Sensory Organs of Eunicida (Annelida)

Abstract

The taxon <strong>Eunicida</strong> comprise marine annelids exhibiting a remarkable variation in the number and morphology of <strong>prostomial sensory organs</strong>. Currently seven different families of Eunicida are accepted: Eunicidae, Onuphidae, Dorvilleidae, Oenonidae, Lumbrineridae, Histriobdellidae, and Hartmaniella. Most representatives of Eunicida are <strong>highly mobile predators</strong> with a <strong>complex nervous system</strong>, showing <strong>significant interspecific and interfamilial variations</strong> in the number and morphology of antennae, palps, buccal lips, eyes, and nuchal organs. This diversity makes Eunicida an ideal model to study the following key questions: (1) To what extent is the brain morphology affected by the presence and variation of prostomial sensory organs? and (2) Do differences in the number and shape of these sensory organs reflect phylogenetic relationships and ecological adaptations? It could be assumed that different brain morphologies become apparent between the families, that deliver not only insight into the different processing of sensory information but also help to provide clues about phylogenetic relationships and ecological adaptation. Further, the lateral organs were investigated to gather information, if they can be used as supporting character in assessing phylogenies. Using a comparative morphological approach - including immunohistochemistry, µCT, TEM, SEM, paraffin histology, and semi-thin sectioning techniques - we show that while the <strong>gross morphology of the central nervous system is largely conserved</strong>, the <strong>brain's microanatomy varies in association with the presence or absence of sensory organs</strong>. In Eunicidae and Onuphidae (Eunicoidea), many well-developed sensory appendages are present correlating with distinct commissural and tract arrangements in the brain. In contrast, Oenonidae and Lumbrineridae (Oenonoidea) exhibit reduced prostomial sensory structures but share unique neuroanatomical features such as lateral neuropils, a dorsoventrally extended brain and similar nuchal organs, supporting a sister-group relationship. <br/> These findings align with recent phylogenetic hypotheses and suggest a secondary reduction of antennae in several lineages. Furthermore, the sensory organ morphology reflects ecological adaptation: Species in epibenthic, light-rich habitats retain complex sensory structures, while endobenthic taxa show reductions consistent with life in low-light environments. This study provides new insights into the evolution of sensory systems, nervous system plasticity, and phylogenetic relationships within Eunicida and Annelida.