Muscular system in polychaetes (Annelida)

The structure of the polychaete muscular system is reviewed. The muscular system comprises the muscles of the body wall, the musculature of the parapodial complex and the muscle system of the dissepiments and mesenteries. Various types of organisation of the longitudinal and circular components of the muscular body wall are distinguished. In Opheliidae, Polygordiidae, Protodrilidae, Spionidae, Oweniidae, Aphroditidae, Acoetidae (=Polyodontidae), Polynoidae, Sigalonidae, Phyllodocidae, Nephtyidae, Pisionidae, and Nerillidae circular muscles are lacking. It is hypothesised that the absence of circular muscles represents the plesiomorphic state in Annelida. This view contradicts the widely accepted idea of an earthworm-like musculature of the body wall comprising an outer layer of circular and an inner layer of longitudinal fibres. A classification of the various types of parapodial muscle construction has been developed. Massive and less manoeuvrable parapodia composed of many components like those of Aphrodita are regarded to represent the plesiomorphic state in recent polychaetes. An analysis of the diversity of the muscular structure supports the hypothesis that the primary mode of life in polychaetes was epibenthic and the parapodial chaetae had a protective function.     

Locomotion and fine structure of parapodia in <Emphasis Type="Italic">Myzostoma cirriferum</Emphasis> (Myzostomida)

Most myzostomids are ectocommensals of crinoids on which they move freely. Their locomotion is ensured by five pairs of parapodia located laterally below their trunk. Each parapodium in Myzostoma cirriferum is a conical structure that includes a hook-like chaeta, replacement chaetae and an aciculum. Structure and ultrastructure of the myzostomid chaetae are similar to those of polychaetes: they are formed by a chaetoblast, which gives rise to microvilli where chaetal material is assembled on the outer surface. Myzostoma cirriferum walks on its host. It moves the anterior part, the posterior part or the lateral parts forwards but is able to rotate of 180° on itself. Its locomotion entirely depends on parapodial motions and not on trunk movements. Three pairs of muscles are involved in parapodial motions: parapodium flexor and parapodium extensor, aciculum protractor and aciculum retractor, and hook protractor with conjunctor. A functional model is proposed for explaining the global motion of a parapodium in M. cirriferum that may be extended to all ectocommensal myzostomids.     

Trochonerilla mobilis gen. et sp.n., a meiofaunal nerillid (Annelida, Polychaeta) from a marine aquarium in Moscow

A nerillid species new to science is described from the marine aquarium system of Moscow Zoo. Trochonerilla mobilis gen. et sp.n. is characterized by eight chaetigerous segments, three antennae, absence of palps, parapodia with two bundles of simple chaetae and pygidium with two anal cirri. In contrast to other eight-segmented nerillids, Trochonerilla mobilis are very active and mobile animals. They are also able to swim through the water column by means of the strong ring of cilia on the first chaetigerous segment. Their geographical origin is unknown.     

Chaetal arrangement provides no support for a close relationship of Sabellidae and Sabellariidae (Annelida)

Sabellid and sabellariid polychaetes are regarded as sister groups in a number of recent phylogenetic analyses. This is based mainly on a shared specific arrangement of chaetae referred to as chaetal inversion. Remarkably, the uncini have a notopodial position in the abdomen, whereas capillary chaetae occur in the neuropodia in both taxa in contrast to the situation in putative relatives. However, in sabellids uncini and capillary chaetae change their position completely at the border between thorax and abdomen, whereas uncini are missing in the parathorax of Sabellariidae. Due to this difference the significance of the chaetal inversion for systematics has been subject to discussion for years. Serial semithin sections of parapodia of the Sabellidae Sabella pavonina, Branchiomma bombyx, Fabricia stellaris, and of the Sabellariidae Sabellaria alveolata were studied in order to obtain detailed information on their chaetal arrangement and sites of chaetal origin. SEM investigations and computer-aided 3D-reconstructions provide deep insight into the spatial organization of the rami. Though differing externally, the principal chaetal arrangement and the location of the formative sites turned out to be almost identical within the species of Sabellidae. Most chaetae are aligned in straight transverse rows with a dorsal site of origin within neuropodia and a ventral one in notopodia as is common in sedentary polychaetes. Semicircular and spiral arrangements are revealed to be modified transverse rows. Only in thoracic notopodia does an additional dorsocaudal formative site form distinct rows. The chaetal inversion in Sabellidae is additionally characterized by an abrupt change of capillary chaetae and uncini along with a sudden change of the parapodial morphology at the border between thorax and abdomen. All chaetae of S. alveolata are aligned in transverse rows with the same location of the formative sites as in sabellids and other sedentary polychaetes. However, in contrast to sabellids the chaetae are not inverted across a parathoracic abdominal border. Moreover, there is no inversion of the parapodial structure from parathorax to abdomen and the neuropodial chaetal composition changes gradually from parathorax to abdomen. The chaetal arrangement in Sabellariidae thus cannot be described as inverted along the body-axis as in Sabellidae. Evolutionary steps implied by the assumption of an inverted chaetal pattern in a supposed common ancestor are discussed. It is concluded that the specific chaetal arrangement of Sabellidae and Sabellariidae arose independently and therefore provides no support for a sistergroup relationship of sabellids and sabellariids.     

Anatomy and relationships of the Early Cambrian worm Myoscolex

Dzik, J. (2004). Anatomy and relationships of the Early Cambrian worm Myoscolex . — Zoologica Scripta, 33 , 57–69.
Numerous fossil specimens of Myoscolex ateles Glaessner, 1979 from the late Early Cambrian Emu Bay shale of Kangaroo Island, South Australia with phosphatized organic matter-rich tissues show its muscular body wall penetrated by rows of rod-like structures — possible chaetae. The body wall was composed of an external layer with transverse (circular) fibres. This layer was thickest in lateral parts of the body and very thin dorsally. In the ventro-lateral quarter of the body circumference, a belt of longitudinal fibres extended along the body. Longitudinal fibres also occurred in the dorsal region of the body. Along the venter extended a narrow longitudinal belt of probably oblique cords, crossing themselves perpendicularly. In having a virtually smooth, laterally flattened body, Myoscolex closely resembles the slightly geologically younger Pikaia from the Burgess shale of British Columbia, generally believed to be one of the oldest chordates. Being the oldest probable annelid, at least superficially similar to the opheliid polychaetes, Myoscolex may appear not too distant from the ancestor of the phylum. The lateral body flattening of Myoscolex was apparently an adaptation to swimming by undulation of the body in transverse plane, similar to today's errant polychaetes but without using chaetae or appendages in propulsion.     

The origin of annelids

Annelids are a phylum of segmented bilaterian animals that have become important components of ecosystems spanning terrestrial realms to the deep sea. Annelids are remarkably diverse, possessing high taxonomic diversity and exceptional morphological disparity, and have evolved numerous feeding strategies and ecologies. Their interrelationships and evolution have been the source of much controversy over the past century with the composition of the annelid crown group, the relationship of major groups and the body plan of the ancestral annelid having undergone major recent revisions. There is a convincing body of molecular evidence that polychaetes form a paraphyletic grade and that clitellates are derived polychaetes. The earliest stem group annelids from Cambrian Lagerstätten are errant, epibenthic polychaetes, confirming that biramous parapodia, head appendages and diverse, simple chaetae are primitive for annelids. Current evidence from molecular clocks and the fossil record suggest that crown group annelids are a Late Cambrian – Ordovician radiation, with clitellates radiating in the Late Palaeozoic. Their body fossil record is largely confined to deposits showing exceptional preservation and is punctuated by the acquisition of hard parts in major groups. The discovery of an Ordovician fossil with soft tissues has shown that machaeridians are in fact a clade of crown polychaetes. They were in existence for more than 200 million years and possess unique calcitic dorsal armour, allowing their mode of life and phylogeny to be interpreted in the context of the annelid body plan. We identify a novel clade of machaeridians, the Cuniculepadida, which exhibit a series of adaptations for burrowing.     

On the ground pattern of Annelida

Annelida, traditionally divided into Polychaeta and Clitellata, are characterized by serial division of their body into numerous similar structures, the segments. In addition, there is a non-segmental part at the front end, the prostomium, and one at the back, the pygidium. New segments develop in a prepygidial proliferation zone. Each segment contains four groups of chaetae made up of β-chitin, a pair of coelomic cavities separated by mesenteries, and septa. The nervous system is a rope-ladder-like ventral nerve cord with a dorsal brain in the prostomium. For the last stem species a trochophore larva and a benthic adult are commonly postulated. There are two conflicting hypotheses describing the systematization of Annelida: the first postulates a sister-group relationship of Polychaeta and Clitellata, the second sees Clitellata as a highly derived taxon forming a subordinate taxon within the polychaetes which, consequently, are regarded as paraphyletic. Depending on the hypothesis, different characters have to be postulated for the stem species of Annelida. Besides segmentation other characters such as nuchal organs, palps and antennae, body wall musculature, cuticle, parapodia as well as structure of the central nervous system and the foregut play an important role in this discussion. Here, the different characters and character states are critically reviewed and analyzed with respect to morphology and function. The consequences for systematization of their phylogenetic interpretation as autapomorphies, synapomorphies or plesiomorphies are outlined. The resulting hypotheses are compared with those relying on molecular data sets.     

An Early Cambrian stem polychaete with pygidial cirri

The oldest annelid fossils are polychaetes from the Cambrian Period. They are representatives of the annelid stem group and thus vital in any discussion of how we polarize the evolution of the crown group. Here, we describe a fossil polychaete from the Early Cambrian Sirius Passet fauna, Pygocirrus butyricampum gen. et sp. nov., with structures identified as pygidial cirri, which are recorded for the first time from Cambrian annelids. The body is slender and has biramous parapodia with chaetae organized in laterally oriented bundles. The presence of pygidial cirri is one of the characters that hitherto has defined the annelid crown group, which diversified during the Cambrian-Ordovician transition. The newly described fossil shows that this character had already developed within the total group by the Early Cambrian.     

Locomotion of Gymnarchus Niloticus： Experiment and Kinematics

In addition to forward undulatory swimming, Gymnarchus niloticus can swim via undulations of the dorsal fin while the body axis remains straight; furthermore, it swims forward and backward in a similar way, which indicates that the undulation of the dorsal fin can simultaneously provide bidirectional propulsive and maneuvering forces with the help of the tail fin. A high-resolution Charge-Coupled Device （CCD） imaging camera system is used to record kinematics of steady swimming as well as maneuvering in G. niloticus. Based on experimental data, this paper discusses the kinematics （cruising speed, wave speed, cycle frequency, amplitude, lateral displacement） of forward as well as backward swimming and maneuvering. During forward swimming, the propulsive force is generated mainly by undulations of the dorsal fin while the body axis remains straight. The kinematic parameters （wave speed, wavelength, cycle frequency, amplitude） have statistically significant correlations with cruising speed. In addition, the yaw at the head is minimal during steady swimming. From experimental data, the maximal lateral displacement of head is not more than 1% of the body length, while the maximal lateral displacement of the whole body is not more than 5% of the body length. Another important feature is that G. niloticus swims backwards using an undulatory mechanism that resembles the forward undulatory swimming mechanism. In backward swimming, the increase of lateral displacement of the head is comparatively significant; the amplitude profiles of the propulsive wave along the dorsal fin are significantly different from those in forward swimming. When G. niloticus does fast maneuvering, its body is first bent into either a C shape or an S shape, then it is rapidly unwound in a travelling wave fashion. It rarely maneuvers without the help of the tail fin and body bending.     

Stroke frequency, but not swimming speed, is related to body size in free-ranging seabirds, pinnipeds and cetaceans

It is obvious, at least qualitatively, that small animals move their locomotory apparatus faster than large animals: small insects move their wings invisibly fast, while large birds flap their wings slowly. However, quantitative observations have been difficult to obtain from free-ranging swimming animals. We surveyed the swimming behaviour of animals ranging from 0.5 kg seabirds to 30 000 kg sperm whales using animal-borne accelerometers. Dominant stroke cycle frequencies of swimming specialist seabirds and marine mammals were proportional to mass(-0.29) (R(2)= 0.99, n = 17 groups), while propulsive swimming speeds of 1-2 m s(-1) were independent of body size. This scaling relationship, obtained from breath-hold divers expected to swim optimally to conserve oxygen, does not agree with recent theoretical predictions for optimal swimming. Seabirds that use their wings for both swimming and flying stroked at a lower frequency than other swimming specialists of the same size, suggesting a morphological trade-off with wing size and stroke frequency representing a compromise. In contrast, foot-propelled diving birds such as shags had similar stroke frequencies as other swimming specialists. These results suggest that muscle characteristics may constrain swimming during cruising travel, with convergence among diving specialists in the proportions and contraction rates of propulsive muscles.     

Swimming mechanisms in nereidiform polychaetes

New observations of the swimming of several species of nereidiform polychaete are presented and the parameters of body motion analysed. It is shown that there are theoretical limits to the values of these parameters outside which propulsion by the wave motion of the body is impossible. Many of the observations do fall outside them. This is because of the effect of the beating parapodia. Kinematic considerations are introduced to show how the wave of parapodial activity is related to the wave motion of the body. Dynamic considerations lead to an estimate of the minimum Young's modulus of the parapodia for pure parapodial swimming, lower values being possible for a combination of body-wave and parapodial swimming. The speeds achieved by different methods of swimming are discussed for the species that have been examined.     

Evolution of body wall musculature

A body wall musculature comprising an outer layer of circularfibers and an inner layer of longitudinal fibers is generallyseen as the basic plan in Annelida. Additional muscles may bepresent such as oblique, parapodial, chaetal, and dorsoventralmuscles. The longitudinal muscle fibers do not form a continuouslayer but are arranged in distinct bands in polychaetes. Mostlythere are four to six bands, usually including prominent ventraland dorsal bands. However, other patterns of muscle band arrangementalso exist. The ventral nerve cord lies between the two ventralbands in certain polychaetes, and is covered by an additionallongitudinal muscle band of comparatively small size. In manypolychaetes with reduced parapodia and in Clitellata a moreor less continuous layer of longitudinal fibers is formed. Clitellatais the only group with a complete layer of longitudinal musculature.Circular fibers are usually less developed than the longitudinalmuscles. However, recent investigations employing phalloidinstaining in combination with confocal laser scanning microscopyrevealed that absence of circular muscles is much more widelydistributed within the polychaetes than was previously known.This necessitates thorough reinvestigations of polychaete musclesystems, and this feature has to be taken into account in furtherdiscussions of the phylogeny and evolution of Annelida.     

Caenorhabditis elegans body mechanics are regulated by body wall muscle tone

Body mechanics in the nematode Caenorhabditis elegans are central to both mechanosensation and locomotion. Previous work revealed that the mechanics of the outer shell, rather than internal hydrostatic pressure, dominates stiffness. This shell is comprised of the cuticle and the body wall muscles, either of which could contribute to the body mechanics. Here, we tested the hypothesis that the muscles are an important contributor by modulating muscle tone using optogenetic and pharmacological tools, and measuring animal stiffness using piezoresistive microcantilevers. As a proxy for muscle tone, we measured changes in animal length under the same treatments. We found that treatments that induce muscle contraction generally resulted in body shortening and stiffening. Conversely, methods to relax the muscles more modestly increased length and decreased stiffness. The results support the idea that body wall muscle activation contributes significantly to and can modulate C. elegans body mechanics. Modulation of body stiffness would enable nematodes to tune locomotion or swimming gaits and may have implications in touch sensation.     

Parapodial glandular organs in Spiophanes (Polychaeta: Spionidae)—studies on their functional anatomy and ultrastructure

Parapodial glandular organs (PGOs) of Spiophanes (Polychaeta: Spionidae) were studied using light and electron microscopy. These organs are found in parapodia of the mid body region, starting on chaetiger 5 and terminating with the appearance of neuropodial hooks (chaetiger 14 or 15 in adult individuals). Large PGOs in anterior chaetigers display different species‐specific types of openings whereas small PGOs in posterior parapodia of the mid body region always open in a simple vertical slit. Each PGO is composed of three main complexes: (1) the glandular sac with several distinct epithelia of secretory cells and secretory cell complexes and the reservoir filled with fibrous material, (2) the gland‐associated chaetal complex (including the region of chaetoblasts and follicle cells, follicular canals, two chaetal collector canals, the combined conducting canal, the chaetal spreader including the opening of the glandular organ with associated type‐1 secretory cells, and the gland‐associated chaetae), and (3) a bilayered musculature surrounding the gland. A considerable number of different cell types are involved in the secretory activity, in the guidance of the gland‐associated chaetae, and in the final expulsion of the fibrous secretion at the opening slit. Among these different cell types the type‐5 secretory cells of the proximal glandular complex with their cup‐shaped microvilli emanating thick microfibrils into the lumen of the glandular sac are most conspicuous. Secretory cells with cup‐shaped microvilli being involved in the production of β‐chitin microfibrils have so far only been reported from some representatives of the deep‐sea inhabiting Siboglinidae (Polychaeta). We suggest that the gland‐associated chaetae emerging from inside the PGOs of Spiophanes are typical annelid chaetae formed by chaetoblasts and follicle cells. Functional morphology implies the crucial role of PGOs in tube construction. Furthermore, the PGOs are discussed in consideration of phylogenetic aspects. J. Morphol., 2012. © 2011 Wiley Periodicals, Inc.     

Glycogen concentrations and endurance capacity of rats with chronic heart failure

The endurance capacities of rats with myocardial infarctions (MI) and of rats having undergone sham operations (SHAM) were tested during a submaximal exercise regimen that consisted of swimming to exhaustion. During this test, a decrement in the endurance capacity of the MI rat was demonstrated as the SHAM rat swam 25% longer than the MI rat (65 +/- 4 vs. 52 +/- 4 min). Glycogen concentrations were measured in the liver and the white gastrocnemius, plantaris, and soleus muscles of SHAM and MI rats that were randomly divided into four subgroups, which consisted of resting control, swim to exhaustion, swim to exhaustion + 24 h recovery, and swim to exhaustion + 24 h recovery + a second swim to exhaustion. The results demonstrated that the glycogen concentrations found in the liver, white gastrocnemius, plantaris, and soleus muscles of the SHAM and MI rats belonging to the resting control groups were similar. After swimming to exhaustion the glycogen concentrations in these tissues were significantly reduced compared with those found in the resting control groups of rats, and after 24 h of recovery the glycogen concentrations in these tissues were again similar to those found in the resting control groups of rats. Since the magnitude of the glycogen depletion in the liver and the white gastrocnemius, plantaris, and soleus muscles was similar in the SHAM and MI rats and because the SHAM rats consistently swam for longer periods of time in each of the experimental groups, it would be logical to assume that the rates of glycogen utilization for the various tissues may have been greater in the MI rat during exercise.(ABSTRACT TRUNCATED AT 250 WORDS)     

Conservation rules, their breakdown, and optimality in Caenorhabditis sinusoidal locomotion

Undulatory locomotion is common to nematodes as well as to limbless vertebrates, but its control is not understood in spite of the identification of hundred of genes involved in Caenorhabditis elegans locomotion. To reveal the mechanisms of nematode undulatory locomotion, we quantitatively analysed the movement of C. elegans with genetic perturbations to neurons, muscles, and skeleton (cuticle). We also compared locomotion of different Caenorhabditis species. We constructed a theoretical model that combines mechanics and biophysics, and that is constrained by the observations of propulsion and muscular velocities, as well as wavelength and amplitude of undulations. We find that normalized wavelength is a conserved quantity among wild-type C. elegans individuals, across mutants, and across different species. The velocity of forward propulsion scales linearly with the velocity of the muscular wave and the corresponding slope is also a conserved quantity and almost optimal; the exceptions are in some mutants affecting cuticle structure. In theoretical terms, the optimality of the slope is equivalent to the exact balance between muscular and visco-elastic body reaction bending moments. We find that the amplitude and frequency of undulations are inversely correlated and provide a theoretical explanation for this fact. These experimental results are valid both for young adults and for all larval stages of wild-type C. elegans. In particular, during development, the amplitude scales linearly with the wavelength, consistent with our theory. We also investigated the influence of substrate firmness on motion parameters, and found that it does not affect the above invariants. In general, our biomechanical model can explain the observed robustness of the mechanisms controlling nematode undulatory locomotion.     

Principle of spiral arrangement of the skeletal muscles of humans and animals

It has been stated long ago, that smooth muscle elements in the vascular walls and other tubular systems in the human being and in the animals demonstrate spiral arrangement. The authors decided to show that there is a spiral formation of the skeletal musculature in the human being and in vertebrata at the level of the whole organism, its parts and separate muscles. By means of successive joining certain muscles, their parts and even separate groups of muscular fasciculi by tendons, aponeuroses, fascia and intermuscular septa, ligaments and bones kinematic chains of muscles have been revealed, those chains that have spiral direction regarding the longitudinal axes of the body and its parts. Two examples of left- and right-hand-screw types of spirals are presented and it is stressed that the spiral principle reflects biological symmetry of structural oppositions--enantiomorphism. A conclusion is made that the spiral form of the skeletal musculature is a universal regularity for the human being and for all vertebrata. The cylindric form of the vertebral body serves as a predestinated moment for this. The spiral twisting of the muscles is the most optimal for ensuring variability of movements and performing adaptive survival of the human being and animals in the Earth gravitational field.     

The relationship between body size and the field potentials generated by swimming crayfish

Aquatic animals generate electrical field potentials which may be monitored by predators or conspecifics. Many crustaceans use rapid, forceful contractions of the flexor and extensor muscles to curl and extend their abdomens during swimming in escape and locomotion. When crayfish swim they generate electrical field potentials that can be recorded by electrodes nearby in the water. In general, it is reasonable to assume that larger bodied crayfish will generate signals of greater amplitude because they have larger muscles. It is not known, however, how activity in particular muscles and nerves combines to produce the compound electrical waveform recorded during swimming. We therefore investigated the relationship between abdominal muscle, body size and the amplitude of nearby tailflip potentials in the freshwater crayfish (Cherax destructor). We found that amplitude was correlated positively with abdominal muscle mass. The mean amplitude recorded from the five smallest and five largest individuals differed by 440 microV, a difference sufficiently large to be of significance to predators and co-inhabitants in the wild.