VISION AND SENSORY PHYSIOLOGY The lateral line systems of three deep-sea fish

The distribution and ultrastructure of the lateral line systems in three taxonomically dispersed deep-sea fish are described: Poromitra capito, Melanonus zugmayeri and Phrynichthys wedli. They are meso- to bathpelagic and are thought to feed on small crustaceans and fish. All possess highly developed lateral line systems, a feature associated with life in the deep sea. Poromitra capito and M. zugmayeri exhibit widened head canals which are connected to the outside by large pores and which contain around 60 large neuromasts. Each neuromast consists of a cupula, shield-shaped mantle and a sensory plate containing hundreds to thousands of hair cells. Direction of sensitivity is in the long axis of the canal (perpendicular to the long axis of the mantle). Depending on their position on the sensory plate, the hair cells have different morphologies. They fall into three basic classes which, from comparison with past work, may be tuned to different frequencies. Alternatively, the various hair cell morphologies could be interpreted as being members of a developmental or growth sequence. Phrynichthys wedli has no canal organs, these being replaced secondarily by many superficial neuromasts placed on prominent papillae in rows which cover much of the 'head' and body. Direction of sensitivity is along the axis of the neuromast row. An extreme proliferation of superficial neuromasts are also found on the heads of P. capito and M. zugmayeri and these are of a type not described before. They consist of stitches, raised on papillae in M. zugmayeri and several mm long in P. capito , in which continuous lines of hair cells, two to three cells wide, are embedded. Direction of sensitivity is perpendicular to the long axis of the stitch. Based on the structure and direction of sensitivity, possible functional implications of all the neuromast types described are compared and discussed.     

New insights into the complex structure and ontogeny of the occipito‐vertebral gap in barbeled dragonfishes (Stomiidae,Teleostei)

In all stomiid genera there is an occipito‐vertebral gap between the skull and the first vertebra bridged only by the flexible notochord. Morphological studies from the early 20th century suggested that some stomiid genera have 1–10 of the anteriormost centra reduced or entire vertebrae missing in this region. Our study reviews this previous hypothesis. Using a new approach, we show that only in Chauliodus, Eustomias and Leptostomias gladiator vertebral centra are actually lost, with their respective neural arches and parapophyses persisting. We present results from a comparative analysis of the number and insertion sites of the anteriormost myosepta in 26 of the 28 stomiid genera. Generally in teleosts the first three myosepta are associated with the occiput, and the fourth is the first vertebral myoseptum. The insertion site of the fourth myoseptum plays an important role in this analysis, because it provides a landmark for the first vertebra. Lack of association of the fourth myoseptum with a vertebra is thus evidence that the first vertebra is reduced or absent. By counting the occipital and vertebral myosepta the number of reduced vertebrae in Chauliodus, Eustomias and Leptostomias gladiator can be inferred. Proper identification of the spino‐occipital nerves provides an additional source of information about vertebral reduction. In all other stomiid genera the extensive occipito‐ vertebral gap is not a consequence of the reduction of vertebrae, but of an elongation of the notochord. The complex structure and ontogeny of the anterior part of the vertebral column of stomiids are discussed comparatively. J. Morphol. 271:1006–1022, 2010. © 2010 Wiley‐Liss, Inc.     

Local selection and population structure in a deep‐sea fish,the roundnose grenadier (Coryphaenoides rupestris)

Local populations within a species can become isolated by stochastic or adaptive processes, though it is most commonly the former that we quantify. Using presumably neutral markers we can assess the time‐dependent process of genetic drift, and thereby quantify patterns of differentiation in support of the effective management of diversity. However, adaptive differences can be overlooked in these studies, and these are the very characteristics that we hope to conserve by managing neutral diversity. In this study, we used 16 hypothetically neutral microsatellite markers to investigate the genetic structure of the roundnose grenadier in the North Atlantic. We found that one locus was a clear outlier under directional selection, with F_ST values much greater than at the remaining loci. Differentiation between populations at this locus was related to depth, suggesting directional selection, presumably acting on a linked locus. Considering only the loci identified as neutral, there remained significant population structure over the region of the North Atlantic studied. In addition to a weak pattern of isolation by distance, we identified a putative barrier to gene flow between sample sites either side of the Charlie‐Gibbs Fracture Zone, which marks the location where the sub‐polar front crosses the Mid‐Atlantic Ridge. This may reflect a boundary across which larvae are differentially distributed in separate current systems to some extent, promoting differentiation by drift. Structure due to both drift and apparent selection should be considered in management policy.     

Observations of gametogenesis in the deep-sea asteroids Paragonaster subtilis and Pseudarchaster parelii (Phanerozonia: Goniasteridae)

Examination of the gametogenic cycle of the deep-sea Goniasterid starfish Paragonaster subtilis and Pseudarchaster parelii suggests that accessory cell activity takes place in a meshwork surrounding the developing oocyte. This meshwork develops as vitellogenesis proceeds and reaches a maximum thickness of 150 μm. Accessory cell activity is distinctly different from that observed in deep-sea Astropectinid and Benthopectinid starfish, suggesting parallel evolution in these phanerozoan asteroids.     

Exploratory deep-sea fishing in the Falkland Islands, south-western Atlantic

Records of demersal deep-sea fish assemblages in waters around the Falkland Islands (Patagonian shelf area) are rare. Twenty deep-water stations to the east and south of the Falkland Islands were sampled by commercial bottom trawl deployed in upper, middle and lower benthopelagic zones (depth range of approximately 500-1000 m). Forty-one species (22 families) of teleost fish were recorded, 10 species (two families) of elasmobranch and one species of agnathan. Different assemblages of fish were found to characterize each depth zone (e.g. Moridae in deeper waters, Bothidae and Rajidae in shallower waters), with diversity being greatest in the mid-zone and biomass greatest in the upper and lower zones. Some species occurred in all zones but showed depth-related abundance. Four species, namely the grenadiers Macrourus carinutus and Coelorhynchus fasciatus , the southern blue whiting Micromesistius australis , and the Patagonian toothfish Dissostichus eleginoides , accounted for 85% by weight of all fish caught. Quantitative sampling of selected species revealed depth-related variations in their population structure. Length-frequency analyses are presented for M. carinatus and D. eleginoides and show a tendency for larger individuals to inhabit deeper water. Discard rates from the commercial catch were sometimes high, particularly for the smaller species, raising concerns about the impact of a fishery on by-catch species. The potential for deep-sea fisheries in Falkland waters is discussed and further studies are suggested in the light of developing oil, gas and fishing industries. The presence of some invertebrate taxa is recorded.     

Homeoviscous theory under pressure: II. The molecular order of membranes from deep-sea fish

The molecular order of brain and liver membranes isolated from deep sea and continental shelf fish species have been estimated and compared using the fluorescence polarization technique in order to determine whether life in a high pressure habitat is associated with an adjustment of membrane order. Fish were trawled at depths between 200 m and 4000 m, liver and brain membranes were fractionated, and fluorescence polarization was measured at 4°C and ambient pressure. Polarization of the brain myelin fraction provided a statistically significant regression with depth of capture ($\text{P<0.001}$) with a slope of ?0.004 km^?1. This change in polarization with depth was sufficient to offset approximately half of the pressure-induced increase in polarization and thus represents the first structural evidence of homeoviscous adaptation to pressure. Polarization of the brain synaptic and liver mitochondrial fraction was not significantly related to depth. This may be due, at least in part, to a high individual variability of polarization compared to laboratory-acclimated freshwater fish.     

Exon-primed, intron-crossing (EPIC) loci for five nuclear genes in deep-sea protobranch bivalves: primer design, PCR protocols and locus utility

We describe PCR primers and amplification protocols developed to obtain introns from conserved nuclear genes in deep-sea protobranch bivalves. Because almost no sequence data for protobranchs are publically available, mollusk and other protostome sequences from GenBank were used to design degenerate primers, making these loci potentially useful in other invertebrate taxa. Amplification and sequencing success varied across the test group of 30 species, and we present five loci spanning this range of outcomes. Intron presence in the targeted regions also varied across genes and species, often within single genera; for instance, the calmodulin and β-tubulin loci contained introns with high frequency, whereas the triose phosphate isomerase locus never contained an intron. In introns for which we were able to obtain preliminary estimates of polymorphism levels in single species, polymorphism was greater than traditional mitochondrial loci. These markers will greatly increase the ability to assess population structure in the ecologically important protobranchs, and may prove useful in other taxa as well.     

Community structure of deep-sea demersal fish in the North Aegean Sea (northeastern Mediterranean)

The spatial structure and seasonal changes of the demersal fish assemblages on the continental shelf (100–200 m) and upper slope (200–500 m) in the North Aegean Sea (Northern Aegean and Thracian Seas, northeastern Mediterranean, Greece) were analysed. Seasonal experimental trawl surveys, carried out from summer 1990 to autumn 1993, provided a total of 151 demersal fish species. Analysis of 259 bottom trawls showed the existence of four groups associated with the continental shelf and the upper slope; each group was dominated by a small number of species. The bathymetric distribution of the species, established using measures of the centre of gravity and habitat width, revealed that most of the species had a wide distributional range within the study area, although a few were restricted to the greatest depths. Density, biomass, species richness and diversity decreased significantly with depth, and were also indicative of distinctive characteristics between these fish assemblages. Mean fish weight exhibited two different trends: a bigger-deeper phenomenon at the continental shelf and a smaller-deeper phenomenon at the upper slope. The variability in assemblage structure was determined mainly by depth and, to a lesser extent, by season and geographical location. For some species, results suggest a pattern of gradual species replacement along the depth gradient coupled with ontogenetic habitat shifts.     

Dimorphism, parasitism, and sex revisited: modes of reproduction among deep-sea ceratioid anglerfishes (Teleostei: Lophiiformes)

Sexual parasitism, a remarkable mode of reproduction unique to some members of the deep-sea anglerfish suborder Ceratioidei, in which males are dwarfed and become permanently attached to much larger females, is described in a number of previously unreported specimens and taxa. Although generally attributed to all ceratioids, and despite more than a sevenfold increase in the number of known parasitized females in collections throughout the world in the past 50 years, the phenomenon is surprisingly confined to few taxa within the suborder. To date, permanently attached males have been found in only 5 of the 11 ceratioid families, 10 of the 35 genera, and 23 of the 160 recognized species. Notes on taxonomic content, available material, occurrence of sexual parasitism, gravid females and ripe males, the development of eyes and nostrils of the males, the ability of males to capture and ingest food independently of the female, occurrences of multiple attachments of males to a single female, and the nature of the fusion between coupled males and females are given for each ceratioid family. This information is then summarized and discussed. Evidence is presented to reaffirm the presence of three reproductive modes: obligatory parasitism, facultative parasitism, and temporary nonparasitic attachment. Additional evidence is provided to reaffirm the hypothesis that sexual parasitism has evolved separately at least three and perhaps five or more times within the suborder.     

Genetic panmixia and demographic dependence across the North Atlantic in the deep-sea fish,blue hake (Antimora rostrata)

The efficient investment of resources and effort into conservation strategies depends on the accurate identification of management units. At the same time, understanding the processes by which population structure evolves requires an understanding of the conditions under which panmixia may exist. Here, we study a species with an unusual, apparently sex-biased pattern of distribution, and test the hypothesis that distribution processes associated with this pattern (for example, congregating at a single dominant spawning site or periodic mixing during reproduction) could lead to panmixia over a large geographic range. Using 13 microsatellite markers, we compared 393 blue hake (Antimora rostrata) from 11 sample sites across a geographic range of over 3000 km, and found no evidence of population structure. We estimated current effective population size and found it to be large (∼15 000) across the sampled area. In addition, we used simulation models to test expectations about demographic correlation among populations and our ability to detect relevant levels of gene flow. All data were consistent with the interpretation of long-range panmixia.     

Sea warming and fish distribution: the case of the small pelagic fish, Sardinella aurita, in the western Mediterranean

This study analyses the temporal and spatial changes in abundance and distribution of the warm water species round sardinella (Sardinella aurita) in the western Mediterranean over the last decades in relation to sea water temperature. In the western Mediterranean basin (1950–2003), a significant positive relationship was found between round sardinella landings and temperature anomalies. Along a latitudinal gradient off the Mediterranean Iberian coast (1989–2004), a gradual increase in species abundance was observed from south to north, with a certain time lag going northwards, associated with the increase in sea water temperature. The abundance of round sardinella in the two warmest and southernmost areas was positively and significantly correlated with sea surface temperature registered during the start of gonad maturation the previous year. In addition, the positive relationship established between water temperature and abundance of round sardinella in the coldest and northernmost study area demonstrates that there is a temperature limit for the distribution of this species in the western Mediterranean. In addition, this study analyses round sardinella larvae distribution and abundance in the summers of 2003 and 2004, and conducts a comparison with the situation 20 years ago (summer 1983). Results show a marked increase in larval abundance during the last decades and the present appearance of larvae in the northernmost study areas, where they did not occur 20 years ago. This indicates the successful reproduction of round sardinella in the northern part of the Mediterranean, where the species has expanded, confirming its establishment in the area.     

Adaptive radiation in African weakly electric fish (Teleostei: Mormyridae: Campylomormyrus): a combined molecular and morphological approach

We combined multiple molecular markers and geometric morphometrics to revise the current taxonomy and to build a phylogenetic hypothesis for the African weakly electric fish genus Campylomormyrus. Genetic data (2039 bp DNA sequence of mitochondrial cytochrome b and nuclear S7 genes) on 106 specimens support the existence of at least six species occurring in sympatry. We were able to further confirm these species by microsatellite analysis at 16 unlinked nuclear loci and landmark-based morphometrics. We assigned them to nominal taxa by comparisons to type specimens of all Campylomormyrus species recognized so far. Additionally, we showed that the shape of the elongated trunk-like snout is the major source of morphological differentiation among them. This finding suggests that the radiation of this speciose genus might have been driven by adaptation to different food sources.     

Bathymetric and geographic population structure in the pan-Atlantic deep-sea bivalve Deminucula atacellana (Schenck, 1939)

The deep-sea soft-sediment environment hosts a diverse and highly endemic fauna of uncertain origin. We know little about how this fauna evolved because geographic patterns of genetic variation, the essential information for inferring patterns of population differentiation and speciation are poorly understood. Using formalin-fixed specimens from archival collections, we quantify patterns of genetic variation in the protobranch bivalve Deminucula atacellana, a species widespread throughout the Atlantic Ocean at bathyal and abyssal depths. Samples were taken from 18 localities in the North American, West European and Argentine basins. A hypervariable region of mitochondrial 16S rDNA was amplified by polymerase chain reaction (PCR) and sequenced from 130 individuals revealing 21 haplotypes. Except for several important exceptions, haplotypes are unique to each basin. Overall gene diversity is high (h = 0.73) with pronounced population structure (Phi(ST) = 0.877) and highly significant geographic associations (P < 0.0001). Sequences cluster into four major clades corresponding to differences in geography and depth. Genetic divergence was much greater among populations at different depths within the same basin, than among those at similar depths but separated by thousands of kilometres. Isolation by distance probably explains much of the interbasin variation. Depth-related divergence may reflect historical patterns of colonization or strong environmental selective gradients. Broadly distributed deep-sea organisms can possess highly genetically divergent populations, despite the lack of any morphological divergence.     

Morphometric and molecular characterisation of specimens of Lepidapedon Stafford, 1904 (Digenea: Lepidapedidae) from the deep-sea fish Mora moro (Risso) (Teleostei: Moridae) in the western Mediterranean

In a study of the parasites of the deep-sea fish Mora moro (Risso) (Gadiformes: Moridae) off the Mediterranean coasts of Catalonia and the Balearic Islands (Spain), we were able to distinguish two morphs of specimens belonging to Lepidapedon Stafford, 1904 (Digenea: Lepidapedidae). This material is herein described and illustrated. Comparative sequence analyses using partial mitochondrial nad1 sequences revealed that the material assigned to one of these morphs can be considered conspecific with the material identified as Lepidapedon desclersae Bray & Gibson, 1995 from the same host. However, the published nad1 sequence for L. desclersae was generated from a specimen ex M. moro from the North East Atlantic. Examination of the voucher specimens associated with this sequence revealed that both the North East Atlantic and the Mediterranean specimens ex M. moro differ from L. desclersae as described from its type-host, Lepidion eques (Günther), in the anterior extent of the vitelline fields which is further posterior, reaching only to the posterior margin of the external seminal vesicle in L. desclersae, versus being at the mid-level of this organ and reaching the posterior margin of the ventral sucker. Therefore, we have tentatively assigned the material characterised here, both morphologically and molecularly as Lepidapedon sp. Acquisition of additional sequences for both nad1 mitochondrial and 28S rRNA genes of L. desclersae from material ex Lepidion spp. is required in order to determine whether the observed morphometric variation reflects host-related or inter-specific differences. The second morph of Lepidapedon from M. moro is described and distinguished on morphometric grounds, such as the position of the most anterior vitelline follicles, which reach to the anterior margin of the ventral sucker. Its identity is commented upon, but, in view of the fact that there were few specimens and no molecular data available, it is not named.     

Chromosome evolution in the erythrinid fish, Erythrinus erythrinus (Teleostei: Characiformes)

The genus Erythrinus belongs to the family Erythrinidae, a neotropical fish group. This genus contains only two described species, Erythrinus erythrinus being the most widely distributed in South America. Six samples of this species from five distinct Brazilian localities and one from Argentina were studied cytogenetically. Four groups were identified on the basis of their chromosomal features. Group A comprises three samples, all with 2n = 54 chromosomes, a very similar karyotypic structure, and the absence of chromosome differentiation between males and females. One sample bears up to four supernumerary microchromosomes, which look like 'double minute chromosomes' in appearance. Groups B-D comprise the three remaining samples, all sharing an X(1)X(1)X(2)X(2)/X(1)X(2)Y sex chromosome system. Group B shows 2n = 54/53 chromosomes in females and males, respectively, and also shows up to three supernumerary microchromosomes. Groups C and D show 2n=52/51 chromosomes in females and males, respectively, but differ in the number of metacentric, subtelocentric, and acrocentric chromosomes. In these three groups (B-D), the Y is a metacentric chromosome clearly identified as the largest in the complement. The present results offer clear evidence that local samples of E. erythrinus retain exclusive and fixed chromosomal features, indicating that this species may represent a species complex.     

Africa's Clarias gariepinus (Teleostei: Clariidae) appears in rivers in Brazil

As a major tributary of the Nyl River, and ultimately for the Nylsvley wetland, the Olifantspruit, Limpopo province, South Africa, was investigated during the summer (high flow) and winter (low flow) of 2007 at three sites. This preliminary study used physical and biological indicators to determine the quality of the water that ultimately flows into Nylsvley. Significant seasonal differences were found. The observed differences relate to habitat structural differences and human activities in the catchment. Key findings included higher water temperatures during the high-flow period than during the low-flow period. The higher temperatures were accompanied by lower concentrations of dissolved oxygen at all sites. In total, 13 fish species were recorded, with most being found in the pool biotopes. The Nantes site, located nearest to the Nylsvley wetland, had the highest number of fish species, possibly indicating a linkage between these two systems. The most common macroinvertebrate families were the Baetidae, Aeshnidae and Coenagrionidae. It can be assumed that this river acts as an important refuge area for fish and macroinvertebrates from the Nyl River, and therefore it is important to conserve the Olifantspruit.     

Sperm production in an extremophile fish,the cave molly (<Emphasis Type="Italic">Poecilia mexicana</Emphasis>, Poeciliidae,Teleostei)

A prominent trade-off in life history theory and evolution balances the costs of reproduction with those of basic somatic needs. Hence, reproductive efforts may be reduced in environments where additional energy is required for somatic maintenance. Here, we investigated male sperm stores in Atlantic mollies (Poecilia mexicana) from a sulfidic cave and several sulfidic and non-sulfidic surface habitats. We found significant differences among populations in the number of sperm stripped per male, which was also correlated with differences in gonad weights. The largest sperm stores were detected in males from non-sulfidic surface creeks, while males from a partially sulfidic surface system had lower sperm counts, and males from completely sulfidic systems, surface as well as subterranean, had even fewer available sperm. We conclude that the extreme environmental conditions in sulfidic habitats appear to constrain male sperm production, since hydrogen sulfide as a naturally occurring toxin requires energy-demanding adaptations. Furthermore, we examined sperm counts of lab-reared cave and surface mollies in response to energy limitation. Males from stock populations were placed under high and low food treatments for a 2-week period and then stripped of sperm. Sperm counts of surface mollies tended to be reduced by low food availability, whereas sperm counts of cave mollies did not significantly vary between food treatments, which likely points towards a higher starvation resistance in cave mollies.