Yolk-sac Larvae Research Articles

Ontogenetic development of a rare liparid, Paraliparis dipterus, collected from Suruga Bay, Japan, with notes on its reproduction

Adult liparid specimens [17.2–47.2 mm standard length (SL)], together with larvae and juveniles (5.6–16.5 mm SL), collected from near-bottom depths of 174–802 m in inner Suruga Bay, southern Japan, were identified as Paraliparis dipterus, previously known only from the holotype. Descriptions of the ontogenetic development of the species, together with their gut contents, are provided, along with notes on ovarian eggs. The present species can be easily distinguished from nine other Japanese species of Paraliparis by the combination of 54–58 dorsal-fin rays, 48–54 anal-fin rays and 6 caudal-fin rays. Ontogeny is characterized by the acquisition of general adult characters by the flexion stage (e.g., fin ray numbers, except pectoral fin, and most body proportions) and indistinct transformation thereafter, with some characters changing gradually throughout the juvenile to adult stages (e.g., elongating upper and lower pectoral-fin rays), as in other known species of Paraliparis. Radiolaria, Harpacticoida and other copepods dominated the gut contents. The months when both yolk-sac larvae and adults possessing “developed” ovarian eggs (≥0.9 mm in diameter) were collected indicated that P. dipterus spawns year-round.

Sublethal effects of alizarin complexone marking on Baltic cod ( Gadus morhua) eggs and larvae

Standard, single-exposure alizarin complexone- (ALC) marking was conducted on early life stages of Baltic cod ( Gadus morhua L.) to examine acute and chronic effects. Embryos and yolk sac larvae were marked using different concentrations of ALC (0, 50, 100 and 200 mg l − 1 ). Experiments included control groups for treatment and handling effects. In agreement with previous studies, long-lasting, distinct otolith marks were produced by immersion of yolk sac larvae in ≥ 50 mg l − 1 ALC for 24 h. Mortality of eggs and larvae was low during the marking procedure. Hatching success of ALC marked embryos was significantly reduced and hatching was delayed with increasing ALC concentration. Growth rates of larvae through 21 dph (exogenous feeding stage) were significantly reduced in ALC marked fish compared to controls. Biochemical condition (RNA–DNA ratio) was not affected. Subtle changes in activity and metabolism were indicated by reduced first feeding success and yolk absorption rates. Our results reveal that batch marking of finfish early life stages using ALC can have not only significant, acute impacts on survival but, despite long-standing assumptions to the contrary, also sublethal effects on other vital parameters like growth. We discuss these acute and chronic effects and give recommendations for assessment of life stage- and species-specific ALC immersion marking procedures.

Relationship between oxygen consumption, growth and survival of larval fish

This work aimed to evaluate the relationship between morphological development, oxygen consumption and reduced mortality in larval fish. We measured the resting metabolic rate (RM), specific metabolic rate (SMR) and the change in the total length during the larval stage of four fish species. Resting metabolism decreased from hatching to mouth opening, and then increased after mouth opening. The changes in the SMR were variable during the larval stage. After hatching, there was no increase in SMR in yolk-sac larvae. However, SMR increased between mouth opening and the onset of notochord flexion and then decreased during notochord flexion before finally stabilizing. We observed two peaks in mortality during the larval period of all species: between mouth opening to the onset of notochord flexion and from the completion of notochord flexion to the juvenile stage based on the per cent mortality and the number of dead fish collected from the bottom of the rearing tank. Interestingly, the changes in SMR coincided with these periods of mortality. We hypothesize that larvae require more energy during these periods of larval development and are thus more susceptible to mortality when energy is insufficient. Thus, it is important to supply enough nutrition to larvae in during early development to prevent mass larval mortality.

Efficacy of florfenicol in the treatment of bacterial infections in halibut, Hippoglossus hippoglossus (L.), larvae

At early life stages, Atlantic halibut larvae are susceptible to bacterial infections often caused by opportunistic Vibrio species (Bergh, Nilsen & Samuelsen 2001). Eggs and yolk-sac larvae are dominated by non-fermentive rods belonging to the Cytophaga/ Flexibacter/Flavobacterium group. During first feeding, the intestinal biota gradually changes towards a fermentive biota dominated by the Vibrio/Aeromonas group (Bergh, Naas & Harboe 1994). As the larvae possess no specific immune system when hatched, vaccination is not an option at this stage (Vadstein, Mo & Bergh 2004; Bjornsdottir, Johannsdottir, Coe, Smaradottir, Agustsson, Sigurgisladottir & Gudmundsdottir 2009). Therefore, when an infection occurs, one possibility for the farmer is to apply antibacterial agents for treatment. Antibacterial agents may be administered to fish larvae either by bath or orally via the feed (Bergh et al. 2001; Samuelsen, Nerland, Jorgensen, Schroder, Svasand & Bergh 2006). However, as bath treatment was found to be more costly and less effective (Duis, Hammer, Beveridge, Inglis & Braum 1995; Gapasin, Nelis, Chair & Sorgeloos 1996; Katharios, Smullen & Inglis 2005), administration via live feed organisms was considered a better choice, and bioencapsulation of a number of antibacterial agents in Artemia sp. nauplii and rotifers, Brachionus plicatilis, was investigated (Mohney, Lightner, Williams & Bauerlein 1990; Verpraet, Chair, Leger, Nelis, Sorgeloos & Deleenheer 1992; Touraki, Rigas & Kastritsis 1995; Gapasin et al. 1996; Gomez-Gil, Cabanillas-Ramos, Paez-Brambila & Roque 2001; Cook & Rust 2002; Mejia, Barrera, Figueroa, Castro & Andrade 2007; Langdon, Nordgreen, Hawkyard & Hamre 2008; Roiha, Otterlei & Samuelsen 2010b,c). Furthermore, various studies have shown that uptake of antibacterial agents in fish larvae is feasible using administration via live feed. Studies have been performed on European sea bass, Dicentrarchus labrax (L.), turbot, Scophthalmus maximus (L.), and cod, Gadus morhua L., larvae, applying Artemia nauplii enriched with florfenicol, flumequine, oxolinic acid, sarafloxacin and a combination of trimethoprim (TMP) and sulphamethoxazole (SMX) at a ratio of 1:5 (Chair, Romdhane, Dehasque, Nelis, De Leenheer & Sorgeloos 1991; Chair, Dehasque, Sorgeloos, Nelis & De Leenheer 1995; Gapasin et al. 1996; Touraki, Mourelatos, Karamanlidou, Kalaitzopoulou & Kastritsis 1996; Touraki, Niopas & Kastritsis 1999; Roiha, Otterlei, Litlabo & Samuelsen 2010a; Touraki, Niopas, Ladoukakis & Karagiannis 2010). The purpose of this study was to investigate the possibility of treatment of bacterial infections in Atlantic halibut, Hippoglossus hippoglossus (L.), larvae using Artemia franciscana nauplii as carriers of florfenicol. In 2009, high and unexpected mortalities of halibut larvae were observed in a number of Journal of Fish Diseases 2011, 34, 927–930 doi:10.1111/j.1365-2761.2011.01307.x

Some biological characteristics of early larvae Dacodraco hunteri (Notothenioidei: Channichthyidae) in the western Ross Sea

Ichtyoplankton surveys were carried out in the western Ross Sea by the R/V Italica in the austral summer 1996 and 1997–1998 to study species composition and spatial distribution of larval stages of fish. One of the most abundant icefish caught was Dacodraco hunteri, a poorly known channichthyid inhabiting the high-Antarctic Zone. Based on 382 yolk-sac larvae and 13 preflexion larvae, the study was focused to estimate spatial distribution and abundance, as well as diet and growth rate. The pigmentation pattern and some morphometric measurements were also recorded for comparative purposes. The specimens were caught in relatively restricted areas located in Terra Nova Bay and north of the Ross Ice Shelf on the Challenger and Joides Basins. The standardized abundance of early larvae ranged between 0.03 and 1.72 individuals 10−3 m3 in 1996 and 0.16–4.53 individuals 10−3 m3 in 1997–1998, respectively. Fitting a linear model to the mean length increase in larvae collected in subsequent catch dates, the growth rate was estimated to be approximately 0.11 mm/day. Based on back calculation of growth rate and presumed hatch size of 11 mm, larval hatching probably took place in mid-December. Diet of preflexion larvae consisted exclusively of larvae of the pelagic nototheniid Pleuragramma antarcticum, a key species of the high-Antarctic pelagic food web. Hence, D. hunteri probably plays a more important role than previously thought in the pelagic community of the Ross Sea.

Differential immune response of rainbow trout ( Oncorhynchus mykiss) at early developmental stages (larvae and fry) against the bacterial pathogen Yersinia ruckeri

Innate immune factors play a crucial role in survival of young fish especially during early stages of life when adaptive immunity is not fully developed. In the present study, we investigated the immune response of rainbow trout ( Oncorhynchus mykiss) larvae and fry at an early stage of development. We exposed 17 and 87° days post hatch larvae and fry (152 and 1118 degree days post hatch; avg. wt. 70 and 770 mg, respectively) to the bacterial pathogen, Yersinia ruckeri for 4 h by bath challenge. Samples were taken at 4, 24, 72 and 96 h post exposure for qPCR and immunohistochemical analyses to elucidate the immune response mounted by these young fish. Larvae showed no mortality although infected larvae at 48 h post exposure showed hyperaemia in the mouth region and inflammation on the dorsal side of the body. Gene expression studies showed an up-regulation of iNOS and IL-22 in infected larvae 24 h post exposure but most of the investigated genes did not show any difference between infected and uninfected larvae. Immunohistochemical studies demonstrated a high expression of IgT molecules in gills and CD8 positive cells in thymus of both infected and uninfected larvae. Infection of rainbow trout fry with Y. ruckeri, in contrast, induced a cumulative mortality of 74%. A high expression of cytokines (IL-1β, TNF-α, IL-22, IL-8 and IL-10), acute phase proteins (SAA, hepcidin, transferrin and precerebellin), complement factors (C3, C5 and factor B), antimicrobial peptide (cathelicidin-2) and iNOS was found in infected fry when compared to the uninfected control. IgT molecules and mannose binding lectins in gills of both infected and uninfected fry were detected by immunohistochemistry. The study indicated that early life stages (yolk-sac larvae), merely up-regulate a few genes and suggests a limited capacity of larvae to mount an immune response by gene regulation at the transcriptional level. Based on the observed clearance of bacteria and lack of mortality it could be speculated that larvae may be covered by protective shield of different immune factors providing protection against broad range of pathogens. However, the increased susceptibility of older fry suggests that Y. ruckeri may utilize some of the immune elements to enter the naive fish. The up-regulation of iNOS and IL-22 in the infected larvae implicates an important role of these molecules in immune response at early developmental stages. A dense covering of surfaces of gill filaments by IgT antibody in the young fish suggest a role of this antibody as innate immune factor at early developmental stages.

Fine-scale observations of the predatory behaviour of the carnivorous copepod Paraeuchaeta norvegica and the escape responses of their ichthyoplankton prey, Atlantic cod (Gadus morhua)

Paraeuchaeta norvegica (8.5 mm total length) and yolk-sac stage Atlantic cod larvae (4 mm total length) (Gadus morhua) larvae were observed in aquaria (3 l of water) using silhouette video photography. This allowed direct observations (and quantitative measurement) of predator–prey interactions between these two species in 3-dimensions. Tail beats, used by cod larvae to propel themselves through the viscous fluid environment, also generate signals detectable by mechanoreceptive copepod predators. When the prey is close enough for detection and successful capture (approximately half a body-length), the copepod launches an extremely rapid high Reynolds number attack, grabbing the larva around its midsection. While capture itself takes place in milliseconds, minutes are required to subdue and completely ingest a cod larva. The behavioural observations are used to estimate the hydrodynamic signal strength of the cod larva’s tail beats and the copepod’s perceptive field for larval fish prey. Cod larvae are more sensitive to fluid velocity than P. norvegica and also appear capable of distinguishing between the signal generated by a swimming and an attacking copepod. However, the copepod can lunge at much faster velocities than a yolk-sac cod larva can escape, leading to the larva’s capture. These observations can serve as input to the predator–prey component of ecosystem models intended to assess the impact of P. norvegica on cod larvae.

Stochastic feeding of fish larvae and their metabolic handling of starvation

Developmental patterns of yolk-sac larvae are well captured by the standard deb model: (i) when feeding is delayed post birth the size at which post-feeding growth begins is reduced but the rate of growth post-feeding is unaffected and (ii) maternal effects (initial energy in egg) show up as differences in condition at birth and maximum length of non fed individuals. We extended the standard deb model in two ways to account for starvation. (I): if somatic maintenance can no longer be paid structure is also mobilized to cover the costs, but at an extra cost–conversion efficiency of structure to energy. Death occurs if structure reaches a fraction of the maximum at the onset of shrinking. (II): if maturity maintenance can no longer be paid then maturity level decays exponentially (rejuvenation). Hazard due to rejuvenation is proportional to the difference between maturity and the maximum maturity at the onset of rejuvenation. We performed Monte Carlo simulation studies which treat feeding as a random process to evaluate the contribution of the metabolic handling of starvation to early teleost life history. The simulations suggest that food density strongly impacts growth, energy reserves, mineral fluxes, hazard and mortality from shrinking. Environmental factors can soon override maternal induced differences between individuals. Moreover in the low food density, simulated individuals from eggs of lower caloric content experience mortality from shrinking earlier than their counterparts issued from higher energy eggs. Empirically observed patterns of real data, i.e. high scatter in respiration in combination with low scatter in lengths, can be expected when the metabolism is treated as a deterministic system while behaviourally controlled input is stochastic. At low food densities where mortality from shrinking reaches 10% almost all individuals experience hazard due to rejuvenation. This hazard is difficult to access experimentally but represents moments of heightened susceptibility to pathogens and toxicants and could be ecologically significant.

Modeling transport and survival of anchoveta eggs and yolk–sac larvae in the coastal zone off central-southern Chile: Assessing spatial and temporal spawning parameters

A hydrodynamic model was coupled to an individual-based model (IBM) initialized with conditions observed during 2004 to examine the influence of spatial and temporal spawning parameters on transport and survival of Engraulis ringens in central-southern Chile. Processes modeled were transport, growth, and mortality of the early stages. The results showed that spatial and temporal variations in spawning modulated the survival of anchoveta eggs and yolk–sac larvae. The resultant transport of the early stages had an important south–north direction. In terms of connectivity, the greatest transfer of eggs and larvae to the pre-recruitment zone came from four spawning zones. Retention occurred in all spawning zones and was strongest in the Gulf of Arauco and at Lebu– Corral. Mortality due to temperature and advective losses was more intense at higher latitudes ( Lebu– Corral). Observed and simulated distributions of eggs and larvae were compared, and the differences are discussed herein. A conceptual model is proposed, and we discuss the relevance of IBMs as useful tools with which to assess spawning zones for purposes of management.

Seasonal and annual variation in Chilean hake Merluccius gayi spawning locations and egg size off central Chile

Variability in Chilean hake reproductive tactics off central Chile was assessed by analyzing ichthyoplankton samples from nine oceanographic cruises (1996–2005) and through experimental trials with early life stages (eggs, yolk-sac larvae) during the main (austral spring) and secondary (late summer–early autumn) spawning seasons. Abundant eggs in the plankton (1300–2000 eggs per 10 m 2) and historical adult reproductive data showed the highest reproductive activity in austral spring, with large egg aggregations near shelf break (50–100 m depth). Large, recently spawned eggs (1.15–1.20 mm diameter) were advected nearshore by coastward subsurface flows in the spring upwelling season. Experimental trials indicated that recently hatched larvae (3.4–3.5 mm) consumed their yolk-sac (0.17–0.41 mm 3) in 3–4 days at 10–12 °C; plankton sampling indicated that larval hake remained at mid-depth (50–100 m) without showing daily vertical migrations until completing their caudal fin formation (∼15 mm). During the secondary reproductive peak, hake spawned nearshore, when smaller eggs (0.95–1.13 mm) and recently hatched larvae (2.2–2.6 mm notochord length) occurred in surface waters (0–10 m depth). Their relatively large yolk-sac volumes (0.57 ± 0.11 mm 3) provided endogenous nourishment for at least 5 days at 10 °C, according to experiments. In the field, preflexion larvae occurred mainly in the mixed layer (0–25 m) and started ontogenetic daily vertical migrations at 7 mm. A strong decline occurred after 2002 in the adult Chilean hake biomass (estimated by hydroacoustic surveys) and body size, coinciding with variations in spawning locations (more coastward in early spring 2004 and 2005) and decline in egg size. Thus, recent variations in Chilean hake reproductive tactics may reflect an indirect effect of declines in the parental population size.

Egg size-dependent expression of growth hormone receptor accompanies compensatory growth in fish.

Large egg size usually boosts offspring survival, but mothers have to trade off egg size against egg number. Therefore, females often produce smaller eggs when environmental conditions for offspring are favourable, which is subsequently compensated for by accelerated juvenile growth. How this rapid growth is modulated on a molecular level is still unclear. As the somatotropic axis is a key regulator of early growth in vertebrates, we investigated the effect of egg size on three key genes belonging to this axis, at different ontogenetic stages in a mouthbrooding cichlid (Simochromis pleurospilus). The expression levels of one of them, the growth hormone receptor (GHR), were significantly higher in large than in small eggs, but remarkably, this pattern was reversed after hatching: young originating from small eggs had significantly higher GHR expression levels as yolk sac larvae and as juveniles. GHR expression in yolk sac larvae was positively correlated with juvenile growth rate and correspondingly fish originating from small eggs grew faster. This enabled them to catch up fully in size within eight weeks with conspecifics from larger eggs. This is the first evidence for a potential link between egg size, an important maternal effect, and offspring gene expression, which mediates an adaptive adjustment in a relevant hormonal axis.

Thresholds for short-term acid and aluminum impacts on Atlantic salmon smolts

Although the negative effects of acid and aluminum (Al) on smolt development have been known for some time, the thresholds for impact of short-term exposure of several days that may occur during episodic acidification have not been systematically examined. In order to determine the levels of acid and Al that impact juvenile Atlantic salmon, smolts and yolk-sac larvae were exposed to three pH levels (6.0, 5.7, and 5.3) and four added Al levels (0, 40, 80 and 175μg/L total Al) for 48h. Following this treatment, 10 smolts were sampled in freshwater and another 10 were subjected to a 24h seawater challenge (35ppt). Survival of yolk-sac larvae was >96% in all acid and Al treatments. All smolts died within 48h at pH 5.3, 175μgL−1 Al. There were some mortalities in freshwater at pH 5.3, 80μgL−1 Al and pH 5.7, 175μgL−1 Al, and further mortalities when these fish were transferred to seawater. Mortalities in these groups were associated with decreased plasma chloride in freshwater and higher plasma chloride in seawater, indicating that these smolts had lost seawater tolerance. Gill Na+/K+-ATPase (NKA) activity decreased at pH 5.7, 175μgL−1 Al in freshwater, and further decreases were observed at more moderate pH and Al exposures after transfer to seawater. Hematocrit and plasma glucose were the most sensitive physiological responses, increasing at all Al treatments at pH 5.7 and 5.3 in freshwater. There was no detectable increase in gill Al levels at pH 6.0 with added Al, whereas substantial increases in gill Al were observed in all added Al groups at pH 5.7 and 5.3. Our results demonstrate a critical interaction between acid and Al in their effects on smolts, and that episodic acidification events will negatively impact smolt survival in freshwater and after seawater entry.

Effects of substrate condition on habitat use and survival by white sturgeon (Acipenser transmontanus) larvae and potential implications for recruitment

To understand links between substrate and recruitment of white sturgeon ( Acipenser transmontanus ), I evaluated the effects of substrate condition on larval drift, hiding, and predation between hatch and 15 days posthatch (dph). Over porous substrates (small gravel = 1.2–1.9 cm; medium gravel = 2.5–5.0 cm; cobble = 10–15 cm), rapid interstitial hiding was observed from 0 to 6 dph at low water velocity (4 cm·s–1), whereas larvae drifted in response to nonporous substrates (sand < 0.2 cm; embedded cobble). Velocities of 20 cm·s–1 led to significantly lower drift only at 1 dph over small gravel. Hiding occurred an average of 2.0–13.3 s after release at 0–6 dph. Predation by sculpins ( Cottus spp.) on larval sturgeon also decreased significantly in response to porous substrates at 1 dph. The strongest expression of increased hiding and decreased predation when small gravel was available suggests that yolksac larvae prefer small interstitial spaces created by that substrate. Considering behavioural responses in preferred natural spawning habitat suggests yolksac larvae predominantly hide in the vicinity of spawning locations. Identification of strong effects of substrate condition on age-specific drift and survival suggests that substrate degradation may contribute to recruitment limitations for sturgeon.

Spatial structure and bottom characteristics of the only remaining spawning area of Chinese sturgeon in the Yangtze River

This study was performed to determine the spatial structure and bottom characteristics of the only remaining spawning area of Chinese sturgeon (Acipenser sinensis) located just downstream of the Gezhouba Dam. During the period 1996–2003, three methods were employed, including (i) the examination of the stomachs of opportunistic predatory fishes that feed on sturgeon eggs, (ii) tracking the mature sturgeons by ultrasonic tags and (iii) capturing eggs and yolk-sac larvae (ELS) at and near the river bottom by D-shaped bottom drift nets to monitor the spawning activities of A. sinensis. It was found that A. sinensis selected permanent locations to mate. There were two mating locations: an upstream mating area (UMA), in the tailrace of the Dajiang Hydroelectric Station and about 870 m far from the Gezhouba Dam (area, 0.1 km2), and a downstream mating area (DMA), about 2500 m from Gezhouba Dam (area, 0.3 km2). The DMA was the chief mating area and was used every year. The incubation area (for dispersal of fertilized eggs to incubate) was range from 870 m to 7 km from the dam. Generally, the total length of the spawning area was 5–7 km length (area 2.0–2.7 km2) with the range slightly varied in different spawning activities. A relief map of the riverbed and a bottom hydroacoustic survey of the DMA found a deep depression and rocky substrate. These may be important characteristics of geomorphology for the spawning area of A. sinensis.

The effects of temperature change on the hatching success and larval survival of largemouth bass Micropterus salmoides and smallmouth bass Micropterus dolomieu

In this study, the effects of abrupt temperature change on the hatching success and larval survival of eggs, yolk-sac larvae (YSL) and larvae above nest (LAN), for both largemouth bass Micropterus salmoides and smallmouth bass Micropterus dolomieu were quantified. Temperature had a significant effect on hatching success and time to 50% mortality, with large heat shocks causing accelerated mortality. The temperature changes shown to influence survival of all life stages, however, were beyond what is typically experienced in the wild. Micropterus salmoides had greater egg hatching success rates and increased survival rates at YSL and LAN stages, relative to M. dolomieu. Additionally, egg hatching success and survival of LAN varied across nests within the study. These findings suggest that temperature alone may not account for variations in year-class strength and may emphasize the need for protection of the nest-guarding male Micropterus spp. to ensure recruitment.

Effects of salinity and temperature on the growth, survival, whole body osmolality, and expression of Na +/K + ATPase mRNA in red porgy ( Pagrus pagrus) larvae

Atlantic red porgy, Pagrus pagrus, is an important reef fish species in the Mediterranean and the snapper–grouper complex off the southeastern United States. Red porgy is a viable candidate for aquaculture with high market value and the ability to spawn freely in captivity. The objective of this study was to examine the combined effects of temperature and salinity on eggs, yolk-sac larvae, and early feeding-stage larvae of red porgy to day 16 post-hatching (d16ph). To determine the optimal temperature and salinity conditions for culture, embryos were reared under four temperatures (17, 19, 21, and 23 °C) and two salinities (24 and 34 g/L) in a 4 × 2 factorial design. Significant effects of temperature and salinity on growth (notochord length, wet and dry weight), survival, whole body osmolality, and expression of Na +/K + ATPase were observed with minimal interactive effects. Under both salinities, growth increased with increasing temperature. On d16ph, wet weights at 21 and 23 °C (2.03 and 2.91 mg, respectively) were significantly higher than at 17 and 19 °C (0.20 and 0.69 mg). Salinity had no effect on growth at any temperature, but had a significant effect on survival to d16ph, with greater survival at 24 g/L (18.4%) than at 34 g/L (6.77%). Salinity significantly affected whole body osmolality on d2ph and d11ph, with 24 g/L having lower whole body osmolality than 34 g/L on both days. Temperature significantly affected whole body osmolality on d6ph, with no clear trends and on d17ph, with 17 °C (534 mOsm/kg) higher than all the higher temperatures (396–411 mOsm/kg). After increasing each tank to 44 g/L to create a sublethal salinity challenge on d16ph, larvae from 34 g/L treatments did not show an increase in levels of Na +/K + ATPase mRNA after 24 or 48 h post transfer. However, fish in the 24 g/L treatments showed a significant increase in expression of Na +/K + ATPase mRNA after 24 h followed by a decrease after 48 h. Within the ranges tested, a temperature of 23 °C and a salinity of 24 g/L appear to be optimal for culture of red porgy embryos, yolk-sac, and first-feeding stage larvae to d16ph.

Improvement in the feeding activity, early growth and survival of Pacific bluefin tuna Thunnus orientalis larvae fed a casein peptide-based microdiet supplemented with inosine monophosphate

Two feeding experiments were conducted to determine the effect of a 5′-inosine monophosphate (IMP)-supplemented casein peptide-based microdiet (MD) on the feeding activity, growth and survival of Pacific bluefin tuna (PBT) larvae. PBT larvae [total length: 14.0 mm (Exp. 1) or 13.3 mm (Exp. 2)] were fed a casein peptide-based MD supplemented with or without 3% IMP for 12 days. As a negative control, a limited number of yolk-sac larvae of spangled emperor Lethrinus nebulosus were fed only for the first 7 days and starved for the next 5 days. A sufficient amount of yolk-sac larvae of spangled emperor was fed in the Exp. 2. All fish in the control group had died by 9 or 10 days after the experiment. Extensive mortality was observed in fish fed the MD. The MD was only found in the stomach contents of PBT fed the MD + IMP. Of the fish fed an IMP-supplemented diet, 50% survived for 12 days after the initiation of both feeding experiments. In addition, significantly higher growth than that observed during the first 7 days was seen in the PBT in the IMP treatment after 12 days of the feeding experiment. These results suggest that the supplementation of casein peptide-based MD with IMP could improve the feeding activity, survival and growth performance of PBT.

Lipids and fatty acids as indicators of egg condition, larval feeding and maternal effects in Cape hakes (Merluccius paradoxus and M. capensis)

Cape hakes, Merluccius paradoxus and M. capensis, are important gadoid fish that are commercially harvested in the Benguela Current system off Namibia and South Africa. The aim of this study was to elucidate the nutritional condition and feeding preferences of their larvae. Hake eggs and larvae were sampled in austral spring of two consecutive years, 2007 and 2008, off the west coast of South Africa. They were identified to species using genetics, and total lipid content and fatty acid (FA) composition were analysed for each individual egg and larva to compare the condition of different early life stages of both hake species. Higher abundances of M. paradoxus eggs and larvae were consistently found compared to M. capensis. In both species, eggs contained wax esters (WE) and had significantly higher lipid content per dry mass than larvae. Lipid content as well as FA composition changed with the developmental stage of larvae. Quantities of essential fatty acid (EFA) increased with feeding of larvae due to dietary lipid incorporation. In 2007, yolk-sac larvae contained significantly lower total lipids than in 2008. It is argued that this was due to reduced lipid transfer by the spawning females to the eggs. These findings indicate that maternal effects are important in determining condition of hake larvae and that this may have an effect on their survival and subsequent recruitment.

The invasive ctenophore Mnemiopsis leidyi poses no direct threat to Baltic cod eggs and larva

Since its invasion in to the Baltic Sea in 2006, the ctenophore Mnemiopsis leidyi has been suspected of serious predation on the early life stages of Baltic cod (Gadus morhua callarias L.) due to a temporal and spatial overlap in the most important cod spawning ground, the Bornholm Basin. We conducted laboratory incubation experiments and video observations to quantify feeding rates on Baltic cod eggs and larvae. Ingestion rates increased with cod larvae concentrations up to 8 prey L−1, beyond which ingestion remained constant. Neither Mnemiopsis size nor egg concentration (1–16 prey L−1) affected feeding rates on cod eggs. Observed feeding rates pooled from all experiments conducted at nonsaturating prey concentrations were low, with the highest volume‐specific clearance on < 4.5‐d‐old yolk‐sac larvae (0.05 ± 0.02 L (mL Mnemiopsis)−1 h−1), and lower rates on 4.5‐8‐d‐old larvae (0.02 ± 0.02 L (mL Mnemiopsis)−1 h−1) and eggs (0.02 ± 0.03 L (mL Mnemiopsis)−1 h−1). When offered Artemia salina and cod eggs simultaneously, Mnemiopsis passively selected against cod eggs. Video recordings showed that eggs did not trigger the capture response that Mnemiopsis shows toward motile prey, and ingested eggs were often ejected (88%, n = 8). Applying our clearance rates to in situ abundances of cod eggs, larvae, and Mnemiopsis for the peak of the spawning season, we demonstrate that the predation pressure of the invasive ctenophore is negligible. We conclude that Mnemiopsis constitutes no direct threat to the Baltic cod population.

Detection of DNA damage in yolk-sac larvae of the Japanese Medaka, Oryzias latipes, by the comet assay

This study was set up to determine the suitability of the early life stage (ELS) alkaline comet assay for the detection of DNA strand breaks induced by genotoxicants in whole organism. This assay was performed on cells of medaka 2 days posthatch (dph). An efficient procedure for cell dissociation using enzymatic and mechanical digestion was developed. This protocol ensures 80% viability of cells and low DNA damage background. Cells from 2 dph medaka larvae were exposed in vitro to model genotoxicants, hydrogen peroxide, cadmium, and fluoranthene, followed by comet assay analysis. Results show a significant increase in the percentage of DNA damage of dissociated cells by all the tested compounds when compared to controls. The assay was also performed in vivo on medaka larvae (2 dph) exposed for 24 h to waterborne cadmium or fluoranthene. Significant induction of DNA damage levels were observed following larvae exposure to cadmium and fluoranthene at concentrations of 0.1 and 50 μM, respectively. This study demonstrates that cells of embryo life stage medaka respond to known DNA damaging agents and that the ELS comet assay may be a useful biomarker to detect DNA strand breakage in whole body of pluricellular organism induced by a range of agents. This technique may provide a sensitive, nonspecific endpoint of genotoxicity as part of ELS toxicity test.