Mitochondrial DNA,morphology, and the phylogenetic relationships of Antarctic icefishes (Notothenioidei: Channichthyidae)

The Channichthyidae is a lineage of 16 species in the Notothenioidei, a clade of fishes that dominate Antarctic near-shore marine ecosystems with respect to both diversity and biomass. Among four published studies investigating channichthyid phylogeny, no two have produced the same tree topology, and no published study has investigated the degree of phylogenetic incongruence between existing molecular and morphological datasets. In this investigation we present an analysis of channichthyid phylogeny using complete gene sequences from two mitochondrial genes (ND2 and 16S) sampled from all recognized species in the clade. In addition, we have scored all 58 unique morphological characters used in three previous analyses of channichthyid phylogenetic relationships. Data partitions were analyzed separately to assess the amount of phylogenetic resolution provided by each dataset, and phylogenetic incongruence among data partitions was investigated using incongruence length difference (ILD) tests. We utilized a parsimony-based version of the Shimodaira-Hasegawa test to determine if alternative tree topologies are significantly different from trees resulting from maximum parsimony analysis of the combined partition dataset. Our results demonstrate that the greatest phylogenetic resolution is achieved when all molecular and morphological data partitions are combined into a single maximum parsimony analysis. Also, marginal to insignificant incongruence was detected among data partitions using the ILD. Maximum parsimony analysis of all data partitions combined results in a single tree, and is a unique hypothesis of phylogenetic relationships in the Channichthyidae. In particular, this hypothesis resolves the phylogenetic relationships of at least two species (Channichthys rhinoceratus and Chaenocephalus aceratus), for which there was no consensus among the previous phylogenetic hypotheses. The combined data partition dataset provides substantial statistical power to discriminate among alternative hypotheses of channichthyid relationships. These findings suggest the optimal strategy for investigating the phylogenetic relationships of channichthyids is one that uses all available phylogenetic data in analyses of combined data partitions.     

Antarctic icefishes (Channichthyidae): a unique family of fishes. A review, Part I

Icefish or white-blooded fish are a family of species, unique among vertebrates in that they possess no haemoglobin. With the exception of one species which occurs on the southern Patagonian shelf, icefish live only in the cold-stable and oxygen-rich environment of the Southern Ocean. It is still questionable how old icefish are in evolutionary terms: they may not be older than 6 Ma, i.e. they evolved well after the Southern Ocean started to cool down or they are 15–20 Ma old and started to evolve some time after the formation of the Antarctic Circumpolar Current. Individuals of most icefish species with the exception of species of the genus Champsocephalus have been found down to 700–800 m depth, a few even down to more than 1,500 m. Icefish have been shown to present organ-level adaptations on different levels to compensate for the ‘disadvantages’ of lacking respiratory pigments. These include a low metabolic rate, well perfused gills, increased blood volume, increased cardiac output, cutaneous uptake of oxygen, increased blood flow with low viscosity, enlarged capillaries, large heart, and increased skin vascularity. Biological features, such as reproduction and growth, are not unique and are comparable to other notothenioids living in the same environment. Icefish produce large yolky eggs which have a diameter of more than 4 mm in most species. Consequently, the number of eggs produced is comparatively small and exceeds 10,000–20,000 eggs in only a few cases. With the exception of species of the genus Champsocephalus which mature at an age of 3 to 4 years, icefish do not attain maturity before they are 5–8 years old. Spawning period of most icefish species is autumn-winter. The incubation period spans from 2 to 3 months in the north of the Southern Ocean to more than 6 months close to the continent. Growth in icefish to the extent it is known is fairly rapid. They grow 6–10 cm in length per annum before they reach spawning maturity. Icefish feed primarily on krill and fish. Some icefish species were abundant enough to be exploited by commercial fisheries, primarily in the 1970s and 1980s with Champsocephalus gunnari as the main target species. Most stocks of this species had been overexploited by the beginning of the 1990s, some had further declined due to natural causes. Other species taken as by-catch species in fisheries were Chaenocephalus aceratus, Pseudochaenichthys georgianus, and Chionodraco rastrospinosus. Chaenodraco wilsoni was the only species exploited on a commercial scale in the high-Antarctic. Part II will be published in the following issue. DOI 10.1007/s00300-005-0020-6.     

Antarctic icefishes (Channichthyidae): a unique family of fishes. A review, Part II

Icefish or white- blooded fish are a family of species unique among vertebrates in that they possess no haemoglobin. With the exception of one species which occurs on the southern Patagonian shelf, icefish live only in the cold-stable and oxygen-rich environment of the Southern Ocean. It is still questionable how old icefish are in evolutionary terms: they may not be older than 6 Ma, i.e. they evolved well after the Southern Ocean started to cool down or they are 15–20 Ma old and started to evolve some time after the formation of the Antarctic Circumpolar Current. Individuals of most icefish species with the exception of species of the genus Champsocephalus have been found down to 700–800 m depth, a few even down to more than 1,500 m. Icefish have been shown to present organ-level adaptations on different levels to compensate for the ‘disadvantages’ of lacking respiratory pigments. These include a low metabolic rate, well perfused gills, increased blood volume, increased cardiac output, cutaneous uptake of oxygen, increased blood flow with low viscosity, enlarged capillaries, large heart, and increased skin vascularity. Biological features, such as reproduction and growth, are not unique and are comparable to other notothenioids living in the same environment. Icefish produce large yolky eggs which have a diameter of more than 4 mm in most species. Consequently, the number of eggs produced is comparatively small and exceeds 10,000–20,000 eggs in only a few cases. With the exception of species of the genus Champsocephalus which mature at an age of 3 to 4 years, icefish do not attain maturity before they are 5–8 years old. Spawning period of most icefish species is autumn–winter. The incubation period spans from 2 to 3 months in the north of the Southern Ocean to more than 6 months close to the continent. Growth in icefish to the extent it is known is fairly rapid. They grow 6–10 cm in length per annum before they reach spawning maturity. Icefish feed primarily on krill and fish. Some icefish species were abundant enough to be exploited by commercial fisheries, primarily in the 1970s and 1980s with Champsocephalus gunnari as the main target species. Most stocks of this species had been overexploited by the beginning of the 1990s, some had further declined due to natural causes. Other species taken as by-catch species in fisheries were Chaenocephalus aceratus, Pseudochaenichthys georgianus, and Chionodraco rastrospinosus. Chaenodraco wilsoni was the only species exploited on a commercial scale in the high-Antarctic. Part I was published in the preceding issue of Polar Biology. DOI 10.1007/s00300-005-0019-z.     

Morphological Characteristics of Sailfish Pike Channichthys velifer (Channichthyidae) from the Kerguelen Islands (Southern Ocean)

Journal of Ichthyology - Based on a representative sample set (41 specimens), features of external morphology, seismosensory system, and axial skeleton of the sailfish pike Channichthys velifer...     

Nitric oxide synthase type I (nNOS), vascular endothelial growth factor (VEGF) and myoglobin-like expression in skeletal muscle of Antarctic icefishes (Notothenioidei: Channichthyidae)

The Antarctic icefishes Channichthyidae lack haemoglobin and are thought to lack myoglobin (Mb) in their skeletal muscle as well. Due to the absence of both respiratory pigments, icefishes may present a variety of physiological adaptations in their skeletal muscles. In mammals, molecular responses to limiting oxygen availability in the skeletal muscle include, among others, the over expression of nitric oxide synthases (NOS), such as type I (neuronal nNOS) and type III (endothelial eNOS), as well as the vascular endothelial growth factor (VEGF). In this paper, we evaluated by western blot analysis whether the skeletal muscle of haemoglobin-less icefishes expresses in a constitutive manner higher levels of the type I and type III NOS isoforms and VEGF. Our results demonstrate that haemoglobin-less icefish of the family Channichthyidae do indeed present higher expression of the type I NOS isoform compared with red-blooded Antarctic fish species of other families of the same suborder Notothenioidei. In contrast, VEGF was not over-expressed. Moreover, we show that some icefish species, thought previously to lack Mb in oxidative muscles, actually present Mb-like immunoreactivity in their skeletal muscle.     

Diet of the Antarctic starry skate Amblyraja georgiana (Rajidae, Chondrichthyes) at South Georgia (Southern Ocean)

Stomach contents were identified from 206 Antarctic starry skate (Amblyraja georgiana) that were collected during three groundfish surveys (September 2007, April 2008 and January 2009) at South Georgia, Southern Ocean. The diet of A. georgiana varied with skate size and between years. Preferred prey included fish (particularly for larger individuals) and Antarctic krill, Euphausia superba, as well as amphipods, polychaetes and other benthic fauna. The skate A. georgiana appears to be an opportunistic predator, and the clear presence of Antarctic krill in this demersal predator’s diet may indicate a benthic habit of this euphausiid species, which has hitherto mainly been considered as occupying a purely pelagic niche.     

Taxonomic studies of the Antarctic icefish genus Cryodraco Dollo, 1900 (Notothenioidei: Channichthyidae)

Based on a meristic and morphometric study of 101 specimens, we recognise 2 valid species in the Antarctic channichthyid genus Cryodraco: Cryodraco antarcticus Dollo, 1900 and C. atkinsoni Regan, 1914. Although the species overlap in most meristic and morphometric characters, we have distinguished several reliable characters for diagnosis and identification. Multidimensional scaling, a nonparametric multivariate technique, clearly separates the two species on the basis of pelvic fin length, head length, number of second dorsal fin rays and origin of the lower lateral line relative to the anal fin rays. We provide a revised identification key to the species of Cryodraco. From a zoogeographical point of view, C. antarcticus has a circum-Antarctic distribution whereas C. atkinsoni is largely confined to the East Antarctic Zoogeographic Province.     

Distribution of surface zooplankton and seabirds across the Southern Ocean

Surface zooplankton and seabird densities and community composition in the Atlantic (between Cape Town and Sanae) and Pacific (between New Zealand and the Ross Sea) sectors of the Southern Ocean are described and related to oceanographic features. Samples were collected during two return voyages aboard the MV Benjamin Bowring as part of the Transglobe Expedition (1979–1981). High abundances of surface zooplankton and seabirds were consistently observed within the main frontal systems of the Southern Ocean. Generally, on a mesoscale significant correlations between surface temperature and the distribution of zooplankton or seabirds were observed. On a macroscale, the geographical positions of the zooplankton and seabird communities coincided with specific water masses. The results of this study suggest that appropriate food availability rather than water temperature is important for the determination of seabird distribution. The ecological importance of the recently described frontal zone associated with the northern boundary of the maximum winter expansion of sea ice is confirmed by biological data obtained in this study.     

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.     

Kinetics of dissolution of Antarctic diatom frustules and the biogeochemical cycle of silicon in the Southern Ocean

Summary In order to simulate the fate of biogenic silica generated in the surface waters of the Southern Ocean, the dissolution of silica frustules was studied for seven natural assemblages of diatoms, collected during summer 1984 in the Indian sector, and two typical Antarctic diatoms (Nitzschia cylindrus and Chaetoceros deflandrei), following the procedure of Kamatani and Riley (1979). For mean summer conditions in the surface waters of the Southern Ocean (2-3d^-1 for the natural assemblages. The silica frustules trapped by fecal pellets and by gelatinous aggregates, and rapidly transported through the cold waters of the Circumpolar Current, reach the sea bottom of either the continental shelves of the abysses without loosing much of the initial amount of silica (less than 10%). A model based on Stokes' law, modified to take in account of non ideal conditions and of the upwelling rate, is used in order to simulate the fate of silica of unaggregated particles settling down in the cold waters of the Antarctic Divergence. It supports the ideas that 1-the cycle of siliceous particles which radii are <2 m (i.e., of a part of the nanoplankton) is completely achieved in the surface layer, 2-although the biogenic silica of large unaggregated particles (radii over 25 m) may reach the seabottom (within one month to a few years) without complete dissolution, the main explanation for the accumulation of biogenic silica on Antarctic abysses remains transport by fecal pellets and gelatinous aggregates.     

Distribution and ecology of <Emphasis Type="Italic">Chaenocephalus aceratus</Emphasis> (Channichthyidae) around South Georgia and Shag Rocks (Southern Ocean)

Chaenocephalus aceratus (Family Channicthyidae) is one of the dominant species of demersal fish living on the South Georgia shelf where it is caught in low numbers as by-catch in the mackerel icefish and Antarctic krill commercial fisheries. Data collected during 14 demersal fish surveys, from 1986 to 2006, are analysed to investigate biomass, distribution, growth and diet. Biomass estimates from a swept area method ranged from 4,462 to 28,740 tonnes on the South Georgia and Shag Rock shelves although few fish were caught at Shag Rocks. Analysis of length frequency data indicated that growth was fast in the first five years with males and females attaining lengths at first spawning of 440 mm TL and 520 mm TL. The diet was comprised of fish and crustaceans, with an ontogenetic shift in diet from Euphausia superba and mysids to benthic fish and decapods observed to begin at 250 mm TL. In larger fish (>500 mm TL) the diet was dominated by fish. C. aceratus diet is sufficiently different from the other species of channichthyids around South Georgia to suggest that these species have undergone resource partitioning.     

The trophodynamics of Southern Ocean Electrona (Myctophidae) in the Scotia Sea

The Scotia Sea is one of the most productive regions of the Southern Ocean, but its surface waters are experiencing a rapid increase in temperature, which may be changing the behaviour and distribution of many myctophids and their prey species. Electrona antarctica and Electrona carlsbergi are two of the most abundant myctophids in the region, but their ecology is poorly understood and their response to ongoing environmental change is difficult to determine. This study investigated spatial and temporal patterns in their abundance, population structure and diets using mid-water trawl nets deployed across the Scotia Sea during spring, summer and autumn. E. antarctica was the most numerically abundant species (0.09–0.21 ind. 1,000 m^?3), with greatest concentrations occurring in the sea-ice sectors. E. carlsbergi occurred in more northern regions, comprising densities of 0.02–0.11 ind. 1,000 m^?3. There was evidence of seasonal variation in depth distribution, size-related sexual dimorphism and size-specific vertical stratification for both species. Latitudinal trends in sex ratio and female body size were apparent for E. antarctica. Its diet varied between regions, seasons and size classes, but overall, Euphausia superba, Metridia spp. and Themisto gaudichaudii were the dominant prey items. E. carlsbergi appeared not to recruit in the Scotia Sea. Its diet was dominated by copepods, particularly Rhincalanus gigas and Metridia spp., but regional, seasonal and ontogenetic variations were evident. This study contributes to our understanding of how mid-water food webs are structured in the Southern Ocean and their sensitivity to ongoing environmental change.     

Genetic homogeneity of krill (Euphausia superba Dana) in the Southern Ocean

Summary Development of a comprehensive picture of the genetic population structure of the Antarctic krill (Euphausia superba) has been hampered by a lack of genetic data from two major areas of the species' distribution, the Bellingshausen Sea and the Ross Sea. Evidence from earlier studies of a discrete Bellingshausen Sea population was based on anomalous allele frequencies in two sample sets that were collected near the west coast of the Antarctic Peninsula rather than in the Bellingshausen Sea proper. In this paper we describe the first biochemical genetic data obtained on krill from the central Bellingshausen Sea and from the Ross Sea. Analyses of eight polymorphic loci in samples from these two areas have failed to provide any evidence of population structuring within the Pacific sector of the Southern Ocean, and have indicated that Pacific sector krill cannot be genetically discriminated from Atlantic sector krill or Indian Ocean sector krill. These findings further support the hypothesis of a single circumpolar breeding population of Antarctic krill.     

Potential Climate Change Effects on the Habitat of Antarctic Krill in the Weddell Quadrant of the Southern Ocean

Antarctic krill is a cold water species, an increasingly important fishery resource and a major prey item for many fish, birds and mammals in the Southern Ocean. The fishery and the summer foraging sites of many of these predators are concentrated between 0° and 90°W. Parts of this quadrant have experienced recent localised sea surface warming of up to 0.2°C per decade, and projections suggest that further widespread warming of 0.27° to 1.08°C will occur by the late 21^st century. We assessed the potential influence of this projected warming on Antarctic krill habitat with a statistical model that links growth to temperature and chlorophyll concentration. The results divide the quadrant into two zones: a band around the Antarctic Circumpolar Current in which habitat quality is particularly vulnerable to warming, and a southern area which is relatively insensitive. Our analysis suggests that the direct effects of warming could reduce the area of growth habitat by up to 20%. The reduction in growth habitat within the range of predators, such as Antarctic fur seals, that forage from breeding sites on South Georgia could be up to 55%, and the habitat’s ability to support Antarctic krill biomass production within this range could be reduced by up to 68%. Sensitivity analysis suggests that the effects of a 50% change in summer chlorophyll concentration could be more significant than the direct effects of warming. A reduction in primary production could lead to further habitat degradation but, even if chlorophyll increased by 50%, projected warming would still cause some degradation of the habitat accessible to predators. While there is considerable uncertainty in these projections, they suggest that future climate change could have a significant negative effect on Antarctic krill growth habitat and, consequently, on Southern Ocean biodiversity and ecosystem services.     

Plankton community respiration in Bransfield Strait (Antarctic Ocean) during austral spring

Community respiration (R) was determined in Bransfield Straitfrom oxygen changes in water samples incubated in borosilicatebottles maintained at in situ temperature. The respiratory electrontransport system (ETS) activity of seawater communities wasalso measured from the same samples. Both data sets were relatedby the regression equation: log R (mg O₂ m^–3 day^–1)=0.462+0.730xlogETS activity mg O₂ m^–3 day^–1) (r=0.80, n=23). Fromthis equation and 37 ETS activity depth profiles, we calculatedthe integrated (0–100 m) community respiration as beingin the range 1.2–4.5 g O₂ m^–2 day^–1 (mean=2.2).These values do not differ significantly from other publishedresults for the Arctic and Antarctic Oceans. Assuming a respiratoryquotient of unity, the areal respiration ranges between 0.45and 1.69 g C m^–2 day^–1 (mean=0.8). This would representan important sink for the primary production reported for BransStrait. The spatial distribution of community respiration showedhigher values associated with the warmer and phytoplankton-richwaters outflowing from Gerlache Strait into Bransfield Strait,and with the front that separates Bellingshausen Sea watersfrom Weddell Sea waters. We suggest that this pattern of distributionmay be related to the transport of organic matter by the BransfieldCurrent along the front.     

Phylogeography of the Chionodraco genus (Perciformes,Channichthydae) in the Southern Ocean

Antarctic fish of the suborder Notothenioidei represent one of the most notable examples of adaptive radiation in the marine environment. The evolutionary relationships between and within the eight families of this suborder have been well established by numerous studies, whereas the microevolutionary processes of notothenioid species remain largely unexplored. In the present paper we investigated the evolutionary relationships between three closely related species of the genus Chionodraco (family Channichthyidae), namely Chionodraco hamatus, Chionodraco rastrospinosus, and Chionodraco myersi by analysing portions of the mitochondrial genome (D-loop and 16S rRNA). The taxonomic status of C. hamatus and C. rastrospinosus as separate species has been questioned because of the limited number of key morphological characters that distinguish these two taxa. Our results, based on the analysis of several specimens belonging to both morphological groups revealed a small genetic differentiation among haplotypes, however, a clear separation between the two nominal species emerged since all individuals of each of the two taxa clustered together in distinct monophyletic groups. C. myersi appeared more distantly related in the phylogenetic analysis. For one species, C. hamatus, sampling was carried out at three different geographic locations in the area of the Ross Sea and Weddell Sea. The results showed that the partition of the genetic variation within this species is not compatible with the hypothesis of panmixia as gene flow between populations was significantly reduced.     

Pigment distribution in the Pacific region of the Southern Ocean (autumn 1995)

Phytoplankton distribution patterns are still largely unknown for the Pacific region of the Southern Ocean. Pigment distributions were determined by HPLC on 40-m samples collected from the mixed layer during the ANTXII/4 cruise in March–May 1995 aboard RV “Polarstern”. A transect was covered (90°W, from 51°S to 70°S), crossing the Subantarctic Front in the north, the Polar Front, and the Southern Polar Front in the south. Coinciding with high concentrations of silicate, diatoms dominated in the Antarctic waters south of the Polar Front. North of the Polar Front, silicate concentrations dropped to values less than 10 μM. In this area flagellates (Prymnesiophyceae and green algae) were the dominant phytoplankton group. Nutrient depletion of the surface waters near the Southern Polar Front indicated formerly enhanced productivity. These findings confirmed previous observations by the British Sterna expedition, which described locally elevated chlorophyll a biomass near the southern boundary of the Southern Polar Front. We propose a role for supply of bioavailable iron via the front, and emphasise the importance of frontal systems for phytoplankton productivity in the Southern Ocean. Received: 11 June 1997 / Accepted: 16 November 1997     

Variability in the inter-island environment of the Prince Edward Islands (Southern Ocean)

Variability in the oceanographic parameters and macrozooplankton and micronekton composition, densities and distributional patterns were investigated during a repeat survey conducted between the Prince Edward Islands in April 1998. Results of this study demonstrated the occurrence of pronounced water pulses along the inter-island trench. The location of the Subantarctic Front to the north of the island plateau, through its interactions with the island group, appeared to have a marked effect on the mesoscale dynamics of physical and biological parameters between and around the islands. Seawater temperature and salinity accounted for >40% of the variation in the zooplankton distribution during the trench studies. A total of 41 macroplankton and micronekton taxa, consisting of subantarctic, subtropical and Antarctic species, were identified. Numerical analyses revealed two major groupings of stations corresponding to an offshore and inshore region. Although there was no evidence for quantitative differences in macroplankton densities between the inshore surveys, offshore plankton biomass was at least three- to eightfold higher than during the trench surveys. The importance of water pulses in carrying stocks of large plankton between the islands appeared to be minimal, at least during the time when the investigation took place. Received: 1 November 1999 / Accepted: 12 February 2000