Temporal dynamics of oocyte development, plasma sex steroids and somatic energy reserves during seasonal ovarian maturation in captive Murray cod Maccullochella peelii peelii

The temporal dynamics of oocyte growth, plasma sex steroids and somatic energy stores were examined during a 12 month ovarian maturation cycle in captive Murray cod Maccullochella peelii peelii under simulated natural photothermal conditions. Ovarian function was found to be relatively uninhibited in captivity, with the exception that post-vitellogenic follicles failed to undergo final maturation, resulting in widespread pre-ovulatory atresia. Seasonal patterns of oocyte growth were characterised by cortical alveoli accumulation in March, deposition of lipids in April, and vitellogenesis between May and September. Two distinct batches of vitellogenic oocytes were found in Murray cod ovaries, indicating a capacity for multiple spawns. Plasma profiles of 17β-oestradiol and testosterone were both highly variable during the maturation period suggesting that multiple roles exist for these steroids during different stages of oocyte growth. Condition factor, liver size and visceral fat stores were all found to increase prior to, or during the peak phase of vitellogenic growth. Murray cod appear to strategically utilise episodes of high feeding activity to accrue energy reserves early in the reproductive cycle prior to its deployment during periods of rapid ovarian growth.     

Age-related changes in ovarian characteristics, plasma sex steroids and fertility during pubertal development in captive female Murray cod Maccullochella peelii peelii

Age-related changes in ovarian development characteristics and plasma sex steroids in female Murray cod were examined throughout their second, third and fourth years of life to better understand the physiological and endocrine processes associated with puberty in this species in captivity. Spawning performance of 2+ and 3+ year old females was also assessed to identify ontogenetic differences in egg fertility. Puberty was acquired in 38% of 1+ year old females and 100% of age 2+ females. By age 3+, all females had developed full (adult) reproductive function. Ovarian development in pubertal fish was characterised by a rapid transition between cortical alveoli and lipid droplet oogenic phases, coinciding with significantly lower plasma 17β-oestradiol in age 2+ females (p < 0.05). Mean mature oocyte diameter (2.44 mm), post-fertilisation viability (30.80%) and hatchability (0.99%) of eggs from age 2+ females were significantly reduced relative to age 3+ adults (2.81 mm, 84.89% and 23.58%, respectively). Ovaries of pubertal Murray cod exhibited both vitellogenic and ovulatory capacities, yet functional abnormalities during secondary oocyte growth are likely to have contributed to poor egg fertility and consequently, evaluations of age-at-first maturity based on the presence of advanced ovarian stages may overestimate the reproductive potential of younger broodstock populations.     

Blood cells of Murray cod Maccullochella peelii peelii (Mitchell)

Analysis of the peripheral blood cells of Murray cod Maccullochella peelii peelii identified seven distinct subpopulations including a novel basophilic cell. Haematological reference ranges were established to facilitate future diagnostic blood sampling of this fish.     

Fatty acid metabolism in the freshwater fish Murray cod (Maccullochella peelii peelii) deduced by the whole-body fatty acid balance method

The whole-body fatty acid balance method was used to investigate the fatty acid metabolism in Murray cod (Maccullochella peelii peelii) fed diets containing canola (CO) or linseed oil (LO). Murray cod were able to elongate and desaturate both 18:2n-6 and 18:3n-3. In fish fed the CO diet, 54.4% of the 18:2n-6 consumed was accumulated, 38.5% oxidized and 6.4% elongated and desaturated to higher homologs. Fish fed the LO diet accumulated 52.9%, oxidized 37% and elongated and desaturated 8.6% of the consumed 18:3n-3. The overall roles of n-6 fatty acids appeared more important in Murray cod compared to other freshwater species. Murray cod also showed a preferential order of utilization of C18 fatty acid for energy production (18:3n-3 > 18:2n-6 > 18:1n-9). Moreover, it is demonstrated that an increase in dietary 18:3n-3 is directly responsible of increased desaturase activity and augmented saturated fatty acid accumulation in the fish body. The present study also suggests that, in the context of the possible maximization of the natural ability of fish to produce long chain polyunsaturated fatty acids, the whole-body approach can be considered well suited and informative and Murray cod is a suited candidate to fish oil replacement for its diets.     

Multi-scale habitat selection by Murray cod Maccullochella peelii peelii in two lowland rivers

This study provides information on habitat selection by the threatened Murray cod Maccullochella peelii peelii at two spatial scales in the Ovens and Murray Rivers in south-eastern Australia. Both adult (>450 mm total length, L _T) and age 0 year (<150 mm L _T) M. p. peelii selected macro and microhabitats based on structural variables. At the macrohabitat scale, adults selected channel habitats in the river, floodplain channels at high floods and within Lake Mulwala, whereas the floodplain proper was avoided. Adult and age 0 year fish selected similar microhabitats regardless of site or hydrologic conditions, and selection was primarily influenced by the presence of higher loadings of structural woody habitat, higher c.v. in depth, more overhanging vegetation, shallower comparative depths and lower water velocities, closer to the bank. Age 0 year M. p. peelii appeared to select shallower habitats with greater amounts of structural woody habitat, closer to the river bank than adult fish.     

Factorial aerobic scope is independent of temperature and primarily modulated by heart rate in exercising Murray cod (Maccullochella peelii peelii)

Several previous reports, often from studies utilising heavily instrumented animals, have indicated that for teleosts, the increase in cardiac output (Vb) during exercise is mainly the result of an increase in cardiac stroke volume (V(S)) rather than in heart rate (fH). More recently, this contention has been questioned following studies on animals carrying less instrumentation, though the debate continues. In an attempt to shed more light on the situation, we examined the heart rates and oxygen consumption rates (Mo2; normalised to a mass of 1 kg, given as Mo2kg) of six Murray cod (Maccullochella peelii peelii; mean mass+/-SE = 1.81+/-0.14 kg) equipped with implanted fH and body temperature data loggers. Data were determined during exposure to varying temperatures and swimming speeds to encompass the majority of the biological scope of this species. An increase in body temperature (Tb) from 14 degrees C to 29 degrees C resulted in linear increases in Mo2kg (26.67-41.78 micromol min(-1) kg(-1)) and fH (22.3-60.8 beats min(-1)) during routine exercise but a decrease in the oxygen pulse (the amount of oxygen extracted per heartbeat; 1.28-0.74 micromol beat(-1) kg(-1)). During maximum exercise, the factorial increase in Mo2kg was calculated to be 3.7 at all temperatures and was the result of temperature-independent 2.2- and 1.7-fold increases in fH and oxygen pulse, respectively. The constant factorial increases in fH and oxygen pulse suggest that the cardiovascular variables of the Murray cod have temperature-independent maximum gains that contribute to maximal oxygen transport during exercise. At the expense of a larger factorial aerobic scope at an optimal temperature, as has been reported for species of salmon and trout, it is possible that the Murray cod has evolved a lower, but temperature-independent, factorial aerobic scope as an adaptation to the largely fluctuating and unpredictable thermal climate of southeastern Australia.     

The effects of turbidity, prey density and environmental complexity on the feeding of juvenile Murray cod Maccullochella peelii

Juvenile Murray cod Maccullochella peelii exhibited a type II functional response while preying on blackworms Lumbriculus variegatus, and the parameters of the type II model did not differ significantly between clear (0 NTU) and turbid (150 NTU) treatments. Further experiments showed that vision may not be necessary for prey detection and capture by juvenile M. peelii; consumption of inanimate prey was not significantly different between light and dark (<1 × 10(-4) μE m(-2) s(-1)) trials. These results imply that the sensory physiology of M. peelii is well adapted to a turbid visual environment. In addition, habitat complexity increased the food consumption rate of juvenile M. peelii, perhaps by relaxing innate predator avoidance behaviours that depress foraging in more open environments.     

Biomanipulation: a review of biological control measures in eutrophic waters and the potential for Murray cod Maccullochella peelii peelii to promote water quality in temperate Australia

Biomanipulation is a method of controlling algal blooms in eutrophic freshwater ecosystems. The most common approach has been to enhance herbivores through a reduction of planktivorous fish and introduction of piscivorous fish. The method was originally intended to reduce grazing pressure on zooplankton, thereby increasing grazing pressure on phytoplankton to increase water clarity and promote the growth of aquatic macrophytes. Biomanipulation has received considerable attention since it was proposed in 1975 where innovative approaches and explanations of the processes have been developed. Although many successful biomanipulation exercises have been conducted internationally, it has received comparatively little attention in the Southern Hemisphere and has not been trialled in the southern temperate climate of South Australia. This is a review to speculate upon the criteria for and against the application of biomanipulation in southern temperate Australia using the native species Murray cod (Maccullochella peelii peelii) and to suggest future research.     

Variable stocking effect and endemic population genetic structure in Murray cod Maccullochella peelii

Microsatellite markers were utilized to examine the genetic structure of Murray cod Maccullochella peelii throughout its distribution in the Murray--Darling Basin (MDB) of eastern Australia, and to assess the genetic effects of over three decades of stocking hatchery-reared fingerlings. Bayesian analysis using the programme Structure indicated that the species is largely genetically panmictic throughout much of its extensive range, most probably due to the high level of connectivity between catchments. Three catchments with terminal wetlands (the Lachlan, Macquarie and Gwydir), however, contained genetically distinct populations. No stocking effects were detected in the catchments that were genetically panmictic (either because of low genetic power or lack of effects), but the genetically differentiated Gwydir and Macquarie catchment populations were clearly affected by stocking. Conversely, there was no genetic evidence for survival and reproduction of stocked fish in the Lachlan catchment. Therefore, stocking of M. peelii throughout the MDB has resulted in a range of genetic effects ranging from minimal detectable effect, to substantial change in wild population genetic structure.     

Histiostoma papillata sp. n. (Acari: Histiostomatidae), a mite attacking fish in Australia

Histiostoma papillata sp. n. (Acari: Histiostomatidae) is described from Victoria, Australia. It was found in the fins and gills of juvenile Murray cod, Maccullochella peelii peelii (Mitchell), where it appeared to inflict injuries thought to be responsible for the mortality of fish in a diet trial.     

Cytodifferentiation of ovarian follicle cells during oocyte growth and maturation

Inasmuch as 17α,20β-diOHprog was identified as the maturation-inducing hormone, we now have two known biologically important mediators of oocyte growth and maturation in salmonids, estradiol-17β and 17α,20β-diOHprog. It is now established that the granulosa cells are the site of production of these two mediators, but production by the ovarian follicle depends on the provision of precursor steroids by the thecal cell (two-cell type hypothesis). A dramatic switch in the steroidogenic pathway from estradiol-17β to 17α,20β-diOHprog occurs only in ovarian follicle cells immediately prior to oocyte maturation. This switch is a prerequisite step for the growing oocyte to enter the maturation phase. Resolution of the molecular events regulating this switch will provide new insight into the hormonal events regulating oocyte growth and maturation.     

Ontogenetic patterns of habitat use by fishes within the main channel of an Australian floodplain river

The ontogenetic patterns of habitat use by a community of fishes in the main channel of the Broken River, an Australian lowland river, was investigated. Stratified sampling was conducted fortnightly across six habitat types throughout the spring‐summer period within the main channel. As predicted by the 'low flow recruitment hypothesis', backwaters and still littoral habitats were important nursery habitats for most species. These habitats were found to be used by some species throughout all stages of their life cycle, while other species showed clear ontogenetic shifts in habitat preference. Only one species, Murray cod Maccullochella peelii peelii , was never found in backwaters. This study confirms the significance of main channel habitats in the rearing of larvae of some riverine fish species, and emphasizes the importance of considering the habitat requirements of all stages of a fish's life cycle in the management and restoration of rivers and streams.     

Diet of Murray cod (<Emphasis Type="Italic">Maccullochella peelii peelii</Emphasis>) (Mitchell) larvae in an Australian lowland river in low flow and high flow years

Researchers have hypothesised that influxes of pelagic zooplankton to river channels after floods and high flows are necessary for strong recruitment of some native fish species, including Murray cod (Maccullochella peelii peelii) (Mitchell), in the Murray–Darling river system, Australia. This study investigated the composition of the diet and gut fullness of drifting Murray cod larvae weekly during two spawning seasons with contrasting flows, to determine if pelagic zooplankton comprised a greater proportion of the gut contents and guts were fuller in a high flow (2000) than in a low flow (2001) year. Gut fullness and yolk levels of 267 larvae were ranked, and prey identified to family level. Approximately 40 and 70% of individuals had been feeding in 2000 and 2001, respectively. Gut fullness increased with declining yolk reserves. Larvae in both the years had an almost exclusively benthic diet, irrespective of the flow conditions at the time. Substantial inundation of dry ground in 2000, albeit restricted to in-channel benches, anastomosing channels and oxbow lakes, did not lead to an influx of pelagic, floodplain-derived zooplankton subsequently exploited by Murray cod larvae. These results have the implications for the management of regulated temperate lowland rivers: high flows cannot automatically be assumed to be beneficial for the fish larvae of all species and their food resources, and caution should be exercised with the timing of flow releases.     

Cytochemical characterisation of the leucocytes and thrombocytes from Murray cod (Maccullochella peelii peelii,Mitchell)

Cytochemistry has proven effective in differentiating specific cell lineages and elucidating their functional properties. This study utilised a range of cytochemical techniques to further investigate the leucocyte populations from Murray cod, an iconic Australian teleost fish species. This analysis provided clear insight into the structure and function of the leucocytes from this fish, which were found to be broadly similar to those of other fish species. However, some important differences were identified in Murray cod, such as the presence of naphthol AS chloroacetate esterase activity in the heterophil population, positive staining for periodic acid Schiff's, alkaline phosphatase and Sudan black B in the lymphocyte population, and a prevalent population of myeloid precursor cells.     

The neuroendocrine regulation of the human ovarian cycle.

The menstrual cycle is now thought to be mainly determined by the ovary itself, which sends various signals to the pituitary and the hypothalamus. The hypothalamus is an autonomous pacemaker, with a pulse frequency that is modulated by ovarian signals; in turn, it is indispensable to ovarian function. In women, the ovarian cycle produces a single mature oocyte each month from puberty to menopause. This follicle is rescued from atresia, the genetically controlled ovarian apoptosis (or "programmed cell death"), involving 99.9% of the follicles. Follicular growth and maturation are mostly independent of gonadotropins from the stage of primordial to antral follicles. A complete intraovarian paracrine system is implied in this gonadotropin-independent follicular growth and in the modulation of the action of gonadotropins in the ovary. Follicle-stimulating hormone (FSH) allows the rescue of a minority of follicles from atresia and is indispensable only for the final maturation of the preovulatory follicle during the follicular phase of the cycle. Luteinizing hormone (LH) is responsible for the final growth of the dominant follicle in the late follicular phase. the induction of ovulation during the LH peak, and the survival of the corpus luteum during the luteal phase. The cyclical variations of gonadotropins are under the control of ovarian steroids (estradiol and progesterone) and peptides (inhibins). The cycle length is determined by the duration of terminal follicular growth and by the fixed life span of the corpus luteum. The ovarian cycle can be monitored as well at the level of target tissues of steroids, such as the endometrium. In fact, the endometrial maturation is synchronized to follicular development, and this synchronization is indispensable for successful implantation of the embryo. The improving knowledge of follicular and endometrial physiology will allow the development of new treatments of infertility, the design of new contraceptive techniques, and a better tolerance of treatments using sex steroids.     

OVARIAN NUTRITIONAL RESOURCES DURING THE REPRODUCTIVE CYCLE OF THE HEMATOPHAGOUS DIPETALOGASTER MAXIMA (HEMIPTERA: REDUVIIDAE): FOCUS ON LIPID METABOLISM

In this study, we have analyzed the changes of the ovarian nutritional resources in Dipetalogaster maxima at representative days of the reproductive cycle: previtellogenesis, vitellogenesis, as well as fasting‐induced early and late atresia. As expected, the amounts of ovarian lipids, proteins, and glycogen increased significantly from previtellogenesis to vitellogenesis and then, diminished during atresia. However, lipids and protein stores found at the atretic stages were higher in comparison to those registered at previtellogenesis. Specific lipid staining of ovarian tissue sections evidenced remarkable changes in the shape, size, and distribution of lipid droplets throughout the reproductive cycle. The role of lipophorin (Lp) as a yolk protein precursor was analyzed by co‐injecting Lp‐OG (where OG is Oregon Green) and Lp‐DiI (where DiI is 1,10‐dioctadecyl‐3,3,30,30‐tetramethylindocarbocyanine) to follow the entire particle, demonstrating that both probes colocalized mainly in the yolk bodies of vitellogenic oocytes. Immunofluorescence assays also showed that Lp was associated to yolk bodies, supporting its endocytic pathway during vitellogenesis. The involvement of Lp in lipid delivery to oocytes was investigated in vivo by co‐injecting fluorescent probes to follow the fate of the entire particle (Lp‐DiI) and its lipid cargo (Lp‐Bodipy‐FA). Lp‐DiI was readily incorporated by vitellogenic oocytes and no lipoprotein uptake was observed in terminal follicles of ovaries at atretic stages. Bodipy‐FA was promptly transferred to vitellogenic oocytes and, to a much lesser extent, to previtellogenic follicles and to oocytes of ovarian tissue at atretic stages. Colocalization of Lp‐DiI and Lp‐Bodipy‐FA inside yolk bodies indicated the relevance of Lp in the buildup of lipid and protein oocyte stores during vitellogenesis.     

Polymorphic microsatellite loci for species of Australian freshwater cod, <Emphasis Type="Italic">Maccullochella</Emphasis>

The Australian freshwater cod genus, Maccullochella is represented by three species: Murray cod, M. peelii peelii, eastern freshwater cod, M. ikei, and trout cod, M.macquariensis. Seven novel microsatellite loci from M. ikei and six previously published loci from M. peelii peelii were tested on wild populations of Murray, eastern and trout cod. Levels of polymorphism varied between species with 13 loci polymorphic in Murray cod, 9 in trout cod and 7 in eastern cod. Observed heterozygosities ranged from 0.053 to 0.842. This suite of microsatellite loci will facilitate future studies of the genetic status of wild and hatchery bred populations of Maccullochella.     

Ovarian steroidogenesis and the role of sex steroid hormones on ovarian growth and maturation of the Japanese eel

Three sex steroid hormones, estradiol-17β (E2), 11-ketotestosterone (11-KT), and 17α,20β-dihydroxy-4-pregnen-3-one (DHP), are well established as primary estrogen, androgen, and progestin, respectively, in teleost fish. Japanese eel, Anguilla japonica, would be a suitable candidate to study ovarian steroid physiology of fish because the ovarian growth and steroidogenesis is dormant under laboratory condition but can be induced by administration of exogenous gonadotropic reagents. In this review, we summarized our work on the function and production of sex steroid hormones in the ovary of the Japanese eel during ovarian growth and oocyte maturation artificially induced by treatment with extract of salmon pituitary. In vitro and in vivo assays suggest that 11-KT and E2 play primary roles in previtellogenic and vitellogenic growth of oocytes, respectively, whereas DHP is essential for induction of final oocyte maturation. We also reviewed the correlation between ovarian steroidogenesis to produce these sex steroid hormones, serum titers and gene expression.     

Role of the tidal cycle in the gonadal development and spawning of the tropical wrasse Halichoeres trimaculatus

Many wrasses on coral reefs exhibit daily spawning that peaks around daytime high tides. In this study, we examined tidal-related ovarian development in the threespot wrasse, Halichoeres trimaculatus, a species common on coral reefs in the Indo-Pacific Ocean. When the fish were collected in the morning at different tidal phases, the gonadosomatic index (GSI) and ovarian histology changed; concomitant with increases in GSI towards high tide, a clutch of the most advanced oocytes developed from vitellogenic to maturation stages. Ovulated eggs and post-ovulated follicles (POF) existed in most ovaries around high tide, but only POF remained around ebb tide, suggesting that spawning occurred during or after high tide. We noticed that tidal-related spawning was considerable in the morning and that most ovaries collected on the afternoon high tide exhibited post-spawning features. This suggests that certain labrid species possess plasticity with regard to their spawning time and utilize potent environmental cues to ensure their reproductive success. When pieces of ovary were incubated with precursor steroids, high conversion of testosterone to 17beta-estradiol occurred during high and ebb tides, while that of 17alpha-hydroxyprogesterone to 17alpha,20beta-dihydroxy-4-pregnen-3-one and 17alpha,20beta,21-trihydroxy-4-pregnen-3-one was observed during low and flood tides. Incubation of pieces of ovary with human chorionic gonadotropin resulted in similar fluctuations in the steroid hormones with tidal phase. Production of these steroid hormones correlated with oocyte development in the ovaries and was probably regulated by gonadotropin. These results demonstrate that the daily cycle is fundamental for oocyte development, and that the tidal cycle is superimposed on this process.