Macroscopic and histological analyses of gonads during the spawning season of Chionodraco hamatus (Pisces,Channichthyidae) off Terra Nova Bay,Ross Sea,Southern Ocean

A macroscopic and histological analysis of gonads was carried out during the spawning season of the high-Antarctic channichthyid Chionodraco hamatus in the western Ross Sea. Samples were collected between December and February during several years in the coastal waters of Terra Nova Bay. Gonad maturity stages were described for males and females according to macroscopic and histological scales. Using multi-year data, the estimated length at first spawning of females was about 35 cm TL, very similar to that obtained indirectly for males. Similar to many other high-Antarctic fish, C. hamatus is a summer spawner. The greater part of the stock was indeed in spawning condition between December and February, although a large proportion of females large enough to spawn probably did not spawn in that season. The present data confirm that C. hamatus, as is typical for Antarctic fish, probably spawns a single batch of oocytes once a year. In addition, vitellogenesis is a slow process that extends over at least 1 year. Discrepancies between the macroscopic and histological appearance of gonads were found. These were associated mainly with spent and resting females (maturity stages 5 and 2, respectively). This study demonstrates the importance of histological analysis of gonads in order to confirm the results of the macroscopic analyses routinely carried out in studies of reproductive biology. This is of particular importance in determining size at maturity and spawning stock biomass, for assessment purposes.     

Satellite DNAs,Heterochromatin and sex chromosomes in Chionodraco hamatus (Channichthyidae,Perciformes)

We studied the genome of an antarctic ice fish, Chionodraco hamatus, in order to detect highly repetitive DNAs that may play a role in heterochromatinization processes and sex chromosome differentiation. We used two different experimental approaches. Hybridization of a Bkm probe to genomic DNA showed slight differences between the two sexes. Using restriction enzymes, a Bgl II satellite (pIF) was isolated. In situ hybridization revealed a preferential localization of pIF on the centromeres and the telomeres of most chromosomes, as well as an interstitial band on the long arms of the neo-Y sex chromosome, where probably the hypothetical fusion took place. Dot-blot experiments showed that pIF is still present in species belonging to different families of the same suborder. Though preliminary, our results suggest a conservative nature of this DNA which might have played a definite functional role in the genome of these polar fishes.     

Feeding plasticity of Trematomus newnesi (Pisces, Nototheniidae) in Terra Nova Bay, Ross Sea, in relation to environmental conditions

Forty-three and 49 specimens of the Antarctic fish Trematomus newnesi were collected in the coastal waters of Terra Nova Bay, Ross Sea, during December 1994 and February 1998, respectively. The dietary composition of the two fish samples was determined by means of stomach content analysis and then compared. In general, as reported in most of the previous studies, planktivory appeared to be the main feeding habit of T. newnesi. However, the different environmental conditions occurring in the study area in the two periods investigated, such as the degree of the sea-ice coverage and the related amount of light available below the ice, influenced the food composition of T. newnesi. In December 1994, the whole area investigated was covered by a thick layer of sea ice and the diet of T. newnesi consisted of few prey taxa. Some species that characterize the so-called “cryopelagic habitat”, such as the euphausiid Euphausia crystallorophias and the copepod Metridia gerlachei, were by far the most important prey, followed by amphipods and the pteropod Limacina helicina. Conversely, the ostracod Alacia belgicae and larval stages of fish (Pleuragramma antarcticum) were consumed occasionally. In late summer (February 1998), the lack of sea-ice coverage and changes in the associated fauna influenced the feeding habit of T. newnesi. In this period the food spectrum appeared to be more diverse, revealing an active feeding search in the water column. The bulk of food was composed of several prey groups, such as E. crystallorophias, hyperiid amphipods (Hyperiella dilatata), copepods, L. helicina and several species of fish larvae. Present data provide evidence of a marked feeding plasticity of T. newnesi, in response to diverse environmental conditions that characterize the High-Antarctic Zone. Accepted: 17 July 1999     

Age structure of Chionodraco hamatus (Teleostei,Channichthyidae) samples caught in Terra Nova Bay,East Antarctica

Summary The age composition of Chionodraco hamatus samples caught by fixed nets in Terra Nova Bay during the Austral summer 1987/88 was determined by examination of thin otolith sections. Individual ages were estimated by counting the number of annuli seen in the sections, since we postulated that annuli were laid down yearly. Fish were estimated to be 5–10 years old. Although our apparent ages were not confirmed by other independent estimates, our data seem to compare well with the values reported for other Antarctic fishes. Females in our samples were on average larger than males of the same age and grow somewhat faster, at least over much of the sampled size range.     

Aspects of the biology of the icefish Dacodraco hunteri (Notothenioidei, Channichthyidae) in the Ross Sea, Antarctica

On the basis of five specimens, the icefish Dacodraco hunteri (Notothenioidei, Channichthyidae) is documented for the first time in the Ross Sea, Antarctica. Meristic counts and morphometric measurements are provided for this small, streamlined, laterally compressed species. D. hunteri has a weakly ossified skeleton with considerable cartilage in the skull. It has a partially persistent notochord and reduced amounts of bone in the vertebral column since the centra are incompletely constricted. Its weight in seawater averages only 1.28% of its weight in air and, as one of the lightest notothenioids, D. hunteri is probably a permanent inhabitant of the water column. The diet consists of relatively large specimens of the pelagic nototheniid fish Pleuragramma antarcticum. Accepted: 27 September 1998     

Isolation and characterization of microsatellite loci in the icefish Chionodraco rastrospinosus (Perciformes,Notothenioidea, Channichthyidae)

We characterized eight polymorphic microsatellites in the icefish Chionodraco rastrospinosus (Perciformes, Notothenioidea, Channichthyidae). Microsatellites were isolated from a partial genomic library enriched for an AC motif. Chionodraco rastrospinosus is an endemic species inhabiting southern ocean waters surrounding the Antarctic Peninsula, the South Shetland Islands, and the South Orkney Islands. An excess of homozygotes was observed in seven out of the eight investigated loci; however, presence of null alleles was detected only for three of them suggesting that other factors may act in reducing heterozygosity. These molecular markers will be useful to investigate icefish genetic structure, possibly providing insights on its effective population size and demographic history.     

Identification and localization of α- and β-spectrins in oocytes of three Antarctic teleosts: Trematomus bernacchii, Trematomus newnesi (Nototheniidae) and Chionodraco hamatus (Channichthyidae)

The presence and localization of α- and β-spectrins and of the spectrin cross-linking protein actin were investigated, in previtellogenic oocytes of three species of Antarctic teleosts: the two red-blooded nototheniids, Trematomus bernacchii and Trematomus newnesi , and the channichthyid, the ice fish Chionodraco hamatus . Analyses by western blotting indicated that these species had an unusual abundance of spectrin isoforms and that they were characterized by rather low molecular masses. The immunocytochemistry in situ demonstrated that α- and β-spectrins showed a variable pattern of localization that clearly depended on both the species considered and the stage of oocyte differentiation. In particular, the two Trematomus spp. showed a distribution of spectrins absolutely comparable and rather different from that of C. hamatus . The evidences collected confirmed that channichthyids have isolated early from the group of red-blooded species and suggest that in notothenioids significant changes might have occurred in spectrin genes and in their protein products.     

Reproduction of the Nassau grouper,Epinephelus striatus (Pisces: Serranidae) and its relationship to environmental conditions

Synopsis Spawning aggregations of the Nassau grouper, Epinephelus striatus, occurring at the southern end of Long Island were investigated during the winters of 1987–1988 and 1988–1989. Fish aggregate at two specific sites around the time of the full moon in December and January, possibly not during November and almost certainly not during February. At one site the fish aggregate well inside the shelf edge while at the other they are on the dropoff into deep water. Fish migrate, probably as distinct groups of up to about 500 individuals, to the sites on or before the full moon and remain for several days. There was no significant difference in the size of males and females caught from the aggregations between years or months. Female/male sex ratio was between 5 : 1 and 3 : 1. Courtship occurred in late afternoon with spawning commencing shortly before sunset. Most spawning occurred within 10 min of sunset. Water temperatures were 25.0–25.5°C during a period of gradual decrease towards the annual minimum in February and March. Two color patterns were important in courtship and spawning. The bicolor pattern is a submissive coloration indicating a non-aggressive state acquired by both males and females near the time of spawning. The dark phase is acquired by females who are followed by numerous bicolor fish during courtship and they lead spawning events in this pattern. Spawning occurred among subgroups of the aggregation numbering 3–25 fish; gamete release was well above the bottom. Drogues deployed with the gametes either moved inshore or did not move far away from the shelf edge over the course of several days. Nassau groupers may not strictly be protogynous hermaphrodites and other groupers which form spawning aggregations may not be so either. Latitudinal shifts in spawning time may be related to water temperatures. Currents at aggregation sites do not appear to favor offshore transport of eggs. Questions of whether spawning aggregations should be protected need to be answered.     

Ultrastructure and proteins of the egg chorion of the antarctic fish Chionodraco hamatus (Teleostei, Notothenioidei)

The chorion morphology and protein content of unfertilised eggs of the antarctic icefish Chionodraco hamatus were characterised. Scanning and transmission electron microscopy showed that the chorion of this antarctic species is organised differently from that of non-polar species, with several concentric layers varying in thickness. Purified chorions were dissolved in 8-M urea buffer, and polypeptides were revealed by one- and two-dimensional gel electrophoresis. Glycoproteins were detected using affinity blotting with concanavalin-A. The principal glycoproteins had a molecular weight of 49 and 93 kDa. Purification of the main polypeptides between 40 and 92 kDa was achieved by preparative electrophoresis followed by electroelution of excised bands. The main biochemical composition of C. hamatus chorion was similar to that of other teleost species investigated.     

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.     

Ectoenzymatic ratios in relation to particulate organic matter distribution (Ross Sea,Antarctica)

The results of a study on ectoenzymatic activity (the enzyme activity bound to particles larger than 0.2 micro m) and its relation to organic particle concentration are reported here. The sampling was carried out during the 1994 Antarctic spring, at a fixed station (Station 11) in the polynya of the Ross Sea, an area characterized by quick changes in sea ice cover. The sampling was repeated 4 times over a 20-day time period. The particulate organic matter distribution followed the physical structure of the water column, which depends on ice dynamics and is mainly determined by salinity. In the mixed-water surface layer (0-50 m) the concentrations were higher (on average 65.6 micro gC/L) than in the deeper water layer (50 m-bottom) (on average 19.1 micro gC/L). This distribution and quality, expressed by the protein:carbohydrate ratio, linked the particulate organic matter to the phytoplanktonic bloom which was in progress in the area. We determined the kinetic parameters of the glycolytic and proteolytic ectoenzymes and also the total activity for the proteolytic enzyme, in order to evaluate the contribution of the particle-bound activity. We observed higher values in the surface layer than in the deeper layer. b-Glucosidase activity ranged between 0.03 and 0.92 nmol L(-1) h(-1); b-N-acetylglucosaminidase activity was in the range of 0.04-0.58 nmol (L-1) (h-1). The total proteolytic activity (leucine aminopeptidase) ranged between 0.85 and 33.71 nmol L(-1) (h-1). The ectoproteolytic activity was about 35-60% of the total. The Km values were slightly higher for the proteolytic activity (on average 0.43 micro M for ectoproteolytic activity and 0.58 micro M for total proteolytic activity) than for the b-glucosidase (on average 0.36 micro M) and b-N-acetylglucosaminidase (on average 0.17 micro M), showing no remarkable variations in the water column. The ectoenzymatic ratios and their relationship with particulate organic substrates confirm the close link between organic substrate availability and degradation system response. The significant and positive correlations are not specific and suggest a prompt and efficient systemic response to the input of trophic resources. Nevertheless, changes in ectoenzyme activity and synthesis may act as adaptive responses to changing features of the ecosystem. In particular, variations in the proteolysis:glycolysis ratio depend on the functional features of the ecological system. In our study area this ratio is higher (about 10 or more) during production (particularly autotrophic) and lower (about 5 or less) during degradation/consumption events. The analysis of previous data, collected over a larger area characterized by different environmental conditions due to the changes of the pack ice cover, during the same cruise, confirms the existence of a significant relationship. Furthermore, the analysis of enzyme-uptake systems, expressed as Vmax:Km ratio, suggests that glycolytic ectoenzymes, although poorly expressed, may encourage microconsumers to grow rapidly on a wide range of organic substrates, including the refractory ones such as cellulose and chitin. However, low ectoenzyme potential exploitation rates of available organic substrates (on average about 5% for glycolytic and 12% for proteolytic ectoenzymes) would suggest that, during spring, zooplankton grazing or vertical and lateral transport are likely to play an important role in the removal of organic materials from the system.     

Larval development of Hypsophrys nicaraguensis (Pisces: Cichlidae) under laboratory conditions

The cichlid Hypsophrys nicaraguensis is a popular fish known as butterfly, and despite its widespread use as pets, little is known about its reproductive biology. In order to contribute to this knowledge, the study describes the relevant larval development characteristics, from adult and larval cultures in captivity. Every 12h, samples of larvae were collected and observed under the microscope for larval stage development, and every 24h morphometric measurements were taken. Observations showed that at 120h, some larvae had swimming activity and the pectoral fins development was visible; at 144h, the dorsal fin appear and all larvae started food intake; at 168h, the formation of anal fins begins, small rudiments of pelvic fins emerge, the separation of caudal fin from anal and dorsal fins starts, and the yolk sac is reabsorbed almost completely; at 288h, the pelvic fins starts to form; at 432h, the rays and spines of dorsal and anal fins can be distinguished, both the anal and the dorsal fins have the same number of spines and rays as in adults. After 480h larvae have the first scales, ending the larval stages and starting the transformation to fingerlings. Larvae were successfully fed with commercial diet.     

Characterization of a Tc1-like transposon in the Antarctic ice-fish,Chionodraco hamatus

We report the presence of Tc1 transposon-like sequences in the Antarctic ice-fish Chionodraco hamatus, belonging to the Notothenioidei. The complete DNA sequence of these transposon-like elements is reduced in length compared to other Tc1 transposons, but it appears to share significant structural similarities with them. It contains a degenerate open reading frame, whose inferred 264 amino acid sequence shares sequence similarity with the 'aspartic acid, aspartic acid (35) glutamic acid' family of transposases, particularly those from Caenorhabditis species (sp.) and Drosophila sp. Southern blot analysis and polymerase chain reaction amplification indicate that Tc1 transposon-like sequences are present in other notothenioid species, though their amount can vary in the different lineages.     

Cytology of lymphomyeloid head kidney of Antarctic fishes Trematomus bernacchii (Nototheniidae) and Chionodraco hamatus (Channicthyidae)

Species that live in extreme conditions have specially adapted physiology and tissue/organ organisation. The adaptation of lymphoid organs to low temperatures in polar species could be an original field of study, indicating how the immune system works under extreme conditions. In fishes, the head kidney is a key organ for immunity and here the cytology of this organ is studied in two common Antarctic species: Trematomus bernacchii and Chionodraco hamatus. Ultrastructural analysis revealed heterogeneity of epithelial cells, with reticular cells, subcapsular- and perivascular-limiting cells. Differences in the size and morphology of epithelial cells were observed between the polar species and warm water species of fish. Intermingled with epithelial cell leucocytes, such as lymphocytes, thrombocytes and macrophages, had comparable morphology in both species, contrary to sharp differences observed in the morphology of erythrocytes and granulocytes. The functional adaptation of the head kidney to the low temperatures of polar water is discussed.     

Distribution and abundance of postlarval and juvenile Pleuragramma antarcticum (Pisces, Nototheniidae) off Terra Nova Bay (Ross Sea, Antarctica)

During the Italian Antarctic Expedition of 1987–1988 zooplankton was collected in Terra Nova Bay (Ross Sea) by a multinet BIONESS (250-μm or 500-μm net mesh size). The early life history of Pleuragramma antarcticum was described from 268 samples. More than 98% of a total of 34,436 fish larvae belonged to P. antarcticum. The mean relative abundance in the whole area for positive 0 to 150-m hauls was 434 ind./100 m³ (±720 SD). Postlarvae were most abundant and frequent in the samples (99.8%) while low concentrations of juveniles were found (n=67). Length of age group 0 ranged from 8 to 20 mm and age group 1+ were from 36 to 53 mm. Average growth rate over a period of 1 year was 0.08 mm per day. Based on modal values, the mean daily increment of P. antarcticum postlarvae in the period 5 January to 2 February (29 days) was about 0.21 mm. Highest abundances occurred near Cape Washington (mean: 2,108 ind./100 m³) while lower densities were recorded in the northern basin of the self (31 ind./100 m³). Horizontal and vertical distribution patterns in Terra Nova Bay seem to be strictly correlated to hydrographic features and different water masses with highest densities associated with the westward flowing current of the limb of the Antarctic coastal current and southern limb of the Ross Sea Gyre. These currents become part of the clockwise gyre in Terra Nova Bay. More than 62% of the postlarvae were collected in well-stratified warm surface water (0–50 m) near the summertime thermocline (20–70 m). The northern part of Terra Nova Bay seems to represent nursery ground of early stages of P. antarcticum and the presence of permanent polynya could provide favourable food conditions for development of the first stages of life. Differing distribution patterns probably reflect an interaction of various parameters including bathymetry, floating ice shelf, hydrographic features such as currents, local eddies and frontal systems, with P. antarcticum postlarval biology (spawning) and ecology (feeding, horizontal and vertical distribution patterns). Received: 10 June 1996 / Accepted: 19 July 1997     

The biology of the icefish Cryodraco antarcticus Dollo, 1900 (Pisces, Channichthyidae) in the southern Scotia Arc (Antarctica)

The icefish Cryodraco antarcticus is common in deeper waters of the southern Scotia Arc and the high-Antarctic zone. A number of biological features of this species are presented, with new information collected from recent scientific surveys of the South Shetland Islands and South Orkney Islands. The species is closely related to Chaenocephalus aceratus, but can be distinguished by a number of meristic characters. The two species are similar in size, colouration and body shape, and have a number of aspects of their life-cycle in common, such as reproduction, length-weight relationship and diet. These two species appear to occupy a very similar niche in the southern Scotia Arc ecosystem. However, Cryodraco antarcticus is less abundant and uses different spatial components of shelf areas, replacing Chaenocephalus aceratus in deeper water and high-Antarctic regions.     

Structure and membrane interactions of chionodracine,a piscidin-like antimicrobial peptide from the icefish Chionodraco hamatus

Chionodracine (Cnd) is a 22-residue peptide of the piscidin family expressed in the gills of the Chionodraco hamatus as protection from bacterial infections. Here, we report the effects of synthetic Cnd on both Psychrobacter sp. TAD1 and Escherichia coli bacteria, as well as membrane models. We found that Cnd perforates the inner and outer membranes of Psychrobacter sp. TAD1, making discrete pores that cause the cellular content to leak out. Membrane disruption studies using intrinsic and extrinsic fluorescence spectroscopy revealed that Cnd behaves similarly to other piscidins, with comparable membrane partition coefficients. Membrane accessibility assays and structural studies using NMR in detergent micelles show that Cnd adopts a canonical topology of antimicrobial helical peptides, with the hydrophobic face toward the lipid environment and the hydrophilic face toward the bulk solvent. The analysis of Cnd free energy of binding to vesicles with different lipid contents indicates a preference for charged phospholipids and a more marked binding to native E. coli extracts. Taken with previous studies on piscidin-like peptides, we conclude that Cnd first adsorbs to the membrane, and then forms pores together with membrane fragmentation. Since Cnd has only marginal hemolytic activity, it constitutes a good template for developing new antimicrobial agents.     

Respiratory responses of Oreochromis niloticus (Pisces, Cichlidae) to environmental hypoxia under different thermal conditions

The oxygen uptake ( V O₂), breathing frequency ( f _R), breath volume ( V _S.R), gill ventilation ( V G) and oxygen extraction (%) from the ventilatory current of four groups of Oreochromis niloticus during graded hypoxia were measured under the following acclimation temperatures: 20. 25. 30 and 35°C. The critical oxygen tensions ( P O₂), determined from V O₂ v. P O₂ of inspired water at each experimental temperature were, respectively. 19±1±3±1. 18±0±4±9, 29±7± 4±1 and 30±2± 0.6 mmHg. The f _R remained nearly constant during the reductions of O₂ at all the temperatures studied. V G increased discretely from normoxic levels until the P O₂ was reached, below which it assumed extremely high values (17-fold higher or more). The increases observed in V G resulted, at all the acclimation temperatures, in an elevation in V _S.R rather than in f _R. The extraction of O₂ decreased gradually from normoxia until the P O₂ was reached, below which an abrupt reduction of extraction was recorded, except at 35°C when fish showed a gradual reduction in extraction just below the tension of 80 mmHg.