Morphology of a new interstitial Psammodrilus (Psammodrilidae, Annelida) from Sardinia, Italy

Psammodrilus curinigallettii n. sp., the first Mediterranean endemic psammodrilid, is here described combining light, scanning electron, and confocal laser scanning microscopy. The new species is characterized by the presence of autapomorphic segmental diamond glands and a unique combination of the following features: small size, simple pharyngeal apparatus, head entirely ciliated with no distinct collar region, presence of no more than three uncini in each abdominal parapodium. Confocal laser scanning microscopy studies of the F-actin stained musculature revealed (i) a dense grid of longitudinal muscles surrounded by regularly arranged circular muscles along the entire trunk, (ii) oblique muscles inserting at the uncini in the abdomen, (iii) muscularized thoracic cirri, and (iv) a limited pharyngeal muscular diaphragm in the posterior peristomium. Immunoreactivity against tubulin revealed two pairs of fully ciliated nephridia opening in segments 7 and 8. The lack of a well-defined collar region, small size, and low number of chaetae resembles traits found in juvenile forms of larger congeners, indicating a progenetic origin of P. curinigallettii n. sp. from larger ancestors.