Structure and development of free neuromasts in barramundi, Lates calcarifer (Block)

This study was conducted to clarify the development of free neuromasts with growth of the barramundi, Lates calcarifer. A pair of free neuromasts was observed behind the unpigmented eyes in newly hatched eleutheroembryos with a mean total length of 1.93 mm, and two-hour-old eleuthero-embryos could respond to an approaching pipette. At 2 days after hatching, the egg yolk sac was mostly consumed, the eyes were pigmented, and the larvae commenced feeding on rotifers. Free neuromasts increased in number with growth and commenced developing into canal neuromasts in barramundi 15 days old with a mean total length of 8.07 mm. The average length of the major axis of the trunk free neuromasts attained approximately 12.9-15.5 microm, and the number of sensory cells was 15.4-17.5 at 15-20 days old. Developed cupulae of free neuromasts were observed in 1-day-old eleutheroembryos. The direction of maximum sensitivity of free neuromasts, determined from the polarity of the sensory cells, coincided with the minor axis of the lozenge-shaped outline of the apical surface of the free neuromasts. The polarity of trunk neuromasts was usually oriented along the antero-posterior axis of the fish body, but a few had a dorso-ventral direction. On the head, free neuromasts were oriented on lines tangential to concentric circles around the eye.     

Development of sensory organs in larvae of African catfish Clarias gariepinus

African catfish Clarias gariepinus hatched with morphologically immature features; however, sensory organs developed rapidly with fish growth. Although the eyes of newly hatched larvae were immature without pigment, in 2 day‐old larvae, the retina of the eyes had already developed except for the rod cells. No free neuromasts were observed in newly hatched larvae. In 1 day‐old larvae, however, free neuromasts were observed on the head and trunk. Free neuromasts increased with larval growth. Newly hatched larvae had simple round‐shaped otic vesicles; however, all sensory epithelia of the inner ear were observed until the larvae were 3 days old. Two day‐old larvae swam horizontally, had sharp teeth, commenced ingesting rotifers and also artificial feed (small‐size pellets) under both light and dark conditions; by then the larvae already had many taste buds. Three day‐old larvae showed negative phototaxis and cannibalism by eating their conspecifics. Most of the free neuromasts observed in this study had the peculiar feature of many microvilli around the sensory cells on the apical surface. Detected free neuromasts as ordinary type lateral‐line organs were not observed in previous reports in teleosts. In 10 day‐old larvae, there were two lines of free neuromasts on the flank and lower edge of the trunk; presumptive canal neuromasts were oval shaped and had begun to sink under the skin. The direction of maximum sensitivity of the neuromasts was parallel with the longitudinal axis of their elliptical apical surface.     

Morphogenesis of free neuromasts in the larvae of brown-marbled grouper Epinephelus fuscoguttatus

Newly hatched larvae had one pair of free neuromasts behind the eyes. As the larvae grew, free neuromasts increased in number. The apical surface of sensory epithelium widened and subsequently elongated. The number of sensory hair cells increased and the directions of maximum sensitivity became both anteroposterior and dorsoventral on the trunk. Before notochord flexion, only the anteroposterior type was observed. After notochord flexion, two types of neuromasts were observed on the trunk. On the head, the orientation of free neuromasts formed a tangential line to concentric circles around the eyes and nostrils. Free neuromasts on the head could therefore receive stimuli from various angles from predators or zooplanktons. This suggests that these free neuromasts play a role in compensating for a dead angle of vision, and an important role in detecting zooplankton under scotopic vision. Canal organs were observed on the head and operculum in 40-d-old animals.     

Development of lateral line organs in leptocephali of the freshwater eel Anguilla japonica (Teleostei,Anguilliformes)

A study of the ontogeny of the lateral line system in leptocephali of the Japanese eel Anguilla japonica reveals the existence of three morphologically different types of lateral line organs. Type I is a novel sensory organ with hair cells bearing a single kinocilium, lacking stereocilia, distributed mainly on the head of larvae, and morphologically different from typical superficial neuromasts of the lateral line system. Its developmental sequence suggests that it may be a presumptive canal neuromast. Type II is an ordinary superficial neuromast, common in other teleost larvae, which includes presumptive canal neuromasts that first appear on the trunk and accessory superficial neuromasts that later appear on the head and trunk. Type III is a very unusual neuromast located just behind the orbit, close to the otic vesicle, with radially oriented hair cells, suggesting that these serve as multiple axes of sensitivity for mechanical stimuli. The behavior of larval eels suggests that the radially oriented neuromasts may act as the sole mechanosensory organ until the ordinary superficial neuromasts develop. The finding that larval eels possess a well-developed mechanosensory system suggests the possibility that they are also capable of perceiving weak environmental mechanical stimuli, like other teleost larvae.     

Development of free and canal neuromasts and their directions of maximum sensitivity in the larvae of ayu,Plecoglossus altivelis

Morphological changes in free neuromasts are reported from larvae of the Ayu,Plecoglossus altivelis. In newly-hatched larvae, free neuromasts were already recognizable in both the head and trunk. During larval growth, the number of free neuromasts increased, and the number of its sensory cells 2 days after hatching was constant. In the trunk, two types of free neuromasts, one with maximum sensitivity in the antero-posterior direction and the other with maximum sensitivity in the dorso-ventral direction, were observed. The former type predominated. In the head, free neuromasts were located around the eye and nose, their directions of maximum sensitivity forming lines tangential to concentric circles about the eye and nose. Distinct changes in free neuromasts occurred during the formation of the canal organ. The canal organ was first observed in the head region 64 days after hatching and in the trunk region 100 days after hatching. Concomitant with the formation of the canal organ, the profile of the cupulae of the free neuromasts changed from a flat bar to semispherical. Sensory cells in the canal neuromasts did not differ morphologically from those in the free neuromasts. It is considered that there is a close relationship between the sensitivity of the neuromast and the shape of the cupula, i.e., that the free neuromasts are adapted to slow water flow, as in lakes and the sea, while the neuromasts in the canal organ are adapted to rapid water flow.     

Organization of the superficial neuromast system in goldfish, Carassius auratus

Distribution, morphology, and orientation of superficial neuromasts and polarization of the hair cells within superficial neuromasts of the goldfish (Carassius auratus) were examined using fluorescence labeling and scanning electron microscopy. On each body side, goldfish have 1,800-2,000 superficial neuromasts distributed across the head, trunk and tail fin. Each superficial neuromast had about 14-32 hair cells that were arranged in the sensory epithelium with the axis of best sensitivity aligned perpendicular to the long axis of the neuromast. Hair cell polarization was rostro-caudal in most superficial neuromasts on trunk scales (with the exception of those on the lateral line scales), or on the tail fin. On lateral line scales, the most frequent hair cell polarization was dorso-ventral in 45% and rostro-caudal in 20% of the superficial neuromasts. On individual trunk scales, superficial neuromasts were organized in rows which in most scales showed similar orientations with angle deviations smaller than 45 degrees . In about 16% of all trunk scales, groups of superficial neuromasts in the dorsal and ventral half of the scale were oriented orthogonal to each other. On the head, most superficial neuromasts were arranged in rows or groups of similar orientation with angle deviations smaller than 45 degrees . Neighboring groups of superficial neuromasts could differ with respect to their orientation. The most frequent hair cell polarization was dorso-ventral in front of the eyes and on the ventral mandible and rostro-caudal below the eye and on the operculum.     

Directional cell migration establishes the axes of planar polarity in the posterior lateral-line organ of the zebrafish

The proper orientation of mechanosensory hair cells along the lateral-line organ of a fish or amphibian is essential for the animal's ability to sense directional water movements. Within the sensory epithelium, hair cells are polarized in a stereotyped manner, but the mechanisms that control their alignment relative to the body axes are unknown. We have found, however, that neuromasts can be oriented either parallel or perpendicular to the anteroposterior body axis. By characterizing the strauss mutant zebrafish line and by tracking labeled cells, we have demonstrated that neuromasts of these two orientations originate from, respectively, the first and second primordia. Furthermore, altering the migratory pathway of a primordium reorients a neuromast's axis of planar polarity. We propose that the global orientation of hair cells relative to the body axes is established through an interaction between directional movement by primordial cells and the timing of neuromast maturation.     

Development of the lateral line system in the sea bass

Using light and electron microscopy, a study of the development of the lateral line system of the sea bass Dicentrarchus labrax , from embryo to adult, revealed that the first free neuromasts appeared on the head shortly before hatching and multiplied during the larval stage. They were aligned on the head and trunk in a pattern which corresponded to the location of future canals. The transition to the juvenile stage marked the start of important anatomical changes during which head and trunk canals were formed successively. Neuromasts, with a cupula and consisting of standard sensory cells and supporting cells, were characterized by bidirectional polarity. The exact location of the first neuromast formed in the embryo was identified and its differentiation monitored from primordium to eruption. This neuromast was distinguishable from the others by its radial polarity. Correlations were made between the development of the lateral line system and the behaviour of the sea bass.     

Metamorphosis-related changes in the lateral line system of lampreys, <Emphasis Type="Italic">Petromyzon marinus</Emphasis>

Lamprey metamorphosis leads to considerable changes in morphology and behavior. We have recently reported that larval lampreys possess a functional lateral line system. Here we investigated metamorphic morphological changes in the lateral line system using light and electron microscopy. Functional modifications were studied by recording the trunk lateral line nerve activity of larvae and adults while stimulating neuromasts with approximately sinusoidal water motion. We found a general re-patterning of neuromasts on the head and trunk including an increase in numbers, redistribution within the pit lines, and shifts of the pit lines relative to external features. The trunk lateral line nerve response was qualitatively similar in adults and larvae. Both showed two neuronal populations responding to opposite directions of water flow. Magnitude of the response increased monotonically with stimulus amplitude. At low frequencies, the response lag relative to the stimulus maximum was approximately 220°, and the gain depended approximately linearly on frequency, confirming that superficial neuromasts are velocity detectors. Changes in phase lag with increasing stimulus frequency were steeper in larvae, suggesting slower afferent conductance. The response gain with frequency was smaller for adults, suggesting a narrower frequency discrimination range and decreased sensitivity. These changes may be adaptations for the active lifestyle of adult lampreys.     

Rheotaxis in larval zebrafish is mediated by lateral line mechanosensory hair cells

The lateral line sensory system, found in fish and amphibians, is used in prey detection, predator avoidance and schooling behavior. This system includes cell clusters, called superficial neuromasts, located on the surface of head and trunk of developing larvae. Mechanosensory hair cells in the center of each neuromast respond to disturbances in the water and convey information to the brain via the lateral line ganglia. The convenient location of mechanosensory hair cells on the body surface has made the lateral line a valuable system in which to study hair cell damage and regeneration. One way to measure hair cell survival and recovery is to assay behaviors that depend on their function. We built a system in which orientation against constant water flow, positive rheotaxis, can be quantitatively assessed. We found that zebrafish larvae perform positive rheotaxis and that, similar to adult fish, larvae use both visual and lateral line input to perform this behavior. Disruption or damage of hair cells in the absence of vision leads to a marked decrease in rheotaxis that recovers upon hair cell repair or regeneration.     

Sensory structures at the surface of fish skin. II. Lateralis System

Scanning electron microscopy shows the form of the cupulae of free neuromasts in two species of teleost fish, and gives information about the organization of the free neuromasts in teleosts and lampreys. In lampreys some neuromasts were found to lack the surrounding moat and the flanking hillocks characteristic of the lateral line organs previously described in these fish. In all cases, the sensory cells had the kinocilium aligned with respect to the stereocilia on the longer axis of the neuromast surface, thus enabling the direction of effective stimulation of the free neuromasts to be deduced from their morphological arrangement.     

Sensory development and concurrent behavioural changes in Atlantic croaker larvae

Development of the visual and mechanosensory systems of Atlantic croaker larvae was examined and compared to behavioural performance in order to determine sensory system functionality. Early larvae were characterized by a photopic visual system with a high density of cone photoreceptors, but no rods or photoreceptor summation on to higher order cells. Acuity of small larvae was comparable to other species. The mechanosensory system of early larvae consisted of a relatively high density of free neuromasts on the head and body. By approximately 10 mm total length ( L _T), many sensory attributes of larvae had changed. Visual acuity had improved to levels slightly better than in other species. Rods began forming and summation ratios increased, indicating development of scotopic vision. Cephalic lateral-line canals began to enclose at 15 mm L _T and were complete by 24 mm. The trunk lateral-line began to enclose at 30 mm and was complete by 40 mm. Behavioural performance was tested using an artificial predatory stimulus under different conditions to isolate the roles of vision and mechanoreception. Both sensory systems were necessary to elicit the highest levels of responsiveness. Responsiveness of larvae <6mm was equally influenced by visual and neuromast input, while responsiveness of larvae > 6 mm was largely influenced by visual input. Both vision and mechanoreception influenced ontogenetic changes in reactive distance of larvae. When there was no visual input, reactive distances increased slightly through ontogeny, indicating the use of mechanoreception. Only when mechanoreception was blocked, the ontogenetic increase in reactive distances was greater than that of larvae with all sensory systems intact. These results indicate that Atlantic croaker larvae use both sensory systems throughout the larval period, but the relative importance of these sensory systems changes with ontogeny.     

A default mechanism of spindle orientation based on cell shape is sufficient to generate cell fate diversity in polarised Xenopus blastomeres

The process of oriented divisions of polarised cells is a recurrent mechanism of cell fate diversification in development. It is commonly assumed that a specialised mechanism of spindle alignment into the axis of polarity is a prerequisite for such systems to generate cell fate diversity. Oriented divisions also take place in the frog blastula, where orientation of the spindle into the apicobasal axis of polarised blastomeres generates inner and outer cells with different fates. Here, we show that, in this system, the spindle orients according to the shape of the cells, a mechanism often thought to be a default. We show that in the embryo, fatedifferentiative, perpendicular divisions correlate with a perpendicular long axis and a small apical surface, but the long axis rather then the size of the apical domain defines the division orientation. Mitotic spindles in rounded, yet polarised, isolated Xenopus blastula cells orient randomly, but align into an experimentally introduced long axis when cells are deformed early in the cell cycle. Unlike other systems of oriented divisions, the spindle aligns at prophase, rotation behaviour is rare and restricted to small angle adjustments. Disruption of astral microtubules leads to misalignment of the spindle. These results show that a mechanism of spindle orientation that depends on cell shape rather than cortical polarity can nevertheless generate cell fate diversity from a population of polarised cells.     

Surface and subsurface structures of neuromasts in tadpoles of the crab-eating frog, Rana cancrivora

Neuromast structure in Rana cancrivora larvae was observed by scanning and transmission electron microscopy. Neuromast units, each being composed of two or three neuromasts, are arranged in several well-defined lines in the head, body, and tail regions. The structure of neuromasts in these three regions is basically identical. The neuromast is composed of sensory, sustentacular, and mantle cells. The top of each neuromast has a hillocklike appearance, and is surrounded by four to six epidermal cells with tight intercellular junctions. Long kinocilia and many stereocilia occur in the apex of the neuromasts and are surrounded by numerous microvilli. Numerous granules are present on the apical portions of the mantle and the sustentacular cells. Four or five trapeziform mantle cells are connected closely with each other to form the shell of the neuromast. Large intercellular spaces occur between the mantle cells and the cells of the inner epidermal layers, and between the cells of the inner epidermal layer. Thus, at the apical parts of the neuromast intercellular junctions are tight and the intercellular spaces are more dilated in more basal areas. Morphologically the neuromasts of R. cancrivora larvae resemble those of generalized pond anurans, based on the grouping of Lannoo (Journal of Morphology 191:115-129, 1987a), although larvae of this species inhabit brackish water.     

Afferent Neurons of the Zebrafish Lateral Line Are Strict Selectors of Hair-Cell Orientation

Hair cells in the inner ear display a characteristic polarization of their apical stereocilia across the plane of the sensory epithelium. This planar orientation allows coherent transduction of mechanical stimuli because the axis of morphological polarity of the stereocilia corresponds to the direction of excitability of the hair cells. Neuromasts of the lateral line in fishes and amphibians form two intermingled populations of hair cells oriented at 180° relative to each other, however, creating a stimulus-polarity ambiguity. Therefore, it is unknown how these animals resolve the vectorial component of a mechanical stimulus. Using genetic mosaics and live imaging in transgenic zebrafish to visualize hair cells and neurons at single-cell resolution, we show that lateral-line afferents can recognize the planar polarization of hair cells. Each neuron forms synapses with hair cells of identical orientation to divide the neuromast into functional planar-polarity compartments. We also show that afferent neurons are strict selectors of polarity that can re-establish synapses with identically oriented targets during hair-cell regeneration. Our results provide the anatomical bases for the physiological models of signal-polarity resolution by the lateral line.     

The lateral line system in larvalIchthyophis (Amphibia: Gymnophiona)

Summary The lateral line systems of larval caecilians of the genusIchthyophis possess two types of elements, free neuromasts and ampullary organs. Free mechanoreceptive neuromasts are typical of those found in other vertebrates, and are arranged in series roughly homologous to neuromast groups in many other fishes and amphibians. In contrast to other amphibians,Ichthyophis larvae possess only one paired, dorsal body series of neuromasts. Regional specialization of neuromasts is evident inIchthyophis. Premaxillary and anterior head neuromasts are the largest in size and total cell number. Overall, size and total cell numbers are correlated with depth of epidermis. Neuromasts on the anterior sides of the head occur in slight grooves and have apical tips situated farther below the level of the epidermis and with greater apical indentation. These features probably provide increased protection against abrasion. Apparently abnormal neuromasts are frequently found among the neuromast series. Such neuromasts contain fewer cells that lack normal apical extension, producing a sunken effect similar to that of the ampullary organ elements. The ampullary organs ofIchthyophis are morphologically similar to those found in various freshwater fishes and known to function as electroreceptors. These organs are not observed in the lateral line systems of members of other amphibian orders (Urodela and Anura), and we suggest that they function as electroreceptors. The sunken neuromasts of theIchthyophis lateral line system may parallel the possible evolutionary development of pit organs from normal neuromasts.     

Morphological development of larval cobia Rachycentron canadum and the influence of dietary taurine supplementation

The morphological development of larval cobia Rachycentron canadum from 3 days post hatch (dph) until weaning (27 dph) was examined using S.E.M. Two groups of fish were studied: a control group (CF), reared under standard feeding protocol, and a group in which prey items were enriched with supplemental taurine (4 g l(-1) day(-1) ; TF). TF fish grew faster (P < 0·001), attained greater size (mean ±s.e. 55·1 ± 1·5 v. 33·9 ± 1·0 mm total length) and had better survival (mean ±s.e. 29·3 ± 0·4 v. 7·1 ± 1·2 %) than CF fish. Canonical variance analysis confirmed findings with respect to differences in growth between the treatment groups with separation being explained by two cranial measurements. S.E.M. revealed that 3 dph larvae of R. canadum (in both groups) possess preopercular spines, superficial neuromasts on the head and body, taste buds in the mouth, an olfactory epithelium which takes the form of simple concave depressions, and primordial gill arches. Gill filaments start to form as early as 6 dph and lamellae buds are visible at 8 dph in both groups. In CF fish, the cephalic lateral line system continues its development at 12-14 dph with invagination of both supra- and infraorbital canals. At the same time, a thorn-like or acanthoid crest forms above the eye. At 14 dph, invaginations of the mandibular and preopercular canals are visible and around 22 dph enclosure of all cranial canals nears completion. In CF larvae, however, completely enclosed cranial canals were not observed within the course of the trial, i.e. 27 dph. In TF larvae, grooves of the cephalic lateral line system form 4 days earlier than observed in CF larvae of R. canadum (i.e. at 8 dph), with enclosure commencing at 16 dph, and completed by 27 dph. Along the flanks of 6 dph larvae of either treatment, four to five equally spaced neuromasts delineate the future position of the trunk lateral line. As myomeres are added to the growing larvae, new neuromasts appear such that at 16 dph a neuromast is associated with each myomere. By 27 dph, the trunk lateral line starts to invaginate in CF larvae, while it initiates closure in TF larvae. These findings elucidate important features of the larval development of R. canadum and show that dietary taurine supplementation benefits larval development, growth and survival in this species. Moreover, they suggest a conditional requirement for taurine in larval R. canadum.     

Larval ontogeny and morphology of the fire-tailed gudgeon, Hypseleotris galii (Eleotridae)

The larval ontogeny of Hypseleotris galii is described and illustrated. 'Premature' yolk sac larvae hatch with unpigmented eyes and no mouth. 'Late' yolk sac larvae hatch with pigmented eyes and a functional mouth. Hatching glands are distributed on the head and ventral surface of the body. The yolk is absorbed and the larvae begin feeding 6 days after hatching. Larval development is completed 74 days after hatching. Hypseleotris galii larvae have six pairs of naked neuromasts: two pairs on the head and four pairs on the body. The significance of these results to developmental strategies in Hypseleotris species is discussed.     

Neuromast formation in the prehatching embryos of the Japanese flounder (Paralichthys olivaceus)

The present paper clarifies the initial development of the lateral line organs in the embryonic Japanese flounder, Paralichthys olivaceus. The first appearances of lateral line primordia, and the proliferation, distribution and morphological development of the free neuromasts, including nerve ending formation: establishment of hair cell innervations via the formation of synapses, were examined by light microscopy, scanning and transmission electron microscopy. The first pair of neuromast primordia appeared in the otic region ≈ 30 h prior to hatching and subsequently differentiated into free neuromasts, otic neuromasts, after ≈ 8 h. At hatching, a pair of free neuromasts and three pairs of neuromast primordia were present on the head, and three pairs of neuromast primordia were present on the trunk. The hair cell polarity of the otic neuromast until just prior to hatching was radial, but not bi‐directional. The typical afferent and efferent nerve endings in the otic neuromasts had formed by the time of hatching, suggesting that the otic neuromasts are functional prior to hatching. The three neuromast primordia located on each side of the trunk were derived from a long, narrow ectodermal cell cluster and erupted through the epidermis after hatching.     

Larval development of Brachiopod <Emphasis Type="Italic">Coptothyris grayi</Emphasis> (Davidson, 1852) (Brachiopoda,Rhynchonelliformea)

The larval development of the Brachiopod Coptothyris grayi (Davidson, 1852) from the Sea of Japan is described for the first time. Ciliated blastula proved to represent the first free-swimming stage. The blastopore is initially formed as a rounded hole stretching later along the anteroposterior axis. The larva is first divided into two lobes (the apical lobe and the trunk); the mantle lobe is formed later as two lateral folds. Two pairs of seta bundles appear in the late stage larvae. The apical larval lobe in brachiopods is supposed to match the pre-oral lobe and anterior part of the trunk with tentacles in phoronids.