Identification of Antarctic fish eggs by surface structure as shown by the eggs of Trematomus eulepidotus (Teleostei: Nototheniidae)

Summary Oocytes and incubated eggs of the antarctic nototheniid fish Trematomus eulepidotus were observed with a scanning electron microscope to investigate whether their surface structures show a similar pattern. Oocytes were taken from the ovary or were stripped from running females. For comparison, incubated eggs of stages I and IV (classification after Apstein 1909) were sampled from a bottom trawl catch. We found no significant differences in surface pattern and interpore distances. This finding is of main importance for the determination of unknown incubated eggs from ichthyoplankton samples by SEM. T. eulepidotus exhibited a micropyle of type III in unfertilized eggs. Some incubated eggs were covered by hyphae of fungi, which resemble attaching-filaments of substrate spawning fishes.     

Larval fish distribution and abundance in the Antarctic Peninsula region and adjacent waters

The spatial distribution, species composition and abundance of ichthyoplankton in the Bransfield Strait and adjacent waters were studied during two cruises in the Antarctic spring 1991/92 and summer 1992/93 seasons. A multiple plankton net (Bioness) and a Bongo net were used to collect samples at 35 stations in 1991/92 and 75 stations in 1992/93. Early larval stages (14 species) and juveniles (13 species) representing the known Bransfield Strait ichthyofauna were present in the water column. The nototheniids predominated in the entire study area. The greatest species diversity was found in the uppermost 200 m of the water column in the Bransfield Strait. Notothenia gibberifrons and Nototheniops larseni dominated in spring, whilst in summer Pleuragramma antarcticum dominated in association with N. larseni. The dominant species in the Gerlache Strait were P. antarcticum and Notothenia kempi, while P. antarcticum and Trematomus scotti were predominant in the Bellingshausen Sea area.     

Envelope surface ultrastructure and specific gravity of artificially fertilized Pacific cod Gadus macrocephalus eggs

The envelope surface ultrastructure and specific gravity of artificially fertilized eggs of the Pacific cod Gadus macrocephalus were examined. The unfertilized, demersal and slightly adhesive eggs of G. macrocephalus were almost spherical and had no oil globules. Wrinkled envelope surface with elaborated hexagonal reticulated patterns and type I micropyle were observed under a scanning electron microscope. The adhesiveness of the eggs was lost at the blastodermal‐cap stage after fertilization. The micropylar canal was sealed by secretion of the perivitelline fluid, and the entire surface became rough. Numerous bacilli were deposited at the micropyle and the outer envelope surface at the late germ‐ring stage and at the embryo five‐eighths around the yolk stage. The micropyle was completely deformed at the embryo seven‐eighths around the yolk stage. The specific gravity of the fertilized G. macrocephalus eggs ranged from c. 1·0316 to 1·0454. These values, however, sharply decreased towards the end stages of egg development to produce pelagic larvae. The ultrastructural changes in the micropyle and envelope surface of the G. macrocephalus eggs protected the embryo from microorganism infections and mechanical stress during the long incubation period. The adhesiveness and specific gravity of the eggs influenced their dispersion potential.     

Fine structures of the micropyles of pelagic eggs of some marine fishes

The micropyles and their surrounding structures of pelagic fish eggs of nine species were examined and compared using a scanning electron microscope (SEM). The surface of the egg membrane possesses uniformly distributed pores. However,Lateolabrax japonicus possesses a number of small knobs. The region of the micropyle of fish eggs examined is characterized by a circular elevation in which the pores are larger than those on the other parts of the egg membrane. In contrast,Kareius bicoloratus egg has no elevation and possesses uniform pores to the edge of the micropylar canal. The funnel-like structure is seen in the opening of the micropyle inSardinops melanostictus, Saurida elongata, Sillago japonica, Parapristipoma trilineatum, Hypodytes rubripinnis andK. bicoloratus. These specific features of the micropyle may be used for the identification of pelagic eggs.     

Winter aspects of the ichthyoplankton community in Antarctic Peninsula waters

Summary Ichthyoplankton was sampled from the Antarctic Peninsula area of the South Polar Ocean in early winter (May and June 1986). A total of 153 eggs from two species and 1368 larvae or juvenile stages from 12 species were obtained. These included pelagic species, and demersal species with a long pelagic larval or juvenile phase. Most abundant were larvae of Pleuragramma antarcticum and Notothenia kempi, and eggs of Notothenia neglecta. The distribution of notothenioid and paralepidid larvae was apparently unaffected by ice cover, whereas myctophid larvae were confined to ice-free waters. Areas where newly hatched Chionodraco hamatus occurred coincided with dense aggregations of Euphausia superba (Krill) furcilia larvae which is a potential food resource during winter. The hatching of icefish larvae during winter is apparently independent of the seasonal production cycle. Epipelagic eggs of Notothenia neglecta were found during the spawning season, which suggests that eggs ascend to the surface after demersal spawning and that development takes place near the sea surface during winter. Larvae of Notothenia kempi were chiefly confined to shelf and slope waters to the west of the Antarctic Peninsula, with larger larvae found in coastal shelf areas. Pleuragramma antarcticum occurred in the coastal waters off the Biscoe Islands, in the Gerlache Strait, and in the northern Bransfield Strait. The smallest larvae were found in the northern Bransfield Strait, whereas those at the Biscoe Islands and in Gerlache Strait waters were larger and of a similar size. A cyclonic gyre to the west of the Antarctic Peninsula observed in the austral summer was likely to have affected the larval drift of Pleuragramma antarcticum and Notothenia kempi. Differences in the timing of spawning and hatching and the vertical distribution of these larvae will lead to different transport and spatial distribution patterns. It is hypothesized that early winter conditions do not imply severe limitations on the year-class success of larval fish. Dispersal and increased mortality may occur during the second half of the winter.     

The effect of habitat temperature on serum antifreeze glycoprotein (AFGP) activity in Notothenia rossii (Pisces: Nototheniidae) in the Southern Ocean

The marble notothen, Notothenia rossii, is widely distributed around the waters of sub-Antarctic islands in the Southern Ocean and is exposed to different temperatures that range from ?1.5 to 8 °C. This study investigates whether the different environmental conditions experienced by N. rossii at different latitudes in the Southern Ocean affect the levels of its blood serum antifreeze glycoprotein (AFGP). N. rossii specimens were collected from four localities, including the Ob’ Seamount in the Indian Ocean sector, and South Georgia Island, South Shetland Islands and Dallman Bay in the Atlantic Ocean sector. Serum AFGP activity was determined in terms of thermal hysteresis, i.e. the difference between the equilibrium melting and non-equilibrium freezing points (f.p.s.). Among the four populations, the Ob’ Seamount specimen had the lowest serum AFGP activity (0.44 °C) and concentration (4.88 mg/mL), and the highest non-equilibrium f.p. (?1.39 °C). These results are consistent with the warmer, ice-free waters around the Ob’ Seamount. The other three higher latitude populations have 2–3 times greater serum AFGP activity and concentration, and much lower non-equilibrium f.p.s. In contrast, the physiological profiles of serum AFGP size isoforms revealed that all N. rossii populations, including the Ob’ Seamount specimen, possess an extensive complements of AFGP proteins. Isoform variation was observed, especially in the large size isoforms (AFGPs 1–5), when compared to AFGP of the high Antarctic Dissostichus mawsoni. The lower levels of AFGP and the absence of some of the large isoforms are likely responsible for higher non-equilibrium f.p.s. of the Ob’ seamount specimen.     

Functional antifreeze glycoprotein genes in temperate-water New Zealand nototheniid fish infer an Antarctic evolutionary origin

The fish fauna of the Antarctic Ocean is dominated by five endemic families of the Perciform suborder Notothenioidei, thought to have arisen in situ within the Antarctic through adaptive radiation of an ancestral stock that evolved antifreeze glycoproteins (AFGPs) enabling survival as the ocean chilled to subzero temperatures. The endemism results from geographic confinement imposed by a massive oceanographic barrier, the Antarctic Circumpolar Current, which also thermally isolated Antarctica over geologic time, leading to its current frigid condition. Despite this voluminous barrier to fish dispersal, a number of species from the Antarctic family Nototheniidae now inhabit the nonfreezing cool temperate coasts of the southern continents. The origin of these temperate-water nototheniids is not completely understood. Since the AFGP gene apparently evolved only once, before the Antarctic notothenioid radiation, the presence of AFGP genes in extant temperate-water nototheniids can be used to infer an Antarctic evolutionary origin. Genomic Southern analysis, PCR amplification of AFGP genes, and sequencing showed that Notothenia angustata and Notothenia microlepidota endemic to southern New Zealand have two to three AFGP genes, structurally the same as those of the Antarctic nototheniids. At least one of these genes is still functional, as AFGP cDNAs were obtained and low levels of mature AFGPs were detected in the blood. A phylogenetic tree based on complete ND2 coding sequences showed monophyly of these two New Zealand nototheniids and their inclusion in the monophyletic Nototheniidae consisted of mostly AFGP-bearing taxa. These analyses support an Antarctic ancestry for the New Zealand nototheniids. A divergence time of approximately 11 Myr was estimated for the two New Zealand nototheniids, approximating the upper Miocene northern advance of the Antarctic Convergence over New Zealand, which might have served as the vicariant event that lead to the northward dispersal of their most recent common ancestor. Similar secondary northward dispersal likely applies to the South American nototheniid Paranotothenia magellanica, which has four AFGP genes in its DNA, but not to the sympatric nototheniid Patagonotothen tessellata, which does not appear to have any AFGP sequences in its genome at all.     

Eggs and larvae of Nototheniops larseni from the Spongocoel of a Hexactinellid sponge near Hugo island,Antarctic Peninsula

Summary A mass of adhesive fish eggs was found within the spongocoel of a hexactinellid (glass) sponge, Rosella nuda, collected by bottom trawl near Hugo Island, Antarctic Peninsula in early September. Eggs were hatched in the ship's laboratory and the larvae identified as Nototheniops larseni (Lonnberg). Eggs and early larvae are described. Potential implications of the use of sponges by antarctic fishes as refuges and spawning sites are discussed.Contribution No. 795 from the School of Fisheries University of Washington, Seattle, Wash     

Genetic identification of fish eggs collected in Sendai Bay and off Johban,Japan

Mitochondrial DNA typing of 52 fish eggs collected in Sendai Bay and off Johban revealed ten haplotypes of the 225–251 bp region of 16S ribosomal DNA. Comparing with known sequences of fishes, we identified them as Trachurus japonicus (Carangidae), Pagrus major, Evynnis japonica (Sparidae), Paralichthys olivaceus, Pseudorhombus sp. (Paralichthidae), Pennahia argentata (Sciaenidae) and Platycephalus sp. (Platycephalidae). The occurrence of Trachurus japonicus and sparid eggs in the study area indicates significant extensions to the ranges of their spawning sites northward. Our method is useful for identifying fish eggs that are difficult to diagnose by morphology, and to uncover cryptic fish egg assemblages in the field.     

Closure of the micropyle during embryonic development of some pelagic fish eggs

The fine structure of the egg envelope and micropyle was studied in unfertilized and developing eggs of the flounder Paralichthys olivaceus (Temminck & Schlegel), the Alaska pollack Theragra chalcogramma (Pallas), the Japanese tilefish Branchiostegus japonicus (Houttuyn) and the porgy Pagrus major (Temminck & Schlegel). The outer envelope surface of the unfertilized egg was wrinkled, while the inner surface was folded. The micropyle of the unfertilized egg consisted of a shallow vestibule and a distinct canal. The micropylar region of the inner surface of the envelope had a conical- or bowl-shaped protrusion. In developing eggs, the thickness of the envelope decreased and showed smooth outer and inner surfaces which indicated that it had been stretched tangentially at the time of the perivitelline space formation. The lumen of the micropylar canal was invariably occupied with envelope material. We postulate that the blockage of the micropylar canal is a result of the stretching of the envelope. The closure of the micropyle inhibits sperm and external pathogens from penetrating into the perivitelline space and seems to be involved in both the permanent prevention of polyspermy and the protection of the developing embryo from bacterial infection.     

Sperm penetration and transformation of sperm entry site in eggs of the freshwater teleost Rhodeus ocellatus ocellatus

Eggs of bony fishes are enveloped by an egg envelope (chorion) in which a micropyle is present near the animal pole. Therefore, sperm penetration into the eggs is limited to the sperm entry site (SES), a region of plasma membrane just beneath the micropyle. In rose bitterling eggs, the SES transforms from a tuft of microvilli into a swollen mass (SM) that continues to plug the micropyle after sperm penetration. The present observations using the rose bitterling Rhodeus ocellatus ocellatus were conducted to examine: 1) whether or not sperm penetration is necessary for formation of the SM and 2) whether or not actin microfilaments are involved in the formation of the SM. Water activation without sperm transformed the SES from a tuft of microvilli into the SM, although it took a longer time for the transformation and the SMs were smaller than in the case of inseminated eggs. The SES presumably has the ability to transform into the SM upon activation of eggs in the present species. Cytochalasin B, which acts on actin microfilaments, did not prevent formation of the SM, irrespective of insemination or activation. The present observations suggest that sperm penetration is not necessary for SM formation and actin microfilaments do not participate in SM formation. © 1996 Wiley-Liss, Inc.     

Age,growth and reproductive biology ofNotothenia squamifrons gunther, 1880 from the Indian sector of the Southern Ocean

Summary The growth ofNotothenia squamifrons is analysed using scale reading of specimens from various isolated areas of the Indian sector of the Southern Ocean (Kerguelen, Crozet and Marion Islands; Ob, Lena and Kara-Dag seamounts). Studying reproductive biology reveals that spawning is annual and occurs during austral spring. Hatching takes place in summer. Scaling is set during the autumn. The smallest scaled fish measure 3.8 cm in total length. The data mentioned above, the check number on the scales and the sampling dates lead to the identification of the different age groups. Differences in growth rate appear between the studied areas. The fastest growth is observed around the Kerguelen Islands where the oldest specimens are 19 years old and reach 52 cm in total length. On the other hand, in the Crozet population,N. squamifrons does not live more than 12 years for a maximum length of 30 cm. Scale reading in fish from the Ob and Lena seamounts shows a minor decrease in growth during the winter season when compared to that in the other areas. The hydrographical conditions of this sector of the Southern Ocean may, in part, explain these differences: especially temperature and plankton biomass seem to effect growth rate. The cold antarctic waters and the highest plankton productivity, inducing the optimal growth rate, are found in the Kerguelen Islands.     

Morphology of unfertilized mature and fertilized developing marine pelagic eggs in four types of multiple spawning flounders

Starry flounder Platichthys stellatus, spotted halibut Verasper variegates, turbot Scophthalmus maximus, and Japanese flounder Paralichthys olivaceus are four commercially cultivated multiple spawning flounders that spawn pelagic eggs. Through appropriate light and scanning electron microscope processing, the shape and surface structures (such as micropyle, pores, pore density, and paten) of unfertilized mature and fertilized developing eggs of the four species were observed and measured. First, individual or intraspecific comparisons of the surface structures of eggs at different developmental stages were made. Second, interspecific differences among the four species at the same developmental stage of unfertilized mature eggs were statistically computed and analyzed through one-way analysis of variance and hierarchical cluster analysis. Eggs of the same species collected at different stages of development tend to be different in morphology. Smoothing of the convoluted egg envelope surface and closure of the micropyle to serve as a final step of the polyspermy-preventing reaction are common after fertilization. Based on detailed morphology of micropyle of just-mature fertilizable eggs, turbot, starry flounder, and Japanese flounder each have a micropyle with a long canal but no distinct micropylar vestibule, type III of Riehl and Götting (Arch Hydrobiol 74:393–402, 1974). In contrast, spotted halibut has a micropyle with a distinct flat micropylar vestibule and a long canal, type II. Envelope surface microstructures, especially those in the micropyle region, are useful characters for egg identification among the four species. Cluster analysis using selected egg characters indicated the highest similarity between turbot and Japanese flounder and that starry flounder is obviously more similar to turbot and Japanese flounder than to spotted halibut.     

The use of poly-L-lysine to facilitate examination of sperm entry into pelagic, non-adhesive fish eggs

The fish egg is surrounded by a thick envelope called the chorion. The fertilizing spermatozoon enters the egg through a canal-like structure in the chorion, the micropyle. Examination of micropyle at fertilization is difficult if eggs are large and have no distinct landmarks surrounding the micropyle, or if they are positively buoyant in water. Eggs of many commercially important fishes (e.g., flounder, sea bream and eel) are buoyant in water or only slightly adhere to solid objects (e.g., sands, rock and water plants), which makes observation of spermatozoa at fertilization difficult. Here, we report that such eggs can be firmly attached to plastic and glass dishes that have been previously coated with poly-L-lysine. These adhering eggs can be fertilized and develop normally on the dishes. Observations of micropyles of three fish species, before and after sperm entry are presented.     

Characterization of the intestinal microbiota of two Antarctic notothenioid fish species

The fish fauna of the Southern Ocean is dominated by species of the perciform suborder Notothenioidei, which constitute 46% of fish species and 90% of biomass. Notothenioids have undergone rapid morphological and ecological diversification and developed physiological adaptations to a cold, highly oxygenated environment. Microbes inhabiting animal intestines include those that perform essential nutritional functions, but notothenioid gut microbial communities have not been investigated using cultivation-independent approaches. We analyzed bacterial 16S rRNA gene sequences obtained from the intestinal tract of Notothenia coriiceps and Chaenocephalus aceratus, which differ in their pelagic distribution and feeding strategies. Both samples showed dominance of Gammaproteobacteria (mostly Vibrionaceae), as has been reported for temperate teleost species. Both samples showed low diversity relative to that reported for other fish microbiota studies, with C. aceratus containing fewer OTUs than N. coriiceps. Despite the small sample size of this preliminary study, our findings suggest that Antarctic notothenioids carry a gut microbiota similar in composition to that of temperate fish, but exhibiting lower species-level diversity. The omnivorous N. coriiceps individual exhibited greater diversity than the exclusively carnivorous C. aceratus individual, which may indicate that increasing herbivory in fish leads to gut microbe diversification, as found in mammals. Lastly, we detected members of taxa containing known microbial pathogens, which have not been previously reported in Antarctic notothenioid fish.     

An analysis of temporal variability in abundance, diversity and growth rates within the coastal ichthyoplankton assemblage of South Georgia (sub-Antarctic)

The study of spatial and temporal distribution and diversity of ichthyoplankton (fish eggs and larvae) can provide fisheries-independent information on the population dynamics and recruitment processes of marine fish species. Ichthyoplankton studies in the Southern Ocean have to date been largely constrained to the summer months. We analysed ichthyoplankton data collected from a year round, long term (2002–2008), plankton trawl sampling programme in a large fjord system (Cumberland Bay) at South Georgia, sub-Antarctic (54.25°S, 36.5°W) to assess temporal changes in larval fish diversity and abundance. Larvae of 22 species, representing nine families, were identified although three, Krefftichthys anderssoni (Myctophidae), Lepidonotothen nudifrons/Trematomus hansoni (Nototheniidae) and Champsocephalus gunnari (Channichthyidae), dominated abundance in all years. Significant seasonal and interannual differences in the larval fish assemblage were revealed by multivariate analyses. Estimates of larval growth are provided for five abundant species. Considerable inter-specific differences in relative larval growth rate were recorded but interannual variability within species was small. However, in the commercially important C. gunnari, multiple larval cohorts, representing a protracted spawning season, were observed to grow at different rates, and this may be related to temperature and/or food availability. A comparison with historical growth data from South Georgia suggests there has been little change in growth rate for the main species over the last three decades.     

Seasonal aspects of the biology and diet of nearshore nototheniid fish at Potter Cove,South Shetland Islands,Antarctica

Summary Approximately 1000 specimens belonging to eight fish species were collected at Potter Cove, King George Island, South Shetland Islands, from August 1985 to May 1986. This study deals with the dominant species Notothenia neglecta, Notothenia gibberifrons, Trematomus newnesi and Notothenia rossii marmorata. Age and size structure of the fish were analyzed using scale and otolith readings. Notothenia neglecta was the most abundant species. It spawns in the austral autumn. Juvenile N. rossii marmorata migrate offshore when sexually mature. Over eight hundred stomach contents were analyzed. The four species studied were generally benthophagous. However, in summer T. newnesi and N. rossii marmorata, carried out vertical migrations, feeding on pelagic organisms. Gammarid amphipods constituted the main food in all four species. Algae were consumed regularly throughout the year and we suggest that they are intentially eaten by the fish, rather than by accident. Two 48 hour sampling periods, carried out in summer of 1987, showed that N. neglecta was more active during the day.     

Biology and population dynamics of Notothenia rossii rossii from the Kerguelen Islands (Indian sector of Southern Ocean)

Summary Recent cruises on board research vessels and trawlers have followed the adult fishes of the Notothenia rossii rossii Richardson, 1844 population from the Kerguelen Islands. The food of this Nototheniidae being mainly planktonic, the movements of shoals are connected with the high productivity areas. In the Southern Ocean these areas are localized around the Antarctic Convergence. At the Kerguelen Islands the irregular configuration of the shelf and seasonal variations in water masses change the positions of the productive areas. For this reason the adult population can be to the north or south off the archipelago. Spawning only gathers the mature fishes in the southeast of Kerguelen during winter. At that time this species is very vulnerable to fishing efforts. The population dynamics of this Antarctic fish is studied with fishery statistics and biological data. A decline of the average length and catches per unit effort are observed from one year to the next.

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Resumé De récentes missions à bord de navires de recherche et de chalutiers ont permis de suivre la population adulte de Notothenia rossii rossii Richardson, 1844 des îles Kerguelen. Le régime alimentaire de ce Nototheniidae étant principalement de nature planctonique, les mouvements des bancs de poissons sont liés aux zones de haute productivité. Dans l'océan austral ces zones sont localisées au niveau de la convergence antarctique. Aux îles Kerguelen la topographie du plateau continental et les variations saisonnières de l'hydrologie modifient continuellement la position des zones de productivité. Pour cette raison la population adulte peut se trouver soit au nord soit au sud de l'archipel. Seule la ponte réunit les poissons matures dans le sud-est des îles Kerguelen pendant l'hiver. A cette période l'espèce est très vulnérable à un effort de pêche. La dynamique de la population de ce poisson antarctique est étudiée à partir des statistiques de pêche et des données biologiques. Une baisse de la longueur moyenne de la population ainsi qu'une diminution des valeurs de la prise par unité d'effort sont observées pendant la période considérée.

     

Pelagic fish distribution and ontogenetic vertical migration in common mesopelagic species off Lützow-Holm Bay (Indian Ocean sector,Southern Ocean) during austral summer

The horizontal and vertical distributions of fish were examined off Lützow-Holm Bay in the Indian Ocean sector of the Southern Ocean during midnight sun in January 2005. Fish were sampled from six discrete depth layers (0–2,000 m). The most abundant fish in layers from the surface to 200 m were larval stages of Electrona antarctica and Notolepis coatsi. In layers from 200 to 2,000 m, fish assemblages were relatively uniform among all stations and were dominated by E. antarctica (juvenile–subadult), Cyclothone microdon, and Bathylagus antarcticus. Cluster analysis revealed three epipelagic communities related to water temperature and salinity. An ontogenetic habitat shift to deeper layers was apparent for E. antarctica, N. coatsi, and B. antarcticus. Preferences for warm waters were observed in E. antarctica (larvae) and N. coatsi (preflexion to flexion larvae), although they were distributed across a broad range of temperature and salinity in epipelagic zones.     

Differential effects of organic and inorganic mercury on the micropyle of the eggs of Fundulus heteroclitus

Synopsis The eggs of Fundulus heteroclitus from Piles Creek (PC), a polluted environment, were compared to those from a reference population from relatively unpolluted waters on eastern Long Island (LI). We have shown in a previous study that PC embryos had a higher resistance to meHg (methylmercuric chloride) than LI embryos but not to Hg (inorganic mercuric chloride). Twenty minute exposures of eggs prior to insemination likewise showed PC eggs were more tolerant to 1.0mg l^-1 meHg but more susceptible to 1.0mg l^-1 Hg and also remained viable in clean sea water for a longer time than LI eggs. Scanning electron microscopy revealed that the reduced insemination success after egg exposure to meHg was due to artificial activation, due to rupture of cortical vesicles and blockage of the micropyle. On the other hand, inorganic mercury produced very different morphological effects. Eggs exposed to Hg showed a swelling of the micropylar lip and a decrease in its diameter, which may have led to the reduction in insemination.