How Do Prostomial Sensory Organs Affect Brain Anatomy? Phylogenetic Implications in Eunicida (Annelida)

Abstract

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.

Palps across the tree – the neuronal innervation and development of sensory head appendages in Annelida

“…The buccal lips – structures situated ventrally at the anterior tip of the amphinomid head - are innervated from longitudinal nerves solely emanating from distinct lip nerve roots, which are part of the vrcc . Although, we cannot rule out the contribution of nerves emanating from the drcc , our data seems to resemble observations made for Eunicidae and Hyalinoecia tubicola (Onuphidae) as well ( Zanol, 2010 ; Kuhl et al, 2022 ). Nevertheless, further data are needed to support a homology of these structures in the different taxa.…”

Jaw morphology and function in Drilonereis cf. filum (Oenonidae, Annelida)

“…cf. mutans obtained in addition to data on the structure of the brain and innervation of the appendages of the head end of the body (Kuhl et al, 2022; Zanol, 2010). These authors studied the nervous system structure of the anterior part of the Arabella cf.…”

“…iricolor and A. cf. mutans according to Zanol (2010), Kuhl et al (2022) and D. cf. filum are similar, but a detailed comparison is difficult because the data were obtained using different methods.…”

“…Based on the data obtained, two groups of nerves associated with the stomatogastric nervous system were identified, that is, paired esophageal nerves, which then pass into the stomatogastric nerves, and paired pharyngeal nerves. Kuhl et al (2022) described jaw nerves (more dorsal) and stomatogastric nerves (more ventral). We reconstructed only a fragment of the stomatogastric nervous system of D. cf.…”

“…The available data are fragmentary information on A. iricolor and A. cf. mutans obtained in addition to data on the structure of the brain and innervation of the appendages of the head end of the body(Kuhl et al, 2022; F I G U R E 14 Drilonereis cf. filum, scheme of the jaw apparatus and ventral pharyngeal organ structure and function.…”