the body. The organism appears to be optimally adapted to life within the burrow of the shrimp. The water current generated by the shrimp compensates the requirement for the phoronid to possess diagonal muscles for specific adjustments of the lophophore. The combination of cross-striated and smooth longitudinal muscles facilitates robust and sustained contractions in response to threats. Additionally, circular sphincters likely function to prevent hemorrhage when the head region is injured due to the shrimp's movements.