A crustacean statocyst with only three hairs: Light and scanning electron microscopy

Standard histological and SEM techniques have been used to examine the pair of statocyst organs located in the telson of the isopod, Cyathara polita. Each organ is formed as an invagination of the dorsal cuticle of the telson. The invagination narrows to form a stalk between the statocyst and dorsal surface. A canal courses longitudinally through this stalk and forms a continuous channel between the lumen of the cyst and the external environment. On the luminal floor of each statocyst, there are three pits; each correlates with a nodule protruding from the ventro-medial wall. From each pit, a single, bifurcating hair projects dorsally to contact the single concretion within the statocyst lumen. No other static organs have been found in this animal. Thus, maintenance of equilibrium in this species appears to be under the control of but six hairs, three in each statocyst. Innervation of each statocyst is provided by a branch of a nerve which connects anteriorly with the last abdominal ganglion.     

Structure and function of the Nautilus statocyst.

The two equilibrium receptor organs (statocysts) of Nautilus are avoid sacks, half-filled with numerous small, free-moving statoconia and half with endolymph. The inner surface of each statocyst is lined with 130,000-150,000 primary sensory hair cells. The hair cells are of two morphological types. Type A hair cells carry 10-15 kinocilia arranged in a single ciliary row; they are present in the ventral half of the statocyst. Type B hair cells carry 8-10 irregularly arranged kinocilia; they are present in the dorsal half of the statocyst. Both type of hair cells are morphologically polarized. To test whether these features allow the Nautilus statocyst to sense angular accelerations, behavioural experiments were performed to measure statocyst-dependent funnel movements during sinusoidal oscillations of restrained Nautilus around a vertical body axis. Such dynamic rotatory stimulation caused horizontal phase-locked movements of the funnel. The funnel movements were either in the same direction (compensatory funnel response), or in the opposite direction (funnel follow response) to that of the applied rotation. Compensatory funnel movements were also seen during optokinetic stimulation (with a black and white stripe pattern) and during stimulations in which optokinetic and statocyst stimulations were combined. These morphological and behavioural findings show that the statocysts of Nautilus, in addition to their function as gravity receptor organs, are able to detect rotatory movements (angular accelerations) without the specialized receptor systems (crista/cupula systems) that are found in the statocysts of coleoid cephalopods. The findings further indicate that both statocyst and visual inputs control compensatory funnel movements.     

Structure of recent and fossil mysid statoliths (Crustacea,Mysidacea)

Statoliths of 61 Recent species representing all subfamilies of Mysidae were studied with special emphasis on internal structure. In addition 5 samples of fossil statoliths from Miocene deposits were examined. Species of Boreomysinae and Rhopalophthalminae show simple roughly spherical organic statoliths, with setae originating from the sensory cushion and anchored in the statolith with distal branches extending shortly below the surface. All other subfamilies possess mineralized statoliths of greater structural complexity, with differentiation in core and mantle, where each part may consist of up to three layers. Habitus is hemispherical to discoidal. External gross structures are dorsal tegmen, ventral fundus, and the ambitus forming the outer toroidal to semi-toroidal circumference. Setae penetrate the mantle through mineralic canals and insert on the surface of the core. As suggested by congeneric species of Schistomysis, there is no principal structural difference between statoliths mineralized with fluorite compared to vaterite. However, vaterite statoliths tend to be more often of moruloid appearance and are exceptional by showing a central conical hole (the hilum) or a central cavity in certain forms. These structures are typical of fossil calcite statoliths. In vaterite and fluorite statoliths, the mantle shows radially arranged (= spherulitic) crystal aggregates. Such arrangements are badly preserved in fossil calcite statoliths. In large extant statoliths, concentric structures, mainly in the form of superficial striation and/or concentric microstrata, are visible in coexistence with radial aggregates. Stratification is possibly due to stratified deposition of the nonmineralized gland product, while the spherulitic structure is indicative of subsequent radial growth of crystal aggregates. The structure of accessory fluorite statoliths in the statocyst of Mesopodopsis slabberi leads to the hypothesis that mantle material is formed by secretions of the caudal statocyst gland. After demineralization of fluorite, vaterite and calcite statoliths, an organic template remains showing most essential morphological features of the statolith. From this we conclude that the structure of the statolith is (almost) entirely matrix mediated. © 1993 Wiley-Liss, Inc.     

Ultrastructural aspects of the 'statocyst' of Xenoturbella (Deuterostomia) cast doubt on its function as a georeceptor

The "statocyst" in the enigmatic worm Xenoturbella is a structure containing motile flagellated cells. It is situated inside the subepidermal membrane complex (between epidermis and muscular layers) in the anterior end of the body. The motile cells contain a lipophilic refractile body ("statolith"), and a series of vesicles from small dense core vesicles presumably formed from the refractile body to large vesicles with dense aggregates of filamentous tubules that become exocytized through secretion. It is unlikely that the statocyst is a georeceptor (true statocyst); maybe it has an endocrine function.     

Fine structural study of the statocysts in the veliger larva of the nudibranch,Rostanga pulchra

Summary The two statocysts of the veliger larva of Rostanga pulchra are positioned within the base of the foot. They are spherical, fluid-filled capsule that contain a large, calcareous statolith and several smaller concretions. The epithelium of the statocyst is composed of 10 ciliated sensory cells (hair cells) and 11 accessory cells. The latter group stains darkly and includes 2 microvillous cells, 7 supporting cells, and 2 glial cells. The hair cells stain lightly and each gives rise to an axon; two types can be distinguished. The first type, in which a minimum of 3 cilia are randomly positioned on the apical cell membrane, is restricted to the upper portion of the statocyst. The second type, in which 9 to 11 cilia are arranged in a slightly curved row, is found exclusively around the base of the statocyst. Each statocyst is connected dorso-laterally to the ipsilateral cerebral ganglion by a short static nerve, formed by axons arising from the hair cells. Ganglionic neurons synapse with these axons as the static nerve enters the cerebral ganglion. The lumen of the statocyst is continuous with a blind constricted canal located beneath the static nerve.A diagram showing the structure of the statocyst and its association with the nervous system is presented. Possible functions of the statocyst in relation to larval behavior are discussed.     

Free clathrin triskelions are required for the stability of clathrin-associated adaptor protein (AP-2) coated pit nucleation sites.

In this study image correlation spectroscopy was used to demonstrate the presence of two populations of clathrin in situ, on intact cells. In the periphery of the cell approximately 35% of the clathrin triskelions are free within the cytosol while approximately 65% are in large aggregates, presumably coated pits. Although endocytosis is inhibited at low temperature, free clathrin triskelions are still present and small AP-2 aggregates (of approximately 20 proteins), or coated pit nucleation sites, are still observed. Following hypertonic treatment, or cytoplasmic acidification, free clathrin triskelions within the cytosol are depleted and all of the clathrin becomes associated with the membrane. Under these conditions coated pit associated AP-2 remains while the smaller AP-2 aggregates, or coated pit nucleation sites, dissociate. This indicates that the stabilization of AP-2 coated pit nucleation sites requires the presence of free clathrin triskelions within the cytosol. Furthermore, this indicates that free clathrin is required for the early stages of coated pit formation and presumably the continuation of the clathrin-mediated endocytic process. We also provide indirect evidence that AP-2 binding to the membrane in coated pit nucleation sites may be regulated in part by binding to internalization-competent membrane receptors.     

A Fine Structural Analysis of the Statocyst in Turbellaria Acoela

Ferrero, E. (Istituto di Biologia Generale, Universith di Pisa, Pisa, Italy.) A fine structural analysis of the statocyst in Turbellaria Acoela. Zool. Scr. 2 (1): 5–16, 1973.—The fine structure of the statocyst components in the acoelan Convoluta psammophila is described, namely: capsule, parietal cells, lithocyte, and the statolith. The absence of ciliary structures, the highly developed endoplasmic reticulum of the lithocyte and the layered texture of the statolith are remarkable. A functional interpretation of the muscles inserted on the statocyst and of the nerve bundle running nearby is suggested. The morphological similarities and differences between the acoelan statocyst and the statocyst of lower and higher invertebrate phyla are discussed.     

Some aspects of the fine structure of the statocysts of the molluscsPecten andPterotrachea

Summary The statocyst ofPecten is composed of hair cells and supporting cells. The hair cells bear kinocilia and microvilli at their distal ends and the supporting cells bear microvilli. The cilia have a 9+2 internal filament content, and arise from basal bodies that have roots, basal feet and microtubular connections. Two different ciliary arrangements are described, one with a small number of cilia arranged in a ring, and another with many more cilia arranged in rows. Below the hair cells are probable synapses. A ciliated duct connects to the lumen of the static sac and passes through the centre of the static nerve. The hair cells in the statocyst ofPterotrachea bear kinocilia and microvilli. The possible importance of cilia and microvilli in the transduction process is discussed.We would like to thank ProfessorJ. Z. Young for bringing specimens ofPterotrachea from Naples and also the staff of the Stazione Zoologica for the provision of specimens, Dr.M. Land for providing specimens ofPecten, the Science Research Council (U.K.) for providing the electron microscope used in much of the study and also for a grant to one of us (V.C.B.), and Mrs.J. Parkers and Mr.R. Moss and Mrs.J. Hamilton for much photographic and technical assistance.     

The structure of some cephalopod statoliths

Summary The statoliths of Sepia officinalis, Octopus vulgaris, Alloteuthis subulata and Taonius megalops have a smooth outline, but an irregular shape. They have projections and indentations. The statoliths from a pair of statocysts are usually quite similar in size and shape, and the general pattern is probably maintained throughout the size range of the species. Statoliths from large animals are marginally larger than those from smaller ones. The statolith usually occupies only a small part of the cavity of the statocyst, and it is situated in the anterior part of the statocyst. They are joined to the macula by hairs extending from it. These hairs are very delicate and easily broken during preparation of the specimens. The hairs are much longer and narrower than the receptor cilia of the macula. The receptor cilia are enclosed within holes in the tangled hairlike anchoring fibrils.The statolith is made up of crystalline subunits, the statoconia. The crystals vary in size, they are usually elongated, hexagonal with pointed ends. The statolith consists of a closely packed mass of these crystals, sometimes they are irregularly arranged, where in others they are stacked with their long axes parallel. In Sepia officinalis and Taonius megalops, the crystals are arranged in regular shaped packets and these packets of crystals are stacked together. These larger subunits are not always arranged in a regular way, and their major axes can be organised in several different ways. The size and outline of these large subunits do vary in different parts of the statolith.The external surface of the statolith is macroscopically smooth. Over some parts there is a surface layer covering the rod-like crystals that make up the major bulk of the stone. In other regions, the surface is rough at a microscopic level, the roughness is produced by the exposed ends of the filamentous crystals. The crystals are composed of calcium carbonate in the form of aragonite.I wish to thank Professor J.Z. Young, FRS, for considerable help, advice and encouragement throughout this study. Dr. A. Boyde generously allowed me to use his scanning electron microscope and gave freely of his expertise and time. Dr. J. Fitch kindly gave me some fossil statoliths and Dr. J. Elliott examined them with his x-ray diffraction apparatus. Dr. Marion Nixon helped me to collect and prepare the specimens. Mrs. E. Bailey, Miss P. Stephens and Mr. R. Moss provided the expert technical assistance     

Effects of leg movements on the synaptic activity of descending statocyst interneurons in crayfish,Procambarus clarkii

Crustacean postural control is modulated by behavioral condition. In this study, we investigated how the responses of descending statocyst interneurons were affected during leg movements. Intracellular recording was made from an animal whose statoliths had been replaced with ferrite grains so that statocyst receptors could be activated by magnetic field stimulation. We identified 14 morphological types of statocyst-driven descending interneurons. Statocyst-driven descending interneurons always showed an excitatory response to statocyst stimulation on either ipsilateral or contralateral side to the axon. The response of each statocyst-driven descending interneuron to statocyst stimulation was differently modulated by leg movements in different conditions. During active leg movements, six statocyst-driven descending interneurons were activated regardless of whether a substrate was provided or not. In other two statocyst-driven descending interneurons, the excitatory input during leg movements was stronger when a substrate was provided than when it was not. One statocyst-driven descending interneuron received an excitatory input only during leg movements on a substrate, whereas another statocyst-driven descending interneuron did not receive any input during leg movements both on a substrate and in the air. These results suggest that the descending statocyst pathways are organized in parallel, each cell affected differently by behavioral conditions.Abbreviations EMG electromyogram - NGI nonspiking giant interneuron - SDI statocyst-driven descending interneuron     

Functional significance of protein assemblies predicted by the crystal structure of the restriction endonuclease BsaWI

Type II restriction endonuclease BsaWI recognizes a degenerated sequence 5′-W/CCGGW-3′ (W stands for A or T, ‘/’ denotes the cleavage site). It belongs to a large family of restriction enzymes that contain a conserved CCGG tetranucleotide in their target sites. These enzymes are arranged as dimers or tetramers, and require binding of one, two or three DNA targets for their optimal catalytic activity. Here, we present a crystal structure and biochemical characterization of the restriction endonuclease BsaWI. BsaWI is arranged as an ‘open’ configuration dimer and binds a single DNA copy through a minor groove contacts. In the crystal primary BsaWI dimers form an indefinite linear chain via the C-terminal domain contacts implying possible higher order aggregates. We show that in solution BsaWI protein exists in a dimer-tetramer-oligomer equilibrium, but in the presence of specific DNA forms a tetramer bound to two target sites. Site-directed mutagenesis and kinetic experiments show that BsaWI is active as a tetramer and requires two target sites for optimal activity. We propose BsaWI mechanism that shares common features both with dimeric Ecl18kI/SgrAI and bona fide tetrameric NgoMIV/SfiI enzymes.     

Comparative morphology of statocysts in the Plathelminthes and the Xenoturbellida

The general fine-structural organization of statocysts in Catenulida, Nemertodermatida, Acoela, Proseriata, Lurus (Dalyellioida), and Xenoturbella are summarized. In lithophorous (statocyst-bearing) members of the Catenulida, the statocysts exhibit a few parietal cells and one or several movable statoliths within a spacious intracapsular cavity. Statocysts in the Nemertodermatida have several parietal cells and two lithocytes, each equipped with one statolith, whereas those of the other acoelomorphan taxon, the Acoela, always have two parietal cells and one movable lithocyte. The statocysts of lithophorous members of the Proseriata represent more sophisticated systems: each has two clusters of accessory cells in addition to several parietal cells and a voluminous lithocyte in which the statolith is movable. In catenulids and proseriates, processes of outer neurons penetrate the capsule of the statocyst, whereas such innervations have not been found in the Nemertodermatida and Acoela. I conclude that the different types of statocysts have evolved independently within the Plathelminthes. Xenoturbella displays an intraepidermal statocyst with many monociliary parietal cells and several mobile cells (lithocytes) within the central cavity of the statocyst. Each of these mobile cells carries a statolith-like structure and one prominent cilium. The statocyst of Xenoturbella does not correspond to any type of plathelminth statocyst.     

Efferent activity in theOctopus statocyst nerves

Summary Single unit electrophysiological recordings were obtained from efferent fibres in the statocyst nerves ofOctopus vulgaris. A preparation comprising the CNS and a single statocyst was employed. 42% of the efferents displayed a level of resting activity; transient changes in this activity occurred at irregular intervals.The responses of the efferent units were examined during sinusoidal oscillations of the statocyst at stimulus frequencies between 0.01–1 Hz, and amplitudes up to 35°. 84% of the units showed activity synchronised with the imposed oscillations; the time taken to establish this response varied for different units (Fig. 1).The lowest stimulus frequency at which a unit could be entrained varied for different units, with those units with a resting level of activity having the lowest thresholds. The peak firing frequency of the efferents was found to increase with increasing stimulus frequency or amplitude (Fig. 3). However, the change in firing frequency was much smaller than that reported for the statocyst afferents to similar stimuli.The efferent units of the posterior crista nerve were found to respond to clockwise or anticlockwise rotations (Fig. 4), with the individual units having unipolar responses. The phase response of the units changed little with increasing stimulus amplitude but an increase in phase lag occurred with an increase in the stimulus frequency (Fig. 5). The form of this relationship (Fig. 6) was similar to that reported for the statocyst crista afferents.The principal source of the input to the efferents in these experiments was shown to be afferents from the contralateral statocyst. These results are discussed and compared with data from the vertebrate semicircular canal system.     

Interaction of an Fe derivative of TMAP (Fe(TMAP)OAc) with DNA in comparison with free-base TMAP

We investigated the interaction of meso-tetrakis (N-para-methylanilium) porphyrin (TMAP) in its free base and Fe(II) form (Fe(TMAP)OAc) as a new derivative, with high molecular weight DNA at different ionic strengths, using various spectroscopic methods and microcalorimetry. The data obtained by spectrophotometery, circular dichroism (CD), fluorescence quenching and resonance light scattering (RLS) have demonstrated that TMAP association with DNA is via outside binding with self-stacking manner, which is accompanied with the "end-on" type complex formation in low ionic strength. However, in the case of Fe(TMAP)OAc, predominant mode of interaction is groove binding and after increasing in DNA concentration, unstable stacking-type aggregates are formed. In addition, isothermal titration calorimetric measurements have indicated the exothermic process of porphyrins binding to DNA, but the exothermisity in metal derivative of porphyrin is less than the free base. It confirmed the formation of a more organized aggregate of TMAP on DNA surface. Interactions of both porphyrins with DNA show high sensitivity to ionic strength. By addition of salt, the downfield CD signal of TMAP aggregates is shifted to a higher wavelength, which indicates some changes in the aggregates position. In the case of Fe(TMAP)OAc, addition of salt leads to changes in the mode of binding from groove binding to outside binding with self-stacking, which is accompanied with major changes in CD spectra, possibly indicating the formation of "face-on" type complex.     

Modification of statocyst input to local interneurons by behavioral condition in the crayfish brain

Posture control by statocysts is affected by leg condition in decapod crustaceans. We investigated how, in the crayfish brain, the synaptic response of local interneurons to statocyst stimulation was affected by leg movements on and off a substratum. The magnetic field stimulation method permitted sustained stimulation of statocyst receptors by mimicking body rolling. The statocyst-driven local interneurons were classified into four morphological groups (Type-I–IV). All interneurons except Type-IV projected their dendritic branches to the parolfactory lobe of the deutocerebrum where statocyst afferents project directly. Type-I interneurons having somata in the ventral-paired lateral cluster responded invariably to statocyst stimulation regardless of the leg condition, whereas others having somata in the ventral-unpaired posterior cluster showed response enhancement or suppression, depending on the cell, during leg movements on a substratum, but no response change during free leg movements off the substratum. The synaptic responses of Type-II and IV interneurons were also affected differently by leg movements depending on the substratum condition, whereas those of Type-III remained unaffected. These findings suggest that the statocyst pathway in the crayfish brain is organized in parallel with local circuits that are affected by leg condition and those not affected.     

Ontogeny, anatomy and attachment of the dentition in mosasaurs (Mosasauridae: Squamata)

Aspects of mosasaur dental ontogeny are well preserved in many fossils of these giant marine squamates. Replacement teeth on the tooth-bearing elements (TBEs) first appear as small enamel crowns positioned posterolingual to the attached tooth (posterolabial for the pterygoid). Several developing crowns, of progressively larger size, are aligned in rows relating to a specific tooth position. The crowns rest in a dental groove that varies in width and depth depending on the TBE. The crown closest to the attached tooth is always the largest and is found in a small resorption pit. As resorption proceeds, the pit expands in volume (cementum and alveolar bone), and the crown increases in size and settles into the pit. Once mature crown size is achieved, the dentine root and cementum portion of the root develop rapidly, the attached tooth is lost and the replacement tooth erupts out of the alveolus. Mosasaurid teeth develop along a 'zig-zag'-shaped movement path: horizontally along the dental groove, down into the alveolus, and up and out of the alveolus prior to attachment to the alveolar wall. At no point in mosasaurid tooth development are the crowns observed in a horizontal position. The mosasaurid dental lamina appears to have been a continuous strip of dental epithelium as it is in other squamates. Mosasaurid tooth attachment is thecodont (histologically and geometrically) not subpleurodont. Most aspects of mosasaurid tooth attachment and ontogeny are autapomorphic for the group.  © 2007 The Linnean Society of London, Zoological Journal of the Linnean Society , 2007, 149 , 687–700.     

The structure and growth of the statocyst in the Australian crayfish Cherax destructor

The morphology of the statocyst of the Australian crayfish Cherax destructor was examined using scanning electron microscopy. It resembles in general structure, size, and position the statocysts of crayfish described previously, and the size and distribution of the fields of setae on the floor of the capsule are similar but not the same. Over the size range examined, the relationship between the carapace length, the length of the basal antennular segment, the diameter of the statocyst capsule, and the total number of setae are all linear. The number and position of setae on the floor of the statocyst capsule were mapped for animals in two size classes (small, ca. 20 mm; large, ca. 50 mm) to test for changes in their arrangement during growth. The change in the ratio of setal number to statocyst size between the two size classes was about three times greater for the anterior setal field than for the other fields. We propose that differential development of the setal fields may be related to changes in the force-monitoring requirements of the animals as they increase in size, but this remains to be experimentally tested.     

The comparative morphology of pit organs in elasmobranchs

The pit organs of elasmobranchs (sharks, skates and rays) are free neuromasts of the mechanosensory lateral line system. Pit organs, however, appear to have some structural differences from the free neuromasts of bony fishes and amphibians. In this study, the morphology of pit organs was investigated by scanning electron microscopy in six shark and three ray species. In each species, pit organs contained typical lateral line hair cells with apical stereovilli of different lengths arranged in an “organ‐pipe” configuration. Supporting cells also bore numerous apical microvilli taller than those observed in other vertebrate lateral line organs. Pit organs were either covered by overlapping denticles, located in open grooves bordered by denticles, or in grooves without associated denticles. The possible functional implications of these morphological features, including modification of water flow and sensory filtering properties, are discussed. J. Morphol. 2009. © 2009 Wiley‐Liss, Inc.