Wasp predation and wasp-induced hatching of red-eyed treefrog eggs

Eggs often suffer high levels of predation and, compared with older animals, embryos have few options available for antipredator defence. None the less, hatchlings can escape from many predators to which eggs are vulnerable. I studied early hatching as an antipredator defence of red-eyed treefrog embryos, Agalychnis callidryas, in response to egg predation by social wasps (Polybia rejecta). Red-eyed treefrogs attach their eggs to vegetation overhanging water, where they are exposed to arboreal and aerial predators. Wasps attacked half the egg clutches and killed almost a quarter of the eggs I monitored at a natural breeding site in Panama. Hatching tadpoles fall into the water, where they face aquatic predators. As predicted from improved survival of older hatchlings with aquatic predators, most undisturbed eggs hatched relatively late. However, many younger embryos directly attacked by wasps hatched immediately. Embryos attacked by wasps hatched as much as a third younger than the peak undisturbed hatching age, and most hatching embryos escaped. Thus hatching is an effective defence against wasp predation, and plasticity in hatching stage allows embryos to balance risks from stage-specific egg and larval predators. Wasp-induced hatching is behaviourally similar to the snake-induced hatching previously described in A. callidryas, but occurs in fewer eggs at a time, congruent with the scale of the risk. Individual embryos hatch in response to wasps, which take single eggs, whereas whole clutches hatch in response to snakes, which consume entire clutches. Embryos of A. callidryas thus respond appropriately to graded variation in mortality risks. Copyright 2000 The Association for the Study of Animal Behaviour.     

Hatch or wait? A dilemma in reptilian incubation

Animals often form groups to reduce the risk of predation through the per capita dilution of their individual predation risk. The advantages of grouping also influence the timing of reproduction in many species. In particular, synchrony in the timing of births may have evolved as a predator-avoidance strategy as it dilutes the risk of predation upon vulnerable newborn and naive young. Eggs of an Australian freshwater turtle, Emydura macquarii , can hatch synchronously despite developmental asynchrony among eggs of a clutch and hatchlings have a reduced predation risk by emerging from the nest as a group. Developmental asynchrony within clutches was induced to reflect natural nests by dividing clutches and incubating them at either 25°C or 30°C. Some eggs were then reunited with their clutch-mates and hatching occurred synchronously in some of these groups. In groups where synchronous hatching did not occur, less advanced eggs still hatched earlier than the normal incubation period. Synchrony occurred because the less advanced eggs hatched up to five days earlier than the control embryos. We conclude that the less advanced embryos within a clutch either accelerate their development or hatch prematurely to ensure synchrony of hatching and hatchling group formation may facilitate emergence from the nest and dilute predation risk.     

Response of diapausing eggs hatching to changes in temperature and the presence of fish kairomones

It has been hypothesized that the production of diapausing eggs in Daphnia can be induced by fish kairomones. A population of Daphnia could survive severe predation using this predator avoidance strategy. However, in changing environments, diapausing eggs experience various temperature conditions, and hatchlings at emergence may be exposed to the same predation risks as their mothers. Therefore, staying in diapause or an immediate response upon hatching to available environmental information could be important for hatchling survival. For this study, we investigated the impact of water temperature (10, 15, 20, and 25°C) in the presence and absence of fish kairomones (Lepomis macrochirus) on the hatching success of resting eggs (D. galeata). Results show that no diapausing eggs hatched at the lowest temperature (10°C), and the highest hatch percentage occurred at 15°C. Although higher water temperatures reduced hatching success, diapausing eggs hatched more quickly. The number of hatchlings was significantly higher after exposure to fish kairomones, and this was more noticeable at higher temperatures (20 and 25°C). The present results suggest that the diapausing eggs were produced as a predator avoidance strategy in Daphnia; however, the presence of fish works as a positive signal to increase hatchlings when the diapausing stage is terminated.     

Effects of vitamin B1 (thiamine) deficiency in lake trout (Salvelinus namaycush) alevins at hatching stage

The objectives of this study were to examine the relationship between thiamine concentrations in unfertilized eggs and yolksac individuals of lake trout (Salvelinus namaycush), along with any associated histopathological changes in the tissues of alevins at the hatching stage. We address these questions in a lake trout population from different spawning grounds of Lake Michigan (North and South), known for compromised survival due to early mortality syndrome (EMS). However, a dichotomous forage base of lake trout spawning stocks, with a dietary thiaminase-rich alewife in the North, and dietary low-thiaminase round goby in the South, provides the basis for the assumption that different diets may lead to differences in severity of EMS between different stocks. Lake trout eggs of 18 females were collected and fertilized individually with the sperm of several males. The eggs, eyed embryos and newly-hatched alevins were sampled to examine thiamine utilization during embryogenesis. Progenies of females with low (< 0.73 nmol/g) and high (> 0.85 nmol/g) levels of thiamine were chosen for histological studies. The obtained results showed that total thiamine levels in the body and yolk of eyed embryos and alevins at hatching were influenced by thiamine levels of unfertilized eggs and it decreased during embryogenesis (to 51% in eyed embryos and 28% in newly-hatched alevins in comparison to unfertilized eggs). The survival of lake trout until hatching stage does not correlate with the thiamine level, however it was affected by collection site and was significantly higher in fish from the South site (Julian's Reef). At the hatching stage, no pathological changes were observed in the brain, olfactory lobe, retina or liver in embryos regardless of thiamine concentrations in unfertilized eggs. It has been concluded that an enhanced thiamine requirement for the fast muscle mass growth near the swim-up stage is responsible for overt and histopathological signs of EMS. Current study confirms earlier findings that lake trout suffering from EMS can be successfully treated by immersion in thiamine solution as late as at the swim-up stage.     

Water quality during egg incubation influences yolk-sac fry sodium kinetics in Atlantic salmon, Salmo salar L.: a possible mechanism of adaptation to acid waters

Atlantic salmon ( Salmo salar L.) fry hatched from eggs transferred from high-Na to low-Na water during the eyed stage of development had a significantly higher V_max and lower K_m (P <0.01) of the sodium uptake mechanism than fry hatched from eggs incubated entirely in low-Na or high-Na water.
Fry hatched from eggs transferred to acid, high aluminium water during the eyed stage of development had a similar V_max and K_m to fry hatched from eggs incubated entirely in high- or low-Na water. Eggs incubated continuously in acid, high aluminium (low-Na) water produced fry with significantly lower K_m and V_max values than fry hatched from eggs incubated continuously in low-Na water. Eggs and fry in acid, high aluminium water continually lost sodium and mortality was 100% at 5 5 M O degree-days (2–3 weeks after hatching).
The results are discussed with respect to the influence of perivitelline fluid ion activities in eggs in acid, high aluminium water on the kinetic characteristics of sodium uptake in yolk-sac fry. A possible mechanism for the long-term adaptation of teleosts in acidified natural waters is also proposed.     

Developmental Expression of D-Galactoside-Binding Lectin in Sea Urchin (Anthocidaris crassispina) Eggs

The spatial and temporal expression of a sea urchin (Anthocidaris crassispina) egg lectin (SUEL) during early embryogenesis was studied using antiserum raised against SUEL. Western blotting analysis revealed the presence of SUEL in all stages so far examined, from unfertilized eggs to gastrula stage embryos. Immunofluorescence and immunoelectron microscopic observation showed that SUEL was stored in small electron-dense granules which migrated to the cortex within 10 min after fertilization. SUEL was localized in the cortical cytoplasm of the blastomere during cleavage stages and subsequently migrated to the outer surface of the embryo, including the invaginated portion of the gastrula. Immunoelectron microscopic study indicated that SUEL was deposited in the hyaline layer at least at the mid gastrula stage. Migration of SUEL to the cortex was significantly reduced by treatment with cytochalasin B, suggesting that actin filaments play an important role in this translocation. Exogenously added SUEL was adsorbed at the surface of unfertilized eggs and hatched embryos, but not to embryos with fertilization membrane. Lactose inhibited this adsorption, suggesting the presence of an endogenous glycoligand(s) specific for SUEL on the surface of unfertilized eggs and in the hyaline layer. We conclude that SUEL is secreted at a certain stage of embryogenesis and specifically adsorbed to the hyaline layer. Temporal changes in extraembryonic matrices caused by SUEL seem to play an important role in developmental morphogenesis.     

Preservation of rainbow trout (Oncorhynchus mykiss) eyed eggs using a perfluorochemical as an oxygen carrier

The objective of this study was to maintain the viability of chilled rainbow trout (Oncorhynchus mykiss) eyed eggs during storage using oxygenated perfluorochemical (PFC). Three trials were conducted using eggs at 161, 180 or 217 degree days (days from fertilization x incubation temperature in degrees C). A separate trial was conducted for 147 degree day eggs that were not at the eyed stage. For each trial, eggs were stored in a moisture-saturated atmosphere at 1 degrees C in PFC, water, and 1:1 combinations of PFC and PBS, PFC and 0.3 M glucose, PFC and mineral oil, or PFC and water. The PFC was oxygenated before each trial and all media were oxygenated at weekly intervals during the storage period. Eggs from each trial were also incubated without storage to provide Day 0 results. After 3 and 5 weeks of storage, eggs from each medium were incubated at 10 degrees C until hatch. Hatching percentage was expressed as a percentage of Day 0 results. The percentage of normal alevins that hatched was also determined. There were interactions (P < 0.01) between stage of development and treatment for hatching percentage after 3 and 5 weeks of storage. After 3 weeks of storage, eggs stored at 161, 180, or 217 degree days without PFC had hatching rates of 0-14.3% but eggs stored in any medium with PFC had hatching percentages from 75.1 to 106.4% of Day 0 values. After 5 weeks of storage, eggs stored at 161 degree days in PFC plus PBS or PFC plus water, and eggs stored at 217 degree days in PFC or PFC plus water, had higher (P < 0.05) hatching percentages than eggs stored in any of the other media. Eggs stored at 161 degree days for 5 weeks in PFC and water had a higher (P < 0.05) percentage of normal alevins hatching than eggs stored in PFC and PBS. Because of their early developmental stage, eggs stored at 147 degree days had low hatching percentages, except eggs stored for 3 weeks in PFC or PFC plus PBS. Chilling eyed eggs of rainbow trout to 1 degrees C and storing them in water with PFC as an oxygen carrier can preserve their viability for 5 weeks.     

Elevated water temperature impairs fertilization and embryonic development of whitefish Coregonus lavaretus

The adverse effects of high temperatures on the early life stages of anadromous whitefish Coregonus lavaretus were experimentally examined by assessing fertilization success, the percentage of developmental abnormalities, cumulative mortality and the rate of embryogenesis across a range of temperatures. Temperatures ≥ 7° C increased the proportion of unfertilized and abnormally dividing eggs, deformed embryos and consequent mortality. The higher the temperature, the more severe were the effects. When eggs were fertilized and constantly incubated at various temperatures, the effective level for 50% of the eggs and embryos (EL50) of temperature was 7·6° C at the developmental stage when eye pigmentation was visible. Fewer developmental abnormalities and a lower cumulative mortality rate were observed when embryos were exposed to high temperatures from the later, gastrula stage, than from fertilization or the four‐cell stage. Irrespective of retarded development in terms of day‐degrees (i.e. the sum of daily mean temperatures), a high incubation temperature reduced the development time of C. lavaretus, leading to earlier hatching, and hatched fry were shorter than at the reference temperature of 4–5° C. Global warming will particularly pose risks for stenothermic species such as C. lavaretus, with early life stages being especially susceptible. Thus, relatively small increases and fluctuations in river water temperatures during the spawning season of this anadromous species may have substantial negative impacts on its recruitment and population persistence.     

The effect of light and darkness on hatching in the pomacentrid Abudefduf saxatilis

Synopsis Reports that pomacentrid embryos hatch after dusk are confirmed by photic manipulation of sergeant major eggs. Embryos placed in the dark for 20 minutes or longer prior to their normal hatching after sunset hatched, whereas controls held in light did not hatch. Percent of hatched embryos correlated with increasing exposure to darkness up to one hour after which no further improvement in hatching was observed. Embryos maintained in continuous light during their normal twilight hatching period did not hatch. Also, embryos exposed to 60 minutes of darkness, if interrupted by one minute of light every 10 minutes did not hatch. The percent hatch in dark treatments varied significantly between nests and, in some treatments, correlated negatively with the size of the egg clumps (number of eggs per clump) tested. To initiate hatching in the presence of light required intensities of 0.03 lux or less. These low intensities are not reached until about 20 minutes after sunset on the reef where the embryos occur. We conclude that hatching for some embryos occurs about 30 minutes after sunset but for most is not completed until at least one hour after sunset. Hatching therefore takes place at a time long after potential diurnal fish predators have refuged in the reef structure.     

Patterns in the abundance,phenology, and hatching of the resting egg stage of the invasive zooplankter <Emphasis Type="Italic">Bythotrephes longimanus</Emphasis>: implications for establishment

To examine how dormancy contributes to the establishment and persistence of Bythotrephes longimanus, we investigated resting egg production and hatching in relation to the demography of the planktonic stage and environmental conditions in Island Lake Reservoir (USA). During a 3-year study, the largest contribution to the egg bank occurred in autumn and most eggs hatched in spring, but we also detected some resting egg production and hatching in summer. The difference between summer and late autumn densities of eggs in sediments averaged 47% (range 0–98%) for 18 sites throughout the reservoir, which was similar to experimental estimates of in situ hatching fraction of 67% for eggs in the spring and summer following their production. Based on emergence traps, neonates hatch in the field during May and June. We estimated mortality rates of 64% for resting eggs and embryos, and 59% for newly emerged neonates. Although hatching fraction saturated at the same level, eggs incubated offshore hatched later than those nearshore where water temperature was warmer and light was detectable at the sediment surface. Low dissolved oxygen concentration did not significantly reduce hatching fraction but resulted in some eggs that initiated development but failed to hatch. Collectively, our results demonstrate substantial annual turnover in the resting egg bank of B. longimanus and high mortality of resting eggs during recruitment from the egg to the first molt of the planktonic stage. These patterns suggest that propagule pressure in the form of resting eggs requires large numbers for establishment, and that considerable post-establishment resting egg production is necessary for inter-annual persistence.     

Tetracycline tagging in coregonid embryos and larvae

Coregonus peled (Gmelin) embryos at the eyed stage were immersed in tetracycline hydrochloride (TC) solution (600 mg l⁻¹) and fluorescent marks on the otoliths from fish were identified under a UV light microscope. Three weeks after treatment, when the larval fish had hatched, multiple primordia of the sagitta had fluorescent bands whose locations suggest that calcification begins before formation of the otolith's core. Upon hatching, increments were few or absent, and daily increments started forming after hatching. In coregonid larvae immersed in TC solution immediately after hatching, the fluorescent band was only 1 day's growth increment wide in the otolith. The age of larvae based on ring counts after marking corresponded to the actual age, but further studies are required. Triple marks due to 35-h immersions in TC solution separated by 5–7 day feeding periods were clearly readable in the otoliths. One month after treatment the fluorescent marks were identified under UV light on otoliths from every fish. In treated embryos, 38% of fish had marks 3 months after TC immersion, whereas in treated larvae 60–80% of fish grown up to 38.7 mm were marked.     

泥鳅和白鲢胚胎不同发育时期耗氧量的研究

泥鳅和白鲢胚胎发育时的耗氧量是随着发育进程而以指数函数方式在增长,其耗氧曲线显示出卵裂期、原肠期、神经胚期、肌肉效应期以及出膜期是5个呼吸代谢水平较高的时期,其中又以原肠期与出膜期对孵化环境溶氧的需求尤为突出。胚胎在发育过程中纳入氧的快慢与强弱,都直接或间接地反映出新陈代谢的速度以及营养物质不断地被氧化、消耗和能量传递的状况。因此,鱼类胚胎发育时耗氧的高低,无疑是其胚胎发育状况的一种生理指标。     

Fertilization of eggs of zebrafish, Danio rerio, by intracytoplasmic sperm injection

To evaluate the potential for fertilization by sperm injection into fish eggs, sperm from zebrafish, Danio rerio, were microinjected directly into egg cytoplasm of two different zebrafish lines. To evaluate physiological changes of gametes on the possible performance of intracytoplasmic sperm injection (ICSI), four different combinations of injection conditions were conducted using activated or nonactivated gametes. From a total of 188 zebrafish eggs injected with sperm in all treatments, 31 (16%) developed to blastula, 28 (15%) developed to gastrula, 10 (5%) developed abnormally to larval stages, and another 3 (2%) developed normally and hatched. The highest fertilization rate (blastodisc formation) was achieved by injection of activated spermatozoa into nonactivated eggs (35%). Injections were most effective when performed within the first hour after egg collection. Flow cytometric analysis of the DNA content of the developing ICSI embryos revealed diploidy, and the use of a dominant pigment marker confirmed paternal inheritance. Our study indicates that injection of a single sperm cell into the cytoplasm of zebrafish eggs allows fertilization and subsequent development of normal larvae to hatching and beyond.     

Development, surface exposure, and embryo behavior affect oxygen levels in eggs of the red-eyed treefrog, Agalychnis callidryas

Oxygen stress can slow development, induce hatching, and kill eggs. Terrestrial anamniote embryos face a potential conflict between oxygen uptake and water loss. We measured oxygen levels within eggs to characterize the respiratory environment for embryos of the red-eyed treefrog, Agalychnis callidryas, a Neotropical frog with arboreal egg masses and plastic hatching timing. Perivitelline oxygen partial pressure (Po2) was extremely variable both within and among eggs. Po2 increased with air-exposed surface of the egg and declined over the developmental period before hatching competence. Through the plastic hatching period, however, average Po2 was stable despite continued rapid development. Development was synchronous across a wide range of perivitelline Po2 (0.5-16.5 kPa), and hatching-competent embryos tolerated Po2 as low as 0.5 kPa without hatching. The variation in Po2 measured over short periods of time within individual eggs was as great as that measured across development or surface exposure, including sharp transients associated with embryo movements. There was also a strong gradient of Po2 across the egg from superficial to deep positions. Ciliary circulation of fluid within the egg is clearly insufficient to keep it mixed. Embryos may maintain development under hypoxic conditions by strategic positioning of respiratory surfaces, particularly external gills, to exploit the patchy distribution of oxygen within their eggs.     

Patterns of Embryo Mortality in a Demersally Spawning Coral Reef Fish and the Role of Predatory Fishes

The biological significance of embryo mortality in demersally spawning coral reef fishes is poorly understood. Here we describe patterns of variation in embryo mortality in Pomacentrus amboinensis (Pomacentridae) at Lizard Island (Great Barrier Reef). The aim was to determine whether numbers of embryos hatched substantially differed from egg production, and if so, identify whether predatory fishes were a source of embryo mortality. Spawning success (number of eggs laid), embryo mortality (proportion of embryos that died prior to hatching) and number of embryos hatching were estimated from daily maps of clutches laid on artificial surfaces (PVC tiles) defended by nesting males. Patterns of variation in eggs laid, embryo mortality and numbers of embryos hatched were examined at three spatial scales: (1) among widely-spaced locations around the island; (2) between adjacent reef slope and patch reef habitats occupied by P. amboinensis at a single location; and (3) among different males within these two habitats. The embryo mortality was extremely high, with a mean of 25.9pm ± 6.2% (S.E.) for 4 locations examined in 1994 and a mean of 69.2pm ± 2.9% for two habitats surveyed in 1995. There were no significant differences in embryo mortality among locations or habitats in either year. This meant that spatial patterns in the number of embryos hatching reflected differences in the number of eggs laid on tiles. Embryo mortality was extremely variable on the scale of individual territories, with embryo mortality commonly ranging from 0% to 100%. Much of the mortality could be attributed to diurnal predatory fishes, especially the wrasse Thalassoma lunare. However, variation in predator densities did not explain spatial patterns in embryo mortality rates. Both solitary and group predatory behaviour was observed, with groups often causing 100% embryo mortality. The level of embryo mortality observed suggests that predation prior to hatching may have a substantial effect on the reproductive output of populations of this demersal-nesting fish.     

Factors affecting the uptake of DMSO by the eggs and embryos of medaka,Oryzias latipes

High performance liquid chromatography (HPLC) was used to assess the uptake dynamics of the cryoprotectant DMSO by intact unfertilized eggs (stage 0), 8-cell (stage 5) and eyed embryos (stage 30) of medaka, Oryzias latipes, the relation of the internal concentration (Cin) of DMSO with fertilization and survival rates, and the effects of several factors on these processes. The factors examined were: cryoprotectant concentration (0.6, 1.2, 1.9 and 2.5 M), impregnation time (1, 3, 5, 10, 15 and 20 min), temperature (0, 5 and 20 degrees C), hydrostatic pressure (0 and 50 atm), and the osmotic conditions of the materials (normal or partially dehydrated). Cryoprotectant permeation, estimated from the initial rates of DMSO uptake, was higher in embryos than in eggs and increased with embryonic development; however, the DMSO Cin in eyed embryos reached a plateau at 1-5 min and could not be increased by prolonging impregnation. The highest fertilization and survival rates for any given DMSO Cin were obtained with high concentrations and short times of impregnation rather than low concentrations and long impregnation times. Application of hydrostatic pressure (50 atm) and exposure for 3 min to a 1 M trehalose solution prior to impregnation induced a substantial increase in the DMSO Cin of 8-cell embryos in comparison to untreated controls with no significant effect on survival. Hydrostatic pressure also promoted DMSO uptake in unfertilized eggs, but with rapid loss of viability, and was ineffective in eyed embryos. The uptake of DMSO and its toxicity to 8-cell embryos were directly proportional to the temperature of impregnation. The results of this study reveal important interactions between cryoprotectant concentration, impregnation time and the developmental stage (or type) of the materials and provide evidence that hydrostatic pressure, temperature of impregnation and the osmotic conditions of the materials can be manipulated to increase the uptake of cryoprotectant by fish eggs and embryos.     

The effect of early and late hatching on the escape response of walleye pollock (Theragra chalcogramma) larvae

Hatching of fish eggs fertilized at the same time occurs overa period of several days. Differences in the escape responseof fish larvae during the hatching period have not hithertobeen studied. In this study, the escape response of walleyepollock (Theragra chalcogramma) larvae over the hatching periodwas examined. Escape speed, response to multiple touches witha fine probe, response to water currents generated by a predatorand predation by euphausiids (Thysanoessa inermis) and amphipods(Pleusirus secorrus) were measured in the laboratory. Otolithmeasurements of field-collected larvae support a broad hatchingperiod for walleye pollock eggs in the sea similar to that observedin the laboratory. The escape response of walleye pollock larvaewas affected by rank in the order of hatching, thus with respectto predation, hatching order may affect the survival of larvaein the sea. Early hatching larvae were smaller, less sensitiveto tactile stimulation, had a slower, weaker escape responseand higher laboratory rates of predation mortality than thosethat hatched later.     

Effects of high gravity on amphibian development.

In order to clarify the possible effects of high gravity environments on eggs and developing embryos, Rana rugosa and Xenopus laevis fertilized eggs and early embryos were raised in 2 G, 5 G, 7 G and 10 G up to the hatched tadpole stage. The results showed that: (1) High gravity significantly retarded the development of eggs and embryos beginning treatment before the blastula stage and induced various abnormalities, including two heads and microcephally suggesting that high gravity is apt to disrupt the animal-vegital axis. On the other hand, embryos beginning treatment after the gastrula stage showed a striking increase in the number of normal-appearing feeding tadpoles. (2) Autopsy revealed that brains, notochords and muscles were reduced in development and differentiation for embryos and tadpoles developed in high gravity. (3) It seems likely that the system for hydrogen peroxide detoxification develops abnormally in high gravity-treated embryos and tadpoles, which probably results in oxidative stress, leading to considerable cell damage.     

Position of egg within a clutch is linked to size at hatching in a demersal tropical fish

Size variation among propagules is ubiquitous and small initial differences in size can be critical to survival, particularly in taxa where initial survival is variable and strongly size-dependent. Despite this, the sources of size variation among fish at hatching are rarely investigated. This study examined spatial position within egg clutches as a source of size variation at hatching of the benthic spawning fish Amphiprion melanopus. We quantified within-clutch size variation at hatching and found that newly hatched larvae from the periphery (5 mm from edge) of 2-dimensional clutches were smaller in standard length, cranial depth, eye diameter and body area (7%, 8%, 4% and 11%, respectively) than larvae from the interior positions within clutches. To investigate the source of this variation, sizes of embryos at different locations with clutches were measured within 2 h of fertilisation (8 d before hatching). Newly laid embryos from the clutch periphery were smaller in length and volume than embryos from the clutch interior (> 2% and 4-6%, respectively). These eggs from the periphery also had a 33% lower rate of oxygen consumption than did embryos from the clutch interior, throughout development. The relationships between position within a clutch and egg size, oxygen consumption and larval size imply that size variation in larval fish at hatching is partly generated during early embryogenesis, either from maternal endowment or maternal nest design, and was amplified throughout development.     

Length of herring larvae in relation to age and time of hatching

The lengths of newly hatched herring Clupea harengus larvae were generally larger than the lengths of embryos released from eggs on the same day. Over subsequent days, the daily increase in length was higher in groups that were released from the eggs as opposed to groups that hatched naturally. Thus by the end of the hatching period (18 days after fertilization) there were no significant differences in mean lengths.