Differentiation of swimming muscles and gills, and development of anaerobic power in the larvae of cyprinid fish (Pisces, Teleostei)

Structure and function of the larval teleost fish gill

The fish gill is a multifunctional organ that is important in multiple physiological processes such as gas transfer, ionoregulation, and chemoreception. This characteristic organ of fishes has received much attention, yet an often-overlooked point is that larval fishes in most cases do not have a fully developed gill, and thus larval gills do not function identically as adult gills. In addition, large changes associated with gas exchange and ionoregulation happen in gills during the larval phase, leading to the oxygen and ionoregulatory hypotheses examining the environmental constraint that resulted in the evolution of gills. This review thus focuses exclusively on the larval fish gill of teleosts, summarizing the development of teleost larval fish gills and its function in gas transfer, ionoregulation, and chemoreception, and comparing and contrasting it to adult gills where applicable, while providing some insight into the oxygen vs ionoregulatory hypotheses debate.

Tissue dynamics of potential toxic elements in the Pacific hake (Merluccius productus): distribution and the public health risk assessment

This study aimed to establish the distribution of As, Cd, Pb, Cu, and Zn in the main tissues (muscle, liver, gonads, and gills) of the Pacific hake (Merluccius productus) from the northern Gulf of California to establish baseline bioavailability levels in the northern stock. The results for Pb and Cd were the lowest in the studied tissues (Pb < 0.005 mg kg^-1 in the liver and gonads and 1.43 mg kg^-1 for Cd in the liver), followed by levels of Cu and As (muscle > liver > gonads > gills) and Zn with the most abundant levels in all the tissues. The sex of the organisms was not a factor that influenced the bioaccumulation and distribution of the potential toxic elements (PTEs) nor total length, except for As in gills and Cd in muscle and the liver. Important interactions among Zn and non-essential elements were established. The Pacific hake intake of PTEs was probably through the diet via bioaccumulation of the elements in their prey and less by pollution of the water column. In the muscle, a major distribution and storage of As, Zn, and Pb were observed, but in the liver, higher loads were from Cd and Cu. The maximum tolerable weekly intake must be very high to be at health risk for the essential elements and Cd. However, the population might be at risk for Pb and As consumption if more than 124 g of M. productus in adults and 35 g in children are consumed per week. Further investigations are required to understand the dynamics of PTEs in M. productus as it could be proposed as a biomonitor species.

Interacting Effects of Cell Size and Temperature on Gene Expression, Growth, Development and Swimming Performance in Larval Zebrafish

Cell size may be important in understanding the thermal biology of ectotherms, as the regulation and consequences of cell size appear to be temperature dependent. Using a recently developed model system of triploid zebrafish (which have around 1.5-fold larger cells than their diploid counterparts) we examine the effects of cell size on gene expression, growth, development and swimming performance in zebrafish larvae at different temperatures. Both temperature and ploidy affected the expression of genes related to metabolic processes (citrate synthase and lactate dehydrogenase), growth and swimming performance. Temperature also increased development rate, but there was no effect of ploidy level. We did find interactive effects between ploidy and temperature for gene expression, body size and swimming performance, confirming that the consequences of cell size are temperature dependent. Triploids with larger cells performed best at cool conditions, while diploids performed better at warmer conditions. These results suggest different selection pressures on ectotherms and their cell size in cold and warm habitats.

Triploidy in zebrafish larvae: Effects on gene expression, cell size and cell number, growth, development and swimming performance

There is renewed interest in the regulation and consequences of cell size adaptations in studies on understanding the ecophysiology of ectotherms. Here we test if induction of triploidy, which increases cell size in zebrafish (Danio rerio), makes for a good model system to study consequences of cell size. Ideally, diploid and triploid zebrafish should differ in cell size, but should otherwise be comparable in order to be suitable as a model. We induced triploidy by cold shock and compared diploid and triploid zebrafish larvae under standard rearing conditions for differences in genome size, cell size and cell number, development, growth and swimming performance and expression of housekeeping genes and hsp70.1. Triploid zebrafish have larger but fewer cells, and the increase in cell size matched the increase in genome size (+ 50%). Under standard conditions, patterns in gene expression, ontogenetic development and larval growth were near identical between triploids and diploids. However, under demanding conditions (i.e. the maximum swimming velocity during an escape response), triploid larvae performed poorer than their diploid counterparts, especially after repeated stimuli to induce swimming. This result is consistent with the idea that larger cells have less capacity to generate energy, which becomes manifest during repeated physical exertion resulting in increased fatigue. Triploidy induction in zebrafish appears a valid method to increase specifically cell size and this provides a model system to test for consequences of cell size adaptation for the energy budget and swimming performance of this ectothermic vertebrate.

Biomechanics of swimming in developing larval fish

Most larvae of bony fish are able to swim almost immediately after hatching. Their locomotory system supports several vital functions: fish larvae make fast manoeuvres to escape from predators, aim accurately during suction feeding and may migrate towards suitable future habitats. Owing to their small size and low swimming speed, larval fish operate in the intermediate hydrodynamic regime, which connects the viscous and inertial flow regimes. They experience relatively strong viscous effects at low swimming speeds, and relatively strong inertial effects at their highest speeds. As the larvae grow and increase swimming speed, a shift occurs towards the inertial flow regime. To compensate for size-related limitations on swimming speed, fish larvae exploit high tail beat frequencies at their highest speeds, made possible by their low body inertia and fast neuromuscular system. The shifts in flow regime and body inertia lead to changing functional demands on the locomotory system during larval growth. To reach the reproductive adult stage, the developing larvae need to adjust to and perform the functions necessary for survival. Just after hatching, many fish larvae rely on yolk and need to develop their feeding systems before the yolk is exhausted. Furthermore, the larvae need to develop and continuously adjust their sensory, neural and muscular systems to catch prey and avoid predation. This Review discusses the hydrodynamics of swimming in the intermediate flow regime, the changing functional demands on the locomotory system of the growing and developing larval fish, and the solutions that have evolved to accommodate these demands.

Skeletal muscle growth dynamics and the influence of first-feeding diet in Atlantic cod larvae (Gadus morhua L.)

Dynamics between hypertrophy (increase in cell size) and hyperplasia (increase in cell numbers) of white and red muscle in relation to body size [standard length (SL)], and the influence of the first-feeding diets on muscle growth were investigated in Atlantic cod larvae (Gadus morhua). Cod larvae were fed copepod nauplii or rotifers of different nutritional qualities from 4 to 29 days post hatching (dph), Artemia nauplii from 20 to 40 dph and a formulated diet from 36 to 60 dph. The short period of feeding with cultivated copepod nauplii had a positive effect on both muscle hyperplasia and hypertrophy after the copepod/rotifer phase (19 dph), and a positive long term effect on muscle hypertrophy (60 dph). The different nutritional qualities of rotifers did not significantly affect muscle growth. We suggest here a model of the dynamics between hyperplasia and hypertrophy of red and white muscle fibre cells in relation to cod SL (4 to 30 mm), where the different red and white muscle growth phases clearly coincided with different metamorphosis stages in cod larvae. These shifts could be included as biomarkers for the different stages of development during metamorphosis. The main dietary muscle effect was that hypertrophic growth of red muscle fibres was stronger in cod larvae that were fed copepods than in larvae that were fed rotifers, both in relation to larval age and size. Red muscle fibres are directly involved in larval locomotory performance, but may also play an important role in the larval myogenesis. This can have a long term effect on growth potential and fish performance.

Summary: Hyperplastic and hypertrophic growth dynamics of red and white muscle were strongly related to cod larval size and corresponded with the metamorphosis process. First-feeding diet quality can prolong effects on muscle growth potential in cod larvae.

The zebrafish myotome contains tonic muscle fibers: morphological characterization and time course of formation

It is long known that the skeletal muscle of teleost fish contains muscle fibers which are in all probability of a tonic type according to morphological criteria. However, the evidence for the existence of teleost tonic fibers is still confined to a very small number of species, and knowledge concerning their ontogeny and possible functions is even more restricted. A remarkable deficit in this context is that it is not even exactly known whether the zebrafish, which is widely used to study vertebrate developmental biology, has such fibers, or how they arise. The present study demonstrates the existence of tonic fibers in the zebrafish myotome. They are identical with a fiber population previously termed "red muscle rim" fibers. A combined histochemical, immunocytochemical, and ultrastructural approach is used to characterize the morphology and development of these fibers. This study provides a basis for using the zebrafish model system in the future research on the developmental regulation and the functions of tonic fibers.

Persistent effects of incubation temperature on muscle development in larval haddock (Melanogrammus aeglefinus L.)

Muscle development and growth were investigated in haddock larvae(Melanogrammus aeglefinus L.) incubated under controlled temperatures(4, 6, 8°C) and reared post-hatch through yolk-dependent and exogenous-feeding stages in a 6°C post-hatch environment. Changes in cell number and size in superficial and deep myotomes within the epaxial muscle were investigated for 28 days following hatch. Distinct and significant differences in muscle cellularity following separate developmental strategies were observed in superficial and deep myotomes. The number of superficial myofibres increased with time and, although not in a manner proportional to temperature during the first 21 days post hatch (d.p.h.), there was observed a trend during the final 7 days of greater mean cell size that was strongly associated with increased temperature. In addition, there was an apparent correspondence between increased temperature and increased size between 21 and 28 d.p.h. Among all temperature groups the superficial myotome not only demonstrated a consistent unimodal myofibre-size distribution but one that increased in range proportional to temperature. In the deep muscle, myotomes from higher incubation temperatures had a broader range of fibre sizes and greater numbers of myofibres. The onset of a proliferative event,characterized by a significant recruitment of new smaller myofibres and a bimodal distribution of cell sizes, was directly proportional to incubation temperature such that it occurred at 14 d.p.h. at 8°C but not until 28 d.p.h. at 4°C. The magnitude of that recruitment was also directly proportional to temperature. Following hatch, those embryos from the greatest temperature groups had the largest mean deep muscle size but, as a result of the proliferative event, had the smallest-sized cells 28 days later. The muscle developmental and growth strategy as indicated by sequential changes in cellularity and cell-size distributions between myotomes in response to temperature are also discussed in light of whole animal growth and development.

Physiological properties of zebrafish embryonic red and white muscle fibers during early development

The zebrafish is a model organism for studies of vertebrate muscle differentiation and development. However, an understanding of fish muscle physiology during this period is limited. We examined the membrane, contractile, electrical coupling, and synaptic properties of embryonic red (ER) and white (EW) muscle fibers in developing zebrafish from 1 to 5 days postfertilization. Resting membrane potentials were -73 mV in 1 day ER and -78 mV in 1 day EW muscle and depolarized 17 and 7 mV, respectively, by 5 days. Neither fiber type exhibited action potentials. Current-voltage relationships were linear in EW fibers and day 1 ER fibers but were outwardly rectifying in some ER fibers at 3 to 5 days. Both ER and EW fibers were contractile at all ages examined (1 to 5 days) and could follow trains of electrical stimulation of up to 30 Hz without fatiguing for up to 5 min. Synaptic activity consisting of miniature endplate potentials (mEPPs) was observed at the earliest ages examined (1.2-1. 4 days) in both ER and EW fibers. Synaptic activity increased in frequency, and mEPP amplitudes were larger by 5 days. Miniature EPP rise times and half-widths decreased in ER fibers by 5 days, while EW fiber mEPPs showed fast kinetics as early as 1.2-1.4 days. ER and EW muscle fibers showed extensive dye coupling but not heterologous (red-white) coupling. Dye coupling decreased by 3 days yet remained at 5 days. Somites were electrically coupling, and this allowed filtered synaptic potentials to spread from myotome to myotome. It is concluded that at early developmental stages the physiological properties of ER and EW muscle are similar but not identical and are optimized to the patterns of swimming observed at these stages.

Cytochrome P4501A (CYP1A) in teleostean fishes. A review of immunohistochemical studies

Cytochrome P4501A monooxygenase has an important function in the biotransformation of many xenobiotics, including polynuclear aromatic hydrocarbons, and planar organochlorine compounds. The metabolism can lead to detoxification or activation to reactive intermediates. Exposure of fish leads to a receptor-mediated induction of CYP1A gene expression. The induction response can be quantitatively analysed by means of molecular techniques (RT-PCR, Northern Blotting), immunochemical approaches (ELISA, Western Blotting), and enzymatic methods (7-ethoxyresorufin-O-deethylase, EROD) at the catalytical level. Immunohistochemical studies have provided qualitative information on cell and tissue distribution of CYP1A in teleost fish. The liver is the major organ of CYP1A activity in fish, but the enzyme is additionally expressed in numerous extrahepatic organs, including kidney, alimentary canal, heart, gills, olfactory system, gonads, brain and endocrine tissues. In many tissues, the vascular endothelia show a strong CYP1A immunoreactivity. As indicated from immunohistochemical studies with fish embryos and larvae, the typical cell and tissue distribution of CYP1A is established early during fish ontogeny.

Temperature and family effects on muscle cellularity at hatch and first feeding in Atlantic salmon (Salmo salarL.)

Muscle cellularity was investigated in alevins from five families of Atlantic salmon (Salmo salar L.) reared at variable ambient temperatures averaging 4.3 °C and in water heated to ca. 8 °C. At hatch, fish reared at 8 °C had fewer muscle fibres and myonuclei per myotome and lower mean fibre cross-sectional areas than fish reared at ambient temperature. The total cross-sectional area of white muscle was 40% less in the group reared at 8 °C than in the group reared at ambient temperature. Muscle cellularity and response to temperature varied among families and there was evidence of interactions with temperature and developmental stage. The number of red and white muscle fibres approximately doubled between hatch and first feeding. At hatch, red muscle fibres stained with an antibody to fast myosin light chains, but expression was gradually switched off as development proceeded. Following hatch, alevins reared at 8 °C were more effective in translating yolk into muscle than those reared at ambient temperature, so towards the end of yolk resorption there were no significant differences in fibre number or cross-sectional area.

Metabolic enzyme activities in larvae of the African catfish, Clarias gariepinus: changes in relation to age and nutrition

The influence of ontogeny and nutrition on metabolic enzyme activities in larvae of the African catfish, Clarias gariepinus, was studied. After start of exogenous feeding, the larvae were reared for 10 days under three different nutritional conditions: Artemia nauplii, a dry starter diet, and starvation. The live feed gave the best growth (96 mg within 10 days) whereas the dry diet resulted in low growth (33 mg). This growth difference was reflected in larval RNA and DNA concentrations, but not in the levels of soluble protein. Enzymes representing the following aspects of metabolism have been analysed: NADPH generation (G6PDH, ME), glycolysis (PFK, PK), gluconeogenesis (FDPase), amino acid catabolism (GOT, GPT) and oxidative catabolism (CS). All enzymes were present from the start of exogenous feeding onwards, but their maximum specific activities displayed different developmental patterns. In catfish larvae fed on Artemia, G6PDH and ME activities steadily increased with age and weight of the larvae. CS levels remained, after an immediate enhancement upon onset of exogenous feeding, on a rather stable plateau. The amino acid-degrading enzymes GOT and GPT showed maximum levels at days 3-5 of feeding or at a body weight of 10-20 mg, but decreased thereafter. Activities of PFK, PK and FDPase showed low initial levels, and increased significantly with age and size. Based on the ontogenetic patterns of metabolic enzymes, in C. gariepinus larvae an early and a late developmental phase can be distinguished. During the early phase, the glycolytic and gluconeogenetic capacities are low, whereas they are enforced during the later phase. The oxidative capacity is high both during the early and the late phase. The metabolic changes in catfish development coincide with other major ontogenetic events, e.g., alterations of muscle organization, gill morphology, respiration and stomach structure and function. Rearing catfish larvae on a dry diet instead of Artemia partly altered the developmental pattern described: The ontogenetic elevation of CS, PFK and FDPase was delayed and the early peak in GOT and GPT activities was not realized. Particularly during the early developmental phase, the enzyme behaviour of the larvae fed on dry food was similar to that of starved larvae.

Energetics of fish larvae, the smallest vertebrates

In this review recent findings on the energetics of fish larvae are presented, highlighting some of the physiological problems linked to small body size. The existence of a mass-independent phase of specific metabolic rate is confirmed but it is pointed out that in young fish ontogenetic transitions of metabolic scaling have so far been documented only for the routine level of activity. Maximum metabolic rate is limited by mitochondrial density in the swimming muscles which scales with a mass exponent of approximately 0.9. Mitochondrial density in the swimming muscles of a species of fish, from larva to adult, covers about the same range as mitochondrial density in the skeletal muscles of mammals. However, the aerobic capacity (power density) of mitochondria is one order of magnitude lower in fish than in mammals. Energy metabolism in embryos and early larvae of fish is almost entirely aerobic. Anaerobic power in the fast muscle fibres is low after hatching but increases during the transition from larva to juvenile with a mass exponent greater than one. In hypoxic water fish larvae swim more economically (i.e. their cost of transport is lower) than in normoxic water. If the rate of growth exceeds a critical threshold (about 10% d-1) fish larvae are capable of increasing the apparent efficiency of growth, probably by reducing the costs of other energy-consuming functions of maintenance.