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.     

Population differentiation decreases with depth in deep-sea bivalves

The deep sea is the largest ecosystem on Earth. Recent exploration has revealed that it supports a highly diverse and endemic benthic invertebrate fauna, yet the evolutionary processes that generate this remarkable species richness are virtually unknown. Environmental heterogeneity, topographic complexity, and morphological divergence all tend to decrease with depth, suggesting that the potential for population differentiation may decrease with depth. To test this hypothesis, we use mitochondrial DNA (16S rRNA gene) to examine patterns of population differentiation in four species of protobranch bivalves (Nuculoma similis, Deminucula atacellana, Malletia abyssorum, and Ledella ultima) distributed along a depth gradient in the western North Atlantic. We sequenced 268 individuals from formalin-fixed samples and found 45 haplotypes. The level of sequence divergence among haplotypes within species was similar, but shifted from between populations at bathyal depths to within populations at abyssal depths. Levels of population structure as measured by phiST were considerably greater in the upper bathyal species (N. similis = 0.755 and D. atacellana = 0.931; 530-3834 m) than in the lower bathyal/abyssal species (M. abyssorum = 0.071 and L. ultima = 0.045; 2864-4970 m). Pairwise genetic distances among the samples within each species also decreased with depth. Population trees (UPGMA) based on modified coancestry coefficients and nested clade analysis both indicated strong population-level divergence in the two upper bathyal species but little for the deeper species. The population genetic structure in these protobranch bivalves parallels depth-related morphological divergence observed in deep-sea gastropods. The higher level of genetic and morphological divergence, coupled with the strong biotic and abiotic heterogeneity at bathyal depths, suggests this region may be an active area of species formation. We suggest that the steep, topographically complex, and dynamic bathyal zone, which stretches as a narrow band along continental margins, plays a more important role in the evolutionary radiation of the deep-sea fauna than the much more extensive abyss.     

Can the source–sink hypothesis explain macrofaunal abundance patterns in the abyss? A modelling test

Low food availability is a major structuring force in deep-sea benthic communities, sustaining only very low densities of organisms in parts of the abyss. These low population densities may result in an Allee effect, whereby local reproductive success is inhibited, and populations are maintained by larval dispersal from bathyal slopes. This slope–abyss source–sink (SASS) hypothesis suggests that the abyssal seafloor constitutes a vast sink habitat with macrofaunal populations sustained only by an influx of larval ‘refugees'' from source areas on continental slopes, where higher productivity sustains greater population densities. Abyssal macrofaunal population densities would thus be directly related to larval inputs from bathyal source populations. We evaluate three predictions derived from the SASS hypothesis: (i) slope-derived larvae can be passively transported to central abyssal regions within a single larval period, (ii) projected larval export from slopes to the abyss reproduces global patterns of macrofaunal abundance and (iii) macrofaunal abundance decreases with distance from the continental slope. We find that abyssal macrofaunal populations are unlikely to be sustained solely through influx of larvae from slope sources. Rather, local reproduction probably sustains macrofaunal populations in relatively high-productivity abyssal areas, which must also be considered as potential larval source areas for more food-poor abyssal regions.     

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.     

Metagenetic community analysis of microbial eukaryotes illuminates biogeographic patterns in deep-sea and shallow water sediments

Microbial eukaryotes (nematodes, protists, fungi, etc., loosely referred to as meiofauna) are ubiquitous in marine sediments and probably play pivotal roles in maintaining ecosystem function. Although the deep-sea benthos represents one of the world's largest habitats, we lack a firm understanding of the biodiversity and community interactions amongst meiobenthic organisms in this ecosystem. Within this vast environment, key questions concerning the historical genetic structure of species remain a mystery, yet have profound implications for our understanding of global biodiversity and how we perceive and mitigate the impact of environmental change and anthropogenic disturbance. Using a metagenetic approach, we present an assessment of microbial eukaryote communities across depth (shallow water to abyssal) and ocean basins (deep-sea Pacific and Atlantic). Within the 12 sites examined, our results suggest that some taxa can maintain eurybathic ranges and cosmopolitan deep-sea distributions, but the majority of species appear to be regionally restricted. For Operationally Clustered Taxonomic Units (OCTUs) reporting wide distributions, there appears to be a taxonomic bias towards a small subset of taxa in most phyla; such bias may be driven by specific life history traits amongst these organisms. In addition, low genetic divergence between geographically disparate deep-sea sites suggests either a shorter coalescence time between deep-sea regions or slower rates of evolution across this vast oceanic ecosystem. While high-throughput studies allow for broad assessment of genetic patterns across microbial eukaryote communities, intragenomic variation in rRNA gene copies and the patchy coverage of reference databases currently present substantial challenges for robust taxonomic interpretations of eukaryotic data sets.     

Biogeographic and bathymetric ranges of Atlantic deep-sea echinoderms and ascidians: the role of larval dispersal

Dispersal plays an important role in the establishment and maintenance of biodiversity and, for most deep-sea benthic marine invertebrates, it occurs mainly during the larval stages. Therefore, the mode of reproduction (and thus dispersal ability) will affect greatly the biogeographic and bathymetric distributions of deep-sea organisms. We tested the hypothesis that, for bathyal and abyssal echinoderms and ascidians of the Atlantic Ocean, species with planktotrophic larval development have broader biogeographic and bathymetric ranges than species with lecithotrophic development. In comparing two groups with lecithotrophic development, we found that ascidians, which probably have a shorter larval period and therefore less dispersal potential, were present in fewer geographic regions than elasipod holothurians, which are likely to have longer larval periods. For asteroids and echinoids, both the geographic and bathymetric ranges were greater for lecithotrophic than for planktotrophic species. For these two classes, the relationships of egg diameter with geographic and bathymetric range were either linearly increasing or non-monotonic. We conclude that lecithotrophic development does not necessarily constrain dispersal in the deep sea, probably because species with planktotrophic development may be confined to regions of high detrital input from the sea surface. Our data suggest that more information is necessary on lengths of larval period for different species to accurately assess dispersal in the deep sea.     

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.     

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.     

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.     

The biodiversity of the deep Southern Ocean benthos

Our knowledge of the biodiversity of the Southern Ocean (SO) deep benthos is scarce. In this review, we describe the general biodiversity patterns of meio-, macro- and megafaunal taxa, based on historical and recent expeditions, and against the background of the geological events and phylogenetic relationships that have influenced the biodiversity and evolution of the investigated taxa. The relationship of the fauna to environmental parameters, such as water depth, sediment type, food availability and carbonate solubility, as well as species interrelationships, probably have shaped present-day biodiversity patterns as much as evolution. However, different taxa exhibit different large-scale biodiversity and biogeographic patterns. Moreover, there is rarely any clear relationship of biodiversity pattern with depth, latitude or environmental parameters, such as sediment composition or grain size. Similarities and differences between the SO biodiversity and biodiversity of global oceans are outlined. The high percentage (often more than 90%) of new species in almost all taxa, as well as the high degree of endemism of many groups, may reflect undersampling of the area, and it is likely to decrease as more information is gathered about SO deep-sea biodiversity by future expeditions. Indeed, among certain taxa such as the Foraminifera, close links at the species level are already apparent between deep Weddell Sea faunas and those from similar depths in the North Atlantic and Arctic. With regard to the vertical zonation from the shelf edge into deep water, biodiversity patterns among some taxa in the SO might differ from those in other deep-sea areas, due to the deep Antarctic shelf and the evolution of eurybathy in many species, as well as to deep-water production that can fuel the SO deep sea with freshly produced organic matter derived not only from phytoplankton, but also from ice algae.     

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.     

Osmotic and ionic hemolymph concentrations of bathyal and abyssal amphipods of Lake Baikal (Siberia) in relation to water depth

We studied the adaptive variations of the hemolymph concentrations in relation to water depth and pressure using deep-dwelling amphipods from Lake Baikal. Hemolymph osmolality was determined in six bathyal and abyssal species immediately after capture when values come closest to the habitat concentrations. In three species, hemolymph osmolalities correlated positively with depth of capture. Prevalent ions in the hemolymph are sodium and chloride. Lactate, our indicator for capture stress, was highest after trawling (2–6 mM) and lowest after retrieval from cages (0–0.6 mM). Acclimation to different pressure was studied by exposing the specimens to different water depths over several days. Hemolymph concentrations did not change after acclimation to surface pressure in the sublittoral Acanthogammarus albus, a native also to shallow water, but decreased by 30–80 mosmol/kg H₂O in the bathyal and abyssal species Acanthogammarus grewingki, Acanthogammarus reicherti, and Parapallasea lagowskii. Similarly, hemolymph osmolality decreased in A. reicherti and P. lagowskii originating from deep water, when acclimated to reduced water depth, and, in A. reicherti hemolymph osmolality reached its original high value when returned to the depth of capture. Higher hemolymph osmolalities and NaCl concentrations, demonstrated here for the first time, may provide selective advantages to abyssal species. Accepted: 24 August 2000     

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.     

深海钻探计划(DSDP)31航次296站晚新生代介形类

本文研究了深海钻探(DSDP)31航次296站晚新生代介形类动物群的性质及其古海洋学意义.此站钻孔上部上新统至全新统以超微浮游生物软泥和粘土为主的16块岩芯中,共分析获得介形类化石8属11种,计有:Poseidonamicus major Benson, P. anteropunctatus Whatley et al., P. punctatus Whatley et al., Pennyella dorsoserrata (Brady), Henryhowella sp., Pterygocythere mucronalatum (Brady), Abyssocythere sp., Abyssocythereis sulcatoperforata (Brady), Pelecocythere sp., Krithe sp. 1和Krithe sp.2.这些介形类属种均为冷海域深海区介形类分子.由此表明,西北太平洋边缘地区在晚新生代曾为一深海区.在第四纪,其深度可能和现今296站的深度大致相当;在上新世,其深度可能更深一些.研究结果证实,深海底栖介形类属种的分布具全球性;在相当长的地质时期内,介形类属种的形态和壳饰都非常稳定,无明显变化.同时,进一步证实,介形类个体大小变化与深度相关,同一种介形类壳体随水域深度加深而增大.     

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.     

Deep-Sea Biodiversity in the Mediterranean Sea: The Known,the Unknown,and the Unknowable

Deep-sea ecosystems represent the largest biome of the global biosphere, but knowledge of their biodiversity is still scant. The Mediterranean basin has been proposed as a hot spot of terrestrial and coastal marine biodiversity but has been supposed to be impoverished of deep-sea species richness. We summarized all available information on benthic biodiversity (Prokaryotes, Foraminifera, Meiofauna, Macrofauna, and Megafauna) in different deep-sea ecosystems of the Mediterranean Sea (200 to more than 4,000 m depth), including open slopes, deep basins, canyons, cold seeps, seamounts, deep-water corals and deep-hypersaline anoxic basins and analyzed overall longitudinal and bathymetric patterns. We show that in contrast to what was expected from the sharp decrease in organic carbon fluxes and reduced faunal abundance, the deep-sea biodiversity of both the eastern and the western basins of the Mediterranean Sea is similarly high. All of the biodiversity components, except Bacteria and Archaea, displayed a decreasing pattern with increasing water depth, but to a different extent for each component. Unlike patterns observed for faunal abundance, highest negative values of the slopes of the biodiversity patterns were observed for Meiofauna, followed by Macrofauna and Megafauna. Comparison of the biodiversity associated with open slopes, deep basins, canyons, and deep-water corals showed that the deep basins were the least diverse. Rarefaction curves allowed us to estimate the expected number of species for each benthic component in different bathymetric ranges. A large fraction of exclusive species was associated with each specific habitat or ecosystem. Thus, each deep-sea ecosystem contributes significantly to overall biodiversity. From theoretical extrapolations we estimate that the overall deep-sea Mediterranean biodiversity (excluding prokaryotes) reaches approximately 2805 species of which about 66% is still undiscovered. Among the biotic components investigated (Prokaryotes excluded), most of the unknown species are within the phylum Nematoda, followed by Foraminifera, but an important fraction of macrofaunal and megafaunal species also remains unknown. Data reported here provide new insights into the patterns of biodiversity in the deep-sea Mediterranean and new clues for future investigations aimed at identifying the factors controlling and threatening deep-sea biodiversity.     

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.     

Ocean-scale patterns of 'biodiversity' of Atlantic asteroids determined from taxonomic distinctness and other measures

We examine patterns of ‘gamma’ (within-rcgion) and ‘beta’ (between-region) diversity from analysis of a presence/absence dataset for species of asteroids encompassing the whole Atlantic Ocean partitioned into 26 regions. Absolute species numbers (a poor measure of biodiversity) in shallow coastal areas and the deep sea are the same, although species richness per area for two well-sampled regions suggests, qualitatively, that coastal areas may be more speciose. Taxonomic distinctness (A*), an index which is markedly less sample-size dependent than other common diversity measures, shows no significant association with geographic area and no clear pattern with depth, suggesting an absence of latitudinal and coastal/deep trends. Cluster analysis shows that distinctive faunal assemblages are most evident in shelf/ shallow waters, where six groupings separate recognizably acccording to geographical location. Three of these arc the southernmost regions of the Atlantic (southeast of S. America, S. Angola/S. Africa & Tristan da Cunha/Gough Island) and arc characterized by their isolation and high levels of endemism. As depth increases so does the amount of faunal similarity between regions. This indicates that beta diversity is highest in shelf regions and lowest in lower bathyal/abyssal regions. Our results may support the contention which questions the emerging paradigm that the deep sea has exceptionally high diversity. It is evident, however, that comparisons (e.g. between coasts and the deep sea) are problematic and can depend very much on the element(s) of biodiversity measured, sampling methods and the spatial scales (e.g. alpha, beta or gamma diversity) over which assessment is made. Any wider conclusions should therefore be drawn cautiously, particularly since assessment is made of only one faunal group. Other findings include significant correlation between the depth range of asteroids and their geographical range. The utility of low-resolution datasets is reviewed. It is concluded that within limitations they can be of value for determining broad (e.g. regional, ocean-scale and global-scale) patterns of diversity and community structure, especially when analysed using measures relatively uninfluenced by sample size.     

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.