The neuromuscular system of Pycnophyes kielensis (Kinorhyncha: Allomalorhagida) investigated by confocal laser scanning microscopy.

Andreas Altenburger

doi:10.1186/s13227-016-0062-6

Abstract

BackgroundKinorhynchs are ecdysozoan animals with a phylogenetic position close to priapulids and loriciferans. To understand the nature of segmentation within Kinorhyncha and to infer a probable ancestry of segmentation within the last common ancestor of Ecdysozoa, the musculature and the nervous system of the allomalorhagid kinorhynch Pycnophyes kielensis were investigated by use of immunohistochemistry, confocal laser scanning microscopy, and 3D reconstruction software.ResultsThe kinorhynch body plan comprises 11 trunk segments. Trunk musculature consists of paired ventral and dorsal longitudinal muscles in segments 1–10 as well as dorsoventral muscles in segments 1–11. Dorsal and ventral longitudinal muscles insert on apodemes of the cuticle inside the animal within each segment. Strands of longitudinal musculature extend over segment borders in segments 1–6. In segments 7–10, the trunk musculature is confined to the segments. Musculature of the digestive system comprises a strong pharyngeal bulb with attached mouth cone muscles as well as pharyngeal bulb protractors and retractors. The musculature of the digestive system shows no sign of segmentation. Judged by the size of the pharyngeal bulb protractors and retractors, the pharyngeal bulb, as well as the introvert, is moved passively by internal pressure caused by concerted action of the dorsoventral muscles. The nervous system comprises a neuropil ring anterior to the pharyngeal bulb. Associated with the neuropil ring are flask-shaped serotonergic somata extending anteriorly and posteriorly. A ventral nerve cord is connected to the neuropil ring and runs toward the anterior until an attachment point in segment 1, and from there toward the posterior with one ganglion in segment 6.ConclusionsSegmentation within Kinorhyncha likely evolved from an unsegmented ancestor. This conclusion is supported by continuous trunk musculature in the anterior segments 1–6, continuous pharyngeal bulb protractors and retractors throughout the anterior segments, no sign of segmentation within the digestive system, and the absence of ganglia in most segments. The musculature shows evidence of segmentation that fit the definition of an anteroposteriorly repeated body unit only in segments 7–10.Electronic supplementary materialThe online version of this article (doi:10.1186/s13227-016-0062-6) contains supplementary material, which is available to authorized users.

Highlights

Kinorhynchs are ecdysozoan animals with a phylogenetic position close to priapulids and loriciferans
Animals were sorted under a dissecting microscope and fixed for 1 h at room temperature in 4% paraformaldehyde in 0.1 M phosphate buffer (PB)
P. kielensis moves by everting the introvert (Fig. 1c; Additional file 1)

Summary

Introduction

Kinorhynchs are ecdysozoan animals with a phylogenetic position close to priapulids and loriciferans. The body plan of kinorhynchs comprises an eversible head (introvert) with several rings of scalids, a protrusible mouth cone, a neck, and a trunk with 11 segments (called zonites in older literature) [10, 11]. Kinorhynchs use their introvert to move in between sand grains on the sea floor and to feed on bacteria [12, 13]. The musculature and nervous system of members of the cyclorhagid kinorhynch genus Echinoderes have been investigated previously [14, 15]. These studies found a combination of segmented and unsegmented features in the myoanatomy and nervous system of kinorhynchs

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