The development of the retinomotor reactions in larval plaice (Pleuronectes platessa, L.) and turbot (Scophthalmus maximus, L.)

The development of the retinomotor reaction was examined histologically in the larvae of plaice (Pleuronectes platessa, L.) and turbot (Scophthalmus maximus, L.). In plaice retinal pigment migration is present from hatching, but, in turbot, does not occur until just before metamorphosis. In both species rods first develop at metamorphosis. Plaice show cone migration, but this does not occur in turbot. During dark-adaptation retinal pigment contraction started at ≈ 100 lux in all stages of plaice. Before metamorphosis pigment was fully contractedimmediately below 1000 lux, but after metamorphosis full contraction did not occur until the light intensity had fallen to < 1.0 lux. The thresholds for cone migration in plaice showed similar changes, although the actual light intensities were about an order of magnitude lower. In turbot there was always an abrupt transition between light- and dark-adaptation; the thresholds for this fell from ≈ 10 lux at the start of metamorphosis to between 1.0 and 10 lux at the end. The lower threshold light intensities in turbot compared to plaice may indicate that vision is of greater importance in the feeding of the former species.     

Comparative study on the fatty acid and lipid composition of four marine fish larvae

• 1.1. Fatty acid and lipid class composition were determined in larvae of four marine species: Atlantic halibut (Hippoglossus hippoglossus L.), plaice (Pleuronectes platessa), cod (Gadus morhua) and turbot (Scophthalmus maximus) at hatching and prior to first feeding.
• 2.2. Total fatty acid content decreased in the four species with up to 50% reduction in one of the halibut groups. Docosahexanaoic acid (22:6 n-3) was especially utilized.
• 3.3. Low lipid utilization was found in turbot in relation to the other three species.
• 4.4. Water environmental temperature may explain some of the differences in the fatty acid utilization and the source of metabolic energy between cold water species (halibut, cod, and plaice) and temperate species (turbot), in the period from hatching to prior to first feeding.
• 5.5. Relative amounts of neutral lipids and phospholipids were similar in plaice, cod and halibut, approximately 25% and 75% of total lipids, respectively, and were approximately constant during the yolk-sac stage. Neutral lipids were dominant for turbot at hatching, accounting for 53–55% of the total lipids, while phospholipids predominated prior to first feeding, being 56–59%.
• 6.6. Phosphatidylcholine was catabolized in halibut, plaice and cod but not in turbot, while phosphatidylethanolamine tended to be synthesized in all four species.

     

Measurement of cytochrome P4501A induction in dab (Limanda limanda) and other teleosts with species-specific cDNA probes: isolation and characterisation of dab cDNA and its use in expression studies with β-naphthoflavone-treated fish

Induction of cytochrome P4501A (CYP1A) in fish is an important biomarker in marine monitoring programmes but a number of factors complicate interpretation of data based on catalytic activity. To provide additional analytical tools, we have cloned and sequenced entire (dab) and partial cDNAs (flounder, turbot, sand eel) from several fish species. A detailed analysis comparing the new sequences to those on the database (13 sequences) is presented and identifies an invariant, teleost-specific sequence (195-IVVSVANVICGMCFGRRYDH-214) which might be the basis for production of a species cross-reactive antibody. Northern and slot blots of fish RNA (sand eel, plaice, turbot, flounder and dab) showed extensive cross-species hybridisation with each of the cDNAs (sand eel, plaice, turbot, flounder and dab). The exception was turbot RNA, which only gave adequate hybridisation when the turbot probe was used. Attempts to normalise the hybridisation data to GAPDH mRNA were not satisfactory since there were significant species differences in expression of this gene and expression was suppressed (20–40%) by β-naphthoflavone treatment. The CYP1A probes indicated induction levels relative to untreated dab of: plaice (five-fold); turbot (12-fold); flounder (12-fold); and dab (10-fold). The study demonstrates the relative ease with which species-specific molecular probes can be generated and used.     

On the morphology of the alimentary tract of flatfishes (Pleuronectiformes)

The changes during the life cycle in the relative length of certain parts of the alimentary tract, buccal-pharyngeal cavity, oesophagus-stomach, intestine, rectum—are investigated in the five North Sea flatfish species, viz. turbot, brill, plaice, dab, sole. According to the average ratio between the length of the alimentary tract and the food preference three main groups are distinguished.     

Changes in marine turbot (Scophthalmus maximus) epidermis and skin mucus composition during development from bilateral larvae to juvenile flat fish

Alike other flat fish, marine turbot has the particularity that changes from larvae with bilateral symmetry to adult with asymmetry, in terms of the position of the eyes. As expected, the skin configuration of this species is also affected by the development and transformation suffered by fish during metamorphosis. In this context, changes in the epidermis of marine turbot were studied using conventional staining and histochemical techniques using six lectins (UEA-I, PNA, RCA-I, WGA, Con A and SBA). During development from larvae to juvenile (3–300 days post-hatching), the epidermis increased in both thickness and the number of cell layers. In fact, the simple cuboidal epithelium observed in larvae at day 3 already became stratified at days 10–12, which sequentially increase in thickness with fish development. Turbot epidermis is composed basically of four cell types: epithelial and mucous or secretory cells that are present through the development, and pigmented cells and a type that the authors described as club-like cells that appear during and post-metamorphosis. The Alcian blue-periodic acid Schiff (AB-PAS) histochemical method revealed the presence of neutral glycoconjugates in mucous and club-like cells at post-metamorphic stages of fish. Accordingly, lectin analysis showed mucous cells containing glycoproteins rich in fucose (UEA-I labelling) and glycoconjugates rich in the sequence galactose-N-acetyl galactosamine (PNA and RCA-I labelling) when this cell type appears. Interestingly, melanophores were observed in the dorsal epidermis of post-metamorphic juveniles. This type of cell contains a black-to-brown pigment that provides the skin the typical colour of this fish species. Changes in mucous coat composition were observed during fish development, which was attributed to different roles of the glycoconjugates.     

Expression of myofibrillar proteins and parvalbumin isoforms during the development of a flatfish,the common sole Solea solea: comparison with the turbot Scophthalmus maximus

Developmental changes in myofibrillar protein and parvalbumin isoform composition were investigated in the myotomal muscle of the flatfish Solea solea, characterized by a very brief metamorphic stage. Results were compared with previously obtained data on another pleuronectiform teleost, the turbot (Scophthalmus maximus), displaying prolonged metamorphosis. Electrophoretically measurable changes in myofibrillar proteins and parvalbumins were detected late in the sole, after completion of metamorphosis. In the course of development, muscles showed the usual sequential synthesis of isoforms of the myofibrillar proteins myosin light chain LC2, troponin-T, and troponin-I. An adult parvalbumin isoform (PA III) was found to predominate during sole growth. The two flatfish were characterized by highly species-specific parvalbumin isoforms. Compared with turbot, the profiles of the myofibrillar subunits and parvalbumin isoforms varied little in the course of sole development. The early appearance of adult traits might be correlated with the brevity of metamorphosis of this fish.     

Center of pressure and center of mass behavior during gait initiation on inclined surfaces: A statistical parametric mapping analysis

This study analyzed gait initiation (GI) on inclined surfaces with 68 young adult subjects of both sexes. Ground reaction forces and moments were collected using two AMTI force platforms, of which one was in a horizontal position and the other was inclined by 8% in relation to the horizontal plane. Departing from a standing position, each participant executed three trials in the following conditions: horizontal position (HOR), inclined position at ankle dorsi-flexion (UP), and inclined position at ankle plantar-flexion (DOWN). Statistical parametric mapping analysis was performed over the entire center of pressure (COP) and center of mass (COM) time series. COP excursion did not show significant differences in the medial-lateral (ML) direction in both inclined conditions, but it was greater in the anterior-posterior (AP) direction for both inclined conditions. COP velocities are smaller in discrete portions of GI for the UP and DOWN conditions. COM displacement was greater in the ML direction during anticipatory postural adjustments (APA) in the UP condition, and COM moves faster in the ML direction during APA in the UP condition but slower at the end of GI for both the UP and the DOWN conditions. The COP-COM vector showed a greater angle in the DOWN condition. We observed changes for COP and COM in GI in both the UP and the DOWN conditions, with the latter showing changes for a great extent of the task. Both the UP and the DOWN conditions showed increased COM displacement and velocity. The predominant characteristic during GI on inclined surfaces, including APA, appears to be the displacement of the COM.     

How a GNSS Receiver Is Held May Affect Static Horizontal Position Accuracy

The static horizontal position accuracy of a mapping-grade GNSS receiver was tested in two forest types over two seasons, and subsequently was tested in one forest type against open sky conditions in the winter season. The main objective was to determine whether the holding position during data collection would result in significantly different static horizontal position accuracy. Additionally, we wanted to determine whether the time of year (season), forest type, or environmental variables had an influence on accuracy. In general, the F4Devices Flint GNSS receiver was found to have mean static horizontal position accuracy levels within the ranges typically expected for this general type of receiver (3 to 5 m) when differential correction was not employed. When used under forest cover, in some cases the GNSS receiver provided a higher level of static horizontal position accuracy when held vertically, as opposed to held at an angle or horizontally (the more natural positions), perhaps due to the orientation of the antenna within the receiver, or in part due to multipath or the inability to use certain satellite signals. Therefore, due to the fact that numerous variables may affect static horizontal position accuracy, we only conclude that there is weak to moderate evidence that the results of holding position are significant. Statistical test results also suggest that the season of data collection had no significant effect on static horizontal position accuracy, and results suggest that atmospheric variables had weak correlation with horizontal position accuracy. Forest type was found to have a significant effect on static horizontal position accuracy in one aspect of one test, yet otherwise there was little evidence that forest type affected horizontal position accuracy. Since the holding position was found in some cases to be significant with regard to the static horizontal position accuracy of positions collected in forests, it may be beneficial to have an understanding of antenna positioning within the receiver to achieve the greatest accuracy during data collection.     

Development of the eye in the turbot Psetta maxima (Teleosti) from hatching through metamorphosis

Development of the eyes during the larval and metamorphic stages of the turbot Psetta maxima (Teleosti) was studied using microscopy. Events during differentiation of both eyes occur simultaneously, and no differences between he migrating and no-migrating eye were observed during metamorphosis. At hatching, the eyes are rudimentary, consisting of a neuroepithelial optic cup and a small lens. During larval development, major changes occur in the lens and retina, in which cones are the only photoreceptors. The appearance of rods is delayed until metamorphosis. The outer ocular layers (sclera and choroid) arise during larval development as thin connective layers with little differentiation. These layers undergo important changes just before and during metamorphosis. These results indicate that development of the individual components of the eye occurs at different times. Those of ectodermal origin appear early, providing a simple visual organ during larval life. By metamorphosis, the eye shows adult characteristics, including two types of photoreceptors, a rich choroid vascular supply and ocular structures involved in protecting, shaping, and moving the eye. J Morphol 233:31–42, 1997. © 1997 Wiley-Liss, Inc.     

Seasonal Ichthyodiversity and Growth Patterns of Juvenile Flatfish on a Nursery Ground in the Southern Bight of the North Sea (France)

The demersal fish community on a flatfish nursery ground, of the Southern Bight of the North Sea, was sampled monthly between May 1998 and 1999. The studied coastal area is a multispecific nursery area. Although 32 fish species were caught, only nine species had a major influence on the variation in total densities through the year and can be considered as key species. Juvenile sole and plaice exhibited similar seasonal growth patterns with rapid growth during late spring and summer, a growth arrest during autumn and winter and a restart of growth in March. Comparisons of observed growth in length with predicted maximal growth under optimal food conditions suggested that during the summer, growth of 0-group sole and plaice was only determined by prevailing mean water temperature. During autumn and winter, while growth in length of sole followed model predictions, observed length of plaice was less than model predictions, suggesting growth limitation. Analyses of the factors that may be responsible for differences between observed and expected length growth indicated that autumn and winter growth arrest of plaice was not only related to low winter water temperature. It is suggested that for visual feeders such as plaice, the interaction of decreasing food availability and day length during autumn and winter can reduce the access to food resource and therefore feeding success and growth.     

0-group flatfish growth conditions on a nursery ground (Bay of Canche, Eastern English Channel)

The settlement and growth conditions of three 0-group flatfish species, sole (Solea solea L.), plaice (Pleuronectes platessa L.) and dab (Limanda limanda L.) in the Bay of Canche, French coast of the Eastern English Channel, were investigated during summer 1997. 0-group dab were the more abundant of the three species (75.4%) followed by plaice (21%) and sole (3.6%). Analyses of fish size distribution indicated that settlement of plaice occurred before sole and dab. Growth of sole was faster than that of plaice, which in turn was faster than that of dab (0.68, 0.43 and 0.33 mm d^?1 respectively). Sole differed from plaice and dab by their larger mean length attained at the end of their first summer stay in the nursery ground. Growth performances of 0-group sole and plaice were analysed by comparing estimated growth in the field with predicted maximum growth according to temperature-growth rate models from experimental studies of growth with unlimited food supply. The data of this study suggest that plaice growth may have been limited during late summer, whereas sole growth does appear to have been determined only by water temperature.     

Feeding ecology of juvenile flatfishes of the surf zone of a sandy beach

Prey items of 0- and 1-group plaice Pleuronectes platessa , sole Solea solea , brill Scophthalmus rhombus , turbot S. maximus and dab Limanda limanda of the surf zone of a Belgian sandy beach, included hyperbenthic (e.g. mysids), endobenthic (e.g. polychaetes) and epibenthic (e.g. shrimps) species. Little dietary overlap was observed. If diet overlap did occur, it mainly involved prey species that are dominant in the surf zone of Belgian beaches, such as shrimps and mysids. These results suggest an opportunistic utilization by flatfish of the available food resources in surf zone ecosystems. Also, two strategically different feeding habits could be distinguished between the five flatfish species. Turbot and brill mainly fed on large, highly mobile prey (e.g. fish, mysids) and had a rather narrow prey spectrum, whereas plaice, dab and sole ate more benthic prey (e.g. polychaetes) and had a broader prey spectrum.     

Temperature-independent genome-wide DNA methylation profile in turbot post-embryonic development

The morphological and biological characteristics of ectothermic vertebrates are known to be strongly influenced by environmental conditions, particularly temperature. Epigenetic mechanisms such as DNA methylation have been reported to contribute to the phenotypic plasticity observed in vertebrates in response to environmental changes. Additionally, DNA methylation is a dynamic process that occurs throughout vertebrate ontogeny and it has been associated with the activation and silencing of gene expression during post-embryonic development and metamorphosis. In this study, we investigated genome-wide DNA methylation profiles during turbot metamorphosis, as well as the epigenetic effects of temperature on turbot post-embryonic development. Fish growth and rates of development were greatly affected by rearing temperature. Thus, turbot raised at ambient temperature (18 °C) achieved greater body weights and progressed through development more quickly than those reared at a colder temperature (14 °C). Genome-wide DNA methylation dynamics analyzed via a methylation-sensitive amplified polymorphism (MSAP) technique were not significantly different between animals reared within the two different thermal environments. Furthermore, comparisons between phenotypically similar fish revealed that genome-wide DNA methylation profiles do not necessarily correlate with specific developmental stages in turbot.     

The staining of fish otoliths for age determination

Attempts were made to find methods for the staining of fish otoliths which would give results comparable to those of the Christensen burning technique. A variety of different histological stains and otoliths of different species were experimented with. Otoliths of sole, turbot, brill and scad gave best results when sectioned and stained in acidified Neutral Red, whereas those of cod, hake, whiting, plaice and grey mullet showed up the annuli better when dyed in aqueous Aniline Blue or Toluidine Blue and then sectioned. Small, translucent otoliths such as those of pelagic species may be enhanced by staining in Eosin Y.     

Metamorphosis of <Emphasis Type="Italic">Hydractinia echinata</Emphasis>—natural versus artificial induction and developmental plasticity

Many marine invertebrates reproduce through a larval stage. The settlement and metamorphosis of most of the species are synchronised and induced by environmental organisms, mainly bacteria. The hydrozoan Hydractinia echinata has become a model organism for metamorphosis of marine invertebrates. In this species, bacteria, e.g. Pseudoalteromonas espejiana, are the natural inducers of metamorphosis. Like in other species of marine invertebrates, metamorphosis can be induced artificially by monovalent cations, e.g. Cs⁺. In this study, we present systematic data that metamorphosis—with both inducing compounds, the natural one from bacteria and the artificial one Cs⁺—are indeed similar with respect to (a) the morphological progression, (b) the localisation of the primary induction signal in the larva, (c) the pattern of apoptotic cells occurring during the initial 10 h of metamorphosis and (d) the disappearance of RFamide-dependent immunocytochemical signals in sensory neurons during this process. However, a difference occurs during the development of the anterior end, insofar as apoptotic cells and settlement appear earlier in planulae induced with bacteria. Thus, basically, Cs⁺ may be used as an artificial inducer, mimicking the natural process. However, differences in the appearance of apoptotic cells and in settlement raise the question of how enormous developmental plasticity in hydrozoans actually can be, and how this is related to the absence of malignant devolution in hydrozoans.     

The development and fine structure of ultimobranchial glands in larval anurans

A comparative optical microscopic and ultrastructural study on the ultimobranchial (UB) glands of three common species of Israeli anurans: Bufo viridis, Hyla arborea, and Rana ridibunda during metamorphosis is presented. The UB glands typically consist of a single follicle with a central lumen, though occasionally secondary follicles are present in Hyla and Rana. A single UB cell type is found which appears either in a very electron-dense "dark" form or as a less dense "light" form, though the ratio of dark: light cells from gland to gland at any one stage of metamorphic development is quite variable. By the end of metamorphosis in Bufo and Hyla all the UB cells are usually of the light variety, whereas in Rana the dark cells persist. The organelles of these secretory cells including secretory granules, granular endoplasmic reticulum, free ribosomes, tonofilaments, microtubules, Golgi bodies, and lipid droplets, their distribution, abundance, and possible functions in relation to metamorphosis are described. Apocrine secretion into the central lumen of the gland is also described and discussed.     

Zebrafish puma mutant decouples pigment pattern and somatic metamorphosis

The genetic and developmental bases for trait expression and variation in adults are largely unknown. One system in which genes and cell behaviors underlying adult traits can be elucidated is the larval-to-adult transformation of zebrafish, Danio rerio. Metamorphosis in this and many other teleost fishes resembles amphibian metamorphosis, as a variety of larval traits (e.g., fins, skin, digestive tract, sensory systems) are remodeled in a coordinated manner to generate the adult form. Among these traits is the pigment pattern, which comprises several neural crest-derived pigment cell classes, including black melanophores, yellow xanthophores, and iridescent iridophores. D. rerio embryos and early larvae exhibit a relatively simple pattern of melanophore stripes, but this pattern is transformed during metamorphosis into the more complex pattern of the adult, consisting of alternating dark (melanophore, iridophore) and light (xanthophore, iridophore) horizontal stripes. While it is clear that some pigment cells differentiate de novo during pigment pattern metamorphosis, the extent to which larval and adult pigment patterns are developmentally independent has not been known. In this study, we show that a subset of embryonic/early larval melanophores persists into adult stages in wild-type fish; thus, larval and adult pigment patterns are not completely independent in this species. We also analyze puma mutant zebrafish, derived from a forward genetic screen to isolate mutations affecting postembryonic development. In puma mutants, a wild-type embryonic/early larval pigment pattern forms, but supernumerary early larval melanophores persist in ectopic locations through juvenile and adult stages. We then show that, although puma mutants undergo a somatic metamorphosis at the same time as wild-type fish, metamorphic melanophores that normally appear during these stages are absent. The puma mutation thus decouples metamorphosis of the pigment pattern from the metamorphosis of many other traits. Nevertheless, puma mutants ultimately recover large numbers of melanophores and exhibit extensive pattern regulation during juvenile development, when the wild-type pigment pattern already would be completed. Finally, we demonstrate that the puma mutant is both temperature-sensitive and growth-sensitive: extremely severe pigment pattern defects result at a high temperature, a high growth rate, or both; whereas a wild-type pigment pattern can be rescued at a low temperature and a low growth rate. Taken together, these results provide new insights into zebrafish pigment pattern metamorphosis and the capacity for pattern regulation when normal patterning mechanisms go awry.     

Dietary arginine degradation is a major pathway in ureagenesis in juvenile turbot (Psetta maxima)

Recent studies indicate that urea excretion is responsive to protein intake and that turbot, Psetta maxima, appear to differ from other species by their urea excretion pattern and levels. This study was undertaken to evaluate the influence of dietary nitrogen and arginine on ureagenesis and excretion in turbot. Juvenile turbot (29 g) were fed semi-purified diets containing graded levels of nitrogen (0-8% dry matter) and arginine (0-3% dry matter) for 6 weeks. Growth data showed that turbot have high dietary nitrogen (123 mg/kg metabolic body weight/day) and very low dietary arginine (9.3 mg/kg metabolic body weight/day) requirements for maintenance. Requirements for unit body protein accretion were 0.31 g and 0.15 g for nitrogen and arginine respectively. Post-prandial plasma urea levels and urea excretion rates showed that urea production was significantly (P<0.05) influenced by dietary arginine levels. While hepatic arginase (EC 3.5.3.1) activity increased significantly (P<0.05) with increasing dietary arginine levels, activities of other enzymes of the ornithine urea cycle were very low. Our data strongly suggest that the ornithine urea cycle is not active in the turbot liver and that dietary arginine degradation is a major pathway of ureagenesis in turbot.     

Tidal stream mediation of metapopulation structure in North Sea plaice

Migration is widespread among marine fishes, yet little is known about variation in the migration of individuals within localities. We tested the hypothesis that variation in the migratory behaviour among plaice (Pleuronectes platessa) in the North Sea could be explained by large‐scale differences in the speed and directions of the tidal streams, which the fish use as a transport mechanism. Towards this end, 752 mature female plaice tagged with electronic data storage tags were released at eight locations with contrasting tidal flow properties, between December 1993 and September 1999. The experiment yielded 20 403 days of data from 145 plaice. The position of each fish was determined at intervals throughout the liberty period using the tidal location method. The results show 3 geographically discrete feeding aggregations during the summer, which dispersed over the southern North Sea and Eastern English Channel to spawn during winter. Our results re‐affirmed the major role of the tidal streams in the southern North Sea in structuring plaice dispersion, both by providing transport and guidance, and by delimiting the extent of distribution due to thermal stratification during the summer. These results confirm the prediction that large‐scale variation in migration behaviour can be explained in part by the tidal guidance and transport mechanisms available. They have revealed features of spatial dynamics not previously observed from a century of conventional tagging experiments and illustrate how the study of individual fish can successfully define the migratory characteristics of populations.