Eunicida is a taxon of marine annelids currently comprising the taxa Eunicidae, Onuphidae, Dorvilleidae, Oenonidae, Lumbrineridae, Histriobdellidae and Hartmaniella. Most representatives are highly mobile hunters sharing the presence of a sophisticated nervous system but differ in the number and shape of prostomial sensory organs (0–3 antennae; 0 or 2 palps; 0, 2 or 4 (+2) buccal lips; 0, 2 or 4 eyes; single-grooved or paired nuchal organs). This makes Eunicida an ideal model to study the following questions: Is the brain morphology affected by different specificities of prostomial sensory organs? Do similar numbers and shapes of prostomial sensory organs hint at close phylogenetic relationships among different eunicidan taxa? How can antennae, palps and buccal lips be differentiated? For the investigation of sensory organs and the nervous system, we performed immunohistochemistry, µCT, TEM, SEM, paraffin histology and semi-thin sectioning. Our results show that brain anatomy is mostly affected on a microanatomical level by sensory organs and that similar specificities of sensory organs support the latest phylogenetic relationships of Eunicida. Further, a reduction of antennae in Eunicida can be suggested and hypotheses about the presence of sensory organs in the stem species of Eunicida are made.

Observing the process of growth and differentiation of tissues and organs is of crucial importance for the understanding of the evolution of organs in animals. Unfortunately, it is notoriously difficult to continuously monitor developmental processes due to the extended time they take. Long-term labeling of the tissues of interest represents a promising alternative to raise these pivotal data. In the case of the prototroch, a band of ciliated cells typical of marine, planktotrophic trochophora larvae, we were able to apply a long-term fluorescent vital-staining to the prototroch cells that remains detectable throughout further larval life. We were able to stain ciliated cells of planktonic larvae from different spiralian clades by using long-chain dialkylcarbocyanine dyes that are detectable in different fluorescent emission spectra in combination with a non-ionic surfactant. The larvae survived and developed normally, their ciliated cells retaining the originally applied fluorescent labels. Combined with additional fluorescent staining of the larvae after fixation, we provide an easy, versatile, and broadly applicable method to investigate the processes of the differentiation of epidermal organs in various aquatic larvae.