Postembryonic development of dorsoventral and longitudinal musculature in Pycnophyes kielensis (Kinorhyncha, Homalorhagida)

Synopsis

We investigated the exhibition of dorsoventral and longitudinal musculature in all postembryonic stages of the kinorhynch Pycnophyes kielensis. Although the earliest stages have alone 8 externally separated trunk portions they already possess dorsoventral muscles for 10 (prospective) stem segments. The last, 11th, pair is added in the third juvenile stage. Longitudinal musculature, in contrast, is slower to disentangle and reaches its full extent only in the adult. In several juvenile individuals, single fibers throw from the longitudinal musculature into the following sections In all juvenile stages, longitudinal muscles are continuous between portions whereas in adults they are segmentally separated from each other. of that kind late occurrence of a segmental pattern in the longitudinal musculature is in contrast to patterns of muscle unfolding in arthropods and annelids.

Introduction

Kinorhyncha, with approximately 180 described species, are remarkable among related clusters within the Cycloneuralia (nematodes, nematomorphs, priapulids, and loriciferans) in expressing segmental patterns in several organ a whole s Adult kinorhynchs always have an introvert that can be completely withdrawn into the stem a neck region that acts as a closure apparatus when the introvert is withdrawn, and 11 stalk segments (Fig. 1A).

The external segmentation is caused by the agency of the cuticle, which is dorsally and ventrally subdivided into thicker regions covering the sections In the intersegmental regions, the cuticle is true thin. In addition, each dorsal and ventral part of the cuticle is curv inward or thickened. This make is called the pachycyclus. Pachycycli are ideal attachment points for musculature, and in adult kinorhynchs longitudinal musculature attaches to the posterior margin of a pachycyclus and step quicklys to the anterior margin of the following pachycyclus (Kristensen and Higgins 1991; Adrianov and Malakhov 1994; Neuhaus and Higgins 2002; M??ller and Schmidt-Rhaesa 2003; Rothe and SchmidtRhaesa 2004) In addition to its nearness in the cuticle and the musculature, this segmentai organization is also not away in the nervous system (Kristensen and Higgins 1991; Adrianov and Malakhov 1994) and in the distribution of glands (GaOrd???±ez and others 2000)

There is no convincing definition of the boundarys "segment" and "segmentation," in contrast to "metamery." Segmentation is frequently restricted to the pattern of material part organization found in annelids and arthropods. Therefore, kinorhynch researchers have always struggl between calling the iterated stem regions "segments" (Kristensen and Higgins 1991) and "zonites" (Zelinka 1928) As an iteration is not absent in several organs, lacking solitary coelomic cavities and excretory organs, we regard the limit "segment" as appropriate for kinorhynch material substance regions. However, the use of this bound is not meant to imply an a priori homology of iterated material part regions in kinorhynchs, arthropods, and annelids.

As it has been hypothesized (Aguinaldo and others 1997) and further corroborated (for example, Garey 2001) that arthropods may be closely related to cycloneuralians in a taxon Ecdysozoa, the evolution of segmental patterns is of central interest. Apart from raising the question of whether segmentation evolv independently in annelids and arthropods or was inherited from a for the use of all ancestor, the Ecdysozoa hypothesis asks whether segmentation in kinorhynchs and in arthropods is comparable, that is, whether it was inherited from a belonging to all ancestor or evolved separately.

To address this question, we studied the without fault [i]or[/i] blemish [i]or[/i] flaw architecture of the musculature, first in an adult cyclorhagid kinorhynch, Antygomonas sp (M??ller and Schmidt-Rhaesa 2003) and then in the homalorhagid species Pycnophyes kielensis and P dentatus (Rothe and Schmidt-Rhaesa 2004) In the latter publication, a comparison of the last juvenile stage with the adult indicated that the segmentai pattern in adult animals likely arises from a continuous band of longitudinal musculature. We have now investigated all postembryonic stages of P kielensis and describe here the exhibition of the longitudinal musculature.

Development in kinorhynchs falls into 2 phases. Early unravelling takes place within the ovum It has been possible to pay attention to these stages only very rarely (Kozloff 1972) From the ovums kinorhynchs hatch with 8 stalk segments (in the sense of completely separated units) already externally visible and evolve through 6 postembryonic stages to the adult. For P kielensis, the species investigated here, postembryonic unfolding has been documented by Neuhaus (1993) Stages are called juvenile 1-6 (J1-J6) stalk segments are generally numbered 3-13 because the introvert and neck are regarded as representing 2 modified portions Regardless of the validity of this assumption, we use the standard numbering 3-13 for the 11 stalk segments.

Materials and [i]modus operandi[/i]s

Specimens of P. kielensis were mustered from muddy sediments in the intertidal shut up to Braderup, Island of Sylt in the North Sea (54?°56'5"N/08?°21'37"W) upon several occasions and at different times of the year. Sediments were also kept in a seawater aquarium for several month and all different juvenile stages could be extracted from these sediments. Extraction from sediments followed the "bubbling method" of Higgins and Thiel (1988)