Morphological Organization, Sensory Structures and Catecholaminergic Nervous System of <i>Lindrilus flavocapitatus</i> (Annelida: Protodrilidae)

Protodrilidae is a small family of almost exclusively interstitial annelids that lack parapodia and chaetae and possess a basiepithelial nervous system. This study presents a histological description of Lindrilus flavocapitatus (Uljanin, 1877), a protodrilid species last examined morphologically in the early 20th century, and provides detailed information on the organization of its nervous and sensory systems using histochemical detection of catecholamines (CAs), scanning electron microscopy (SEM), and alpha-tubulin immunolabelling. The epidermal ciliary structures on the head show a species-specific distribution pattern, and SEM reveals three types of ciliary sensory structures, similar to those previously described in other protodrilids. Numerous CA-containing (CAc) cells are found in both central (CNS) and peripheral nervous systems. A spatial correlation between epidermal ciliary structures and CAc cells offers the first direct evidence supporting the sensory function of some known ciliary types and allows hypotheses regarding their sensory modalities. The widespread, mostly diffuse distribution of epidermal CAc cells throughout the trunk, pygidium, and palps suggests a mechanosensory function, although some presumed mechanosensory cells are not catecholaminergic or lack CAs. The presence of CAs in putative phaosomes on the palps also points to a possible role for these neurotransmitters in photoreception. In addition to typical annelid sensory organs such as palpal receptors, nuchal organs, and possible phaosomes, L. flavocapitatus possesses a unique bud-shaped sensory organ and a dorsal ridge-like array of receptor cells, both containing CAs. A prominent CAc gastroesophageal ganglion innervating the complex pharyngeal apparatus of L. flavocapitatus is described for the first time in protodrilids. The results reveal a more differentiated neural and sensory organization in protodrilids than previously recognized. Despite its small body size and a relatively low neuron count, L. flavocapitatus possesses additional CNS regions beyond those common to most annelids and a uniquely organized apical sensory organ.