Fish Larvae Research Articles

Delineating early life stages of Belone belone: analysis of external morphology and postcranial skeletal development of the garfish

During fish larvae development functional, morphological, and physiological adaptations are key in larval survival strategies and can determine mortality bottlenecks. For many fish groups such as needlefish, which play crucial roles in marine food webs, studies on early life stages are almost lacking. Herein, the development, with focus on the postcranial skeleton, of the garfish Belone belone (Linnaeus, 1761) is described and a staging system is introduced. Ten larval and juvenile life stages are proposed in three main phases: yolk sac, larval, and juvenile development. During the yolk sac phase the garfish deplete their yolk reservoirs and finish the development of their unpaired fins. During the larval period the lower jaw elongates and the needlenose stage is reached. In the juvenile phase, the upper jaw elongates until both jaws are almost equally long. Since B. belone larvae develop much of their postcranial skeleton in the late embryonic and yolk sac phases they might not experience a severe bottleneck in this early stage such as many other marine fish species. Therefore, young garfish could be more resilient to environmental changes.

Postprandial Sleep in Short-Sleeping Mexican Cavefish.

Interactions between sleep and feeding behaviors are critical for adaptive fitness. Diverse species suppress sleep when food is scarce to increase the time spent foraging. Postprandial sleep, an increase in sleep time following a feeding event, has been documented in vertebrate and invertebrate animals. While interactions between sleep and feeding appear to be highly conserved, the evolution of postprandial sleep in response to changes in food availability remains poorly understood. Multiple populations of the Mexican cavefish, Astyanax mexicanus, have independently evolved sleep loss and increased food consumption compared to surface-dwelling fish of the same species, providing the opportunity to investigate the evolution of interactions between sleep and feeding. Here, we investigate the effects of feeding on sleep in larval and adult surface fish, and in two parallelly evolved cave populations of A. mexicanus. Larval surface and cave populations of A. mexicanus increase sleep immediately following a meal, providing the first evidence of postprandial sleep in a fish model. The amount of sleep was not correlated to meal size and occurred independently of feeding time. In contrast to larvae, postprandial sleep was not detected in adult surface or cavefish, which can survive for months without food. Together, these findings reveal that postprandial sleep is present in multiple short-sleeping populations of cavefish, suggesting sleep-feeding interactions are retained despite the evolution of sleep loss. These findings raise the possibility that postprandial sleep is critical for energy conservation and survival in larvae that are highly sensitive to food deprivation.

Intestinal DHA-PA-PG axis promotes digestive organ expansion by mediating usage of maternally deposited yolk lipids.

Although the metabolism of yolk lipids such as docosahexaenoic acid (DHA) is pivotal for embryonic development, the underlying mechanism remains elusive. Here we find that the zebrafish hydroxysteroid (17-β) dehydrogenase 12a (hsd17b12a), which encodes an intestinal epithelial-specific enzyme, is essential for the biosynthesis of long-chain polyunsaturated fatty acids in primitive intestine of larval fish. The deficiency of hsd17b12a leads to severe developmental defects in the primitive intestine and exocrine pancreas. Mechanistically, hsd17b12a deficiency interrupts DHA synthesis from essential fatty acids derived from yolk-deposited triglycerides, and consequently disrupts the intestinal DHA-phosphatidic acid (PA)-phosphatidylglycerol (PG) axis. This ultimately results in developmental defects of digestive organs, primarily driven by ferroptosis. Our findings indicate that the DHA-PA-PG axis in the primitive intestine facilitates the uptake of yolk lipids and promotes the expansion of digestive organs, thereby uncovering a mechanism through which DHA regulates embryonic development.

Exploring phyto- and zooplankton community structure, spatial variation, and driving forces shaping the community from a large-scale freshwater dominated estuary

Abstract Plankton is essential to estuarine food webs, providing key food for species like fish larvae, and understanding their dynamics helps us comprehend ecological interactions and energy transfer in aquatic ecosystems. This study examined the plankton community structure, diversity, variability and the environmental variables of water that influenced these communities in a freshwater-dominated estuary along the Northern Bay of Bengal during the dry season. A variety of tools were employed, including a dissecting microscope, a Sedgwick-Rafter cell, and a Hanna multi-parameter meter, to obtain detailed and comprehensive data. From the analyses, in total 37 genera of phytoplankton belonging to 11 classes such as Bacillariophyceae (66%), Coscinodiscophyceae (28%), Fragilariophyceae (1%), Euglenophyceae (1%), Cyanophyceae (1%), others (3%) were recorded. Zooplankton, consisting of multicellular organisms, was represented by 11 genera across five classes, with Monogononta (53%) and Insecta (29%) being the most prevalent. Phytoplankton exhibited a mean abundance of 180,000 ± 20,200 cells L⁻¹ whereas zooplankton showed a mean abundance of 3,000 ± 200 cells L⁻¹. The diversity indices (H’), evenness (J’) and dominance (D) ranged from 1.188 ± 0.015 to 0.19 ± 0.006, 0.9377 ± 0.002 to 0.5118 ± 0.006, 0.365 ± 0.001 to 0.143 ± 0.002 respectively, suggested low to moderate plankton diversity with a suppressed community and moderate to high dominance in the river water. Based on the similarity percentage (SIMPER) and analysis of similarity (ANOSIM), Phytoplankton and zooplankton varied significantly between stations while non-metric multi-dimensional scaling (nMDS) indicated greater consistency in phytoplankton communities across stations compared to zooplankton. Furthermore, Pearson's correlation coefficient (rs) and canonical correspondence analysis (CCA) identified transparency, sulfates, dissolved oxygen, conductivity, TDS, pH, and salinity as major influencers of plankton abundance; nitrate had a moderate effect. Spearman's rank correlations showed significant positive (p < 0.05) correlations between diversity indices and environmental variables.

Modular shadowgraph imaging for zooplankton ecological studies in diverse field and mesocosm settings

AbstractVarious iterations of shadowgraph imaging have been used to quantify zooplankton in situ with high spatial resolution. Because these systems can image relatively large volumes of water, they are especially useful for resolving less common meso‐ or macrozooplankton taxa (< 50 ind. m−3), such as larval fishes and gelatinous animals. However, larger volume imagers are typically integrated with heavy towed vehicles and deployed from research vessels, which introduces high costs and limits sampling approaches. Here we demonstrate that versatile configurations of shadowgraph imaging, including modular benchtop, handheld, and towed, compact vehicle systems (along with customizable software), allow for tailoring sampling to a variety of marine and freshwater settings (including mesocosms). These systems encompass a suite of possible architectures, designed for adapting the imaging depth of field, acquisition rates, sensor configuration, and deployment method to fit a wide range of sampling protocols, with high vertical resolution (~ 5 cm) and adequate taxonomic capabilities for > 0.5 mm organisms. The benchtop system facilitates an interactive approach to observe and quantify zooplankton behaviors and optical properties. Video footage from the benchtop system generates thousands of regions of interest min−1 for target organisms with variable orientations and swimming postures. When used in conjunction with in situ imaging, the benchtop system can build large machine learning training libraries targeted toward rare or morphologically diverse zooplankton, which often includes the larval stages of economically valuable taxa. These modular hardware and software components increase affordability and versatility while broadening the scope of scientific questions addressed by plankton imaging systems.

Effects of acute exposure to freshwater acidification on developing Oryzias latipes

Elevated CO2 in aquatic environments causes weak acidification. When exposed to weak acidification, regardless of life stage, most fishes undergo some degree of hypercapnia (elevation of CO2 in the bloodstream). Hypercapnia negatively affects physiological processes and embryo and larval fish are particularly vulnerable to rises in CO2. The aim of our study was to understand if weak acidification induced by elevated CO2 alters the physiology and behaviour of freshwater Japanese medaka (Oryzias latipes) embryos and larvae. To test this, we treated Japanese medaka embryos and larvae for 24 h with varying levels of weak acidification (pH 7.1 [~482 μatm], 6.4 [~2122 μatm], 6.1 [~3280 μatm], 5.8 [~5306 μatm], and 5.7 [~10,130 μatm]) at two developmental stages (72 hpf and 9 dpf). Following the treatment, heart rate, burst activity (movement within the egg), and survival of embryos were quantified. Swimming activity of hatched larvae was also measured. We observed a statistically significant 2-fold decline in heart rate of embryonic Japanese medaka as pH decreased (P < 0.01). We also found that survival of embryos significantly declined as acidification increased (P < 0.01). Behaviour of larval fish was significantly altered (P < 0.001) but not in a pH dependent manner. Our study suggests that weak acidification can cause negative effects to early life stage physiology and that behaviour can be altered. Our results suggest that if fish develop in weakly acidified freshwater there may be unfavourable impacts, including mortality at the most extreme levels.

Composition of inshore ichthyoplankton community in the Canary islands from a spatial and seasonal perspective: Inshore ichthyoplankton of the Canary Islands

Early life stages of fish are insufficiently studied in most regions around the globe, and consequently, there is an important gap in knowledge on the life cycle of most teleosts. In the Canary Islands (Eastern Central Atlantic), most studies on ichthyoplankton communities are based on traditional net tows in open waters that provide information about early larval stages of pelagic species in the area, while nearshore benthic fish remain highly understudied. In this study, light traps were employed for the first time to assess post-larval stages of neritic fish in this archipelago. A two-year survey was carried out to collect nearshore fish larvae every 6 months at 11 localities from El Hierro, Tenerife and Lanzarote. A total of 3940 fish larvae classified into 13 orders, 28 families and 44 species were collected. These results provide a wide description of the composition of the inshore ichthyoplankton off the Canary Islands, across islands and seasons. Main environmental factors (SST, sea floor orography, oceanographic phenomena …) influencing the population dynamics of this community are discussed. Additionally, the ichthyoplankton assemblages were assessed from the intra-annual perspective, analyzing the species composition and abundances by months and seasons, and providing new insights into reproduction cycles of many common benthic fishes at shallow marine ecosystems of the islands. This study is important to further understand the life cycles of some of the most common fish species in the Canary Islands, unraveling main environmental factors that affect the success of their offspring that sustain populations.

External morphology and growth patterns of larvae and juveniles of Bryconops gracilis (Characiformes, Iguanodectidae) from Amazon basin

During early development, fishes undergo significant changes that influence external morphology and the functioning of internal organs and systems. This often results in gradual variation of the morphological traits of individuals across developmental stages. The investigation of larval and juvenile fish development and growth patterns has pertinent implications for the systematic and ecological elucidation of species. Bryconops gracilis is a medium-sized fish, omnivorous that inhabits lotic and lentic environments with acidic and transparent waters in the Amazon basin. In this study, the early development of B. gracilis is described, until recently a practically unknown species. In terms of development, we used morphological, meristic, and morphometric data to characterize the larvae and juveniles. The individuals were collected in the Curuá-Una River, Amazon basin, Brazil. Fifty-four specimens were examined. Samples include individuals with 3.39 to 21.79mm SL. Yolk-sac larvae have two attachment organs on the dorsal surface of head and body. The larvae of B. gracilis are considered altricial, with a fusiform body, and the intestine reaches the median region of the body. Initially, the mouth is subterminal and becomes isognathic from the postflexion stage on. During the postflexion stage, the most relevant morphological changes occur (e.g., presence of all fins, mouth position similar to adults, increased body pigmentation), making individuals more specialized to explore new habitats and diets and maximize their chances of survival. Furthermore, vertebrae and myomeres are compared and assist with differentiating some Bryconops species at early life stages that occur in sympatry in the Amazon basin. Our results contribute to knowledge about the external morphology of neotropical freshwater fishes, enabling the identification of larvae and juveniles through traditional taxonomy and broadening the perspective on the ontogenetic study of the adipose fin in Characoidei.

Yucatan shelf’s larval density and distribution of Auxis spp. and Caranx crysos are primarily driven by regional upwelling and seasonality

The extensive Yucatan Shelf (YS) in the southern Gulf of Mexico provides habitat for many marine species and supports important fisheries. A striking feature is the regional upwelling along its eastern margin due to the interaction of the Yucatan Current with the western slope of the Yucatan Channel. Coupled with easterly winds, the upwelled water fertilizes the shelf and contributes to high productivity in an otherwise oligotrophic region. However, an understanding of the role that regional upwelling plays on fish spawning and larval fish distributions is limited. We describe the distribution, frequency of occurrence, and densities of two commercially important neritic species (Auxis spp. and Caranx crysos) with contrasting life histories (different adult habitat, neritic vs. neritic and oceanic, and with spawning peak vs. year around spawner). The relationship between larval density and environmental variables, including upwelling indicators, was examined for three oceanographic cruises that covered the entire YS and which were held during summer through fall (2015, 2016 and 2018). None (Auxis spp.) or very few (C. crysos) larvae were caught during the late fall cruise in November. The highest densities of both taxa were found during the summer cruise in July, consistent with their spawning periods. Both species were mostly distributed beyond the 40 m isobath, and the highest densities were found over the central and eastern shelf. Generalized additive models indicated that Auxis spp. and C. crysos densities were correlated with low chlorophyll a concentration (located in the outer shelf) and a shallower 22.5 °C isotherm (a proxy for upwelling), indicating that regional upwelling was associated with higher larval fish densities.

DNA barcode reference library of the fish larvae and eggs of the South China Sea: taxonomic effectiveness and geographic structure

Fish early-stages constitute useful indicators of the states of marine ecosystems, as well as important fishery resources. Given the spectacular phenotypic changes during ontogeny, and the paucity of diagnostic morphological characters at the species level, the identification of fish early-stages is a challenging task. DNA barcoding, the use of the mitochondrial gene of the cytochrome c oxidase subunit I (COI) as an internal species tag, opened new perspectives for the identifications of both larval fish and fish eggs. However, the accuracy of the identifications assisted by DNA barcoding are dependent of the completeness of the DNA barcode reference libraries used to assigned unknown sequences to known species. Here, we built a DNA barcode reference library for 113 species of larval fish and 85 species of fish eggs involving the production of 741 newly generated DNA barcodes from South China Sea (63 localities). Together with 514 DNA barcodes mined from Genbank for 116 species from the South China Sea regions, a reference library including 1255 DNA barcodes for 308 species (248 locations) was assembled. The present study emphasizes the importance of integrating DNA barcoding to large scale inventories of early stages, as DNA-based species delimitation analyses delimited 305 molecular operational taxonomic units (MOTUs) and multiple cases of discordance with morphological identifications were detected. Cryptic diversity is detected with 14 species displaying two MOTUs and a total of 23 species were lumped into 11 MOTUs due to low interspecific divergence and/or mixed lineages.

Effects of diuron and two of its metabolites in biochemical markers and behavior of zebrafish larvae.

Diuron (3-(3,4-dichlorophenyl)-1,1-dimethylurea) is an herbicide used in many crops, including sugar cane cultivation. It is commonly found in aquatic ecosystem and is of high concern due to its ability to persist in the environment. Diuron metabolites include DCA (3,4-dichloroaniline) and DCPMU (3-(3,4-dichlorophenyl-1-methylurea). The objective of this study was to evaluate the effects of diuron and two of its metabolites in zebrafish (Danio rerio) developing embryos, from biochemical to individual level. Activities of the enzymes acetylcholinesterase (AChE), catalase (CAT), glutathione-S-transferase (GST), andlactate dehydrogenase (LDH) and the levels of lipid peroxidation (LPO), swimming activity, and body length were investigated after an exposure of 120h, and the heart rate was determined after 48h of exposure. The range of concentrations tested was 0.003-3.000mg/L diuron, 0.020-1.500mg/L DCA, and 0.020-2.100mg/L DCPMU. Results showed that AChE activity was inhibited by diuron (3.000mg/L) and DCPMU (0.326, 0.828mg/L). However, the swimming activity of fish larvae exposed to diuron or its metabolites was not affected. The CAT was induced by DCPMU, and GST was induced by diuron. This suggests that CAT is acting to cope with oxidative stress induced by DCPMU and GST might have a role in the detoxification of diuron. In addition, larvae exposed to DCA (0.633 and 1.500mg/L) had a reduction in their length, and larvae exposed to diuron (0.754 and 3.000mg/L) and DCA (0.267, 0.633, and 1.500mg/L) presented bradycardia, suggesting cardiotoxicity. Overall, results indicate that diuron, DCA, or DCPMU induces adverse effects during the early phases of zebrafish development, such as the impairment of neurotransmission and cardiovascular function and alterations in antioxidant enzymes and growth. Diuron appeared as more toxic than its metabolites since the lowest LOEC (0.012mg/L) and higher integrated biomarker response (IBR) values were obtained with exposure to this herbicide. Furthermore, as it is fast degraded into DCA and DCPMU, which also affected the zebrafish developing embryos at environmentally relevant concentrations, its use might be of concern in ecosystems that receive agriculture runoff due to their potential adverse effects to aquatic biota.

Transcriptome analysis reveals the importance of phototransduction during the first-feeding in mandarin fish (Siniperca chuatsi).

The mandarin fish (Siniperca chuatsi), as a typical freshwater carnivorous fish, has high economic value. Mandarin fish have a peculiar feeding habit of feeding on other live fry during the first-feeding period, while rejecting zooplankton or particulate feed, which may be attributed to the low expression of zooplankton-associated gene sws1 in mandarin fish. The domesticated strain of mandarin fish could feed on Artemia at 3 days post hatching (dph). However, the mechanism of mandarin fish larvae recognize and forage Artemia as food is still unclear. In this study, we employed transcriptional analysis to identify the representative differential pathways between mandarin fish larvae unfed and fed with Artemia at 3 dph. The comparative transcriptome analysis has unveiled a tapestry of genetic expression, highlighting 403 genes that have been up-regulated and 259 that have been down-regulated, all of which constitute the differentially expressed genes (DEGs). KEGG pathway analysis revealed that the number of differentially expressed genes in the photoconductive signaling pathway was the largest. Next, the Vorinostat (suberoylanilide hydroxamic acid, SAHA) was used to assess whether sws1 induced ingestion of Artemia in mandarin fish larvae. We discovered that SAHA-treated larvae had more food intake of Artemia and up-regulated the transcription level of npy, which might have been associated with the up-regulated of sws1 opsin. Additionally, exposure to 0.5 µM SAHA increased the expression of genes involved in phototransduction pathway. These findings would provide insights on the molecular processes involved in mandarin fish larvae feeding on Artemia at the first-feeding stage.

Isotopic Niches of Fish Larvae During Coral Reef Recruitment in a Caribbean Reef

Isotopic Niches of Fish Larvae During Coral Reef Recruitment in a Caribbean Reef

Species Composition of Fish Larvae and Juveniles in the Nanji Islands, China

To describe the species composition of fish larvae and juveniles in the Nanji Islands, monthly collection was conducted at 12 stations around the Nanji Islands from March 2023 to February 2024 by using horizontal tow at the surface layer. The fish larvae and juveniles were collected by a larva net (1.3 m mouth diameter, 0.5 mm mesh aperture). A total of 6446 fish larvae and juveniles were collected, belonging to 59 species and 54 genera in 11 families. Most of them were preflexion larvae (93.31%). Gobiidae, with eight species, was the most diverse family. Based on the index of relative importance (IRI) result, the dominant species was Sebastiscus marmoratus (Cuvier, 1829), accounting for 84.52% of the total number of samples collected. The common species were Stolephorus commersonii Lacepède, 1803; Omobranchus elegans (Steindachner, 1876); Nibea albiflora (Richardson, 1846); Parablennius yatabei (Jordan &amp; Snyder, 1900); Lateolabrax maculatus (McClelland, 1844); and Odontamblyopus lacepediiand (Temminck &amp; Schlegel, 1845). Other species were all rare species. The highest species number was in September (18 species), and the highest density was in December. Only one fish larva was collected in April. Pielou’s evenness index was highest in November, while Margalef’s richness index and Shannon–Wiener diversity were highest in September. The result of cluster analysis showed that the 11 months (except April) could be divided into four groups. The species number, density, and three diversity indexes showed no significant correlation with temperature and salinity.

The Effect of Parasitization by Trichodinid Ciliates on the Mortality of Cultured Pacific Fat Sleeper larvae (Dormitator latifrons).

Trichodinidae, a prevalent group of protozoan ectoparasites in aquaculture, cause rapid mortality in fish hatcheries. Despite their significance, knowledge about these parasites in farmed fish in South America, especially in native species that are currently being domesticated for aquaculture, remains limited. This study morphologically characterized the Trichodinid species that are infecting Pacific fat sleeper (Dormitator latifrons) larvae and evaluated their impact on larval rearing. Four pairs of broodstock were induced with GnRHA implants and placed in tanks containing 200 L of freshwater, with a water temperature of 28 ± 1.0 °C and a dissolved oxygen level of 4.00 ± 1.23 mg L-1, with partial water exchanges being performed daily. The larvae hatched 7 to 8 h after fertilization and were transferred to tanks containing water with the same quality parameters. Twelve hours post-hatching, the presence of Trichodina was observed. Every 24 h, 60 larvae per tank (n = 180 per treatment) were sedated, and larval wet mounts were prepared, air-dried at room temperature, and impregnated with silver nitrate. Infection parameters and daily mortality were calculated. Trichodina was observed to parasitize the pelvic fins, caudal fins, and heads of fish larvae, which showed lethargy and erratic swimming movements. The Trichodina species showed a daily increase in the infection parameters, and a 58% rate of larval mortality was observed at the fourth day post-hatching (dph) in the infected tanks. In captivity, D. latifrons larvae typically survive up to 7 days post-hatching (dph) before reaching their point of no return due to the lack of adequate diet and feeding regimes. However, our study indicates that Trichodina infestation accelerates mortality, causing infected larvae to die more quickly than uninfected ones.

Transition from endogenous to exogenous feeding in longfin yellowtail Seriola rivoliana larvae under simultaneous effects of daily temperature fluctuation and rotifer Brachionus rotundiformis enrichment.

Temperature and nutrition are suggested as the primary factors affecting larval survival during the transition from endogenous to exogenous feeding in fish. However, little is known about its simultaneous impact during this period. In this study, Seriola rivoliana eggs were subjected to a constant 24°C (CTE) and a daily temperature fluctuation (DTF) between 22.8 and 25.2°C until oil droplet exhaustion (5.5days after hatching). On the other hand, marine fish larvae mostly rely on live feed, with certain nutritional deficiencies such as poor long-chain fatty acids. Thus, rotifer Brachionus rotundiformis enrichment was simultaneously evaluated with temperature using three enrichment diets: Ori-green, S.presso, and a Domestic emulsion. For this purpose, the five experimental groups were established in triplicate using six 100-L tanks with three 10-L containers inside (18 experimental units in total). One hundred eggs were incubated, using a green water system, and 10 rotifers mL-1 were offered at mouth opening. After oil droplet exhaustion, survival was only affected by temperature (P < 0.01), being higher at DTF compared to CTE. At the same stage, Domestic emulsion resulted in bigger larvae than Ori-green. In a further assay at 3.7 DAH, the relative expression of the trypsin gene was higher at Domestic emulsion compared to S.presso and Ori-green. This study indicates that daily temperature fluctuation can improve larval performance and low levels of EPA and DHA in Domestic emulsion enriched rotifers were not critical for Seriola rivoliana at first feeding.

Deep-sea fish reveal alternative pathway for vertebrate visual development.

Vertebrate vision is accomplished by two phenotypically distinct types of photoreceptors in the retina: the saturation-resistant cones for the detection of bright light and the highly sensitive rods for dim light conditions [1]. The current dogma is that, during development, all vertebrates initially feature a cone-dominated retina, and rods are added later [2, 3]. By studying the ontogeny of vision in three species of deep-sea fishes, we show that their larvae express cone-specific genes in photoreceptors with rod-like morphologies. Through development, these fishes either retain this rod-like cone retina ( Maurolicus mucronatus ) or switch to a retina with true rod photoreceptors with expression of rod-specific genes and transcription factors ( Vinciguerria mabahiss and Benthosema pterotum ). In contrast to the larvae of most marine fishes, which inhabit the bright upper layer of the open ocean, the larvae of deep-sea fishes occur deeper, exposing them to a dimmer light environment [4-7]. Spectral maxima predictions from molecular dynamics simulations and environmental light estimations suggest that using transmuted photoreceptors that combine the characteristics of both cones and rods maximises visual performance in these dimmer light conditions. Our findings provide molecular, morphological, and functional evidence for the evolution of an alternative developmental pathway for vertebrate vision.

Effects of the harmful algal bloom toxin, okadaic acid, on the mechanoreceptors of larval anchoveta (Engraulis ringens) under varying environmental conditions

The effect of the combination of marine toxins produced by algal blooms, in conjunction with varying environmental characteristics on organisms in the water column, is a poorly explored research field. Pelagic fish species of commercial importance, such as anchoveta (Engraulis ringens) in central Chile, may be exposed to these combined factors in a climate change scenario. This is observed from documented changes in the length of the upwelling season, frequency of upwelling events, and the increased frequency of atmospheric rivers affecting the southern spawning zone of this species. This study evaluated the integrity of hair cells in neuromasts, mechanosensory organs present in fish larvae, under exposure to different combinations of the algal bloom-produced okadaic acid OA (1 ng mL−1), and two temperature (12 and 14°C), and salinity treatments (historically more frequent: 34 PSU- 12°C; expected: 32 PSU - 14°C). Viable hair cells were counted in newly hatched larvae from the Biobío region, central Chile. Results showed a significant decrease in the average number of viable hair cells per neuromast (from 6.1 ± 1.6 to 4.0 ± 1.2) under lower salinity treatments (32 PSU) compared to normal conditions. Additionally, a seasonal trend was observed with fewer viable cells (from 7.4 ± 1.2 to 4.4 ± 1.1) as the fish species’ reproductive period progressed. The combined effect of OA exposure and modifications with the environmental factors also resulted in a significant decrease of up to 70% in the number of viable hair cells in larvae exposed to OA and high temperatures, indicating damage influenced by the toxin along with a synergistic and/or additive role of temperature. These findings reveal how the lipophilic toxin okadaic acid, produced by harmful algal blooms, interacts with abiotic environmental factors affecting coastal ecologically and socio-economically important organisms. This emphasizes the need to consider multiple factors when studying the effects of marine toxins.

Winter distribution of zooplankton and ichthyoplankton assemblages in the North Sea and the English Channel.

Although zooplankton were extensively studied in the North Sea, knowledge about winter zooplankton assemblages is still scarce, despite potential influence of zooplankton overwintering stocks on seasonal plankton succession and productivity. Furthermore, several economically and ecologically important fish species reproduce during winter contributing to the zooplankton community as passive members (eggs) or predators (larvae). To elucidate on winter zooplankton distribution, abundance and composition in the Southern North Sea and Eastern English Channel, we defined assemblages based on mesozoo- and ichthyoplankton data sampled between January and February 2008 using fuzzy-clustering and indicator species. Mesozoo- and ichthyoplankton (eggs+larvae) were integrated in a common analysis by using a spatial grid adapted to the datasets and defined by means of a geostatistical method developed in agronomics. Potential environmental drivers of assemblage distribution were evaluated by means of GLMM and comparison with data from 2022 facilitated insight about the inter-annual representativeness of the assemblages. Five zooplankton assemblages were found varying with regard to total zooplankton abundance, dominant and indicator taxa. Spatial variability of abiotic (dissolved nutrients, salinity, depth, temperature, organic matter in suspension, chlorophyll a), biotic variables (phyto- and microplankton composition), water masses and fish spawning grounds were revealed as potential drivers of assemblage distribution. Assemblages off the Rhine-Scheldt estuary and in the German Bight harbored the biggest zooplankton overwintering stocks that might influence the grazing pressure on phytoplankton spring production. Assemblages off the Rhine-Scheldt estuary and covering the English Channel and the Southern Bight were found to be of high importance for herring and plaice larvae. Although further analyses suggested inter-annual representativeness of the assemblages found (2008 vs 2022), the assessment of further years would be necessary to account for potential inter-annual variability. Future studies could profit from the assessment of microzooplankton facilitating insight in fish larvae feeding potential and zooplankton overwintering strategies.

Advances in understanding physical and biological controls on eggs and larval distribution in Pacific fisheries: A review

The early stages of fish, comprising eggs and larvae, are exceptionally fragile and sensitive to environmental dynamics and climate change. Pacific Ocean (PO) currents play an important role in shaping the distribution of marine organisms, influencing global climate patterns, heat distribution, coastal temperatures, and nutrient redistribution. These currents reveal significant changes within the climate system. Their variability across different timescales can be attributed to the complex interplay of physical forces. These currents are subjected to diverse anthropogenic factors, exerting detrimental effects on the dispersal of fish larvae. Furthermore, climate change variables, including alterations in tropical PO temperature associated with the ENSO cycle, Atlantic Nino modes influencing equatorial Atlantic temperature, changes in ocean salinity, polar ice cap melting, increasing greenhouse gases, marine heatwaves, and fluctuations in subsurface flows, directly impact the distribution, abundance, and species composition of early life stages. Major Pacific fisheries, such as those targeting Pacific sardines, saury, and anchovies, undergo population booms and declines due to significant alterations in the current dynamics of currents and fronts within the PO. The anticipated intensification of the ENSO cycle, characterized by more frequent and severe El Niño (warm) and La Niña (cold) events as a result of climate change, is expected to significantly impact the early developmental stages of important commercial fish stocks regularly. This review synthesizes the current understanding of the physical and biological parameters driving changes in ocean currents and their implications for early fish dispersion, emphasizing the critical need for research in this area to inform global conservation efforts, fisheries management, and food security.