Body musculature of Beauchampiella eudactylota (Gosse, 1886) (Rotifera: Euchlanidae) with additional new data on its trophi and overall morphology

This study presents the results of confocal laser scanning microscopy and fluorescence‐labelled phalloidin used to visualize the system of body musculature in Beauchampiella eudactylota. Moreover, the poorly known trophi of B. eudactylota are described based on scanning electron microscopy. In total, four paired longitudinal muscles (musculi longitudinales I–IV) and three circular muscles (musculi circulares I–III) were identified. Among these are the musculus longitudinalis ventralis, the musculus longitudinalis dorsalis and the musculus circumpedalis as documented in previous studies for other rotifer species. Compared to other species, B. eudactylota is characterized by the low number of lateral longitudinal muscles and the absence of some longitudinal muscles (musculi longitudinales capitum) and circular muscles (corona sphincter, musculus pars coronalis). Moreover, scanning electron microscopic data on the trophi of B. eudactylota reveal a number of striking similarities to the trophi in some species of Epiphanidae. This suggests that either (1) these similarities represent plesiomorphic characters present both in Epiphanidae and B. eudactylota or (2) they are synapomorphic features of B. eudactylota and some species of Epiphanidae, which would question the monophyly of Euchlanidae.     

The musculature of Squatinella rostrum (Milne, 1886) (Rotifera: Lepadellidae) as revealed by confocal laser scanning microscopy with additional new data on its trophi and overall morphology

Wilts, E.F., Wulfken, D., Ahlrichs, W.H. and Martínez Arbizu, P. 2012. The musculature of Squatinella rostrum (Milne, 1886) (Rotifera: Lepadellidae) as revealed by confocal laser scanning microscopy with additional new data on its trophi and overall morphology.—Acta Zoologica (Stockholm) 93 : 14–27. The monogonont rotifer Squatinella rostrum was investigated with light, scanning electron and confocal laser scanning microscopy to reveal new morphological data on its inner and outer anatomy. In total, the visualized somatic musculature displays five paired longitudinal muscles (musculi longitudinales I–V) and nine circular muscles (musculi circulares I–IX). Compared to other species, S. rostrum is characterized by the absence of several longitudinal and circular muscles (e.g. musculus longitudinalis capitis, corona sphincter and pars coronalis). A reconstruction of the mastax musculature revealed a total number of seven paired and two unpaired mastax muscles. Possibly homologous somatic and mastax muscles in other, thus far investigated rotifers are discussed. Moreover, we provide a phylogenetic evaluation of the revealed morphological characters and suggest possible autapomorphic characters supporting Squatinella and Lepadellidae. Finally, we refer to some striking similarities in the morphology, ecology and way of movement of Squatinella and Bryceella that may indicate a closer relationship of both taxa.     

Organisation of body musculature in Encentrum mucronatum Wulfert, 1936, Dicranophorus forcipatus (O. F. Müller, 1786) and in the ground pattern of Ploima (Rotifera: Monogononta)

Fluorescence-labelled phalloidin in combination with confocal laser scanning microscopy (cLSM) has been used to reconstruct the body musculature in Encentrum mucronatum and Dicranophorus forcipatus in order to gain insight into the architecture of body musculature in representatives of the hitherto uninvestigated Dicranophoridae.

In both species, a system of outer circular and inner longitudinal muscles has been found. In E. mucronatum, seven circular muscles (musculi circulares I–VII) and six paired longitudinal muscles (musculi longitudinales I–VI) have been identified. In D. forcipatus, eight circular muscles (musculi circulares I–VIII) and nine paired longitudinal muscles (musculi longitudinales I–IX) are present. In both species, some of the longitudinal muscles span the whole specimen, while others are shorter and connect head and trunk or foot and trunk. Differences in shape and extension of the circular muscles in both species are related to differences in structure of the trunk integument.

Surveying the literature on rotifer musculature, muscles identified in this study are homologised across Rotifera and given individual names. Based on the study of E. mucronatum and D. forcipatus and previous studies on other rotifers, a system of musculature in the ground pattern of Ploima comprising at least three circular muscles (pars coronalis, corona sphincter, musculus circumpedalis) and three pairs of longitudinal muscles (musculi longitudinales ventrales, musculi longitudinales dorsales and musculi longitudinales capitum) is suggested.     

Combining confocal laser scanning and transmission electron microscopy for revealing the mastax musculature in Bryceella stylata (Milne, 1886) (Rotifera: Monogononta)

The rotiferan jaw apparatus (mastax) is characterized by enormous plasticity and according to morphology and feeding strategy, different mastax types can be distinguished. The cuticular hard parts (trophi) of the mastax are often highly specialized and have both a major taxonomic and phylogenetic relevance. Owing to numerous light and scanning electron microscopic studies, the morphology of the trophi is well known but only few attempts have been made to analyze the morphology and functionality of the mastax as a whole. Particularly, the complex muscular system connecting the individual trophi elements and moving them against each other was disregarded in the past. Therefore, the subject of the present study is a detailed analysis of the mastax musculature of the proalid rotifer Bryceella stylata using a combination of transmission electron and confocal laser scanning microscopic techniques, previously applied for revealing the somatic musculature in rotifers exclusively. Based on ultrathin serial sections and phalloidin-dyed specimens, a total number of six paired and two unpaired individual mastax muscles have been identified for the modified malleate trophi system of B. stylata. Possibly homologous muscles in other, so far investigated rotifer species are discussed as well as functional considerations of the individual mastax muscles and their interaction when moving the trophi elements are suggested.     

Combining confocal laser scanning and transmission electron microscopy for revealing the mastax musculature in Bryceella stylata (Milne, 1886) (Rotifera: Monogononta)

The rotiferan jaw apparatus (mastax) is characterized by enormous plasticity and according to morphology and feeding strategy, different mastax types can be distinguished. The cuticular hard parts (trophi) of the mastax are often highly specialized and have both a major taxonomic and phylogenetic relevance. Owing to numerous light and scanning electron microscopic studies, the morphology of the trophi is well known but only few attempts have been made to analyze the morphology and functionality of the mastax as a whole. Particularly, the complex muscular system connecting the individual trophi elements and moving them against each other was disregarded in the past. Therefore, the subject of the present study is a detailed analysis of the mastax musculature of the proalid rotifer Bryceella stylata using a combination of transmission electron and confocal laser scanning microscopic techniques, previously applied for revealing the somatic musculature in rotifers exclusively. Based on ultrathin serial sections and phalloidin-dyed specimens, a total number of six paired and two unpaired individual mastax muscles have been identified for the modified malleate trophi system of B. stylata. Possibly homologous muscles in other, so far investigated rotifer species are discussed as well as functional considerations of the individual mastax muscles and their interaction when moving the trophi elements are suggested.     

Study of architectonics of rotifer musculature by the method of fluorescence with use of confocal microscopy

Musculature of two species of rotifers Testudinella patina (Testudinellidae) and Platyias patulus (Brachiomidae) was studied in confocal laser scanning microscope (CLSM) using fluorescent-labeled phalloidin. It includes cutaneous, visceral, and cutaneus-visceral musculature. The common pattern of structure of the cutaneous musculature is represented by postcoronal circular or transverse muscles and connected with them 2–3 pairs of retractors of the trunk, dorsolateral muscles (17-4), two pairs or bundles of lateral retractors of the corona, circular muscles of the foot, and 10-2 retractors of the foot. Visceral musculature includes muscles of the mastax of both kinds. Spiral-like muscle of cloaca of the T. patina and associated with it V-shaped one as well as strong dorsolateral retractors consisting of 6 longitudinal muscle bundles are typical of Testudinellidae only. Three pairs of cutaneus-visceral muscles bind the musculature of mastax with the body surface in T. patina. Differences in localization and thickness of some elements of musculature of these species are determined by morphological peculiarities of structure of the corona, mastax, and foot, as well as by the rotifer body shape.     

Musculature of an illoricate predatory rotifer Asplanchnopus multiceps as revealed by phalloidin fluorescence and confocal microscopy

The pattern of muscles in the actively swimming predatory rotifer Asplanchnopus multiceps is revealed by staining with tetramethyl-rhodamine isothiocyanate (TRITC)-labelled phalloidin and confocal scanning laser microscopy (CSLM). The major components of the musculature are: prominent semicircular muscles of the corona; paired lateral, dorsal and ventral retractors in the trunk; a network of six seemingly complete circular muscles and anastomosing longitudinal muscles in the trunk; two short foot retractors, originating from a transverse muscle in the lower third of the trunk. The sphincter of the corona marks the boundary between the head and the trunk. The muscular patterns in rotifers with different lifestyles differ clearly, therefore, the muscular patterns seem to be determined by the mode of locomotion and feeding behaviour.     

Musculature of Notholca acuminata (Rotifera: Ploima: Brachionidae) revealed by confocal scanning laser microscopy

Abstract. The body-wall and visceral musculature of Notholca acuminata was visualized using phalloidin-linked fluorescent dye under confocal laser scanning microscopy. The body-wall musculature includes dorsal, lateral, and ventral pairs of longitudinally oriented body retractor muscles, two pairs of head retractors, three pairs of incomplete circular muscles, which are modified into dorso-ventral muscles, and a single pair of dorsolateral muscles. The visceral musculature consists of a complex of thick muscles associated with the mastax, as well as several sets of delicate fibers associated with the corona, stomach, gut, and cloaca, including thin longitudinal gut fibers and viscero-cloacal fibers, never before reported in other species of rotifers. The dorsal, lateral, and ventral retractor muscles and the incomplete circular muscles associated with the body wall appear to be apomorphies for the Rotifera. Muscle-revealing staining shows promise for providing additional information on previously unrecognized complexity in rotifer musculature that will be useful in functional morphology and phylogenetic analyses.     

External Morphology and Muscle Arrangement of Brachionus urceolaris, Floscularia ringens, Hexarthra mira and Notommata glyphura (Rotifera, Monogononta)

We studied four monogonont rotifers (Brachionus urceolaris, Floscularia ringens, Hexarthra mira, Notommata glyphura) using two different techniques of microscopy: (1) the presence of filamentous actin was examined using phalloidin-fluorescent labelled specimens and a confocal laser scanning microscope (CLSM); (2) external morphology was investigated using a scanning electron microscope (SEM). B. urceolaris, F. ringens, and N. glyphura showed similar patterns of muscle distribution: a set of longitudinal muscles acting as head and foot retractors, and a set of circular muscles. However, the size and distribution of circular muscles differed among these species. H. mira differed from the other species in that it lacked circular muscles but possessed strong muscles that extended into each arm. The study showed that using both CLSM and SEM provides better resolution of the anatomy and external morphology of rotifers than using one of these techniques alone. This can facilitate better understanding of the complicated anatomy of these animals.     

Functional morphology of the muscles in Philodina sp. (Rotifera: Bdelloidea)

Whole-mounts of Philodina sp., a bdelloid rotifer, were stained with fluorescent-labeled phalloidin to visualize the musculature. Several different muscle types were identified including incomplete circular bands, coronal retractors and foot retractors. Based on the position of the larger muscle bands in the body wall, their function during creeping locomotion and tun formation was inferred. Bdelloid creeping begins with the contraction of incomplete circular muscle bands against the hydrostatic pseudocoel, resulting in an anterior elongation of the body. One or more sets of ventral longitudinal muscles then contract bringing the rostrum into contact with the substrate, where it presumably attaches via adhesive glands. Different sets of ventral longitudinal muscles, foot and trunk retractors, function to pull the body forward. These same longitudinal muscle sets are also used in `tun' formation, in which the head and foot are withdrawn into the body. Three sets of longitudinal muscles supply the head region (anterior head segments) and function in withdrawal of the corona and rostrum. Two additional pairs of longitudinal muscles function to retract the anterior trunk segments immediately behind the head, and approximately five sets of longitudinal retractors are involved in the withdrawal of the foot and posterior toes. To achieve a greater understanding of rotifer behavior, it is important to elucidate the structural complexity of body wall muscles in rotifers. The utility of fluorescently-labeled phalloidin for the visualization of these muscles is discussed and placed in the context of rotifer functional morphology.     

Musculature of Seison nebaliae Grube, 1861 and Paraseison annulatus (Claus, 1876) revealed with CLSM: a comparative study of the gnathiferan key taxon Seisonacea (Rotifera)

The somatic muscular systems of two species of Seisonacea (Rotifera), Seison nebaliae and Paraseison annulatus, are described using fluorescently labelled phalloidin in combination with confocal laser scanning microscopy. Their overall muscular arrangement is similar and consists of segmentally organised longitudinal fibres that extend the length of the body and are surrounded by semi-circular (= incomplete) bands. However, differences in the musculature between the two species are present and possibly reflect specific adaptations in feeding strategy and locomotion related to the occupation of individual niches on their host, the leptostracan crustacean N. bipes. For example, S. nebaliae has semi-circular muscles in the head region only, while P. annulatus possesses incomplete circular muscles also in the trunk region; furthermore, there are also differences in the arrangements and number of longitudinal muscles. The muscular systems of all rotifer species examined so far are compared in order to establish the ground pattern of the last putative ancestor as well as to seek for traits of systematic importance. Results from both species corroborate earlier hypotheses on the arrangement of muscles in the putative common ancestor of Rotifera, which suggested an orthogonal arrangement consisting of a series of probably continuous (not segmental) inner longitudinal muscles, surrounded by semi-circular fibres, ventrally opened. However, significant morphological and ecological variations among taxa investigated so far show that a consistent correlation between muscular traits and specific ecological features and/or phylogeny is still far from being clear. Hence, musculature of additional taxa, representing the systematic width and occupying a diverse range of habitats, should be investigated.     

The Musculature of <Emphasis Type="Italic">Testudinella patina</Emphasis> (Rotifera: Flosculariacea), Revealed with CLSM

The musculature of Testudinella patina was visualized using phalloidin-linked fluorescent dye by confocal laser scanning microscopy. The conspicuous broad retractors appear to be made up of five separate fibers, of which three anchor in the neck region whereas two extend into the corona. Besides the broad retractors, a total of five paired longitudinal retractors are present and all of them extend into the corona. Incomplete circular muscles are found in groups in the neck region and in the medial and posterior parts of the trunk. The foot musculature comprises eight thin ventral foot muscles and six thicker dorsal foot muscles that all extend from the foot basis to the distal part of the foot. At the basis of the foot, each of the dorsal foot muscles anchors on a smaller, S-shaped subterminal foot muscle. The foot musculature furthermore comprises one pair of paraterminal foot muscles that each anchors basally on a subterminal foot muscle, extends into the most proximal part of the foot and attaches on one of the dorsal foot muscles. The visceral musculature is composed of extremely delicate fibers and is restricted to an area around and posterior to the foot opening. The presence of incomplete circular muscles supports that these muscles are a basal trait for Rotifera, whereas the morphology of the broad retractors and foot muscles is much more specialized and may be autapomorphic for Testudinella or alternatively for this genus and its closest relatives. The present results stress that revealing muscles by staining may produce new information from even well-investigated species, and that this information may contribute to a better understanding of functional as well as phylogenetic aspects of rotifer biology.     

Musculature in three species of <Emphasis Type="Italic">Proales</Emphasis> (Monogononta,Rotifera) stained with phalloidin-labeled fluorescent dye

The musculature in the rotifer species Proales daphnicola, P. reinhardti and P. fallaciosa was stained with phalloidin-labeled fluorescent dye and compared using confocal laser scanning microscopy. All three species share several homologous muscle systems, but each systems detailed morphology varies among the species both concerning appearance, number and location. The obtained results were compared with data from other rotifers and it was concluded that the muscles pars coronalis and the corona sphincter probably represent conditions in Ploima or Monogononta, while incomplete circular muscles and dorsal and ventral trunk retractors might be part of the eurotatorian ground pattern.     

Behavior,metamorphosis, and muscular organization of the predatory rotifer Acyclus inquietus (Rotifera,Monogononta)

Abstract. The atrochid rotifer, Acyclus inquietus, is a sedentary predator that lives within the colonies of its prey, the rotifer Sinantherina socialis. After larvae infiltrate and become associated with the colony, they secrete a permanent gelatinous tube and undergo metamorphosis to the adult stage. We followed settlement and metamorphosis using bright-field microscopy to document specific larval behaviors after eclosion, and used epifluorescence and confocal microscopy of phalloidin-labeled specimens to visualize some of the morphological changes that occur during metamorphosis. Upon eclosion, larvae possess paired eyespots and a ciliated corona that functions strictly in locomotion. After leaving the parent's gelatinous tube, larvae eventually settle on unoccupied colonies of S. socialis or on other substrates if colonies are unavailable. Settlement involves a period of gliding among colony members before attachment with the foot and the secretion of a gelatinous tube. After settlement, there is a drastic reconfiguration of the corona that involves loss of the eyespots, loss of the coronal cilia, and the formation of the cup-shaped infundibulum, a deep depression in the anterior of the head that leads to the mouth. The development of the infundibulum involves the expansion of tissues around the mouth and is accompanied by a reorientation of the underlying musculature that supplies the infundibulum and allows its use in prey capture. The arrangement of the muscles in the trunk and foot regions, which contain outer circular (complete and incomplete) and inner longitudinal bands, remains unchanged between ontogenetic stages, and reflects the condition characteristic of other rotifers.     

Comparative morphology of the somatic musculature in species of Hexarthra and Polyarthra (Rotifera, Monogononta): Its function in appendage movement and escape behavior

Species of Hexarthra and Polyarthra are freshwater rotifers with well-known escape behaviors that result from interactions with planktonic predators. Both rotifers bear a suite of mobile appendages that function in evasive maneuvers and saltatory jumps through the water column, but the anatomical and functional bases of these actions are poorly understood. Here, we use a combination of phalloidin staining, confocal laser scanning microscopy, and video analysis to describe the morphology of the somatic muscles that supply the mobile appendages in order to understand how they function in escape behavior. Results show that species of Hexarthra, which bear six radially distributed limbs, possess a highly complex trunk musculature that supplies the inside of each limb with its own abductor and adductor muscles, i.e., a direct muscle supply. The singular dorsal and ventral limbs each receive a pair of large abductor and adductor muscles (four muscles total per limb), while the paired dorsolateral and ventrolateral limbs each receives three muscles (two abductors, one adductor per limb). Contraction of the abductor muscles creates a power stroke in the form of an anterior sweep of the limbs, which leads to a three-dimensional tumbling of the rotifer through the water column. Alternatively, species of Polyarthra possess 12 blade-like appendages that are arranged into four equal bundles; each bundle receives an indirect muscle supply that attaches to the shoulder of the paddles. A single longitudinal paddle muscle supplies each dorsolateral bundle, while a pair of longitudinal paddle muscles supplies each ventrolateral bundle. Contraction of these muscles, whether singly or in concert, functions to abduct the paddles in a power stroke, leading to rotation of the body and movement of the rotifer. The recovery stroke is hypothesized to be a multi-step process that begins with reorientation of the appendages prior to adduction, followed by contraction of various muscles to antagonize the paddle muscles. In total, these observations reveal novel complexities in the rotifer muscular system that aids our understanding of the biophysics of predator avoidance in appendage-bearing rotifers.     

Morphology of Floscularia ringens (Rotifera, Monogononta) from egg to adult

Abstract. Floscularia ringens is a cosmopolitan, sessile rotifer (class Monogononta) that lives inside a tube it constructs from numerous small, rounded pellets. Adults of F. ringens produce parthenogenetic eggs that are retained within the tube. Upon hatching, juveniles remain within the maternal tube for a short time completing their development before swimming away. The free-swimming juvenile has a conical body, short foot, small corona, and mastax with trophi, but appears unable to feed. After a short time (<1 day), the young rotifer attaches permanently to a substrate and its morphology changes radically: the corona develops 4 wide lobes and the foot elongates, becoming slender and retractable. Once the corona has developed, the young animal begins to feed by producing filtering currents, and also starts to build its own tube. Here we report 4 new morphological details regarding this species. (1) A specialized epidermal groove is present on the trunk in front of the cloaca. (2) A small hole is located in the center of the inner surface of each pellet of the tube. (3) The muscles inside the foot are U-shaped in transverse section. (4) The size of the trophi remains unchanged during growth of the juvenile into an adult.     

Three-dimensional reconstruction of the musculature of Cossura pygodactylata Jones, 1956 (Annelida: Cossuridae)

The musculature of adult specimens of Cossura pygodactylata was studied by means of F-actin labelling and confocal laser scanning microscopy (CLSM). Their body wall is comprised of five longitudinal muscle bands: two dorsal, two ventral and one ventromedial. Complete circular fibres are found only in the abdominal region, and they are developed only on the border of the segments. Thoracic and posterior body regions contain only transverse fibres ending near the ventral longitudinal bands. Almost-complete rings of transverse muscles, with gaps on the dorsal and ventral sides, surround the terminal part of the pygidium. Four longitudinal bands go to the middle of the prostomium and 5–14 paired dorso-ventral muscle fibres arise in its distal part. Each buccal tentacle contains one thick and two thin longitudinal muscle filaments; thick muscle fibres from all tentacles merge, forming left and right tentacle protractors rooted in the dorsal longitudinal bands of the body wall. The circumbuccal complex includes well-developed upper and lower lips. These lips contain an outer layer of transverse fibres, and the lower lip also contains inner oblique muscles going to the dorsal longitudinal bands. The branchial filament contains two longitudinal muscle fibres that do not connect with the body musculature. The parapodial complex includes strong intersegmental and segmental oblique muscles in the thoracic region only; chaetal retractors, protractors and muscles of the body wall are present in all body regions. Muscle fibres are developed in the dorsal and ventral mesenteries. One semi-circular fibre is developed on the border of each segment and is most likely embedded in the dissepiment. The intestine has thin circular fibres along its full length. The dorsal blood vessel has strong muscle fibres that cover its anterior part, which is called the heart. It consists of short longitudinal elements forming regular rings and inner partitions. The musculature of C. pygodactylata includes some elements that are homologous with similar muscular components in other polychaetes (i.e., the body wall and most parapodial muscles) and several unique features, mostly at the anterior end.     

Rotifer muscles as revealed by phalloidin-TRITC staining and confocal scanning laser microscopy

By combining phalloidin‐TRITC staining with confocal scanning laser microscopy (CSLM), the pattern of the musculature in two species of Rotifera, Euchlanis dilatata unisetata and Brachionus quadridentatus is revealed. The same general muscle pattern prevails in both species. The major components of the body wall musculature are: 1. retractor muscles (5 pairs in E. dilatata unisetata and 3 pairs in B. quadridentatus); 2. Two pairs of dorso‐ventral muscles; 3. Two pairs of perpendicular muscles (in E. dilatata unisetata); 4. retractors of the corona (median, lateral and ventral); 5. Foot retractors. In addition, three pairs of cutaneo‐visceral muscles and visceral muscles (including mastax muscles) are described. The sphincter of the corona was found only in B. quadridentatus. The high degree of muscle differentiation points to a high level of development of rotifer muscular system.     

Muscles of the genital appendages of Megalopygidae (Lepidoptera) and their significance for the family systematics

Nine muscles were found in the genital complex of Megalopygidae: the paired m1, m2(10), m4, m5(7), m6(5), m7(6), the unpaired m20, m21, and the phallobase muscles. This arrangement corresponds to the ground plan of Zygaenoidea with some reductions. The archaic muscles of the “musculi phallici internus” system, located inside the phallotheca and aedeagus, were revealed for the first time in representatives of the entire superfamily Zygaenoidea. We suggest that the traditional interpretation of m21 as the inner longitudinal muscle of the aedeagus should be preserved, while the inner muscles of the phallotheca should be designated as musculi phallobasi interni m36. Division of the family into two subfamilies, Megalopyginae and the more archaic Trosiinae, is supported by our results, and the homology of the modified appendages of the tergal complex and vinculum is refined. A list of autapomorphies of the family is given.