The 'goatee' of goatfish: innervation of taste buds in the barbels and their representation in the brain

Goatfish use a pair of large chin barbels to probe the sea bottom to detect buried prey. The barbels are studded with taste buds but little else is known about the neural organization of this system. We found that the taste buds of the barbel are innervated in a strict orthogonal fashion. The barbel is innervated by a main nerve trunk running in the core of the barbel. A longitudinal nerve bundle originates from the main trunk and, after running a short distance distally, divides into two circumferential nerve bundles (CNB) extending respectively, medially and laterally around the barbel. Approximately 15 CNBs innervate each 1 mm length of barbel. At each transverse level, the CNB innervates two clusters of taste buds, each containing 14 end-organs. The primary taste centre in the brain is similarly extraordinary. The sensory inputs from the barbel terminate in a derived dorsal facial lobe, which has a highly convoluted surface forming a multitude of tubercles. Electrophysiological mapping experiments show that the entire barbel is somatotopically represented in a recurved elongate tubular fashion within the dorsal facial lobe.     

Cutaneous taste buds in cod

The distribution of cutaneous taste buds was determined quantitatively in larvae, juveniles and young adults of cod, using scanning electron microscopy. Changes in these distributions associated with development were followed in laboratory reared fish. Taste buds were first seen on the snout and lips of cod at a total length of 8 mm, and on the barbel at a length of 22 mm. The highest taste bud densities were seen at a length of around 90 mm, and subsequently declined on the barbel and pelvic fins with further growth. In these late 0-group fish, mean taste bud densities over much of the head, e.g. throat, dentary and sides of the snout were <100 mm⁻². On the tip of the snout and the lips, mean densities were in the region of 350–400 mm⁻², while on projecting parts of the fish, especially the barbel, anterior naris flap and extremities of the fins, spot densities occasionally exceeded 1000 mm⁻² at some sites. Mean taste bud diameter increased rapidly from 2.23μ± 0.35 μm (S.D.) at a length of 22 mm to 7.19 ± 0.23 μm at 90 mm length, with a much slower increase to about 8 μm associated with a further doubling in body length. These changes indicate a phase of rapid proliferation and growth in size of cutaneous taste buds in the period preceding the adoption of a benthic habit in their first summer. The presence of high taste bud densities on the barbel and pelvic fins in particular appears to correlate with the known feeding behaviour of cod.     

Fine structure of the nasal barbel of the channel catfish,Ictalurus punctatus

Ultrathin sections of the nasal barbel of the channel catfish, Ictalurus punctatus, were studied in the electron microscope and the fine structure was compared to that of barbels of other teleosts and to the mandibular (dentary) barbels of I. punctatus. While the overall histology of the nasal barbel is similar to that of barbels described previously, this study revealed far greater cellular complexity and variability than was previously reported. A layer of stratified epidermal cells rests upon a connective tissue dermis containing a cartilage rod, a large number of nerve fibers and numerous blood vessels, fibroblasts and pigment cells. Taste buds are present in the epidermal layer. This layer was found to contain probably 16 kinds of cell types, several of which may represent transitional stages, in addition to taste bud cells. Observations were made pertaining to innervation and cell types in the taste buds. A new terminology for designating the barbels of I. punctatus is suggested.     

Methods for the Study of the Zebrafish Maxillary Barbel

Barbels are skin sensory appendages found in fishes, reptiles and amphibians. The zebrafish, Danio rerio, develops two pairs of barbels- a short nasal pair and a longer maxillary pair. Barbel tissue contains cells of ectodermal, mesodermal and neural crest origin, including skin cells, glands, taste buds, melanocytes, circulatory vessels and sensory nerves. Unlike most adult tissue, the maxillary barbel is optically clear, allowing us to visualize the development and maintenance of these tissue types throughout the life cycle. This video shows early development of the maxillary barbel (beginning approximately one month post-fertilization) and demonstrates a surgical protocol to induce regeneration in the adult appendage (>3 months post-fertilization). Briefly, the left maxillary barbel of an anesthetized fish is elevated with sterile forceps just distal to the caudal edge of the maxilla. A fine, sterile spring scissors is positioned against the forceps to cut the barbel shaft at this level, establishing an anatomical landmark for the amputation plane. Regenerative growth can be measured with respect to this plane, and in comparison to the contralateral barbel. Barbel tissue regenerates rapidly, reaching maximal regrowth within 2 weeks of injury.Techniques for analyzing the regenerated barbel include dissecting and embedding matched pairs of barbels (regenerate and control) in the wells of a standard DNA electrophoresis gel. Embedded specimens are conveniently photographed under a stereomicroscope for gross morphology and morphometry, and can be stored for weeks prior to downstream applications such as paraffin histology, cryosectioning, and/or whole mount immunohistochemistry. These methods establish the maxillary barbel as a novel in vivo tissue system for studying the regenerative capacity of multiple cell types within the genetic context of zebrafish.     

Morphometry and microanatomy of the barbels of the common sawshark Pristiophorus cirratus (Pristiophoridae): implications for pristiophorid behaviour

The internal anatomy of the barbels of the common sawshark Pristiophorus cirratus was examined with light microscopy to clarify their sensory role. No sensory structures such as taste buds (chemoreception), ampullae of Lorenzini (electroreception) or free neuromasts (lateral line mechanoreception) could be located in the barbels. The presence of bundles of nerve fibres, however, indicates a tactile function for the barbels. Conveyance of information regarding potentially damaging stimuli (nociception) and temperature (thermoception) cannot be excluded at this stage. It is hypothesized that the barbels are used by P. cirratus to locate prey in both the water column and on the substratum via wake detection and sensing changes in surface texture. The barbels may also be involved in the detection of water currents for rheotaxis. Regression analyses on P. cirratus morphometric data showed that the width of the rostrum at two sections (the barbels and the rostrum tip) does not significantly correlate with total length. The regression analyses also suggested that the barbels of P. cirratus may be lateralised.     

Taste preferences and feeding behavior of the stone loach <Emphasis Type="Italic">Barbatula barbatula</Emphasis> (Balitoridae,Cypriniformes) after partial deprivation of circum-mouth external gustatory and tactile receptors

Comparative study of the feeding behavior and gustatory preferences mediated by extraoral and intraoral gustatory reception in the stone loach Barbatula barbatula in the norm and 1–6 months after extirpation of all three pairs of barbels was performed. It was found that partial loss of external gustatory receptors and the sensory deficit caused by it do not lead to any noticeable disturbances of the ability of fish to evaluate taste properties of food objects (artificial agar-agar pellets containing L-isomers of alanine, lysine, cysteine or an water extract of chironomid larvae) and make an adequate decision of their grasping or ignoring and swallowing or refusing. The extirpation of the barbels does not influence feeding behavior related to determination by fish of the taste properties of pellets and completely retains in its ritual an obligatory preliminary examination of the food object using external gustatory reception. It is suggested that external taste buds having different localization are equally capable of providing for fish a preliminary evaluation of the taste properties of the object. After extirpation of the barbels, the efficiency of grasping pellets decreases in fish, which indicates an important role of tactile reception in the determination of the site of location of the food object and in providing of the accuracy of the hunter’s dart in fish with a weak vision development.     

Size-related mortality due to gnathiid isopod micropredation correlates with settlement size in coral reef fishes

The transition between the planktonic and the benthic habitat is a critical period for the larvae of many demersal marine organisms. Understanding the potential constraints on the timing of this habitat transition, called settlement, is important to understanding their biology. Size-specific mortality can set the limits on lifestyle and help explain ontogenetic habitat shifts. We examined whether size-based mortality risks after settlement may include micropredation by ectoparasites by testing whether survival of settlement-stage fish varies with fish size when exposed to a reef-associated micropredator. Fish (14 species) were exposed to one blood-sucking gnathiid isopod overnight, with appropriate controls; gnathiid feeding success and survival, and fish mortality were recorded relative to fish size. After adjusting for fish relatedness, we found the relationship between fish mortality and size differed with gnathiid exposure: for gnathiid-exposed fish, the mean mortality of the smallest fish was much higher (57%) than unexposed controls (10%), and decreased to ~0% for fish >12 mm standard length (SL); mortality was almost nil in controls. Thus, a predicted optimal size to switch habitat and reduce mortality risk from micropredation should be >12 mm SL. We then asked what species might be at greater risk and if the steep increase in survival at ~12 mm SL might coincide with settlement at larger sizes among fishes. Across 102 other species (32 families), 61% settled at ≥12 mm SL. After adjusting for relatedness, mean fish settlement size was 15.0 mm and this was not significantly different from 12 mm. Thus, settlement size clusters around the minimum fish size threshold our gnathiid experiment predicted would be large enough to survive a gnathiid encounter. These results suggest micropredators may contribute to size-selective mortality during settlement processes and are consistent with the hypothesis that the pelagic phase provides fish an escape from certain micropredators.     

Cutaneous and subcutaneous sensory receptors of the hagfish Myxine glutinosa with special respect to the trigeminal system

The integument of the hagfish Myxine glutinosa is described with respect to the topography and the fine structural organization of the dermal and hypodermal nerve fiber plexus. Both nerve fiber plexuses contain small ganglion cells with axodendritic and axosomatic synapscs. The six barbels of the head (4 nasal and 2 oral barbels) are supplied with about 5600 afferent trigeminal nerve fibers via the right and left ophthalmic nerve. With respect to the topography of the sensory nerve terminals in the barbels different types of receptors are termed the external cuff receptor, internal cuff receptor, and perichondrial receptor. Free nerve terminals occur within the epidermal layer, especially at the tip region of the barbels and in the glassy membrane of the dermis. The hypodermal edge receptor organ extends from the ventral nasal barbel to the oral barbel. A mechanoreceptive function of the different receptor types is discussed. The innervation pattern of the barbel is similar to the innervation of the mammalian sinus hair. In this context, the barbel is a highly differentiated receptor organ able to explore the nearest surroundings with high stereognostic perception. The ganglion cells of the skin seem to represent a part of the peripheral autonomic nervous system, which is involved in the control of secretion mechanisms.     

Early life history and settlement of the slender filefish, <Emphasis Type="Italic">Monacanthus tuckeri</Emphasis> (Monacanthidae), at Calabash Caye,Turneffe Atoll,Belize

Synopsis We examined early life history traits and patterns of settlement of the slender filefish, Monacanthus tuckeri, at Calabash Caye, Turneffe Atoll, Belize. A settlement peak was evident at the new moon, and no settlement occurred at the full moon. However, settlement rates at the quarter moons could not be estimated due to sampling gaps. Many reef fishes show new moon settlement peaks, so M. tuckeri shares some characteristics with the primarily perciform species on coral reefs. Pelagic larval duration was long (mean = 42 days) and variable, suggesting that dispersal patterns might be diverse. Size at settlement was large (mean = 32 mm total length) and also variable. Larval duration and size at settlement were outside of the average values exhibited by reef fishes, but are not beyond the extreme end of the range, and might be explained by association with pelagic debris prior to settlement. There were no differences in overall settlement rates on reef and seagrass habitats, and fish settling to either habitat did not differ in larval duration, size at settlement, or larval growth rate. This suggests that settlement to alternative habitats may be random, or driven by availability of suitable microhabitat, rather than habitat quality or individual traits.     

Divergent requirements for fibroblast growth factor signaling in zebrafish maxillary barbel and caudal fin regeneration

The zebrafish maxillary barbel is an integumentary organ containing skin, glands, pigment cells, taste buds, nerves, and endothelial vessels. The maxillary barbel can regenerate (LeClair & Topczewski 2010); however, little is known about its molecular regulation. We have studied fibroblast growth factor (FGF) pathway molecules during barbel regeneration, comparing this system to a well‐known regenerating appendage, the zebrafish caudal fin. Multiple FGF ligands (fgf20a, fgf24), receptors (fgfr1‐4) and downstream targets (pea3, il17d) are expressed in normal and regenerating barbel tissue, confirming FGF activation. To test if specific FGF pathways were required for barbel regeneration, we performed simultaneous barbel and caudal fin amputations in two temperature‐dependent zebrafish lines. Zebrafish homozygous for a point mutation in fgf20a, a factor essential for caudal fin blastema formation, regrew maxillary barbels normally, indicating that the requirement for this ligand is appendage‐specific. Global overexpression of a dominant negative FGF receptor, Tg(hsp70l:dn‐fgfr1:EGFP)^pd1 completely blocked fin outgrowth but only partially inhibited barbel outgrowth, suggesting reduced requirements for FGFs in barbel tissue. Maxillary barbels expressing dn‐fgfr1 regenerated peripheral nerves, dermal connective tissue, endothelial tubes, and a glandular epithelium; in contrast to a recent report in which dn‐fgfr1 overexpression blocks pharyngeal taste bud formation in zebrafish larvae (Kapsimali et al. 2011), we observed robust formation of calretinin‐positive tastebuds. These are the first experiments to explore the molecular mechanisms of maxillary barbel regeneration. Our results suggest heterogeneous requirements for FGF signaling in the regeneration of different zebrafish appendages (caudal fin versus maxillary barbel) and taste buds of different embryonic origin (pharyngeal endoderm versus barbel ectoderm).     

A morphological key for the identification of southern African Synodontis species Cuvier, 1816 (Teleostei: Mochokidae)

The identification of the southern African Synodontis species is hampered by variable and overlapping morphological characteristics such as colour, pigmentation patterns and morphometric ratios. Species are often misidentified due to the complicated nature of the present identification keys. The morphological key proposed in this note uses simplified characters including the shape of the humeral process, size of the outer mandibular barbel, teeth rows, position of the mouth, basal membrane of the maxillary barbel and length of the maxillary barbels, plus the natural distribution range of certain species to facilitate positive identification of southern African Synodontis specimens.     

Characterization of parvalbumin isotypes in white muscle from the barbel and expression during development

Parvalbumin isotypes PA II, PA III, PA IVa, and PA IVb were isolated by chromatography from trunk white muscle of barbel and physicochemically characterized. Electrospray ionization mass spectroscopy revealed that PA II has a lower molecular weight than the other isotypes and that PA IVa and PA IVb each consist of two subforms. Isotype distribution was studied by polyacrylamide gel electrophoresis. In adult fish, the total parvalbumin titre decreased and the isotype distribution varied from the anterior to the posterior myotomes. In the course of barbel development, the total parvalbumin titre increased rapidly as fish standard length increased from 1·3 to 5 cm; then sloped down gently as the length increased to 60 cm. At least six parvalbumin isotypes were identified, three of which are different forms (a, b, and c) of PA II. These three forms were present together at the larval stage, but PA IIc and chiefly PA IIb appeared as early isotypes, contrary to PA IIa which was present until the adult period. Later PA IVb accounted for up to 90% of the total parvalbumin content; PA III and PA IVa are minor adult isotypes. Temporal and spatial variations in the total parvalbumin titre and in the differential expression of barbel parvalbumin isotypes very likely reflected the functional requirements of the fish axial musculature according to fish size and myotome location. Physiologically, the larval isotypes could promote faster relaxation of fast fibres than the adult isotypes, and hence favour shorter contraction times.     

The annual cycle of intraovarian egg size and fecundity of the barbel Barbus barbus in the River Severn

SUMMARY. The annual cycle of intraovarian egg size in 124 mature barbel caught during a 3-year period was investigated. Storage methods, subsampling and measuring procedures were assessed and found to be reliable. Estimates of fecundity were made on thirty-two barbel containing developing ripening eggs. A new egg separation and counting technique was used modified from the volumetric ratio method described by Bagenal & Braum (1968). Fecundity estimates were found to be accurate by comparison with manual counts. Barbel ovaries contained two egg groups throughout the year. A third large, ripening egg group developed prior to spawning. The diameter of the ripening eggs, measured in arbitrary optical units, was significantly correlated with fork length. Large fish spawned bigger eggs than small fish. Fecundity was logarithmically related to fork length and ovary weight and linearly related to somatic weight. The mean diameter of the ripening intraovarian egg group was also logarithmically related to ovary weight. Heavier ovaries contained proportionally fewer, but larger, ripening eggs than small ovaries.     

Functional properties and projection areas of cutaneous receptors in catfish

Summary Neurophysiological studies were conducted in a species of catfish,Ictalurus nebulosus where specialized living habits are coupled with enlargement of certain nervous system structures. These fish, mainly lake bottom dwellers, recognize food only by touch. The entire body surface contains structures resembling mammalian taste buds; the ganglia of the fifth and seventh nerves (receiving input from barbel and head skin) are much larger than in other fish of similar size; the facial lobes, macroscopically invisible in most fish, are prominent. Early anatomical studies reported that the cutaneous buds, considered chemosensitive, project to the brainstem in parallel with tactile paths, but the two modalities remain separate. The facial lobes, the projection area of the cutaneous buds via the seventh nerve, were considered a pure chemosensory structure. In feeding behavior, the chemosensory system was proposed to distinguish sapid from nonsapid substances, the tactile system to localize the food substances.Response properties of single neurons to mechanical and chemical skin stimulations were determined in three successively activated sites: (1) sensory ganglion, (2) facial lobe, and (3) cerebellum. In each location, the majority of neurons were mechanosensitive; only a tiny fraction responded to chemical stimulation. Contrary to early anatomical studies, the two modalities were intermingled, and, particularly the enlarged facial lobes, cannot be considered pure chemosensory structures.Some degree of topographic organization occurred in the ganglion and facial lobe on comparing input from two peripheral barbel nerves. Among chemosensory neurons, some responded only to one, others to several of the applied test solutions. The latter generated distinctive spike trains in response to different test solutions (Fig. 2).Certain response properties, absent in the ganglion, occurred in the facial lobe after synaptic transmission: convergence of afferent fibers on single neurons, indicated by larger receptive fields and by spatial summation of input from two barbel nerves. Stimulation of a single peripheral nerve gave evidence of temporal summation at short conditioning-testing intervals and of inhibition at longer intervals (Fig. 5), suggesting that in many facial lobe units a single stimulus evokes a sequence of excitation followed by inhibition. The latencies of electrically driven units suggest that input from the barbels and head region project to the brainstem, and from there via a multisynaptic path to the cerebellum.This study was supported by U.S. Public Health Service Grants DE 02152 and DE 00248. I am indebted to Mr. R. Hansen, College of Fisheries, University of Washington, for obtaining the fish from Lake Washington.     

Anatomy and histology of the brain and sense organs of the antarctic plunderfish Dolloidraco longedorsalis (Perciformes: Notothenioidei: Artedidraconidae), with comments on the brain morphology of other artedidraconids and closely related harpagiferids

In the high-latitude shelf waters of Antarctica, fishes in the perciform suborder Notothenioidei dominate the fish fauna and constitute an adaptive radiation and a species flock. The 25 species of notothenioid plunderfishes, comprising four genera of the family Artedidraconidae, contribute substantially to fish species diversity on the high Antarctic shelf. A mental barbel is an autapomorphy for the family. Dolloidraco longedorsalis is the most abundant artedidraconid at depths over 400 m in these waters. In this article we present the anatomy and histology of the brain and special sense organs of Dolloidraco and compare it to the brains of other artedidraconids, closely related harpagiferids, and more generally to other notothenioids. We provide a detailed drawing of the brain and cranial nerves. The brain of Dolloidraco is simple, without external hypertrophy of sensory or motor regions, but contains several unusual features associated with the ventricular system and CSF, including well-developed circumventricular organs, subependymal expansions, and subarachnoid cisterns; and a ventricle in the corpus cerebellum. The brain of Dolloidraco also contains a lobed chief sensory nucleus of the trigeminal nerve that is correlated across species with barbel length. The eyes are large and contain a small choroid rete, a structure previously thought to be absent from members of this family. We document the histology of the duplex retina, olfactory apparatus, cutaneous taste buds, and barbel musculature and innervation. We discuss the role of pedomorphy in producing simplified brain morphologies. We consider the possibility that Dolloidraco is a somatosensory specialist-an unusual feature among vertebrates-and decide that this is unlikely.     

Adaptive variability of the gustatory system receptor part of the carp Cyprinus carpio (Cyprinidae, Teleostei) after chronic anosmia

To confirm the existence in fish of the olfactogustatory interactions of adaptive significance, the study was performed of reactions of the receptor part of the carp gustatory system to the complete olfactory deafferentation with use of the method of scanning electron microscopy. Peculiarities of morphological changes of the taste receptor apparatus of the external and oral localization were revealed, and the dynamic of development of the taste receptor adaptive changes was traced for 1–9 months after production of anosmia. Receptors of the intraoral gustatory system had reactions of low statistical significance which were delayed in time by 5–6 months. The receptor apparatus of the external gustatory system receptors of the intraoral taste system showed pronounced hypertrophic changes that appeared since 2–3 months after the beginning of anosmia. The changes consisted in an increase of the area of the taste bud (TB) sensory field and of their number in the sensory zones of the maxillary and mandibular parts. The taste receptors located in the maxillary barbels and in the mandible gular zone had the most expressed reactions to the olfactory deafferentation. In the fish with anosmia, formation of the additional structures of the “external taste” was detected in the form of epidermal processes under the lower lip, which were covered with taste papillae. Thus, it has been shown that the fish olfactory and gustatory systems are functionally interrelated; under conditions of olfactory deprivation the external gustatory system is able to undergo compensatory morphofunctional changes aimed at vicariation of the lost distant chemical reception.     

The barbel‐like specialization of the pelvic fins in Ophidion rochei (Ophidiidae)

Pelvic fins in Ophidion rochei are reduced to four rod‐like structures situated at the ventral jaws. While the fish is swimming, they make continuous sweeping movements on the bottom. This paper examines and describes the anatomy of the pelvic fins to determine the possible functions of these appendages in relation to the mode of life of this fish species. The pelvic fins of O. rochei show strong similarities with barbels because they have identical sensory cell types, (taste buds, solitary chemosensory cells, and goblet cells), innervations and sensory function. Having nocturnal habits, specialization of pelvic fins in O. rochei corresponds to a supporting role to the life in dark environment. J. Morphol., 2012. © 2012 Wiley Periodicals, Inc.     

The effect of initial larval stocking density on growth and survival in common barbel Barbus barbus (L.)

Aim of the study was to determine the effect of initial larval density on growth and survival of the common barbel Barbus barbus (L.) under controlled conditions. The experiment examined fish density from 20 to 200 ind. L^?1 during intensive rearing at 25°C. Rearing lasted 21 days whereby the average total fish length, weight and survival were recorded. During the entire period fish were fed live nauplii Artemia sp. The data provided the basis for calculating the specific growth rate (SGR), Fulton’s condition index (K), and analysis of the relationship between biomass and the stocking density. The documentation was also used to determine the level of ontogeny. The study data obtained indicated that initial larval density has no effect on survival (over 98% in each treatment), level of ontogeny, or on the rearing indexes. Fish achieved the juvenile stage during the second week of rearing (<23 mm total length). At the end of the experiment fish reached 26.50–27.78 mm TL. SGR ranged from 22.75 to 23.53 (% day^?1). No statistical differences were found among all treatments (P > 0.05). The findings of this study suggest that stocking density in common barbel larvae is not a factor limiting growth or the ontogenetic development rate during initial rearing. This creates considerable potential for improvement in stocking material production under controlled conditions and should have a positive effect on the economic effectiveness of production.