Ultrastructure of the antennal sensilla of aphids

Summary An electron microscopical study of aphid antennal sensilla has revealed two types of trichoid sensilla. Type I, innervated by a single neuron is mechanoreceptive; type II, innervated by three to five neurons is both mechanoreceptive and chemoreceptive with possibly a third function. Johnston's organ in the pedicel comprises a peripheral ring of scolopidia inserted into the joint with the flagellum; two non-peripheral groups of scolopidia lie in the lumen with attachment points in the wall of the third segment. The fine structure of a campaniform sensillum on the pedicel is described together with two homologous and previously unknown sense organs at the joint between the fifth and sixth antennal segments. An unusually placed scolopidium in the lumen of the sixth segment has also been found. The function of this scolopidium is unknown but Johnston's organ, the campaniform sensillum and joint receptors are suggested to act as antennal proprioceptors.The authors thank the Long Ashton Research Station, Bristol for use of the SEM facilities. A.K. Bromley gratefully acknowledges the tenure of a S.R.C. CASE Studentship and thanks Professor L.H. Finlayson for research facilities     

Internal proprioceptive organs of the distal antennal segments in Allacma fusca (L.) (Collembola : Sminthuridae): Proprioceptors phylogenetically derived from sensillum-bound exteroceptors

The internal proprioceptive organs of the distal antennal segments of Allacma fusca (Collembola : Sminthuridae) were examined with light, scanning, and transmission electron microscopy. There are 2 such organs, each belonging to the sensory equipment of a sensory peg. These pegs are part of the sensory organs of the 3rd antennal segment (SO - AIII), and show characteristics of both hair sensilla and chordotonal organs. They have 5 sensory cells of which 4 can be regarded as chemoreceptors, because the walls of the pegs are pierced by pores. The 5th cells are the proprioceptors. Their dendritic outer segments are connected to the base of the SO - AIII peg but show no specializations. Their dendritic inner segments are distally joined to a thecogen cell containing a scolopale. Proximally, they span the hemolymph space within the antenna. The perikaryon of one of the organs is located on a muscle that deflects the 3rd antennal segment (IP - AIII). The perikaryon of the other is attached to the antennal nerve near a point where it is connected to a muscle that deflects the 4th antennal segment (IP - AIV). There is a conspicuous membrane system, associated with microtubules and a prominent ciliary rootlet within the dendritic inner segment of the IP - AIII. The cytological features of the organs are discussed with reference to 2 problems, the phylogeny of chordotonal organs and the process of mechanosensory transduction.     

Structure of the tarso-pretarsal chordotonal organ in the imago of Tineola bisselliella Humm. (Lepidoptera : Tineidae)

The tarso-pretarsal chordotonal organ in the imago of Tineola bisselliella (Lepidoptera .: Tineidae) includes 2 groups of scolopidia. (1) A basal group composed of 2 scolopidia, each with one neuron and one scolopidium with 3 neurons, whose dendrites present a typical structure of cilium; the dendrite becomes dense apically; it is a common characteristic for all the mechanoreceptive dendrites. (2) An apical scolopidium with 3 neurons, whose dendrites have the same size and are covered with a conical cap.The combination of single and triple dendrites is unusual in the limb chordotonal organs. The 2 groups of scolopidia are not in contact. The importance of the support structures and fixation structures is discussed. In the ciliary root region, peculiar desmosomes occur between the dendrite and the scolopale cell. For the apical scolopidium, one attachment cell is seen distally and it appears in close association with the articular membrane.     

Metabolic variability within individual fibres of the cat tibialis posterior and diaphragm muscles

Summary Variance in succinate dehydrogenase activity along the transverse and longitudinal axes of fibres from the cat tibialis posterior and diaphragm muscles was determined in order to estimate the three-dimensional distribution of mitochondria within single fibres. The variance (coefficient of variation) in succinate dehydrogenase activity along the transverse fibre axis was greatest in type IIB fibres from both muscles. Intracellular compartmentalization (i.e. subsarcolemmal vs central core differences in succinate dehydrogenase activity) was observed only in type II fibres from the tibialis posterior; the succinate dehydrogenase activity of the subsarcolemmal region was significantly greater than that of the central core. The extent of succinate dehydrogenase variance along the longitudinal fibre axis was dependent on the total length of the fibre segment analyzed, the muscle, and fibre type. The coefficient variation for short fibre segments, i.e. 40 m, was significantly lower than that for much longer fibre segments (840 m). Significant differences in the coefficient variation for 840 m fibre segments were observed between the diaphragm and tibialis posterior muscles. The longitudinal variance in succinate dehydrogenase activity was higher in diaphragm muscle fibres. The succinate dehydrogenase variance along the longitudinal axis of type II fibres was significantly greater than that of the type I fibre population. These results indicate that mitochondria are heterogeneously distributed within muscle fibres. Possible functional implications of such intrafibre metabolic variance are discussed.     

Paramyosin in invertebrate muscles. I. Identification and localization

By sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunodiffusion, we identified paramyosin in two smooth invertebrate "catch" muscles (Mytilus anterior byssus retractor and Mercenaria opaque adductor) and five invertebrate striated muscles (Limulus telson levator, Homarus claw muscle, Balanus scutal depressor, Lethocerus air tube retractor, and Aequipecten striated adductor). We show that (a) the paramyosins in all of these muscles have the same chain weights and (b) they are immunologically similar. We stained all of these muscles with specific antibody to Limulus paramyosin using the indirect fluorescent antibody technique. Paramyosin was localized to the A bands of the glycerinated striated muscles, and diffus fluorescence was seen throughout the glycerinated fibers of the smooth catch muscles. The presence of paramyosin in Homarus claw muscle, Balanus scutal depressor, and Lethocerus air tube retractor is shown here for the first time. Of the muscles in this study, Limulus telson levator is the only one for which the antiparamyosin staining pattern has been previously reported.     

Acoustic startle/escape reactions in tethered flying locusts: motor patterns and wing kinematics underlying intentional steering

We simultaneously recorded flight muscle activity and wing kinematics in tethered, flying locusts to determine the relationship between asymmetric depressor muscle activation and the kinematics of the stroke reversal at the onset of wing depression during attempted intentional steering manoeuvres. High-frequency, pulsed sounds produced bilateral asymmetries in forewing direct depressor muscles (M97, 98, 99) that were positively correlated with asymmetric forewing depression and asymmetries in stroke reversal timing. Bilateral asymmetries in hindwing depressor muscles (M127 and M128 but not M129) were positively correlated with asymmetric hindwing depression and asymmetries in the timing of the hindwing stroke reversal; M129 was negatively correlated with these shifts. Hindwing depressor asymmetries and wing kinematic changes were smaller and shifted in opposite direction than corresponding measurements of the forewings. These findings suggest that intentional steering manoeuvres employ bulk shifts in depressor muscle timing that affect the timing of the stroke reversals thereby establishing asymmetric wing depression. Finally, we found indications that locusts may actively control the timing of forewing rotation and speculate this may be a mechanism for generating steering torques. These effects would act in concert with forces generated by asymmetric wing depression and angle of attack to establish rapid changes in direction.Abbreviations ASR acoustic startle response - dB SPL decibel sound pressure level (re: 20 Pa RMS) - EMG electromyogram - FWA forewing asymmetry - HWA hindwing asymmetry     

Johnston's organ in Neodiprion sertifer (Insecta: Hymenoptera)

The fine structure of Johnston's organ in the pine sawfly, Neodiprion sertifer, was studied by electron microscopy to determine if there exists a dimorphism in this organ corresponding to the sexual dimorphism in antennal shape and surface area. The organ is made up of scolopidia that are ultrastructurally similar to those of other insects. The scolopidia, identical in both sexes, comprise three sensory cells bearing two types of sensory processes: Two are shorter and smaller in diameter than the third, which extends into the cuticle of the membrane connecting pedicel and flagellum and terminates at an epicuticular invagination. The dendrites and sensory processes are surrounded by two types of enveloping (glial) cells-a scolopale cell and an attachment cell. Other enveloping cells occur at different levels of the scolopidium. Sexual dimorphism is evident only in the numbers of scolopidial groups: Males have more groups with fewer scolopidia, but both sexes possess about the same total number of scolopidia.     

Embryogenesis of the connective chordotonal organ in the pedicel of the American cockroach: Cell lineage and morphological differentiation

Summary The ontogenesis of single scolopidia of the chordotonal organ of the American cockroach, Periplaneta americana, takes about 4 days. At 23% embryogenesis (100% = 30 d) the first anlagen of scolopidia were identified within the epithelium by staining with anti-horseradish peroxidase. Reconstruction of the cell lineage of the scolopidial cells was facilitated by two facts: (i) the arrangement of the cells throughout ontogenesis follows a strict pattern, and (ii) daughter cells are recognizable for several hours after mitosis by the cytoplasmic bridge and midbody joining them. When they separate, the midbody undergoes lysosomal degeneration in one of these cells. The earliest recognizable stage is a pair of cells, one of which (cell 1) encloses the other (cell 2) apically. The enclosing cell becomes the accessory cell. Cell 2 divides, yielding the mother cell (cell 3) of two sensory cells which degenerate later, and cell 2. Cell 2 gives rise to the attachment cell and to cell 2, which in turn produces the scolopale cell and the mother cell (cell 2 2) of a second pair of sensory cells; the latter are the definitive sensory cells. The end result is the total of 5 cells characteristic of the adult scolopidium. Secretion of the scolopale and cap together with the migration of the sensory cell perikarya into the antennal lumen complete development.     

The antennal motor system of the stick insect Carausius morosus: anatomy and antennal movement pattern during walking

The stick insect Carausius morosus continuously moves its antennae during locomotion. Active antennal movements may reflect employment of antennae as tactile probes. Therefore, this study treats two basic aspects of the antennal motor system: First, the anatomy of antennal joints, muscles, nerves and motoneurons is described and discussed in comparison with other species. Second, the typical movement pattern of the antennae is analysed, and its spatio-temporal coordination with leg movements described. Each antenna is moved by two single-axis hinge joints. The proximal head-scape joint is controlled by two levator muscles and a three-partite depressor muscle. The distal scape-pedicel joint is controlled by an antagonistic abductor/ adductor pair. Three nerves innervate the antennal musculature, containing axons of 14-17 motoneurons, including one common inhibitor. During walking, the pattern of antennal movement is rhythmic and spatiotemporally coupled with leg movements. The antennal abduction/adduction cycle leads the protraction/retraction cycle of the ipsilateral front leg with a stable phase shift. During one abduction/adduction cycle there are typically two levation/depression cycles, however, with less strict temporal coupling than the horizontal component. Predictions of antennal contacts with square obstacles to occur before leg contacts match behavioural performance, indicating a potential role of active antennal movements in obstacle detection.     

Rapid formation of myometrial gap junctions during parturition in the unilaterally implanted rat uterus

Summary In uterine smooth muscles, gap junction plaques rapidly form during the final stages of gestation. To investigate the related mechanisms, regional differences in myometrial gap junction development in rat uterus were examined quantitatively during delivery, using thin-section and freeze-fracture techniques in combination with light- and electron microscopy.Examination of implanted and nonimplanted horns in the unilaterally ligated rat bicornuate uteri, revealed no differences in the occurrence of gap junction plaques, but after 2 to 4 pups had been delivered, the contracted segments contained more gap junction plaques than did noncontracted segments examined immediately before delivery. In all segments, gap junctions were found more frequently in the circular muscle layers than in the longitudinal muscle layers. Gap junctions ranged in size from 0.002 m² to 0.52 m², but two-thirds were less than 0.1 m². The frequency of small gap junction plaques (less than 0.1 m²) was higher in the noncontracted segment.These results suggest that gap junctions are dynamic structures, and that their formation is controlled not only by general hormonal factors, possibly involved in gap junction increases in the myometrium before delivery, but also by local factors, possibly related to the contraction, that may accelerate an increase in gap junction formation during delivery.     

Structural organization of two fast,rhythmically active crustacean muscles

Summary The organization of the flagellum abductor muscle and of a scaphognathite levator muscle of the green crab, Carcinus maenas, has been compared quantitatively using light and electron microscopy. These muscles are rhythmically active at relatively high frequencies and for long durations. Fibers of both muscles are interconnected to form fascicles of 50 or more fibers within which there is cytoplasmic continuity. A single muscle is made up of 8–12 fascicles. Individual fibers consist of a peripheral rind of densely packed mitochondria, a thick region of glycogen granules, and myofibrils arranged into scattered central islands. Less than half the volume-density of these muscles is contractile material, the balance being largely mitochondria and glycogen. The fibers within a muscle are structurally similar. They have short sarcomeres (about 2 m), thin to thick filament ratios of about 3:1, and junctions between the sarcoplasmic reticulum and the transverse tubules at the M line. Sarcoplasmic reticulum occupies about 10% of the myofibrillar volume-density. A well developed sarcoplasmic reticulum appears to underlie the capacities of these two muscles for high frequency contraction; extensive mitochondria and glycogen stores should confer fatigue resistance under both aerobic and anaerobic conditions.     

The depressor trochanteris motoneurones and their role in the coxo-trochanteral feedback loop in the stick insect Carausius morosus

The innervation pattern of the coxal part of the depressor trochanteris muscle is described. This muscle is located inside the coxa cavity and is innervated by motoneurones contained in nerve C2. Serial sections of nerve C2 reveal that nerve C2 contains 3 large neurones (8, 5, and 3 m in diameter) in addition to many small neurones. In extracellular nerve recordings from nerve C2 3 large spikes could be recorded, which can easily be classified according to their amplitudes. Combined intracellular muscle recordings and extracellular nerve recordings revealed the physiological characteristics of these motoneurones, which are referred to here as the fast depressor trochanteris (FDTr) motoneurone and the spontaneously active slow depressor trochanteris (SDTr) motoneurone. The third motoneurone could be identified as an inhibitory motoneurone. Because this motoneurone was also found in nerves nl2, nl3, nl5 and in nerve C1 (to the levator trochanteris muscle) it is referred to here as the common inhibitor (CI) motoneurone.The hypothesis that the trochanteral hairplate (trHP) is the only effective feedback transducer for the coxo-trochanteral control loop (Schmitz 1984, 1986) is confirmed by the nerve recordings from nerve C2, because no reflex response was measured after ablation of the trHP. In addition, shaving the trHP reduces the activity of the spontaneously active SDTr motoneurone.The frequency responses of the excitatory depressor motoneurones show that the spontaneous activity of the SDTr motoneurone is modulated by the stimulus over a wide range of stimulus frequencies up to 100 Hz and that the FDTr motoneurone is reflexly activated during the same phase of the stimulus as the SDTr motoneurone. Up to 20 Hz the maximum of the motoneurone activity leads the maximum of the movement by about 60 to 80 deg. This shows that nonlinear highpass filter properties of the coxotrochanteral control system, described on the basis of force measurements in an earlier paper (Schmitz 1986), can be found already on the level of the motoneurones.     

Architecture of the media of the arterial vessels in the dog brain: A scanning electron-microscopic study

Summary The architecture of the media of arterial vessels in dog brain was investigated using scanning electron microscopy. The arrangement and shape of the circularly-oriented smooth muscle cells varied with vessel diameter: The arteries (>100 m in diameter) had 4–10 layers of spindle-shaped smooth muscle cells; the muscular arterioles (30–100 m), 2–3 layers of spindle-shaped smooth muscle cells; the terminal arterioles (10–30 m), a compact layer of spindle-shaped smooth muscle cells with more dominant nodular or rod-like processes and thin lateral processes; and the precapillary arterioles (5–15 m), a less compact layer of branched smooth muscle cells.Longitudinally-oriented muscles were observed in the medio-adventitial border. The distribution and arrangement of these muscles varied with vessel size: in the large arteries (> 300 m in diameter), at the branching sites only; in the small arteries (100–300 m), at both the branching and non-branching sites; in the muscular arterioles, at both the branching and non-branching sites in a reticular arrangement with some muscle cells having an asteroid appearance; in the terminal aterioles, only asteroid-like muscle cells were found at the branching and non-branching sites.     

The innervation pattern of crustacean skeletal muscle

Summary The innervation pattern of distal muscle fibers of the opener muscle of walking legs of crayfish (Astacus leptodactylus) was investigated using methylene-blue staining, cobalt infiltration, and electron microscopy. A quantitative analysis of the entire innervation of single muscle fibers was attempted.It was found that instead of the generally assumed parallel array of numerous excitatory and inhibitory terminals, innervation consists of only a few branched terminals. The branches of excitatory and inhibitory terminals lie side-by-side. Both types are characterized by numerous varicosities (see Fig. 9B). The aggregate length of excitatory as well as inhibitory terminals on one muscle fiber is, on the average, about 1,500 m with a total of 152 varicosities spaced about 10 m apart. The average diameter of the varicosities is 4.26 m, that of the connecting thin segments about 0.5 m. Total terminal surface of motor or inhibitory terminals amounts to about 10,000 m² per muscle fiber. There are approximately 2,000 motor synapses on each muscle fiber, but their average total area is only about 6% of the terminal membrane area, or 0.06% of the (idealized) muscle fiber surface.There are conspicuous differences in the postsynaptic specializations associated with excitatory and inhibitory terminals; these are described in detail.The results are discussed in a functional context and with regard to design and results of electrophysiological experiments.Supported by Sonderforschungsbereich 138 of the Deutsche Forschungsgemeinschaft     

Fine structure of scolopidia in Johnston's organ of female Aedes aegypti compared with that of the male.

The Johnston's organ of the female mosquito, Aedes aegypti, has only three types of scolopidia: types A, B, and C. It lacks the type D scolopidium of the male's organ. The basic structure and the location of each type in the female are similar to the counterparts in the male's organ. A single scolopidium is composed of a scolopale cell, an envelope cell, a long cap, and a third sheath, in addition to the two electron-dense scolopales produced inside the cytoplasm of two satellite cells. Each scolopidium has either two (type A) or three (type B) sensory cells. A type C scolopidium, mononematic in contrast to the amphinematic types A and B scolopidia, has two sensory cells, a microtubular cap cell, two microtubular accessory cells, and a scolopale cell with an intracellular scolopale. Even though the female Johnston's organ has all the components of the male's organ except for the single type D scolopidium, the female's organ shows relatively poorer organization and development. The female has a smaller and thinner basal plate, shorter and thicker prongs, fewer type A sensory cells, and a shorter flagellar flange, in addition to the overall smaller size of the pedicel.The probable function of each scolopidial type is discussed, especially in connexion with the probable identification of a single auditory sensillum in the male.     

Three-dimensional organization of smooth muscle cells in blood vessels of laboratory rodents

Summary Three-dimensional aspects of smooth muscle cells of the microvas-culature were studied ultrastructurally in laboratory rodents by means of serial thin sections and reconstruction of muscle cell models. It was demonstrated that a muscle cell of an arteriole (luminal diameter (LD) 17 m) in hamster striated muscle was spindle-shaped, 70 m long, and wound twice round the vessel axis. The volume of the cell was calculated as 750 m³ and its surface area as 1330 m². A muscle cell in an arteriole (LD 6 m) in the rat retina was irregular in shape, about 22 m long, and had branched processes. The cell volume was calculated as 139 m³ and its surface area as 298 m².     

The peripheral and central nervous organization of the locust coxo–trochanteral joint

The micromorphology of the locust coxo–trochanteral joint was examined in cobalt-stained material. Peripheral nervous system, musculature, and internal proprioceptors—two strand receptors and a muscle receptor organ—of the metathoracic coxa are compared with those of the pro- and mesothoracic legs. The number and position of trochanter levator and depressor motoneurons as well as the central projections of coxal sense organs are described. Evidence for a femoro–tibial strand receptor was obtained by tracing the path of a particular nerve branch.     

Fine structure and metabolism of multiply innervated fast muscle fibres in teleost fish

Summary Both the fast and slow muscle fibres of advanced teleost fish are multiply innervated. The fraction of slow-fibre volume occupied by mitochondria is 31.3%, 25.5% and 24.6%, respectively, for the myotomal muscles of brook trout (Salvelinus fontinalis), crucian carp (Carassius carassius), and plaice (Pleuronectes platessa), respectively. The corresponding figures for the fast muscles of these species are 9.3%, 4.6% and 2.0%, respectively. Cytochrome-oxidase and citrate-synthetase activities in the fast muscles of 9 species of teleost range from 0.20–0.93 moles substrate utilised, g wet weight muscle^-1 min^-1 (at 15° C) or around 4–17% of that of the corresponding slow fibres. Ultrastructural analyses reveal a marked heterogeneity within the fast-fibre population. For example, the fraction of fibres with <1% or >10% mitochondria is 0,4,42% and 36, 12 and 0%, respectively, for trout, carp and plaice. In general, small fibres (<500 m²) have the highest and large fibres (>1,500 m²) the lowest mitochondrial densities. The complexity of mitochondrial cristae is reduced in fast compared to slow fibres.Hexokinase activities range from 0.4–2.5 in slow and from 0.08–0.7 moles, g wet weight^-1 min^-1 in fast muscles, indicating a wide variation in their capacity for aerobic glucose utilisation. Phosphofructokinase activities are 1.2 to 3.6 times higher in fast than slow muscles indicating a greater glycolytic potential. Lactate dehydrogenase activities are not correlated with either the predicted anaerobic scopes for activity or the anoxic tolerances of the species studied. The results indicate a considerable variation in the aerobic capacities and principal fuels supporting activity among the fast muscles of different species. Brook trout and crucian carp are known to recruit fast fibres at low swimming speeds. For these species the aerobic potential of the fast muscle is probably sufficient to meet the energy requirements of slow swimming.     

Quantitative analysis of muscle breakdown during starvation in the marine flatfish Pleuronectes platessa

Summary The present study describes the effects of starvation for a duration of four months on the ultrastructure of skeletal muscles from the marine flatfish (Pleuronectes platessa L.). Starvation is associated with a decrease in resting metabolic rate from 20.1±2.2 to 11.6±1.5mg-O₂/kg/h (P<0.05) and muscle wasting. Median fibre size fell from 700 m² to 500 m² in intermediate (fast oxidative) and from 1,800 m² to 600 m² in starved, white (fast-glycolytic) muscle fibres. In contrast, median fibre size in red (slow oxidative) muscle remained within the range 300–400 m². The fraction of red fibre volume occupied by myofibrils (58.6%) and mitochondria (24.5%) did not change significantly with starvation. There was, however, a decrease in stored lipid (10.7% to 3.2%) and an alteration in the structure of the cristae in mitochondria from red muscle.Atrophy of white muscle fibres is associated with a decrease in both the diameter and fractional volume occupied by myofibrils (85.7% to 61.9% P < 0.01). In a high proportion of white fibres peripheral degeneration of Z-discs is evident causing an unravelling of the thin filament lattice. It is suggested that this allows a partial decrease in myofibril diameter and hence the maintenance of contractile function in muscle from starved fish. In severely degenerating white fibres, disorganised thick and thin filaments and numerous multimembrane lysosome-like vesicles are observed.Starvation results in an increase in the average content of mitochondria in white fibres from 2.2 to 6.7% (P<0.01). In fed plaice mitochondria constitute less than 1% of the volume of the white fibre in 43.5% of the fibres. The proportion of white fibres containing more than 6% mitochondria increases from 6.5% to 58% with starvation.     

Muscle fibre types and metabolism in post-larval and adult stages of notothenioid fish

Summary A histochemical study was carried out on muscle fibre types in the myotomes of post-larval and adult stages of seven species of notothenioid fish. There was little interspecific variation in the distribution of muscle fibre types in post-larvae. Slow fibres (diameter range 15–60 m) which stained darkly for succinic dehydrogenase activity (SDHase) formed a superficial layer 1–2 fibres thick around the entire lateral surface of the trunk. In all species a narrow band of very small diameter fibres (diameter range 5–62 m), with only weak staining activity, occurred between the skin and slow fibre layer. These have the characteristics of tonic fibres found in other teleosts. The remainder of the myotome was composed of fast muscle fibres (diameter range 9–75 m), which stain weakly for SDHase, -glycerophosphate dehydrogenase, glycogen and lipid. Slow muscle fibres were only a minor component of the trunk muscles of adult stages of the pelagic species Champsocephalus gunnari and Pseudochaenichthys georgianus, consistent with a reliance on pectoral fin swimming during sustained activity. Of the other species examined only Psilodraco breviceps and Notothenia gibberifrons had more than a few percent of slow muscle in the trunk (20%–30% in posterior myotomes), suggesting a greater involvement of sub-carangiform swimming at cruising speeds. The ultrastructure of slow fibres from the pectoral fin adductor and myotomal muscles of a haemoglobinless (P. georgianus) and red-blooded species (P. breviceps), both active swimmers, were compared. Fibres contained loosely packed, and regularly shaped myofibrils numerous mitochondria, glycogen granules and occasional lipid droplets. Mitochondria occupied >50% of fibre volume in the haemoglobinless species P. georgianus, each myofibril was surrounded by one or more mitochondria with densely packed cristae. No significant differences, however, were found in mean diameter between fibres from red-blooded and haemoglobinless species. The activities of key enzymes of energy metabolism were determined in the slow (pectoral) and fast (myotomal) muscles of N. gibberifrons. In contrast to other demersal Antarctic fish examined, much higher glycolytic activities were found in fast muscle fibres, probably reflecting greater endurance during burst swimming.