Calcitic sclerites at base of malacostracan pleopods (Crustacea)--part of a coxa.

Verena Kutschera,Gerd Mayer,Andreas Maas,Dieter Waloszek

doi:10.1186/s12862-015-0357-6

Abstract

BackgroundCuticular specialisations such as joints and membranes play an important role in the function of arthropod limbs. This includes sclerotisations and mineral incrustations of cuticular areas to achieve either more rigidity or flexibility. The anterior eight thoracopods of Malacostraca have limb stems comprising a coxa and a basipod, which carries the two rami. Their pleopods, the limbs of the posterior trunk part, have for long been regarded to lack a coxa. Several calcitic sclerites occur in the area between ventral body and limb stem. This raises the questions: do these elements represent specialisations of the membrane due to functional requirements, and do they originate from an originally larger limb portion, i.e., the coxa, or in fact represent it.ResultsWe investigated 16 species of selected malacostracan taxa from all major in-groups. Calcitic sclerites occur in constant numbers and position within a species (no individual variation, and independent of specific modification such as in genital appendages). These are even constant within a supra-specific taxon, which facilitates comparisons. In general the sclerites connect via two pivot joints to the sternite medially and the tergopleura laterally, and two more to the limb stem. Based on this, we reconstructed putative ground-pattern conditions for the sclerites of the examined taxa of Malacostraca.ConclusionsThe pattern of sclerites is characteristic for each monophyletic malacostracan taxon. The highest number of sclerites most likely represents the plesiomorphic state. Reduction of sclerite numbers occurs in Caridoida and its in-groups. Sclerite arrangement in these taxa provides an important character complex for phylogenetic studies. The presence of pivot joints to the body proximally and basipod distally demonstrates the existence of a coxa, which is just slightly less sclerotised, particularly on its posterior side. This can be explained by enhanced flexibility of the pleopods evolved in the course to their major role as swimming devices. Both the pivot joints and the proximal and distal extension of the calcitic sclerites demarcate the minimum area of the coxa. With this, sclerites appear as very valuable also in shedding more light on the putative relationships between Malacostraca, Myriapoda, Insecta, and Remipedia.

Highlights

Cuticular specialisations such as joints and membranes play an important role in the function of arthropod limbs
We examined what appears to be the exclusively membranous area extending between the ventral body surface and the comprehensive pleopod limb stem ("the pleopod-body articulation") in a wide array of malacostracan taxa with Alizarin-Di Dissection (Red) staining and fluorescence microscopy (Figures 1, 2, 3, 4 and 5; Table 1)
Anaspidacea (Figures 2 and 3) Anaspides tasmaniae Thomson, 1893 (Figures 1A, 2A–N; Table 4) Calcified tergopleura borders pleomere laterally (Figure 2A–D); calcified, rhombic plate medially interpreted as sternite (Figure 2C, E, G, I, K, M); st subdivided at plm1 (Figure 2E), undivided plm2–5 (Figure G, I, K, M); st drawn out posteriorly into sternitic wings (Figure 2D)

Summary

Introduction

Cuticular specialisations such as joints and membranes play an important role in the function of arthropod limbs. Several calcitic sclerites occur in the area between ventral body and limb stem This raises the questions: do these elements represent specialisations of the membrane due to functional requirements, and do they originate from an originally larger limb portion, i.e., the coxa, or represent it. A well-developed basal membrane did not appear before the euarthropod level in connection with the development of a rigid, antero-posteriorly flattened stem portion, the basipod, which carried the two rami endopod and exopod [8,9]. This design permitted a more elaborate ex- and intrinsic musculature to extend from inside the body and inserting into the limb stem. With the newly formed basal membrane, interactions could take place between the body and the stiffer limb stem, allowing

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