Spawning Behaviour and Post-Spawning Migration Patterns of Atlantic Bluefin Tuna (Thunnus thynnus) Ascertained from Satellite Archival Tags

Spawning behaviour of Atlantic bluefin tuna (Thunnus thynnus) was investigated using electronic satellite tags deployed in the western Mediterranean spawning ground, around the Balearic Islands (years 2009-2011). All the fish were tagged underwater and released within schools. In general, the fish tagged in the same year/school displayed common migratory trends. Following extended residency around the Balearic Islands, most tagged tuna crossed the Strait of Gibraltar heading for the North Atlantic. Discrepancies between the migratory tracks reconstructed from this and previous electronic tagging studies suggest that the bluefin tuna Mediterranean population may comprise distinct units exhibiting differing migratory behaviours. The diving behaviour varied between oceanic regions throughout the migratory pathways, the shallowest distribution taking place in the spawning ground and the deepest at the Strait of Gibraltar. A unique diving pattern was found on the majority of nights while the fish stayed at the spawning ground; it consisted of frequent and brief oscillatory movements up and down through the mixed layer, resulting in thermal profiles characterized by oscillations about the thermocline. Such a pattern is believed to reflect recent courtship and spawning activity. Reproductive parameters inferred from the analysis of vertical profiles are consistent with those estimated in previous studies based on biological samples.     

Histological investigation on the ovarian cycle of the bluefin tuna in the western and central Mediterranean

The histological analysis of eastern Atlantic bluefin tuna Thunnus thynnus ovaries caught from February to September 1999–2000, made it possible to distinguish the presence of seven oocyte developmental stages and allowed the characterization of six time-dependent ovary maturity stages. The ovaries of mature (fork length, L _F ≥ 110 cm) bluefin tuna were non-active from August (spent period) to March (quiescent period) when they contained only perinucleolar-stage oocytes. Ovary development started in April to early May (recrudescent period) with the appearance of oocytes at the lipid stage. Vitellogenesis appeared in mid-May (ripening period) and post-vitellogenesis occurred in late May to mid-June (pre-spawning period). In late June to early July, hydrated oocytes, a sign of imminent spawning, were found only in specimens caught in Balearic waters. Females ranging between 100 and 110 cm L _F, captured during the recrudescent and ripening periods, had the largest oocytes at the lipid stage, most of which were degenerating. An extensive vitellogenic atresia was observed in the ovaries of five females caught during the spawning period in non-spawning areas.     

A histological investigation of the occurrence of non-reproductive female bluefin tuna Thunnus thynnus in the Mediterranean Sea

The presence of non-reproductive Atlantic bluefin tuna Thunnus thynnus females in the Mediterranean Sea was investigated through histological analysis of the gonads. Three hundred and twenty-six ovary samples were collected from adults captured at different locations in the Mediterranean Sea during the reproductive seasons between 1998 and 2008. Only three specimens were considered to be in a non-reproductive state: two of them were in a reabsorbing state showing ovaries with early vitellogenic oocytes and extensive α and β atresia of vitellogenic follicles; the third showed gonads with perinucleolar oocytes and was considered to be in a resting state. The low occurrence of non-reproductive individuals found in this study makes it unlikely that non-reproductive individuals aggregate with reproductive ones during their migration towards spawning grounds. Further research is suggested in order to investigate the potential presence of non-reproductive individuals on non-spawning grounds during the reproductive season.     

Comparative study of liver vitellogenin gene expression and oocyte yolk accumulation in wild and captive Atlantic bluefin tuna (Thunnus thynnus L.)

The sequence of vitellogenin A (VgA) and vitellogenin B (VgB) cDNAs in Atlantic bluefin tuna (Thunnus thynnus L.) were determined, and vitellogenin expression levels in the liver and oocyte yolk accumulation were compared in wild and captive-reared individuals. Liver and ovary samples were taken from 31 individuals reared experimentally in three commercial Atlantic bluefin tuna fattening sites in the Mediterranean Sea and from 33 wild individuals caught by commercial traps during the fish's migration towards their Mediterranean spawning grounds. The total length of VgA cDNA was 5585 nucleotides and that of VgB was 5267 nucleotides. The identity and similarity between deduced amino acid sequences of VgA and VgB were 60% and 78%, respectively. The Atlantic bluefin tuna VgA and VgB amino acid sequences have high similarities with those of other teleost fishes. Relative levels of VgA and VgB mRNAs were low in April, increased significantly during the reproductive period in May and June, and declined in July. There was a trend towards higher relative levels of VgA and VgB mRNAs in captive fish compared to wild individuals during the reproductive period. The surface occupied by eosinophilic yolk granules in fully vitellogenic oocytes, as well as the frequency of oocytes in late vitellogenesis, was significantly higher in captive compared to wild individuals. The study suggests that the experimental conditions under which Atlantic bluefin tuna individuals were reared allowed the occurrence of normal vitellogenesis, based on gene expression of VgA and VgB in the liver and yolk accumulation in the oocytes. The higher yolk accumulation and frequency of vitellogenic oocytes observed in the ovaries of captive fish suggest that improvements in feeding practices may result in an improved vitellogenic process.     

Genetic identity of YOY bluefin tuna from the eastern and western Atlantic spawning areas

We used 320 young-of-the-year (YOY) specimens of the highly migratory and overfished Atlantic bluefin tuna, Thunnus thynnus, Linnaeus 1758, to evaluate the hypothesis that Atlantic bluefin tuna comprises 2 stocks with spawning grounds in the Gulf of Mexico and in the Mediterranean Sea. Significant genetic differentiation at 8 nuclear microsatellite loci (F(ST) = 0.0059, P = 0.0005) and at the mitochondrial control region (Phi(ST) = 0.0129, P = 0.0139) was detected among YOY Atlantic bluefin tuna captured on spawning grounds in the Gulf of Mexico (n = 40) versus the western (n = 255) and eastern (n = 25) basins of the Mediterranean Sea. The genetic divergence among spawning populations, combined with the extensive trans-Atlantic movements reported for juvenile and adult Atlantic bluefin tuna, indicates a high degree of spawning site fidelity. Recognition of genetically distinct populations necessitates independent management of Atlantic bluefin tuna on spawning grounds and warrants evaluation of the level of mixing of populations on feeding grounds. The genetic pattern might not have been detected unless juvenile specimens or actively spawning adults had been sampled.     

Comparative oocyte morphology and fecundity of five characid species from São Francisco River basin,Brazil

The morphology of vitellogenic oocytes and the batch fecundity of five tropical forage species belonging to the family Characidae, were studied in 104 late‐maturing ovaries. Significant morphological differences between vitellogenic oocytes and ovarian follicles were found. The lowest batch fecundity values were recorded in Hemigrammus marginatus (480 ± 163) and Orthospinus franciscensis (1701 ± 562), which were the smaller species in terms of total length, body weight and oocyte diameter. The highest batch fecundity value was observed in Tetragonopterus chalceus (8384 ± 3944) having the highest GSI and oocyte diameter. Batch fecundity and gonad weight was highly correlated followed by body weight and total length. Relative fecundity was estimated per unit total length, body weight and gonad weight. The wide variation in fecundity observed between the specimens and the species analysed is possibly related to the multiple spawning reproductive strategy of these fishes. Since T. chalceus have higher values of batch and relative fecundity, it is concluded that this species has a higher reproductive potential than the other forage species studied.     

Ovarian structure and batch fecundity in Lophiomus setigerus

The ovarian structure and batch fecundity of anglerfish Lophiomus setigerus were examined from specimens collected in the East China Sea during March 1991 to September 1995. The right and left ovarian lobes were connected at their posterior ends. Stalk-like ovigerous lamellae protruded from the ovarian wall. During the spawning season, gelatinous material was secreted from the epithelia of both the ovigerous lamellae and ovarian wall, and these epithelia showed morphological changes accompanying the ovarian maturation cycle. Tertiary yolk, migratory nucleus, and mature stage oocytes occurred in the ovaries between May and November, when females with postovulatory follicles and developing vitellogenic oocytes were collected also. These results suggested an extended spawning season during which females undergo repeated spawnings. When the most advanced oocytes attained the secondary yolk stage, they formed a batch that separated from the adjacent group of smaller oocytes. Batch fecundity ( F ) in 20 females with secondary yolk stage ovaries was related to total length (L_T, mm) as F= 556.2 L_T^1.157 (300≤L_T≤396).     

Expression of vitellogenin receptor gene in the ovary of wild and captive Atlantic bluefin tuna (Thunnus thynnus)

The cDNA sequences of vitellogenin receptor proteins (VgR(+) and VgR(-)), containing or lacking the O-linked sugar domain, were determined in Atlantic bluefin tuna (Thunnus thynnus L.). VgR(-) gene expression in the ovary was compared in captive-reared and wild Atlantic bluefin tuna during the reproductive cycle. Gonad samples from adult fish were sampled from 2008 to 2010 from stocks reared in captivity at different commercial fattening operations in the Mediterranean Sea and from wild individuals caught either by traditional tuna traps during their migration towards the spawning grounds in the Mediterranean Sea or by the long-line artisanal fishery. In addition, juvenile male and female Atlantic bluefin tuna were sampled from a farming facility, to obtain baseline information and pre-adulthood amounts of VgR(-). The total length of VgR(+) cDNA was 4006 nucleotides (nt) and that of VgR(-) was 3946 nt. Relative amounts of VgR(-) were greater in juvenile females and in those adults having only previtellogenic oocytes (119 ± 55 and 146 ± 26 folds more than juvenile males, respectively). Amounts of VgR(-) were less in individuals with yolked oocytes (ripening stage, May-June) and increased after spawning in July (92 ± 20 and 113 ± 13 folds more than juvenile males in ripening and post-spawning fish, respectively). These data suggest that regulation of VgR(-) is not under oestrogen control. During the ripening period, greater VgR(-) gene expression was observed in wild fish than in fish reared in captivity, possibly because of (a) differences in water temperature exposure and/or energy storage, and/or (b) an inadequate diet in reared Atlantic bluefin tuna.     

Individual Spawning Duration of Captive Atlantic Bluefin Tuna (Thunnus thynnus) Revealed by Mitochondrial DNA Analysis of Eggs

This study presents the first results on Atlantic bluefin tuna (Thunnus thynnus) individual spawning duration and its short-term temporal behavior. The study was based on direct measurements resulting from mtDNA analysis of the offspring of spawners held in transport cages during the 2013 spawning monitoring survey in Balearic waters. The number of females consisted of approximately 259 individuals with an average weight of 186 kg. The survey began on May 22 and ended on July 3. Spawning started on May 30 and was observed every night afterwards. The sampling of eggs for genetic monitoring was conducted for 9 days interspersed from the beginning of spawning to the end of the survey. A total of 946 eggs were analyzed and revealed 129 different haplotypes; 77 of these were not previously detected in the Mediterranean. A total of 69 haplotypes were observed in more than one spawning event and those with higher frequency lasted their maximum possible duration. The haplotypes present at the beginning of spawning were also identified at the end of the sampling, indicating a minimum spawning duration of 34 days, and individual annual fecundity was estimated at around 1290 eggs gr^-1. These results differed from those generally presumed until now and are indicative of a much higher fecundity. Females exhibited a regular spawning schedule but with the capacity to shift the spawning hour during the spawning season. These results were observed for the eastern population of Atlantic bluefin tuna and before extrapolating to the western population, their validity should be proved.     

Evidence of Atlantic bluefin tuna spawning in the Bay of Biscay,north-eastern Atlantic

The spawning grounds of the Atlantic bluefin tuna (Thunnus thynnus) are traditionally considered to be the Gulf of Mexico (Gulf of Mexico) and the Mediterranean Sea (Mediterranean Sea). However, for the western Atlantic, unequivocal evidence of bluefin spawning outside the Gulf of Mexico has been shown. In this study we present the first records of genetically confirmed bluefin larvae in the southern Bay of Biscay (eastern Atlantic). These findings provide evidence of bluefin spawning activity outside the Mediterranean Sea, in the north-eastern Atlantic. However, our results suggest that the bluefin spawning in the Bay of Biscay is a sporadic phenomenon.     

Reproductive ecology of the glacier lanternfish Benthosema glaciale

Myctophids play a key role in the transfer of energy in pelagic food webs, linking zooplankton to higher trophic levels. Data are presented, for the first time for the glacier lanternfish (Benthosema glaciale), on maturity ogives, size-at-maturity and reproductive strategy in terms of ovarian organization and fecundity type in two contrasting environments: North Atlantic and Mediterranean. We analysed 292 females collected in Flemish Cap during early summer 2009 and 2011 and off the Balearic Islands during late autumn 2009 and early summer 2010. The spawning season differed between areas. Size at 50% maturity in Flemish Cap was 47.6 and 49.1 mm in 2009 and 2011, respectively, but significantly smaller, 24.5 mm, in the Balearic sea. B. glaciale is a batch spawner with asynchronous ovarian development and indeterminate fecundity. Batch fecundity showed significant relationships with length and weight, although average relative batch fecundity was low (1031 ± 396 oocytes/gram). We compared two staining techniques: Hematoxylin–eosin and PAS–hematoxylin–metanil yellow with the latter being more useful to detect mature females and spawning indicators in regressing and regenerating females. Our results contribute to a better understanding of the reproductive potential and ecological role of this species, the most abundant myctophid in the northern Atlantic.     

Quantification of ovarian follicles in bluefin tuna Thunnus thynnus by two stereological methods

The numbers of different types of ovarian follicles (developing, degenerating and postovulatory follicles) were estimated in bluefin tuna Thunnus thynnus using two stereological procedures: the model‐based method of Weibel & Gomez, which has become a tool of broad application in the quantification of oocytes in fishes, and the assumption‐free ‘disector’ (sic) method of Sterio. The estimates of developing follicles (follicles containing lipid‐stage, vitellogenic and migratory‐nucleus oocytes) made by the model‐based method tended to be lower than those obtained with the disector, though significant differences were not observed except for vitellogenic follicles. Counts of atretic follicles by the model‐based method were higher than those made using the disector, the differences being remarkable between both techniques, particularly in the case of β‐atresia, where the statistical analysis indicated significantly unequal estimations with the two methods. In contrast, the amount of postovulatory follicles estimated by the disector, which would stand for the realized batch fecundity, was somewhat larger than that calculated with the model‐based method.     

Fecundity of fishes inhabiting coastal and estuarine environments in the southwest Atlantic Ocean

The aim of this study was to estimate the fecundity of six marine fish species from the southwest Atlantic off the coast of São Paulo State, Brazil. In particular, the number of oocytes in the most advanced vitellogenic stage (NDO), batch fecundity and the number of batches that will be produced were estimated. Specimens of longfinger anchovy Anchoa filifera, atlantic anchoveta Cetengraulis edentulus, bay whiff Citharichthys spilopterus, Stellifer brasiliensis, rake stardrum S. rastrifer and southern kingcroaker Menticirrhus americanus were captured bimonthly, from June (2012) to May (2013). The mature ovaries were removed, weighed, fixed in formalin solution and histologically analysed. All histological sections were photographed and the images analysed using advanced image processing techniques. The estimated number of batches that will be recruited to the germinal vesicle migration/hydrated stock ranged from one to four batches. NDO is positively related to ovarian and female size. Small females with low ovary weight produce low NDO. The NDO in A. filifera, C. edentulus and M. americanus may represent the final number of oocytes to be spawned in the next spawning event, which could be used for batch fecundity estimation. This study contributes to a better understanding of the reproduction of species inhabiting shrimp fishery grounds which are caught as by-catch in tropical and subtropical ecosystems. Information on the maturity and fecundity of these populations is an important element in assessing stock status and the degree of fishing pressure these populations are experiencing.     

Spawning Dynamics and Size Related Trends in Reproductive Parameters of Southern Bluefin Tuna,Thunnus maccoyii

Knowledge of spawning behaviour and fecundity of fish is important for estimating the reproductive potential of a stock and for constructing appropriate statistical models for assessing sustainable catch levels. Estimates of length-based reproductive parameters are particularly important for determining potential annual fecundity as a function of fish size, but they are often difficult to estimate reliably. Here we provide new information on the reproductive dynamics of southern bluefin tuna (SBT) Thunnus maccoyii through the analysis of fish size and ovary histology collected on the spawning ground in 1993–1995 and 1999–2002. These are used to refine previous parameter estimates of spawning dynamics and investigate size related trends in these parameters. Our results suggest that the small SBT tend to arrive on the spawning ground slightly later and depart earlier in the spawning season relative to large fish. All females were mature and the majority were classed as spawning capable (actively spawning or non-spawning) with a very small proportion classed as regressing. The fraction of females spawning per day decreased with fish size, but once females start a spawning episode, they spawned daily irrespective of size. Mean batch fecundity was estimated directly at 6.5 million oocytes. Analysis of ovary histology and ovary weight data indicated that relative batch fecundity, and the duration of spawning and non-spawning episodes, increased with fish size. These reproductive parameter estimates could be used with estimates of residency time on the spawning ground as a function of fish size (if known) and demographic data for the spawning population to provide a time series of relative annual fecundity for SBT.     

An autofluorescence-based survey of late follicular atresia in ovaries of a teleost fish (Thunnus thynnus)

In this study, the authors examined late atretic follicles in the ovaries of Atlantic bluefin tuna, Thunnus thynnus (Linnæus 1758), at different times of the year using transmitted light and epifluorescence microscopy. Atresia (degeneration and resorption of developing ovarian follicles) is a natural process involved in fecundity downregulation in teleosts and is substantially enhanced in stressful conditions. Early (α and β) atretic stages of yolked oocytes have a relatively short duration in seasonally reproducing species, whereas later (γ and δ) atretic follicles (LAF) persist for longer time in the ovary, serving as a sign of previous vitellogenic activity. LAF can thus be used as reliable markers of maturity during non-reproductive periods. Lipofuscin granules accumulate in the cytoplasm of LAF cells as a result of lysosomal digestion of oocyte components. Taking advantage of the well-known autofluorescent properties of lipofuscins, LAF may be identified in unstained histological sections under fluorescence microscopy using appropriate excitation and emission wavelengths. The authors explore in this study the applicability of fluorescence microscopy to provide a fast and effective method to assess late atresia in fishes. This method may be particularly useful to determine sexual maturity in individuals sampled long after the spawning season, where LAF are difficult to detect in standard histological sections. Furthermore, LAF autofluorescence provides a rapid way to quantify late atresia in fishes using image analysis.     

Assessment of reproductive potential in multiple-spawning fish with indeterminate fecundity: a case study of yellow sea bream Dentex hypselosomus in the East China Sea

This study examined the spawning season, spawning frequency and batch fecundity of yellow sea bream Dentex hypselosomus in the East China Sea to reassess the previously reported reproductive characteristics of the species. Time-course sampling showed that this species had a diurnal ovarian maturation rhythm. Late tertiary yolk-stage oocytes appeared 2 days before spawning, starting the process of germinal vesicle movement and breakdown. On the day of spawning, ovulation and subsequent spawning occurred in the early morning (0400–0800 hours). Postovulatory follicles disappeared from the ovaries within c. 24 h of ovulation. Seasonal changes in the ovarian conditions indicated that this species spawned more or less throughout the year, with the peak ranging from spring to autumn. The compositions of the developing oocytes and degenerating postovulatory follicles in the ovaries suggested that most females spawned repeatedly over 2 to 3 consecutive days during the peak of the spawning season. Somatic body condition did not have a significant effect on batch fecundity, but there was a significant relationship between batch fecundity and fork length according to spawning status. Females spawning on consecutive days were more fecund than those spawning every other day. The findings show that this species has much greater reproductive potential than previously estimated.     

Reproductive strategy of the European perch,Perca fluviatilis Linnaeus, 1758 (Osteichthyes: Percidae) in the Anzali wetland,southwest Caspian Sea

Temporal variation in reproductive traits of geographically distributed fish is supposed to take place in response to the spatial and environmental variations. With regard to the wide distribution of the European perch in the northern hemisphere, important reproductive traits such as the initiation and duration of the spawning activity are likely to vary in different latitudinal gradients. In this study, reproductive biology of the European perch, Perca fluviatilis, is described, based on 324 specimens caught in the Anzali wetland (southwest Caspian Sea) between June 2008 and May 2009. The gonadosomatic index, oocyte frequency distribution and histological examination suggested a long vitellogenic process (October to February) and a short spawning season (January and February). The size‐frequency distribution of the oocytes showed that this perch is a species with group‐synchronous ovarian development. Ovarian development occurred only in one clutch of oocytes (700–900 μm oocyte diameter) with no indication of maturation of any subsequent clutch in the spawning season. The average of (realized) fecundity (±SD) was estimated to be 16177 ± 5846 eggs in late vitellogenic stage, which was lower than the potential fecundity (17188 ± 6917 eggs). Histological examination of the gonads revealed the existence of atretic oocytes in early vitellogenic stages (October and November). This investigation highlights the temporal variation in the initiation and duration of the reproductive activity of the European perch in this region compared to other geographical regions. The results emphasize the necessity of specific temporal management in fishing of European perch based on spatial differences in reproductive biology.     

Reproductive biology of the armoured catfish Loricariichthys castaneus (Castelnau, 1855) in Lajes reservoir,southeastern Brazil

Sex ratio, gonadal development, breeding season and fecundity of the armoured catfish Loricariichthys castaneus were described to assess its reproductive strategy in a Brazilian tropical reservoir. In total, 226 specimens (199 females and 27 males) were captured from September 2005 to August 2006 and examined in the laboratory. Females outnumbered males and achieved sizes larger than 330 mm TL. Oocyte development, determined by histological analysis, was asynchronous with oocyte size, ranging from pre‐spawning (27–270 μm) to spawning (243–3460 μm), followed by a sharp decrease in the mean oocyte diameter postspawning (590–730 μm) as the spawning proceeded. Spawning occurred throughout most of the year, peaking in August–September and reaching a low in April–May, according to variations in GSI and frequencies of stages of gonadal development. Batch fecundity ranged from 242 to 833 vitellogenic oocytes (relative fecundity = 2.27 oocytes g^?1), averaging 483 oocytes, and was positively related to gonad weight (P = 0.00003). Oocyte diameters ranged from 0.027 to 5.59 mm, with vitellogenic diameters ranging from 2.08 to 5.59 mm. Continuous development of oocytes throughout the year suggests that L. castaneus presents indeterminate fecundity and is a batch‐spawner. These attributes, associated with parental care and a wide reproductive period, correspond to an equilibrium strategy that has proved to be effective in the Lajes reservoir.     

Electronic Tagging of Atlantic Bluefin Tuna (Thunnus thynnus,L.) Reveals Habitat Use and Behaviors in the Mediterranean Sea

We analyzed the movements of Atlantic tuna (Thunnus thynnus L.) in the Mediterranean Sea using data from 2 archival tags and 37 pop-up satellite archival tags (PAT). Bluefin tuna ranging in size from 12 to 248 kg were tagged on board recreational boats in the western Mediterranean and the Adriatic Sea between May and September during two different periods (2000 to 2001 and 2008 to 2012). Although tuna migrations between the Mediterranean Sea and the Atlantic Ocean have been well reported, our results indicate that part of the bluefin tuna population remains in the Mediterranean basin for much of the year, revealing a more complex population structure. In this study we demonstrate links between the western Mediterranean, the Adriatic and the Gulf of Sidra (Libya) using over 4336 recorded days of location and behavior data from tagged bluefin tuna with a maximum track length of 394 days. We described the oceanographic preferences and horizontal behaviors during the spawning season for 4 adult bluefin tuna. We also analyzed the time series data that reveals the vertical behavior of one pop-up satellite tag recovered, which was attached to a 43.9 kg tuna. This fish displayed a unique diving pattern within 16 days of the spawning season, suggesting a use of the thermocline as a thermoregulatory mechanism compatible with spawning. The results obtained hereby confirm that the Mediterranean is clearly an important habitat for this species, not only as spawning ground, but also as an overwintering foraging ground.     

Ultrastructure of oogenesis in the bluefin tuna, Thunnus thynnus

Ovarian ultrastructure of the Atlantic bluefin tuna (Thunnus thynnus) was investigated during the reproductive season with the aim of improving our understanding of the reproductive biology in this species. The bluefin, like the other tunas, has an asynchronous mode of ovarian development; therefore, all developmental stages of the oocyte can be found in mature ovaries. The process of oocyte development can be divided into five distinct stages (formation of oocytes from oogonia, primary growth, lipid stage, vitellogenesis, and maturation). Although histological and ultrastructural features of most these stages are similar among all studied teleosts, the transitional period between primary growth and vitellogenesis exhibits interspecific morphological differences that depend on the egg physiology. Although the most remarkable feature of this stage in many teleosts is the occurrence of cortical alveoli, in the bluefin tuna, as is common in marine fishes, the predominant cytoplasmic inclusions are lipid droplets. Nests of early meiotic oocytes derive from the germinal epithelium that borders the ovarian lumen. Each oocyte in the nest becomes surrounded by extensions of prefollicle cells derived from somatic epithelial cells and these form the follicle that is located in the stromal tissue. The primary growth stage is characterized by intense RNA synthesis and the differentiation of the vitelline envelope. Secondary growth commences with the accumulation of lipid droplets in the oocyte cytoplasm (lipid stage), which is then followed by massive uptake and processing of proteins into yolk platelets (vitellogenic stage). During the maturation stage the lipid inclusions coalesce into a single oil droplet, and hydrolysis of the yolk platelets leads to the formation of a homogeneous mass of fluid yolk in mature eggs.