Detailed reconstruction of the musculature in Limnognathia maerski (Micrognathozoa) and comparison with other Gnathifera

Abstract

Introduction: Limnognathia maerski is the single species of the recently described taxon, Micrognathozoa. The most conspicuous character of this animal is the complex set of jaws, which resembles an even more intricate version of the trophi of Rotifera and the jaws of Gnathostomulida. Whereas the jaws of Limnognathia maerski previously have been subject to close examinations, the related musculature and other organ systems are far less studied. Here we provide a detailed study of the body and jaw musculature of Limnognathia maerski, employing confocal laser scanning microscopy of phalloidin stained musculature as well as transmission electron microscopy (TEM).

Results: This study reveals a complex body wall musculature, comprising six pairs of main longitudinal muscles and 13 pairs of trunk dorso-ventral muscles. Most longitudinal muscles span the length of the body and some fibers even branch off and continue anteriorly into the head and posteriorly into the abdomen, forming a complex musculature. The musculature of the jaw apparatus shows several pairs of striated muscles largely related to the fibularium and the main jaws. The jaw articulation and function of major and minor muscle pairs are discussed. No circular muscles or intestinal musculature have been found, but some newly discovered muscles may supply the anal opening.

Conclusions: The organization in Limnognathia maerski of the longitudinal and dorso-ventral muscle bundles in a loose grid is more similar to the organization found in rotifers rather than gnathostomulids. Although the dorso-ventral musculature is probably not homologous to the circular muscles of rotifers, a similar function in body extension is suggested. Additionally, a functional comparison between the jaw musculature of Limnognathia maerski, Rotifera and Gnathostomulida, emphasizes the important role of the fibularium in Limnognathia maerski, and suggests a closer functional resemblance to the jaw organization in Rotifera.

Keywords: 3D reconstructions; CLSM; F-actin; Jaw apparatus; Mastax; Trophi.

Figure 1

TEM sections of Limnognathia maerski . Muscles highlighted in green. A, transversal section of posterior part. Posterior on the right. B, sagittal section showing the vomit behaviour. C, transversal section of the jaws. The ventral side is on the bottom. D, Close up of muscle attachment on a jaw sclerite, showing the non myoepithelial nature of the jaw muscles. Epidermal cells with blue outlines.

Figure 2

CLSM of phalloidin stained muscle system and light microscopy of Limnognathia maerski . Anterior end is positioned left on all pictures. A: Ventral view, Z-stack of the ventral portion, showing only the muscle system. B: Single section showing CLSM of the dorsal muscle system and the contour of the specimen, visualized with transmitted light. C: Synapsin2 staining of L. maerski, maximum intensity projection of a dorsal substack. Lines show the border of the dorsal cells to which the dorso-ventral muscles attach (illustrated in Figure 4B). advm, anterior dorso-ventral muscles; alm, anterior lateral muscle; cpm, ciliated adhesive pad muscle; fmm, front margin muscle; ldm, lateral dorsal muscle; lvm, lateral ventral muscle; mdm, median-dorsal muscle; mvm, medio-ventral muscle; mn, muscle network; pvm, paramedian ventral muscle; pvm2, posterior lateral muscle; sav1,2, small anterior ventral longitudinal muscles; tdvm, trunk dorso-ventral muscles; vpm, ventral pharyngeal muscles.

Figure 3

CLSM of phalloidin stained muscle system of Limnognathia maerski . Anterior end is positioned left on all pictures. A, Ventral view of the maximum depth intensity projection. B, lateral view reconstruction of a dorso-ventral Z-stack. Same specimen as Figure 2A,B. C, Dorsal view of the isosurface reconstruction of the muscular system. Same specimen as Figure 2A,B. advm, anterior dorso-ventral muscles; alm, anterior lateral muscle; cpm, ciliated pad muscle; fmm, front margin muscles; ldm, lateral dorsal muscle; lvm, Lateral ventral muscle; mdm, medio-dorsal muscle; mn, muscle network; mvm, medio-ventral muscle; pvm, paramedian ventral muscle; pvm2, posterior lateral muscle; sav1,2, small anterior ventral longitudinal muscles; tdvm, trunk dorso-ventral muscles; tpm, transversal posterior muscle; vpm, ventral pharyngeal muscles.

Figure 4

Schematic drawings of the somatic musculature of Limnognathia. maerski . Anterior is on the top. Colors follow Figure 3C. A) Dorsal view of the ventral musculature (colors) relative to body wall and ciliated areas (grey shade). B) Dorsal view of the dorsal musculature (colors) and its attachment sites on dorsal epidermis cells (delimitated in light grey) attachment sites of anterior 5 trunk dorso-ventral muscles are inferred. The mdm, pvm, pvm2, and tdvm are present in A) and B) as they extend ventrally and dorsally. advm, anterior dorso-ventral muscles; alm, anterior lateral muscle; cpm, ciliated adhesive pad muscles; fmm, front margin muscle; ldm, lateral dorsal muscle; lvm, lateral ventral muscle; mdm, medio-dorsal muscle; mn, muscle net; mvm, medio-ventral muscle; pvm, paramedian ventral muscle; pvm2, paramedian ventral muscle 2; sav1,2, small anterior longitudinal muscle; tdvm, trunk dorso-ventral muscle; tdm; trunk posterior-muscle.

Figure 5

TEM sections of Limnognathia maerski . Muscles highlighted in green. A, transversal section of the trunk. Dorsal side on top. B, close up of figure A., showing the ventral musculature. C, D, coronal section the jaws. The red line shows the symmetry axis of the jaws. The front is on the left. The section in C is more ventral than the section in D. as, accessory sclerite; dm, dorsal muscle; ca, cauda; cm, cauda muscle; fib, fibularium; lfm, lateral fibularium main jaw muscle; lm, pharyngeal lamella muscle; lvm, lateral ventral muscle; mfm, median fibularium main jaw muscle; mj, main jaws; mvm, medio-ventral muscle; pvm, paramedian ventral muscle; tdvm, trunk dorso-ventral muscle; vjm, ventral jaw muscle; vlm, ventral lateral muscle; vpm, ventral pharyngeal muscle.

Figure 6

Musculature and reconstruction of the jaw apparatus of Limnognathia maerski in dorsal view. Anterior is on the top for all the pictures. A, B, C: ventral part of the jaw system. D, E, F: median part of the jaw system. G, H, I: dorsal part of the jaw system. A, D, G: CLSM of phalloidin stained muscle system, dorsal view of a projection of a sub sample of the Z-stack. B, E, H: enlightenment of the different muscle systems of the jaws. C, F, I: schematic drawing of the dorsal view of the myoanatomy of the jaw system linked to the cuticular elements in greys. Jaw drawing after Sørensen [6]. as: accessory sclerite; afm: anterior fibularium-main jaw muscle; cm: caudal muscle; dj: dorsal jaws; djm: dorsal jaw muscle; fib: fibularium; lm: pharyngeal lamella muscle; lp, pharyngeal lamella; lfm: lateral fibularium-main jaw muscle; mfm: median fibularium-main jaw muscle; mj: main jaws; pp: pseudo-phalangium; vjm: ventral jaw muscle; vpm: ventral pharyngeal muscles; z-b: Z-bands of the cross striated muscles of the ventral pharyngeal muscle.