Geographical distribution and abundance of postlarval and juvenile Pleuragramma antarcticum (Pisces,Notothenioidei) off the Antarctic Peninsula

Summary The early life history of Pleuragramma antarcticum off the Antarctic Peninsula is described from extensive material collected during four expeditions between 1975/76 and 1982 by means of an RMT 1+8. Postlarvae occurred frequently on the northern continental shelf in February and March except during the 1977/78 season when they were almost absent. Abundance varied mainly between 1.8 and 14.7 postlarvae per 10³ m³ with maxima between 32.6 and 143.3 per 10³ m³. Juveniles were less abundant with 0.1–4.9 and maxima between 9.8 and 28.3 per 10³ m³, but formed a consistent, predominant proportion (83–97%) of the pelagic juvenile fish fauna throughout the years of the study. They were distributed over the whole Bransfield Strait and adjacent waters in the top 200 m. Postlarvae and juveniles showed different patterns of vertical distribution; more than 70% of postlarvae occurred in the top 135 m of the water column whereas 83% of juveniles were encountered between 70 and 200 m with increasing abundance towards the lower depth. Three length groups were identified corresponding to age groups 0, 1, and 2. Hatching was estimated to occur in December. Postlarval distribution patterns and the prevailing transport of water masses suggest that coastal waters south of the tip of the Antarctic Peninsula and in the north-eastern Bellingshausen Sea are probable spawning areas. The Bransfield Strait and adjacent waters represent nursery grounds for the early stages of P. antarcticum from these sites.     

Pleuragramma antarcticum (Pisces,Nototheniidae) as food for other fishes in McMurdo Sound,Antarctica

Summary Nine spcies of notothenioid fishes were captured near the southern limit of their range in ice covered McMurdo Sound, Antarctica. Stomach contents were examined using an occurrence method. Fishes were present in the diets of 8 of 9 species;Dissostichus mawsoni andGymnodraco acuticeps were predominantly piscivorous.Pleuragramma antarcticum was the most common prey fish consumed, being present in stomachs of 4 of 8 species.Pleuragramma was also partially piscivorous (22%) as well as cannibalistic (13%). As evidenced by their presence in the stomach contents of other fishes, all life history stages from 20 mm SL postlarvae to 160 mm SL adultPleuragramma were represented under the ice of the Sound.Pleuragramma was also a major food item for Weddell seals, birds and possibly invertebrates. As a widely distributed species in the pelagic zone,Pleuragramma may be an ecological substitute for euphausiids in the food web of the Sound.     

Shallow-water benthic hydroids from Tethys Bay (Terra Nova Bay, Ross Sea, Antarctica)

The shallow-water hydrozoan Antarctic fauna is still poorly studied, and available knowledge mostly refers to samples gathered by traditional ship-operated gears. By scuba diving in the coastal areas off the Italian Antarctic station “Mario Zucchelli” (Ross Sea, Terra Nova Bay), in the austral summer 2002–2003, a total of 20 hydrozoan species were found, belonging to 10 families and 13 genera. As hypothesized, Anthoathecata (11 species), usually under-represented in collections from indirect sampling gears, are common as also are Leptothecata (9 species). Hydractiniidae and Hydractinia are the dominant family and genus, followed by Haleciidae and Halecium. A new species to science, Halecium exaggeratum sp. nov. is also described. Most species are either endemic to Antarctic waters or restricted to Antarctic/sub-Antarctic areas; only two species have a wider distribution. Material reared in aquaria at the Italian Antarctic Base Mario Zucchelli facilitated knowledge of the life cycle and reproductive biology of several species. In particular, Opercularella belgicae was found to liberate a medusa stage referable to Phialella, and the species is assigned here to that genus, as Phialella belgicae. Also, extraordinary is the complete absence or scant representation of the most typical Antarctic benthic hydroid genera (Antarctoscyphus, Oswaldella, Schizotricha, Staurotheca, and Symplectoscyphus), likely related to the shallow limits of sampling (down to 48 m).     

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     

Shallow- and deep-water mollusc distribution at Terra Nova Bay (Ross Sea, Antarctica)

The aim of this work is to improve the knowledge of the mollusc fauna of Terra Nova Bay (Ross Sea), on the basis of more than 100 stations sampled from 25 to 1100 m depth, during Italian Antarctic Expeditions (austral summers 1987/1988, 1989/1990, 1993/1994, 1994/1995, 1995/1996 and 1997/1998). In shallow waters, gastropod fauna is represented by 31 species, among which Neobuccinum eatoni, Onoba gelida, Powellisetia deserta, Philine cf. apertissima and Austrodoris kerguelenensis are the most abundant, while among bivalves (25 species) the commonest species are Adamussium colbecki, Yoldia eightsi, Laternula elliptica and “Montacuta”nimrodiana. At present, most of the Antarctic mollusc species are known as having circumantarctic distribution and are considered eurybathic. Moreover, in general, only scarce data are available on their substrate preferences, because of their occurrence in scattered sampling stations with different sediment features. In the present study carried out at Terra Nova Bay, based on a relatively high number of stations, a clear zonation of mollusc assemblages is shown, according to depth and sediment features. While gastropods characterise the superficial algal belts dominated by the red algae Iridaea cordata and Phyllophora antarctica, bivalve distribution is wider. Adamussium colbecki is the dominant species in the upper 100 m, both on soft and hard bottoms, if the slope is suitable (density up to 40–60 ind./m²). On coarse sands, this species is frequently accompanied by L. elliptica (density <20 ind./m²), while Y. eightsi characterises organic- enriched bottoms (density 70–80 ind./m²). Below 200 m depth Adacnarca nitens, Limatula hodgsoni, “Montacuta”nimrodiana and Mysella gibbosa are commonly found on relatively coarse sediments, while Thyasira dearborni and Yoldiella ecaudata prefer muddy bottoms. In order to identify bivalve assemblages, multivariate analyses were applied to stations grouped into depth ranges, sediment features and location. While shallow bathymetric ranges (25–200 m) are clearly defined, deeper clusters of stations are more influenced by the high degree of eurybathy of most of the species and by the scattered distribution of the sediment particle sizes. Accepted: 18 September 1999     

Sea urchins, sea stars and brittle stars from Terra Nova Bay (Ross Sea, Antarctica)

Although echinoderms constitute some of the most conspicuous taxa of the Antarctic benthic communities, the echinoderm fauna of Terra Nova has not been described yet. The present study provides the first species list of echinoids, ophiuroids and asteroids from Terra Nova Bay (30–500 m depth) and describes the depth distribution of these species. Preliminary observations of the summer reproductive condition of some of the species are also included.     

Copepods in spring annual sea ice at Terra Nova Bay (Ross Sea,Antarctica)

The aim of this study was to investigate patterns of abundance, distribution, temporal changes and species composition of the dominant ice-associated copepods in the spring annual pack ice, platelet ice and water column at Terra Nova Bay, Ross Sea, during late spring 1997. Ice cores were drilled for temporal and spatial scales. Stephos longipes and Harpacticus furcifer dominated the sea ice meiofauna in terms of numbers in the lower few centimeters of the bottom ice associated with high chlorophyll a and phaeopigment levels. Nauplii dominated the S. longipes population (91.6%) and occurred in extremely high concentrations. In contrast, copepodids were the dominant stages in H. furcifer. How H. furcifer carries out its entire life cycle and how it differs from ecologically similar species such as Drescheriella glacialis should be examined in more detail.     

A new Polygordius (Annelida: Polychaeta) from Terra Nova Bay, Ross Sea, Antarctica

Polygordius antarcticus sp. nov. is described from benthic material collected at depths ranging between 31 and 61 m in Terra Nova Bay, Ross Sea, during the 1987–1988 and 1988–1989 expeditions of the Italian National Research Program in Antarctica. This is the first named species of the genus from Antarctic waters. Although no fully mature specimens were available, the new species appears unique in combining the following features: a regular pygidium, inflated and with a single ring of round adhesive pads, a conical prostomium with short antennae (only half as long as the prostomium) and shallow head fold, and a well-developed circulatory apparatus, with circumoesophageal commissures entering the prostomium and long intersegmental commissures insinuating into the ventrolateral compartments of the trunk. These and other significant features were investigated by light and scanning electron microscopy. In the construction of the head and the scheme of the blood system, the new species resembles Polygordius triestinus Woltereck, described by Hempelmann in 1906, an aberrant species inhabiting muddy, anoxic sediments in the Adriatic Sea, but it differs significantly from it in the shape of the mouth (in P. triestinus the upper lip is hypertrophied and strongly protruding) and the pygidium (in P. triestinus this region is stump-like and non-adhesive). The pygidium of P. antarcticus sp. nov. is encircled by 28–30 small adhesive pads, each with 15–20 glandular openings. Preterminal cirri are lacking, as are perianal appendages. The anus is surrounded by six to seven lobes of which the midventral is largest and longest. A world distribution map of the genus is provided. Accepted: 27 August 1998     

Growth form analysis of epiphytic diatom communities of Terra Nova Bay (Ross Sea, Antarctica)

Diatoms have been long collected from the Southern Ocean but almost no data exist for epiphytic communities, despite their high ecological significance as an important food source in Antarctic coastal food chains. Here, we present a first growth form analysis of diatoms associated with rhodophyte hosts from Terra Nova Bay, Ross Sea, Antarctica. We performed this study to gather baseline information on the species composition of epiphytic diatom communities, determine the influence of some environmental variables on the diatom distribution patterns, and assess the caveats that must be taken into account in terms of sampling design. Macroalgal material was collected during the Italian Antarctic expeditions between 1990 and 2004. Epiphytic diatoms were studied by means of scanning electron microscopy. In terms of growth forms, there were no significant differences between the diatom communities on the different macroalgal host species. Motile (mainly small-celled Navicula perminuta and other Navicula spp.) and adnate (Cocconeis spp.) diatoms dominated the community throughout the study period. Many of the macroalgal blades examined were also covered by epiphytic animals (calcareous bryozoans, hydroids) over most of their surface, with a significant effect on the associated diatom community structure. Our findings suggest that the bio-physicochemical characteristics of each sampling site affected the epiphytic diatom communities more than the substrate type provided by the macroalgal host or the sampling depth.     

Summer coastal zooplankton biomass and copepod community structure near the Italian Terra Nova Base (Terra Nova Bay, Ross Sea, Antarctica)

The structure of the zooplankton biotic community and of copepodpopulation in the coastal area of Terra Nova Bay (Ross Sea,Antarctica) was investigated during the 10th Italian AntarcticExpedition (1994/1995). Zooplankton biotic community consistedmainly of pteropods (Limacina helicina and Clione antarctica),Cyclopoid (Oithona similis), Poecilostomatoid (Oncaea curvata)and Calanoid (Ctenocalanus vanus, Paraeuchaeta antarctica, Metridiagerlachei and Stephos longipes) copepods, ostracods, larvalpolychaetes and larval euphausiids. Zooplankton abundance rangedfrom 48.1 ind m^–3 to 5968.9 ind m^–3, and copepodabundance ranged from 45.2 ind m^–3 to 3965.3 ind m^–3.The highest peak of zooplankton abundance was observed between25 m and the surface and was mainly due to the contributionof O. similis, O. curvata and C. vanus. Zooplankton biomassranged from 5.28 mg m^–3 to 13.04 mg m^–3 dry weight;the maximum value was observed between 25 m and the surface.Total lipid content varied from 216.44 to 460.73 mg g^–1dry weight.     

Biodiversity of cultivable psychrotrophic marine bacteria isolated from Terra Nova Bay (Ross Sea, Antarctica)

A set of 146 Antarctic marine isolates from the Ross Sea was characterized by a combination of molecular techniques in order to determine the degree of inter- and intraspecific variability. Isolates were analyzed by amplified rDNA restriction analysis (ARDRA) using the tetrameric enzyme AluI, resulting in 52 different groups, corresponding to at least 52 different bacterial species, indicating a high degree of interspecific variability. The phylogenetic position of bacteria belonging to some ARDRA groups was obtained by sequencing of 16S rDNA. Random amplified polymorphic DNA (RAPD) analysis, carried out on the largest ARDRA groups, revealed a high intraspecific genetic variability, too. The analysis of plasmid content revealed the existence of horizontal gene transfer between strains belonging to the same and to different species. A comparison of the whole body of morphological, physiological and biochemical data was finally carried out.     

Polychaete populations of the shallow soft bottoms off Terra Nova Bay (Ross Sea, Antarctica): distribution, diversity and biomass

 Diversity, distribution and biomass patterns of polychaetes were studied in the shallow soft bottoms off Terra Nova Bay (Ross Sea, Antarctica). Benthic samples, collected in the austral summer 1989–1990, were sampled using a Van Veen grab at 14 stations distributed between 23- and 194-m depth. A total of 5,768 individuals of polychaetes belonging to 77 species were found. The species richness recorded was high, despite the few biotopes investigated at Terra Nova Bay in this study, especially when compared to that recorded from previous investigations in the same area and from the whole Ross Sea (146 species). Despite the high number of species found, only a few species accounted for 76% of the total abundance: Tharyx cincinnatus (27%), Spiophanes tcherniai (23%), Leitoscoloplos mawsoni (18%) and Laonice weddellia (5%) were mainly distributed in the deeper stations (below 100-m depth), while Aglaophamus ornatus (3%) was the only species abundant in the shallowest stations (above 50-m depth), characterized by gravels and coarse sands. Species richness, diversity and evenness showed an increase with depth. Abundances ranged between 40 to over 12,000 ind/m². Biomass values ranged between 0.9 to 176 g/m² (wet weight). A trophic-guild classification revealed the occurrence of various feeding categories that were, however, numerically dominated by deposit feeders with both discretely motile epiand endobenthic forms, and sessile ones. Both the Bray-Curtis similarity and Correspondence (C.A.) analyses showed stations ordered according to depth and to the local dominance of some species. Both poorly and well-structured polychaete assemblages occurred and they seemed related to depth-related factors, such as degree of environmental disturbance (shallowest bottoms vs deep ones), heterogeneity of the substrate and habitat complexity (gravel and coarse sands vs mixed sediment. The richest and most diverse assemblages of polychaetes were those occurring below 140-m depth, where the substrate was dominated by medium sands, often mixed with a mosaic of rocks and boulders. Received: 30 October 1995/Accepted: 1 May 1996     

A nursery area for the Antarctic silverfish Pleuragramma antarcticum at Terra Nova Bay (Ross Sea): first estimate of distribution and abundance of eggs and larvae under the seasonal sea-ice

Pleuragramma antarcticum is the dominant pelagic fish in the waters of the continental shelf in high Antarctic regions, where it plays a key role in the food web. A nursery ground for eggs of this species was first identified in 2002 in Terra Nova Bay (Ross Sea), where eggs were found trapped in ice platelets under the sea-ice during the spring. As part of a monitoring program aimed at understanding the geographic and temporal characteristics of this nursery ground, the present study reports on surveys carried out in the austral springs of 2005 and 2006 using a simple and effective method for sampling from the sea-ice. These surveys enabled the evaluation of the spatial range of the nursery area of the Antarctic silverfish in the sea-ice of the coastal area of Victoria Land between the Coulman Island and the Drygalski Glacier Tongue. P. antarcticum eggs were concentrated in an area of Terra Nova Bay of about 270?km², encompassing two adjacent sites, Gerlache Inlet and Silverfish Bay. The present results add information on life cycle and hatching period of the Antarctic silverfish and confirm the importance of the Terra Nova Bay as a nursery area for this important species. Moreover, the survey points to the sea-ice cover and platelet ice as important environmental features of the nursery area.     

Diet and trophic niche of Antarctic silverfish Pleuragramma antarcticum in the Ross Sea,Antarctica

The diet of Antarctic silverfish Pleuragramma antarcticum was evaluated by examining stomach contents of specimens collected in the Ross Sea (71°–77° S; 165°–180° E) in January to March 2008. Pleuragramma antarcticum (50–236 mm standard length, L_S) and prey items were analysed for stable‐isotopic composition of carbon and nitrogen. According to index of relative importance (I_RI), which incorporates frequency of occurrence, mass and number of prey items, the most important prey items were copepods (81%I_RI over all specimens), predominantly Metridia gerlachei and Paraeuchaeta sp., with krill and fishes having low I_RI (2·2 and 5·6%I_RI overall). According to mass of prey (M) in stomachs, however, fishes (P. antarcticum and myctophids) and krill dominated overall diet (48 and 22%M, respectively), with copepods being a relatively minor constituent of overall diet by mass (9·9%M). Piscivory by P. antarcticum occurred mainly in the extreme south‐west of the region and near the continental slope. Krill identified to species level in P. antarcticum stomachs were predominantly Euphausia superba (14·1%M) with some Euphausia crystallophorias (4·8%M). Both DistLM modelling (PRIMER‐permanova+) on stomach contents (by I_RI) and stepwise generalized linear modelling on stable isotopes showed that L_S and location were significant predictors of P. antarcticum diet. Postlarval P. antarcticum (50–89 mm L_S) consumed exclusively copepods. Juvenile P. antarcticum (90–151 mm L_S) consumed predominantly krill and copepods by mass (46 and 30%M, respectively). Small adult P. antarcticum (152–178 mm L_S) consumed krill, fishes and copepods (37, 36 and 15%M, respectively). Large adult P. antarcticum (179–236 mm L_S) consumed predominantly fishes and krill (55 and 17%M, respectively), especially in the north (near the Ross Sea slope) and in the SW Ross Sea. Amphipods were occasionally important prey items for P. antarcticum (western Ross Sea, 39%M). General concordance between stomach contents and trophic level of P. antarcticum and prey based on δ¹⁵N was demonstrated. Pleuragramma antarcticum trophic level was estimated as 3·7 (postlarval fish) and 4·1 (fish aged 3+ years).     

The macrophytobenthos of Terra Nova Bay (Ross Sea,Antarctica): results of three Italian scientific expeditions

Abstract

The results of a study on the benthic macroalgal flora and vegetation of Terra Nova Bay (Ross Sea) carried out during the Italian expeditions to Antarctica made in the years 1987/88, 1989/90 and 1993/94, are presented. The flora is rather poor (9 Rhodophyceae, 4 Fucophyceae, 4 Chlorophyceae), but richer than that known from adjacent areas. The vegetation is on the whole characterized by a prevalence of sciaphilous species among which Iridaea cordata, Phyllophora antarctica and Clathromorphum lemoineanum dominating in the upper infralittoral, lower infralittoral and circalittoral zones, respectively. Moreover, the results of a study on biomass of a population of I. cordata as well as of a population of P. antarctica, carried out in the austral summer 1993/94, are reported too. The values of the biomass of the population of I. cordata resulted to vary inversely with respect to those of the population of P. antarctica.     

Trophic ecology of the emerald notothen <Emphasis Type="Italic">Trematomus bernacchii</Emphasis> (Pisces,Nototheniidae) from Terra Nova Bay,Ross Sea,Antarctica

The trophic ecology of the emerald notothen Trematomus bernacchii was investigated using a sample of 284 specimens collected in the coastal waters of Terra Nova Bay (Ross Sea) during the summer season 1990–1991. The stomach-content analysis provided data on feeding habits and trophic niche breadth. Overall, 72 taxa of food items were identified; the most important as prey were infaunal and epifaunal polychaetes, amphipods and molluscs. Food of secondary importance comprised isopods, pycnogonids, fish and fish eggs. Several other taxa, such as euphausiids, mysids, decapods, echinoids, holothurians, priapulids and thaliaceans constituted food eaten in very small amounts and only occasionally. On the basis of their diet, T. bernacchii can be considered generalized feeders, with a wide niche breadth composed almost exclusively of benthic organisms. A cluster analysis was conducted to compare stomach-content data of subsamples selected on the basis of fish size and sampling depth, aiming to determine diet diversity and niche breadth, as well as factors involved in reducing intraspecific competition for food resources. Diet was different for fish sampled in shallow (90 m depth) and deeper waters (>90 m depth): while the first group fed mostly on bivalves (Adamussium colbecki) and amphipods, the latter preferred infaunal-epifaunal polychaetes. Conversely, fish size did not contribute significantly to diet diversification. According to our data and published data on feeding behaviour and sensory ecology, we conclude that in this species the intraspecific competition for food is reduced, either through habitat heterogeneity or different foraging strategies.     

Food and feeding ecology of postlarval and juvenile Pleuragramma antarcticum (Pisces; Notothenioidei) in the seasonal pack ice zone off the Antarctic peninsula

Summary Stomach contents of 275 postlarval and 269 one year old juvenile Pleuragramma antarcticum caught in February 1982, March 1981 and November 1977 in the Bransfield Strait and adjacent waters were investigated. Juveniles in November 1977 fed mainly on calanoid copepods and Oithona spp. The principal food of postlarvae in February 1982 were Oncaea spp., eggs of calanoid copepods and tintinnids whereas the staple food of juveniles in February 1982 consisted of calanoid copepods of which copepodites of Calanus propinquus were most abundant. Eggs of Euphausia superba were frequently ingested by postlarvae and were the main component of the juvenile diet in the Antarctic Sound. Postlarvae in March 1981 preyed on Oncaea spp. and calanoid eggs. There was a shift from feeding on cyclopoid copepods by postlarvae to feeding on calanoids by juveniles. The food particle size increased as a function of fish length. Maximum prey size was controlled by mouth width up to at least 45 mm standard length. Prey selection was apparently food density dependent with an inverse relationship between food abundance and selective feeding. Density dependent size selection may have resulted in resource partitioning among postlarval and juvenile fishes in February. This specific feeding behaviour is likely to be an adaptation to the low zooplankton stocks of the high Antarctic shelf waters to minimize food competition among the youngest age classes. Predation on krill eggs indicated that the frequent association of one year old juvenile Pleuragramma antarcticum with the Antarctic krill is related to local spawning events.     

Gametogenesis and reproductive strategies in some species of the Antarctic fish genus Trematomus (Nototheniidae) from Terra Nova Bay, Ross Sea

Species of the genus Trematomus are the most common nototheniids in the High-Antarctic Zone, and several live sympatrically on the Ross Sea continental shelf. Histological analysis of gonadal development of T. bernacchii, T. hansoni and T. newnesi was carried out on samples collected during the austral summer in the coastal waters of Terra Nova Bay, western Ross Sea. Gonads of both females and males of the three species were described based on macroscopic and histologic scales. Histologically, both males and females of T. bernacchii were in postspawning or in early maturing condition, thus indicating that they recently completed spawning. Conversely, most specimens of T. hansoni were in spawning condition, as evidenced by the histological appearance of gonads and by the high value of gonado-somatic index (GSI) of females between 20 and 36%. In the same period, females of T. newnesi had maturing ovaries, whereas males were at an early developmental stage of maturity. In all species, the testicular structure closely resembled the unrestricted spermatogonial type, although spermatogonia were more concentrated in the blind end of tubules. On the basis of the pattern of oocyte maturity observed within the ovarian lamellae, all Trematomus had a group synchronous ovary, as commonly reported in Antarctic fish. Furthermore, the presence of oocytes showing the commencement of yolk deposition in spent females confirms that vitellogenesis is a slow process, extending over at least 1 year. The reproductive strategies of Trematomus are discussed in terms of timing of the spawning season of each species as a function of the seasonal production cycle and of the interspecific relationships. The spawning activities of the three species investigated appeared to be mismatched with respect to time as well as location within the range of distribution of each.     

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.     

Adaptive morphology of Capulus subcompressus Pelseneer, 1903 (Gastropoda: Capulidae) from Terra Nova Bay, Ross Sea (Antarctica)

Capulus subcompressus Pelseneer, 1903 (Gastropoda: Capulidae) is a small epibiont gastropod living at Terra Nova Bay (Ross Sea) down to 540 m on the calcareous tubes of its unique host, the serpulid Serpula narconensis Baird, 1865. This polychaete forms bush-like aggregates which host a rich microfauna of crustaceans, hydroids and molluscs. In contrast to all other capulids, C. subcompressus shows an evident oval shell aperture, which is due to an allometric growth that can be imputed to the Serpula tube morphology. Since the allometric growth is detectable in all size classes, it could be deduced that the compressed shape of the C. subcompressus shell is the stable result of a significant evolutionary history which binds tightly these two species in Antarctic waters. Accepted: 17 July 1999