Fine structure and ecdysis of mandibular sensilla associated with the lacinia mobilis in Neomysis integer (Leach, 1814) (Crustacea, Malacostraca, Peracarida)

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Highlights

  • Mandibles of adult Neomysis integer were studied using light and electron microscopy.

  • We describe the external and internal structure.

  • Special reference is paid to the lacinia mobilis and characteristics of the ecdysis.

  • Present sensory cells reveal that both laciniae mobiles are mechanosensory organs.

  • Discussion of the classification and a possible homology in Peracarida and Decapoda.

Abstract

The external and internal structures of adult Neomysis integer mandibles were studied using light and electron microscopy with special reference to the lacinia mobilis, a highly specialized appendage on the gnathal edge of many crustaceans. The right and left lacinia mobilis are equipped with ciliary primary sensory cells revealing that both laciniae are also mechanosensory organs in addition to their mechanical function during mastication. A detailed character analyses indicated that the right lacinia is probably a highly derived sensory seta, whereas two alternative interpretations are considered for the left lacinia; it could be a sensillar appendage equipped with two mechanosensory units, or it could be a movable appendage of the incisor process containing two sensilla deprived of external appendages. The ecdysis of the lacinia mobilis corresponds very well to type I sensillar ecdysis, suggesting classification as a sensillar appendage. These features support a possible homology of the right lacinia mobilis in Peracarida and Decapoda, tracing them to an origin as a member of the setal row. Whether the left lacinia mobilis is a sensillum or an appendage with sensilla cannot be resolved presently.

Introduction

The lacinia mobilis is a distinctive structure among various appendages arming the gnathal edges of mandibles in Peracarida. The lacinia mobilis on the left mandible is described as an extremely developed, jointed structure resembling the toothed shape of the adjacent incisor process. The lacinia on the right mandible seems to be smaller and variously shaped but is also articulated (De Jong-Moreau et al., 2001, Mayer et al., 2013). Similar seta-like structures have been described in other eumalacostracan taxa, such as Euphausiacea and Decapoda, but only in larvae (i.a. Weigmann-Haass, 1977, Maas and Waloszek, 2001, Yang, 2005, Dupré et al., 2008). Hypotheses about the origins of these structures, including assumptions about their possible homology, are still not discussed (Dahl and Hessler, 1982, Richter et al., 2002, Geiselbrecht and Melzer, 2010). A wealth of studies has analyzed the external features of the lacinia mobilis, but a transmission electron microscopic (TEM) analysis of the internal features has not been conducted. Richter et al. (2002) presumed that the incisor process, the setal row and the lacinia mobilis, are simple cuticular outgrowths and indicated that detailed histological or electron microscopic comparisons would have questionable value. However, they considered a different origin for the left and the right lacinia mobilis. This is of peculiar interest as mandibles of decapod zoea have a large number of sensory structures as detected by Geiselbrecht and Melzer (2013) and the gnathal edge appendage, also referred to as “lacinia mobilis”, has been shown to be among these sensory structures in these larvae. Given the unresolved question of homology, an electron microscopic analysis of the ultrastructure of a peracarid lacinia mobilis should resolve some of these issues. Therefore, we studied the lacinia mobilis of Neomysis integer with regard to external morphology and ultrastructural features. We also studied histological changes inside the mandible involved in initiating the molting process.

Section snippets

Animals

Living adult specimens of N. integer (Leach, 1814) were purchased from a commercial supplier (Aquarium-Center Wildenauer, Munich, Germany). The animals were collected in the area around the Elbe estuary, North Sea, Germany. Specimens were identified according to Makings (1977). Mandibles of specimens in the intermolt stage and mandibles of specimens showing signs of imminent ecdysis were studied. The molting process can be assigned to stage D0 and D1, indicating early or late apolysis, or D3

External structure of the mandibles

The mandibles of N. integer are composed of two distinct main portions, the mandibular palp and a medially extending coxal endite forming the gnathal edge (Fig. 1). The gnathal edge is armed with different processes and appendages (Figs. 2A and 4A). The molar process is dorsally situated closest to the mouth. It is a flattened oval structure densely covered with small setae. The margin of the gnathal edge becomes narrower ventrally and a series of setae form the “setal row”. The lacinia mobilis

Discussion

The basic form of the mandibles and laciniae mobiles as well as their asymmetry correspond well with the descriptions for Boreomysis inermis and Hemimysis speluncola (De Jong-Moreau et al., 2001) and also for some gammaridean species (Mayer et al., 2013). The strong toothed structure of the lacinia mobilis on the left mandible and a smaller and seta-like structure on the right mandible seem common. Our results agree with most of the features described by Richter et al. (2002) who studied the

Acknowledgments

We thank Heidemarie Gensler and Stefan Friedrich for expert technical assistance and Tobias Lehmann for critically revising the manuscript. This study was partly supported by the Sealife Center (Munich) by funding of laboratory material.

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