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.     

Phylogeography of a pan-Atlantic abyssal protobranch bivalve: implications for evolution in the Deep Atlantic

The deep sea is a vast and essentially continuous environment with few obvious barriers to gene flow. How populations diverge and new species form in this remote ecosystem is poorly understood. Phylogeographical analyses have begun to provide some insight into evolutionary processes at bathyal depths (<3000 m), but much less is known about evolution in the more extensive abyssal regions (>3000 m). Here, we quantify geographical and bathymetric patterns of genetic variation (16S rRNA mitochondrial gene) in the protobranch bivalve Ledella ultima, which is one of the most abundant abyssal protobranchs in the Atlantic with a broad bathymetric and geographical distribution. We found virtually no genetic divergence within basins and only modest divergence among eight Atlantic basins. Levels of population divergence among basins were related to geographical distance and were greater in the South Atlantic than in the North Atlantic. Ocean‐wide patterns of genetic variation indicate basin‐wide divergence that exceeds what others have found for abyssal organisms, but considerably less than bathyal protobranchs across similar geographical scales. Populations on either side of the Mid‐Atlantic Ridge in the North Atlantic differed, suggesting the Ridge might impede gene flow at abyssal depths. Our results indicate that abyssal populations might be quite large (cosmopolitan), exhibit only modest genetic structure and probably provide little potential for the formation of new species.     

A source-sink hypothesis for abyssal biodiversity

Bathymetric gradients of biodiversity in the deep-sea benthos constitute a major class of large-scale biogeographic phenomena. They are typically portrayed and interpreted as variation in alpha diversity (the number of species recovered in individual samples) along depth transects. Here, we examine the depth ranges of deep-sea gastropods and bivalves in the eastern and western North Atlantic. This approach shows that the abyssal molluscan fauna largely represents deeper range extensions for a subset of bathyal species. Most abyssal species have larval dispersal, and adults live at densities that appear to be too low for successful reproduction. These patterns suggest a new explanation for abyssal biodiversity. For many species, bathyal and abyssal populations may form a source-sink system in which abyssal populations are regulated by a balance between chronic extinction arising from vulnerabilities to Allee effects and immigration from bathyal sources. An increased significance of source-sink dynamics with depth may be driven by the exponential decrease in organic carbon flux to the benthos with increasing depth and distance from productive coastal systems. The abyss, which is the largest marine benthic environment, may afford more limited ecological and evolutionary opportunity than the bathyal zone.     

Morphological disparity as a biodiversity metric in lower bathyal and abyssal gastropod assemblages

Studies of deep-sea biodiversity focus almost exclusively on geographic patterns of alpha-diversity. Few include the morphological or ecological properties of species that indicate their actual roles in community assembly. Here, we explore morphological disparity of shell architecture in gastropods from lower bathyal and abyssal environments of the western North Atlantic as a new dimension of deep-sea biodiversity. The lower bathyal-abyssal transition parallels a gradient of decreasing species diversity with depth and distance from land. Morphological disparity measures how the variety of body plans in a taxon fills a morphospace. We examine disparity in shell form by constructing both empirical (eigenshape analysis) and theoretical (Schindel's modification of Raup's model) morphospaces. The two approaches provide very consistent results. The centroids of lower bathyal and abyssal morphospaces are statistically indistinguishable. The absolute volumes of lower bathyal morphospaces exceed those of the abyss; however, when the volumes are standardized to a common number of species they are not significantly different. The abyssal morphospaces are simply more sparsely occupied. In terms of the variety of basic shell types, abyssal species show the same disparity values as random subsets of the lower bathyal fauna. Abyssal species possess no evident evolutionary innovation. There are, however, conspicuous changes in the relative abundance of shell forms between the two assemblages. The lower bathyal fauna contains a fairly equable mix of species abundances, trophic modes, and shell types. The abyssal group is numerically dominated by species that are deposit feeders with compact unsculptured shells.     

Cryptic speciation along a bathymetric gradient

The deep ocean supports a highly diverse and mostly endemic fauna, yet little is known about how or where new species form in this remote ecosystem. How speciation occurs is especially intriguing in the deep sea because few obvious barriers exist that would disrupt gene flow. Geographic and bathymetric patterns of genetic variation can provide key insights into how and where new species form. We quantified the population genetic structure of a protobranch bivalve, Neilonella salicensis, along a depth gradient (2200–3800 m) in the western North Atlantic using both nuclear (28S and calmodulin intron) and mitochondrial (cytochrome c oxidase subunit I) loci. A sharp genetic break occurred for each locus between populations above 2800 m and below 3200 m, defining two distinct clades with no nuclear or mitochondrial haplotypes shared between depth regimes. Bayesian phylogenetic analyses provided strong support for two clades, separated by depth, within N. salicensis. Although no morphological divergence was apparent, we suggest that the depth‐related population genetic and phylogenetic divergence is indicative of a cryptic species. The frequent occurrence of various stages of divergence associated with species formation along bathymetric gradients suggests that depth, and the environmental gradients that attend changes in depth, probably play a fundamental role in the diversification of marine organisms, especially in deep water. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 113 , 897–913.     

Explaining bathymetric diversity patterns in marine benthic invertebrates and demersal fishes: physiological contributions to adaptation of life at depth

Bathymetric biodiversity patterns of marine benthic invertebrates and demersal fishes have been identified in the extant fauna of the deep continental margins. Depth zonation is widespread and evident through a transition between shelf and slope fauna from the shelf break to 1000 m, and a transition between slope and abyssal fauna from 2000 to 3000 m; these transitions are characterised by high species turnover. A unimodal pattern of diversity with depth peaks between 1000 and 3000 m, despite the relatively low area represented by these depths. Zonation is thought to result from the colonisation of the deep sea by shallow‐water organisms following multiple mass extinction events throughout the Phanerozoic. The effects of low temperature and high pressure act across hierarchical levels of biological organisation and appear sufficient to limit the distributions of such shallow‐water species. Hydrostatic pressures of bathyal depths have consistently been identified experimentally as the maximum tolerated by shallow‐water and upper bathyal benthic invertebrates at in situ temperatures, and adaptation appears required for passage to deeper water in both benthic invertebrates and demersal fishes. Together, this suggests that a hyperbaric and thermal physiological bottleneck at bathyal depths contributes to bathymetric zonation. The peak of the unimodal diversity–depth pattern typically occurs at these depths even though the area represented by these depths is relatively low. Although it is recognised that, over long evolutionary time scales, shallow‐water diversity patterns are driven by speciation, little consideration has been given to the potential implications for species distribution patterns with depth. Molecular and morphological evidence indicates that cool bathyal waters are the primary site of adaptive radiation in the deep sea, and we hypothesise that bathymetric variation in speciation rates could drive the unimodal diversity–depth pattern over time. Thermal effects on metabolic‐rate‐dependent mutation and on generation times have been proposed to drive differences in speciation rates, which result in modern latitudinal biodiversity patterns over time. Clearly, this thermal mechanism alone cannot explain bathymetric patterns since temperature generally decreases with depth. We hypothesise that demonstrated physiological effects of high hydrostatic pressure and low temperature at bathyal depths, acting on shallow‐water taxa invading the deep sea, may invoke a stress–evolution mechanism by increasing mutagenic activity in germ cells, by inactivating canalisation during embryonic or larval development, by releasing hidden variation or mutagenic activity, or by activating or releasing transposable elements in larvae or adults. In this scenario, increased variation at a physiological bottleneck at bathyal depths results in elevated speciation rate. Adaptation that increases tolerance to high hydrostatic pressure and low temperature allows colonisation of abyssal depths and reduces the stress–evolution response, consequently returning speciation of deeper taxa to the background rate. Over time this mechanism could contribute to the unimodal diversity–depth pattern.     

Cenozoic record of elongate,cylindrical, deep-sea benthic foraminifera in the North Atlantic and equatorial Pacific Oceans

In the late Pliocene–middle Pleistocene a group of 95 species of elongate, cylindrical, deep-sea (lower bathyal–abyssal) benthic foraminifera became extinct. This Extinction Group (Ext. Gp), belonging to three families (all the Stilostomellidae and Pleurostomellidae, some of the Nodosariidae), was a major component (20–70%) of deep-sea foraminiferal assemblages in the middle Cenozoic and subsequently declined in abundance and species richness before finally disappearing almost completely during the mid-Pleistocene Climatic Transition (MPT). So what caused these declines and extinction?In this study 127 Ext. Gp species are identified from eight Cenozoic bathyal and abyssal sequences in the North Atlantic and equatorial Pacific Oceans. Most species are long-ranging with 80% originating in the Eocene or earlier. The greatest abundance and diversity of the Ext. Gp was in the warm oceanic conditions of the middle Eocene–early Oligocene. The group was subjected to significant changes in the composition of the faunal dominants and slightly enhanced species turnover during and soon after the rapid Eocene–Oligocene cooling event. Declines in the relative abundance and flux of the Ext. Gp, together with enhanced species loss, occurred during middle–late Miocene cooling, particularly at abyssal sites. The overall number of Ext. Gp species present began declining earlier at mid abyssal depths (in middle Miocene) than at upper abyssal (in late Pliocene–early Pleistocene) and then lower bathyal depths (in MPT). By far the most significant Ext. Gp declines in abundance and species loss occurred during the more severe glacial stages of the late Pliocene–middle Pleistocene.Clearly, the decline and extinction of this group of deep-sea foraminifera was related to the function of their specialized apertures and the stepwise cooling of global climate and deep water. We infer that the apertural modifications may be related to the method of food collection or processing, and that the extinctions may have resulted from the decline or loss of their specific phytoplankton or prokaryote food source, that was more directly impacted than the foraminifera by the cooling temperatures.     

Biodiversity and distribution of the genus Gromia (Protista, Rhizaria) in the deep Weddell Sea (Southern Ocean)

We present a survey of morphospecies of Gromia, a genus of testate protists, from bathyal and abyssal depths in the Weddell Sea and adjacent areas of the Southern Ocean. This material represents the most extensive and diverse available collection of deep-sea gromiids so far recorded. The twelve species, nine of which are undescribed, are recognized on the basis of morphological criteria, including the test size and shape, the appearance and structure of the oral capsule, and the characteristics of the test wall. Most species have a single oral capsule, which is circular in plan view with a conical nipple-like shape in lateral view. One morphospecies has three oral capsules. The appearance and structure of the wall displays great variability among Gromia species, ranging from very delicate and transparent with highly reflective highlights to relatively thick with distinct patterns of ridges covering the surface. More often, however, differences in wall structure are more subtle. Most morphospecies were distributed at bathyal depths along the continental margin, but one was sampled at ~4,800 m, representing the first record of an abyssal gromiid. Concurrent with findings from other regions of the World’s oceans, the Weddell Sea gromiids were mostly found in surficial sediments in areas of elevated organic input, suggesting that deep-sea gromiids are likely to play an important role in carbon cycling in bathyal eutrophic regions through the ingestion and degradation of fresh organic matter.     

Testing the depth-differentiation hypothesis in a deepwater octocoral

The depth-differentiation hypothesis proposes that the bathyal region is a source of genetic diversity and an area where there is a high rate of species formation. Genetic differentiation should thus occur over relatively small vertical distances, particularly along the upper continental slope (200–1000 m) where oceanography varies greatly over small differences in depth. To test whether genetic differentiation within deepwater octocorals is greater over vertical rather than geographical distances, Callogorgia delta was targeted. This species commonly occurs throughout the northern Gulf of Mexico at depths ranging from 400 to 900 m. We found significant genetic differentiation (F_ST = 0.042) across seven sites spanning 400 km of distance and 400 m of depth. A pattern of isolation by depth emerged, but geographical distance between sites may further limit gene flow. Water mass boundaries may serve to isolate populations across depth; however, adaptive divergence with depth is also a possible scenario. Microsatellite markers also revealed significant genetic differentiation (F_ST = 0.434) between C. delta and a closely related species, Callogorgia americana, demonstrating the utility of microsatellites in species delimitation of octocorals. Results provided support for the depth-differentiation hypothesis, strengthening the notion that factors covarying with depth serve as isolation mechanisms in deep-sea populations.     

First molecular evidence for underestimated biodiversity of Rhachotropis (Crustacea, Amphipoda), with description of a new species

The crustacean genus Rhachotropis has a worldwide distribution and amongst the largest bathymetric range known from any amphipod genus. DNA barcoding of new material from around New Zealand and the Ross Sea indicated depth-related biogeographic patterns. New Zealand Rhachotropis do not form a monophyletic clade. Species from bathyal depths on the Chatham Rise, east of New Zealand, show lower sequence divergence to bathyal species from California and the Arctic than to abyssal New Zealand species. Species sampled in the Kermadec Trench, north of New Zealand below 5000 m, seem to be more closely related to Ross Sea abyssal species than to the New Zealand shelf species. The worldwide geographic and bathymetric distribution for all Rhachotropis species is presented here. Depth may have a greater influence on phylogeny than geographic distance.Molecular and morphological investigations of Rhachotropis specimens from the Chatham Rise, New Zealand revealed a species new to science which is described in detail, including scanning electron microscopy. This increases the number of described species of Rhachotropis to 60 worldwide.     

BATHYMETRIC PATTERNS OF BODY SIZE IN DEEP-SEA GASTROPODS

The shift to smaller body size in marine invertebrates at the deep-sea threshold and size-depth clines within the deep-sea ecosystem are global biogeographic phenomena that remain poorly understood. We present the first standardized measurements of larval and adult size among ecologically and phylogenetically similar species across a broad and continuous depth range, using the largest family of deep-sea gastropods (the Turridae). Size at all life stages increases significantly with depth from the upper bathyal region to the abyssal plain. These consistent clines may result from selection favoring larger size at greater depths because of its metabolic and competitive advantages. The unusually small size of deep-sea mollusks, in general, may represent an independent evolutionary process that favors invasion by inshore taxa composed of small organisms.     

Genetic structure of the deep-sea coral Lophelia pertusa in the northeast Atlantic revealed by microsatellites and internal transcribed spacer sequences

The azooxanthellate scleractinian coral Lophelia pertusa has a near-cosmopolitan distribution, with a main depth distribution between 200 and 1000 m. In the northeast Atlantic it is the main framework-building species, forming deep-sea reefs in the bathyal zone on the continental margin, offshore banks and in Scandinavian fjords. Recent studies have shown that deep-sea reefs are associated with a highly diverse fauna. Such deep-sea communities are subject to increasing impact from deep-water fisheries, against a background of poor knowledge concerning these ecosystems, including the biology and population structure of L. pertusa. To resolve the population structure and to assess the dispersal potential of this deep-sea coral, specific microsatellites markers and ribosomal internal transcribed spacer (ITS) sequences ITS1 and ITS2 were used to investigate 10 different sampling sites, distributed along the European margin and in Scandinavian fjords. Both microsatellite and gene sequence data showed that L. pertusa should not be considered as one panmictic population in the northeast Atlantic but instead forms distinct, offshore and fjord populations. Results also suggest that, if some gene flow is occurring along the continental slope, the recruitment of sexually produced larvae is likely to be strongly local. The microsatellites showed significant levels of inbreeding and revealed that the level of genetic diversity and the contribution of asexual reproduction to the maintenance of the subpopulations were highly variable from site to site. These results are of major importance in the generation of a sustainable management strategy for these diversity-rich deep-sea ecosystems.     

Biodiversity,biogeography, and connectivity of polychaetes in the world's largest marine minerals exploration frontier

Aim

The abyssal Clarion-Clipperton Zone (CCZ), Pacific Ocean, is an area of commercial importance owing to the growing interest in mining high-grade polymetallic nodules at the seafloor for battery metals. Research into the spatial patterns of faunal diversity, composition, and population connectivity is needed to better understand the ecological impacts of potential resource extraction. Here, a DNA taxonomy approach is used to investigate regional-scale patterns of taxonomic and phylogenetic alpha and beta diversity, and genetic connectivity, of the dominant macrofaunal group (annelids) across a 6 million km² region of the abyssal seafloor.

Location

The abyssal seafloor (3932–5055 m depth) of the Clarion-Clipperton Zone, equatorial Pacific Ocean.

Methods

We used a combination of new and published barcode data to study 1866 polychaete specimens using molecular species delimitation. Both phylogenetic and taxonomic alpha and beta diversity metrics were used to analyse spatial patterns of biodiversity. Connectivity analyses were based on haplotype distributions for a subset of the studied taxa.

Results

DNA taxonomy identified 291–314 polychaete species from the COI and 16S datasets respectively. Taxonomic and phylogenetic beta diversity between sites were relatively high and mostly explained by lineage turnover. Over half of pairwise comparisons were more phylogenetically distinct than expected based on their taxonomic diversity. Connectivity analyses in abundant, broadly distributed taxa suggest an absence of genetic structuring driven by geographical location.

Main Conclusions

Species diversity in abyssal Pacific polychaetes is high relative to other deep-sea regions. Results suggest that environmental filtering, where the environment selects against certain species, may play a significant role in regulating spatial patterns of biodiversity in the CCZ. A core group of widespread species have diverse haplotypes but are well connected over broad distances. Our data suggest that the high environmental and faunal heterogeneity of the CCZ should be considered in future policy decisions.     

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.     

The composition,structure, and distribution of benthic ostracod fauna (Ostracoda,Myodocopa) in Antarctic waters

The benthic fauna of ostracods of the order Myodocopida of Antarctic waters is characterized by high diversity, relative species abundance, and a complicated taxonomic and ecological structure, with a simplified biogeographical structure. This fauna, which is distinguished by a high level of endemicity, although at a low taxonomic rank, includes a great share of deep-sea and subtidal elements. Ostracod populations of High and Low-Antarctic subzones differ qualitatively and quantitatively. A distinct impoverishment of fauna is observed in the region of the Antarctic divergence compared to the more northern areas. The number of species increases with depth to reach its maximum in the lower subtidal zone and on the upper continental slope at depths of 200–500 m. The number of species decreases with increasing depth. Myodocopida have not been yet found in the Antarctic waters deeper than 5000 m.     

Reproduction in marine invertebrates in “stable” environments: the deep sea model

Summary

Studies over the last 15 years have revealed that deep-sea benthic megainvertebrates show a variety of reproductive patterns that are adapted to the deep-sea, an environment in which the fauna occurs at low densities and resources are sparse. In the NE Atlantic the majority of species reproduce year round whilst a limited number of species reproduce on a seasonal basis believed to be entrained by the deposition of surface derived organic material on the deep-sea bed. A third pattern of rapid growth and early reproduction is found in a limited number of species that utilize unpredictable and ephemeral resources in the deep sea. Examination of the fertilization and behavioural biology of species from the bathyal depths suggest some species enhance fertilization success by forming pairs during their breeding season. However, the same concentration of sperm, as seen in shallow water invertebrates, is required for successful fertilization. At least one deep-sea species of echinoid requires high pressure for successful embryogenesis suggesting a depth-related segregation of deep-sea fauna. The origin of megafaunal populations of deep-sea invertebrates in the N. Atlantic is discussed in the light of these new data in relation to varying reproductive patterns and the environmental changes that have occurred during the last deglaciation.     

Ancient Origin of the Modern Deep-Sea Fauna

The origin and possible antiquity of the spectacularly diverse modern deep-sea fauna has been debated since the beginning of deep-sea research in the mid-nineteenth century. Recent hypotheses, based on biogeographic patterns and molecular clock estimates, support a latest Mesozoic or early Cenozoic date for the origin of key groups of the present deep-sea fauna (echinoids, octopods). This relatively young age is consistent with hypotheses that argue for extensive extinction during Jurassic and Cretaceous Oceanic Anoxic Events (OAEs) and the mid-Cenozoic cooling of deep-water masses, implying repeated re-colonization by immigration of taxa from shallow-water habitats. Here we report on a well-preserved echinoderm assemblage from deep-sea (1000–1500 m paleodepth) sediments of the NE-Atlantic of Early Cretaceous age (114 Ma). The assemblage is strikingly similar to that of extant bathyal echinoderm communities in composition, including families and genera found exclusively in modern deep-sea habitats. A number of taxa found in the assemblage have no fossil record at shelf depths postdating the assemblage, which precludes the possibility of deep-sea recolonization from shallow habitats following episodic extinction at least for those groups. Our discovery provides the first key fossil evidence that a significant part of the modern deep-sea fauna is considerably older than previously assumed. As a consequence, most major paleoceanographic events had far less impact on the diversity of deep-sea faunas than has been implied. It also suggests that deep-sea biota are more resilient to extinction events than shallow-water forms, and that the unusual deep-sea environment, indeed, provides evolutionary stability which is very rarely punctuated on macroevolutionary time scales.     

The commonness of rarity in a deep-sea taxon

The generality and drivers of rarity, defined along the axes of geographic range, population size and habitat specificity, have received considerable scientific attention for well over a century. Yet, studies that examine rarity holistically among these three attributes are limited, especially among invertebrate and marine taxa. The perceived paradox of deep-sea species, with often low population size but large geographic ranges, remains poorly resolved and understood. Here I assess seven forms of rarity and their drivers in deep-sea bivalves across the Atlantic Ocean. Rarity appears to be a common trait among deep-sea bivalves, with nearly 85% of the species exhibiting some form of rarity. Bivalves also showed a strong bimodal pattern of very common and very rare species. Geographic range, population size and habitat specificity were all heavily right skewed. Taxonomic superfamilies, body size, energy availability, temperature, depth and latitude, all significantly predicted geographic range, population size and habitat specificity. In a few cases, these patterns were counter to theoretical expectations. The drivers of rarity appear to be predictable from knowledge of the intrinsic biological and extrinsic environmental context of the species. These findings have major implications for deep-sea conversation, especially as anthropogenic threats are increasing.     

Population Differentiation and Species Formation in the Deep Sea: The Potential Role of Environmental Gradients and Depth

Ecological speciation probably plays a more prominent role in diversification than previously thought, particularly in marine ecosystems where dispersal potential is great and where few obvious barriers to gene flow exist. This may be especially true in the deep sea where allopatric speciation seems insufficient to account for the rich and largely endemic fauna. Ecologically driven population differentiation and speciation are likely to be most prevalent along environmental gradients, such as those attending changes in depth. We quantified patterns of genetic variation along a depth gradient (1600-3800m) in the western North Atlantic for a protobranch bivalve ( Nuculaatacellana ) to test for population divergence. Multilocus analyses indicated a sharp discontinuity across a narrow depth range, with extremely low gene flow inferred between shallow and deep populations for thousands of generations. Phylogeographical discordance occurred between nuclear and mitochondrial loci as might be expected during the early stages of species formation. Because the geographic distance between divergent populations is small and no obvious dispersal barriers exist in this region, we suggest the divergence might reflect ecologically driven selection mediated by environmental correlates of the depth gradient. As inferred for numerous shallow-water species, environmental gradients that parallel changes in depth may play a key role in the genesis and adaptive radiation of the deep-water fauna.     

Deep-sea cephalopods of the north-western Mediterranean: indications of up-slope ontogenetic migration in two bathybenthic species

The cephalopod fauna collected during six surveys carried out in the bathyal basin of the north-western Mediterranean is discussed. Samples were taken at depths mainly between 1000 and 2000 m. Ten species were identified. Bathypolypus sponsalis and Neorossia caroli were the commonest species. Small individuals of both these species occurred at greater depths than did larger individuals, suggesting up-slope ontogenetic migration. The depth ranges recorded for all species collected are discussed and compared to the results of previous studies found in the literature.