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 α 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.

Related papers:

title: Marine Benthic Diversity: A Comparative Study
abstract: In this paper a methodology is presented for measuring diversity based on rarefaction of actual samples. By the use of this technique, a within-habitat analysis was made of the bivalve and polychaete components of soft-bottom marine faunas which differed in latitude, depth, temperature, and salinity. The resulting diversity values were highly correlated with the physical stability and past history of these environments. A stability-time hypothesis was invoked to fit these findings, and, with this hypothesis, predictions were made about the diversities present in certain other environments as yet unstudied. The two types of diversity, based on numerical percentage composition and on number of species, were compared and shown to be poorly correlated with each other. Our data indicated that species number is the more valid diversity measurement. The rarefaction methodology was compared with a number of diversity indexes using identical data. Many of these indexes were markedly influenced by sample size. Good agreement was found between the rarefaction methodology and the Shannon-Wiener information function.

title: Biological accomodation in the deep-sea benthos: comparative evidence on the importance of predation and productivity
abstract: Measures of the diversity and relative abundance of gastropod predators are positively and significantly correlated with overall gastropod diversity in 15 epibenthic sled samples collected from 478 to 4862 m along the Gay Head-Bermuda transect. Predator diversity is lowest on the abyssal plain, highest on the lower continental slope and abyssal rise, and relatively low on the upper continental slope. The variation of gastropod predator diversity with depth is similar to those of their most probable prey, the polychaetes and protobranch bivalves. Predator diversity and overall diversity were also related to variation in faunal density with depth. Density decreases exponentially with depth and reflects the average rate of production reaching the deep-sea benthos. Relationships among diversity, predation, and production are consistent with the theory that predators exert a diversifying influence on communities and that the degree to which they are able to do this depends on the rate and stability of production. The results suggest that both predation and productivity are important to biological accomodation in deep-sea communities and that their relative contributions to maintaining diversity vary considerably with depth.

title: Macrofaunal communities within and adjacent to a detritus-rich submarine canyon system
abstract: Macrofaunal abundance, biomass, diversity and species assemblages within Scripps and La Jolla Submarine Canyons are compared with those on the nearby continental shelf and slope. Our primary objective was to examine the effect of detrital aggregates on infaunal communities within canyons. Two submarines, a remotely operated vehicle (ROV), and a Soutar box-corer were used to collect samples. Within the canyons, organic enrichment by macrophyte detritus was evident from canyon heads down to 550 m, and evidence of strong currents (coarse sediment) was found down to 700 m. Infaunal density and biomass were higher in the canyons than outside at all depths where comparative data were available (100–500 m). Infaunal assemblages in canyons were distinct from those at reference stations. Both the canyon and non-canyons samples showed community differentiation with depth. Species diversity was generally high, but decreased with depth outside of canyons and increased with depth within the canyons. Low diversity at shallow depths within the canyon is attributed to a combination of organic enrichment and physical disturbance. Submarine canyons are commonly found to contain distinct species assemblages or higher faunal densities and/or biomass than nearby non-canyon regions at similar depths. Canyons are regular features along most ocean margins and appear to be important as sites of enhanced secondary production, provide diverse habitats, and act as conduits of coastal detritus to the deep-sea.

title: ENVIRONMENTAL INFLUENCES ON REGIONAL DEEP-SEA SPECIES DIVERSITY
abstract: Most of our knowledge of biodiversity and its causes in the deep-sea benthos derives from regional-scale sampling studies of the macrofauna. Improved sampling methods and the expansion of investigations into a wide variety of habitats have revolutionized our understanding of the deep sea. Local species diversity shows clear geographic variation on spatial scales of 100-1000 km. Recent sampling programs have revealed unexpected complexity in community structure at the landscape level that is associated with large-scale oceanographic processes and their environmental consequences. We review the relationships between variation in local species diversity and the regional-scale phenomena of boundary constraints, gradients of productivity, sediment heterogeneity, oxygen availability, hydrodynamic regimes, and catastrophic physical disturbance. We present a conceptual model of how these interdependent environmental factors shape regional-scale variation in local diversity. Local communities in the deep sea may be composed of species that exist as metapopulations whose regional distribution depends on a balance among global-scale, landscape-scale, and small-scale dynamics. Environmental gradients may form geographic patterns of diversity by influencing local processes such as predation, resource partitioning, competitive exclusion, and facilitation that determine species coexistence. The measurement of deep-sea species diversity remains a vital issue in comparing geographic patterns and evaluating their potential causes. Recent assessments of diversity using species accumulation curves with randomly pooled samples confirm the often-disputed claim that the deep sea supports higher diversity than the continental shelf. However, more intensive quantitative sampling is required to fully characterize the diversity of deep-sea sediments, the most extensive habitat on Earth. Once considered to be constant, spatially uniform, and isolated, deep-sea sediments are now recognized as a dynamic, richly textured environment that is inextricably linked to the global biosphere. Regional studies of the last two decades provide the empirical background necessary to formulate and test specific hypotheses of causality by controlled sampling designs and experimental approaches.

title: Hypsometry of ocean basin provinces
abstract: The frequency distribution of depths in ocean basins has been determined using the most recent American and Russian oceanographic charts as sources and a computer for data processing. Data have been complied for individual ocean basins and marginal seas, and also for nine different physiographic provinces, including, for example, ‘ocean basin,’ ‘rise and ridge’ and ‘continental shelf and slope.’ The summary result differs little from a hypsometric curve drawn by Murray and Hjort in 1912. Considering both land and sea, by far the most common levels are approximately at sea level and at about 5 km, the general depth of ocean floor. The compilation by physiographic provinces leads to a new understanding of the hypsometric curve, even though it does not change the shape. The distribution of depths in the ocean basin province is singly peaked and symmetrical. The distribution in the rise and ridge province is similar, but the mean depth is about 1 km less. This can be interpreted as indicating that one province is merely the elevated equivalent of the other, a conclusion which generalizes the field observation that some localized oceanic rises and ridges are formed by ephemeral bulges in the mantle under a normal oceanic crust. Most occurrences of depths between sea level and the deep sea floor have been produced by the formation of rises and ridges rather than by the deposition of sediment derived from continents. The development and collapse of rises and ridges may have caused substantial fluctuations in sea level during geological time.

title: Deep-sea food chains and the global carbon cycle
abstract: Carbon dioxide and other "greenhouse" gases are increasing in the atmosphere due to the burning of fossil fuels, the destruction of rain forests, etc, leading to predictions of a gradual global warming which will perturb the global biosphere. An important process which counters this trend toward potential climate change is the removal of carbon dioxide from the surface ocean by photosynthesis. This process packages carbon in phytoplankton which enter the food chain or sink into the deep sea. Their ultimate fate is a "rain" or organic debris out of the surface-mixed layer of the ocean. On a global scale, the mechanisms and overall rate of this process are relatively little known. The authors of the 25 papers in this volume present their state-of-the-art approaches to quantifying the mechanisms by which the "rain" of biogenic debris nourishes deep ocean life. Prominent deep sea ecologists, geochemists and modellers address relationships between data and models of carbon fluxes and food chains in the deep ocean. An attempt is made to estimate the fate of carbon in the deep sea on a global scale by summing up the utilization of organic matter among all the populations of the abyssal biosphere. Comparisons are made been these ecological approaches and estimates of geochemical fluxes based on sediment trapping, one-dimensional geochemical models and horizontal (physical) input from continental margins. Planning interdisciplinary enterprises between geochemists and ecologists, including new field programmes, are summarized in the final chapter. The summary includes a list of the important gaps in understanding which must addressed before the role of the deep-sea biota in global-scale processes can be put in perspective.

title: Population differentiation decreases with depth in deep-sea gastropods
abstract: The evolutionary processes that have generated the rich and highly endemic deep-sea benthic invertebrate fauna remain largely unstudied. Patterns of geographic variation constitute the most basic and essential information for understanding speciation and adaptive radiation. Here we present a multivariate analysis of phenotypic distance to quantify geographic variation in shell form among populations of eight deep-sea gastropod species distributed along a depth gradient in the western North Atlantic. Our primary aim is to identify regions within the deep sea that may promote population differentiation. The degree of interpopulation divergence is highest at the shelf-slope transition (200 m), and then decreases dramatically with increasing depth across the bathyal region (200–4000 m) to the abyssal plain (>4000 m). Phenotypic change parallels faunal change along the depth gradient, suggesting that the selective regime becomes more uniform with increasing depth. Results indicate that the potential for gastropod radiation within the deep-sea environment (>200 m) varies geographically and is highest in the narrow upper bathyal region.

title: MORPHOLOGICAL DISPARITY AS A BIODIVERSITY METRIC IN LOWER BATHYAL AND ABYSSAL GASTROPOD ASSEMBLAGES
abstract: Studies of deep-sea biodiversity focus almost exclusively on geographic patterns of α-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.

title: Inverse density dependence and the Allee effect.
abstract: The Allee effect describes a scenario in which populations at low numbers are affected by a positive relationship between population growth rate and density, which increases their likelihood of extinction. The importance of this dynamic process in ecology has been under-appreciated and recent evidence now suggests that it might have an impact on the population dynamics of many plant and animal species. Studies of the causal mechanisms generating Allee effects in small populations could provide a key to understanding their dynamics.

title: Population dynamics in two-patch environments: Some anomalous consequences of an optimal habitat distribution
abstract: The effect of dispersal on population size and stability is explored for a population that disperses passively between two discrete habitat patches. Two basic models are considered. In the first model, a single population experiences density-dependent growth in both patches. A graphical construction is presented which allows one to determine the spatial pattern of abundance at equilibrium for most reasonable growth models and rates of dispersal. It is shown under rather general conditions that this equilibrium is unique and globally stable. In the second model, the dispersing population is a food-limited predator that occurs in both a source habitat (which contains a prey population) and a sink habitat (which does not). Passive dispersal between source and sink habitats can stabilize an otherwise unstable predator-prey interaction. The conditions allowing this are explored in some detail. The theory of optimal habitat selection predicts the evolutionarily stable distribution of a population, given that individuals can freely move among habitats so as to maximize individual fitness. This theory is used to develop a heuristic argument for why passive dispersal should always be selectively disadvantageous (ignoring kin effects) in a spatially heterogeneous but temporally constant environment. For both the models considered here, passive dispersal may lead to a greater number of individuals in both habitats combined than if there were no dispersal. This implies that the evolution of an optimal habitat distribution may lead to a reduction in population size; in the case of the predator-prey model, it may have the additional effect of destabilizing the interaction. The paper concludes with a discussion of the disparate effects habitat selection might have on the geographical range occupied by a species.

title: Sources, Sinks, and Population Regulation
abstract: Animal and plant populations often occupy a variety of local areas and may experience different local birth and death rates in different areas. When this occurs, reproductive surpluses from productive source habitats may maintain populations in sink habitats, where local reproductive success fails to keep pace with local mortality. For animals with active habitat selection, an equilibrium with both source and sink habitats occupied can be both ecologically and evolutionarily stable. If the surplus population of the source is large and the per capita deficit in the sink is small, only a small fraction of the total population will occur in areas where local reproduction is sufficient to compensate for local mortality. In this sense, the realized niche may be larger than the fundamental niche. Consequently, the particular species assemblage occupying any local study site may consist of a mixture of source and sink populations and may be as much or more influenced by the type and proximity of other habitats as by the resources and other conditions at the site.

title: Depth-related gradients in size structure and the bathymetric zonation of deep-sea brittle stars
abstract: Brittle stars show rather diffuse zonation patterns with depth in the deep ocean compared to other fauna. Measurements of disk diameters of samples taken from 1973 to 1980 from populations of two species in the Rockall Trough (N.E. Atlantic Ocean) also show marked differences in frequencies of juvenile and adult sizes present at different depths. The breeding population of one of these species, Ophiocten gracilis G. O. Sars, is concentrated at about 1 000 m depth, from where samples show a polymodality in disk-size frequencies indicative of marked seasonal recruitment and von Bertalanffy growth of year classes identified from Gaussian mixtures fitted to the frequencies. A general trend of an increasing proportion of postlarval and juvenile sizes with increasing depth was evident in samples from various depths down to 2 900 m, where a marked annual settlement survives at best only a few months. However, active gametogenesis was detected in specimens as small as 1.4 mm in disk diameter and from as deep as 2 076 m. Models of age structure fitted to the size frequencies indicate a roughly depth-related gradient in decreasing survivorship and possibly also in growth rate of individuals. The population of the other species, Ophiura ljungmani Lyman, is best developed at 2 500 to 3 000 m. O. ljungmani also shows annual recruitment, and models of probable age structure fitted to samples from around 2 900 up to 1 632 m depth indicate a similar depth-related gradient in survivorship, but inconclusive evidence for depth-related differences in growth rate. With both species, certain samples from intermediate depths deviated markedly from the expectation from models of simple bathymetric gradients. This indicates that factors controlling size and age structure in these populations are complex size and age structure in these populations are complex and only indirectly related to depth. Factors controlling the patterns observed may reflect local differences in predation pressure and trophic resources as well as more direct controls such as the effect of hydrostatic pressure on physiological functions.

title: Vertical distribution of macrozoobenthos within the sediment on the continental slope of the Goban Spur area (NE Atlantic)
abstract: Macrofauna density and biomass were estimated in 4 sediment layers (0-1, 1-5, 5-10 and 10-15 cm) from 12 stations ranging from 185 to 4460 m water depth in the Goban Spur area (NE Atlantic). At the same stations profiles of median grain size, % organic carbon and % total nitrogen were measured to a sediment depth of 15 cm. Macrofauna densities decreased exponentially from similar to 8000 m(-2) at 208 m to similar to 550 m(-2) at the deepest stations (3673 and 4460 m). Most of the macrofauna was concentrated in the upper 1 cm of the sediment. The highest proportion (70 to 80%) was in the upper 1 cm found at intermediate depths (670 to 1425 m), whereas at the deeper stations the fauna was more evenly distributed. Polychaeta were the most abundant taxon, with a high proportion of subsurface deposit-feeders at the deep stations and a high proportion of filter-feeders plus surface deposit-feeders at intermediate depth. Biomass values showed a very variable pattern, mainly due to a few extremely large individuals at some of the stations. Excluding these large individuals, a maximum biomass value was observed around 1000 m water depth. At most stations the biomass was concentrated below the upper 1 cm, which was also reflected in the vertical distribution pattern of the mean individual weight, showing small individuals in the upper 1 cm and larger individuals in the deeper sediment layers. Both density and biomass fell within the range of values usually observed at these depths. Only the biomass value at 1034 m was extremely high and concentrated in the upper 1 cm of the sediment. This may be due to pulsed inputs of fresh material at this station, whereas at the deepest stations (3673 and 4460 m) a constant low input of refractory material supports the fauna of mainly deep-living subsurface deposit-feeders. Median grain size decreased with increasing water depth from similar to 90 pm at 208 m to similar to 8 mu m below 2000 m. The vertical profiles showed very homogeneous sediments to a depth of 15 cm at all stations, probably due to bioturbation by the relatively abundant deposit-feeders. In general, the % of total N and organic C in the upper 1 cm of the sediment increased with increasing water depth, but showed a peak at similar to 1000 m. The % C and especially N were lower in the deeper sediment layers at the deeper stations, resulting in C:N ratios of similar to 12 to 15 in contrast to similar to 7 to 8 in the upper 1 cm of the sediment.  vertical distribution; macrozoobenthos; continental slope; grain size; C:N ratio Benthic community; deep; ocean]

title: An introduction to the study of deep-sea benthic faunal assemblages along the Gay Head-Bermuda transect*
abstract: This paper is the introduction to a long-term, detailed study of open ocean, benthic assemblages. Attention will be paid to taxonomy, community structure, physiology, recruitment, patchiness, modes of feeding, zoogeography, and many other related topics. The major area of study is a transect between Massachusetts, U.S.A., and Bermuda, in the North Atlantic Ocean. Quantitative samples were taken with an anchor dredge and were washed through a 0·42-millimeter-aperture screen. The sediments along the transect range from fine sands of terrigenous origin on the upper continental slope, to silty clays and clays composed primarily of pelagically derived calcium carbonate in the Sargasso Sea. With increasing depth. temperature at the stations not only decreases, but shows more restricted fluctuation. Analysis of results shows much greater faunal densities than those reported from previous studies; this probably results from our using a smaller screen aperture. Each region of the transect supports a characteristic number of animals per square meter in a general trend of decreasing density with increasing depth and distance from the continent: outer continental shelf, 6,000–13,000; upper continental slope, 6,000–23,000; lower continental slope, 1,500–3,000; abyssal rise, 500–1,200; abyss under the Gulf Stream, 150–270; abyss in the Sargasso Sea, 31–130; lower Bermuda Slope, 140–300; upper Bermuda Slope, 500–850. From all but three stations, Polychaeta, Crustacea, Pelecypoda and Sipunculoidea combined comprised 85–100% of the fauna, with the former group being particularly important. Pogonophorans, ophiuroids, anemones, solenogastres, gastrods, and scaphopods were also important, but less abundant. The dominance of polychaetes tended to decrease with distance from the continent, while the crustaceans correspondingly increased in importance. Attempts to relate the density of animals to the amount of organics in the sediment provided no clear correlation. It is suggested that this results from the fact that the analytical techniques do not differentiate between labile and more refractory organic compounds.

title: Benthic biomass estimates from the northwestern atlantic ocean and the northern Gulf of Mexico
abstract: Estimates have been made of the biomass and abundance of macrobenthic invertebrates off the Gulf of Mexico and Atlantic coasts of the United States. Based on these estimates, it is concluded that deep-sea life is more abundant in the Atlantic than in the Gulf. Regressions of the logarithm (base 10) of biomass and animal density against depth indicate that the abundance of life followed an exponential decline with depth, Y = ae−bx, where Y is either density of individuals or biomass, x is depth and a is proportional to average surface-water phytoplankton production. The rate of decline (b) can be related to the rate of decrease in phytoplankton production in an offshore direction and the efficiency of water-column heterotrophs at utilizing sinking organic matter. The regressions also indicate, through comparison with the literature, that both benthos and zooplankton follow similar exponential decays in quantity of life with depth. The constants (a) both appear to be functions of surface productivity and it can be inferred that the sources of food for zooplankton and benthos in the deep sea are the same. The seemingly conservative nature of organic matter over depth ranges where there is an exponential decrease in life can probably be attributed to the increase in the relative abundance of the refractory organic compounds with depth.

title: The deep-sea macrobenthos on the continental margin of the northwest Atlantic Ocean
abstract: DSRV Alvin was used to take 62 small box core samples between 203 and 3659 m deep on the continental margin off the northeastern United States. Abundance and wet-weight biomass of total macrobenthic communities decreased sharply with depth. Standing stocks were higher in the Hudson canyon than in adjacent areas of similar depths, but only in the canyon head at upper continental slope depths, where the canyon is geologically ‘inactive’ and is filling with organic-rich, fine-grained sediments. At lower continental slope and upper continental rise depths no such differences could be discerned. Diversity increased directly and gradually with depth and varied inversely with sample size (= animal abundance), but it was not related to the canyon. Assemblages defined using a percent similarity index were ordered across the depth gradient, with a peculiar canyon fauna only in the organic-rich canyon sediments on the upper continental slope. The greatest faunal discontinuity (= least similarity between samples) was encountered between the upper and the lower continental slope and not at the continental shelf break (about 300 m). Percent sand was high in the sediments deeper in the canyon on the upper continental rise, but the fauna had a species composition, community structure, and biomass that could not be considered different from the continental rise outside of the canyon, where the sediments were silt and clay. The canyon on the upper continental slope traps organic-rich sediments. Although the low concentrations of organic matter on the continental rise in the canyon would imply that the organic matter from shallow water does not reach this depth, pore water concentrations of ammonia and nitrate were consistently higher in canyon sediments than outside the canyon on the rise, indicating metabolism (biologic activity) in these sediments is greater in the canyon, presumably a result of labile organic matter being trapped and funneled by the canyon across the continental rise.

title: Benthic community respiration in the N.W. Atlantic Ocean: in situ measurements from 40 to 5200 m
abstract: Benthic community respiration was measured in situ at 9 stations along the Gay Head-Bermuda transect from depths of 40 to 5200 m. Three methods were used; bell jar respirometers, grab respirometers, and free vehicle respirometers. Benthic community respiration rates spanned three orders of magnitude, decreasing from 21.5 ml O2 m-2 h-1 at 40 m in November to 0.02 ml O2 m-2 h-1 at 5200 m. Rates decreased two orders of magnitude between 40 and 1800 m and then significantly declined again between the continental rise (3650 m) and the abyssal plain stations. Predictive equations for benthic community respiration along the transect reflect a strong correlation with depth of water. Of lesser significance are the correlations with water temperature, dissolved oxygen, benthic animal biomass, surface primary productivity and sediment organic matter. Calculations show that annual benthic respiration can utilize 1 to 2% of the surface primary productivity. Of the 2 to 7% organic carbon fixed at the surface which supposedly reaches the bottom, only 15 to 29% is utilized by the benthic community at 2200, 3000, and 3650 m. The energy requirements of other biological components of deep-sea benthic communities, such as benthopelagic and macro-epibenthic animals, not included in these measurements, must also be considered in calculating a balance of carbon.

title: Planktotrophic development in deep-sea prosobranch snails from the western North Atlantic
abstract: The incidence of planktotrophic development in prosobranch snails increases with depth seaward of the continental shelf in the western North Atlantic. This pattern may result from both stronger selection for dispersal ability to track patchy prey resources and relaxed restrictions on dispersal with increasing depth.

title: Protobranch bivalves.
abstract: The subclass Protobranchia comprises more than 600 species of bivalves that occur throughout the world ocean. Mostly deposit feeders in soft sediments, they are abundant in the deep sea. Apomorphies that unite them as a group include gill structure, hinge conformation, shell microstructure, larval development, foot morphology, respiratory pigments, trophic mode and digestion. They are relatively small and highly conserved in form, originating in the Cambrian era. They may represent an ancestral, derived or paraphylectic group of the Bivalvia. The protobranchs include two orders, the Nuculoida and Solemyoida, which previously were classified separately in the subclasses Paleotaxodonta and Cryptodonta, respectively. They are of ecological interest and have a unique functional morphology. They feed mostly under the surface of the sediment with highly modified labial palps, but the degree to which they are selective in diet remains difficult to determine. They are important bioturbators in many soft-sediment assemblages; their feeding and locomotion affects sediment structure and community development. Solemyoids are unusual in inhabiting reducing environments and hydrocarbon seeps and in deriving their nutrition from endosymbiotic chemosynthetic bacteria. A variety of species of protobranchs are found in oceanic trenches, near hydrothermal vents, and in submarine caves. Protobranchs produce a lecithotrophic larval stage, the pericalymma, making their development unique among bivalves. The pericalymma remains in the plankton for a short time and presumably has low dispersal ability. Recruitment may be intermittent. Growth is rapid in post-larvae but decreases with age, though rates may not necessarily be slow, especially in continental shelf species. Life spans are commonly 1 to 2 decades, but deep-sea representatives may grow more slowly and live longer. Bottom fish, seastars and gastropods are their major predators and a few parasites and commensals have been documented. The predominance of protobranchs in deep-sea sediments may be a result of deep-sea origin or displacement from shallow waters by lamellibranchs. Their ability to deposit-feed, digest food extracellularly, and develop by means of lecithotrophic larvae make them particularly well adapted to cold and oligotrophic habitats.

title: Spawning and larval development of the black turban snail Tegula funebralis (Prosobranchia: Trochidae)
abstract: An understanding of spawning and larval development can be fundamental to interpreting the abundance, distribution, and population structure of marine invertebrate taxa. Tegula funebralis (A. Adams, 1855), the black turban snail, has been the focus of numerous ecological studies on the Pacific coast of North America. To date, there are only conflicting and anecdotal reports of spawning, and there is no information on larval or juvenile development for this conspicuous and abundant species. On 19 September 1995, two individuals of T. funebralis were observed free-spawning gametes into seawater in tanks at the Oregon Institute of Marine Biology. Embryos and larvae were subsequently reared to metamorphosis and beyond. Development was pelagic and similar to development described for other trochids, and larvae were observed not to feed at any stage. Larvae began to metamorphose at 5.7 to 6.7 d and settled at 260 μm shell length. Juveniles grew ≃ 10 μm in shell length per day and appeared to feed on detritus. Juveniles lacked some adult diagnostic shell characters, including two columellar nodes and a closed umbilicus. In the field, small (<3 mm) juveniles occurred in the adult habitat on all sampling dates between October and March. Small juveniles were found only under rocks and were most abundant under rocks partially buried in coarse sand, suggesting that juveniles may utilize a specific microhabitat within the adult T. funebralis habitat.

title: Feeding by a “nonfeeding” larva: uptake of dissolved amino acids from seawater by lecithotrophic larvae of the gastropod Haliotis rufescens
abstract: Larvae of the red abalone (Haliotis rufescens Swainson) are functionally incapable of capturing particulate foods. The aim of this study was to determine whether these larvae could acquire energy from seawater in the form of dissolved organic material. Trochophore and veliger larvae were shown to acquire energy by transporting dissolved organic material from seawater. Both larval stages took up all classes of amino acids tested. The influx of radiolabeled alanine represented the net substrate flux, as determined by direct chemical measurement for both trochophore and veliger larvae. Although veliger larvae have a transport system to take up taurine from seawater, a net efflux was observed for this amino acid. The release of taurine occurred independently of the presence of either taurine or other amino acids in the medium. Transported alanine was used in both anabolic and catabolic pathways. The percent of 14C-alanine in the trichloroacetic acid-insoluble fraction (macromolecules) of veliger larvae ranged from 21 to 56% of the total radioactivity in the larvae. No lipid biosynthesis was detected from 14C-labeled alanine. Veliger larvae catabolized 15 to 19% of the total alanine taken up and released it as 14CO2. The metabolic rate (oxygen consumption) and the rate of amino acid uptake were both determined for the same group of veliger larvae. The percent contribution that the uptake of amino acids, from a total concentration of 1.6 μM, made to the metabolic demand of abalone larvae ranged from 39 to 70%. Thus, these lecithotrophic larvae are not energetically independent of their environment, a result which differs from the current view of energy allocation to “nonfeeding” larvae.

title: The High-Velocity Core of the Western Boundary Undercurrent at the Base of the U.S. Continental Rise
abstract: The Western Boundary Undercurrent is a high-velocity, contour-following bottom current that flows southwesterly on the U.S. Atlantic continental margin. A high-velocity core of the Western Boundary Undercurrent is delineated by an analysis of underlying sediments, which are characterized by coarse particle sizes and efficiently aligned magnetic grains in a zone from 4440 meters at the base of the rise to 5200 meters on the adjacent abyssal plain.

title: Benthic storms: temporal variability in a deep-ocean nepheloid layer.
abstract: Time series measurements of light scattering were made for 2(1/2) months at 20 meters above the bottom in the western North Atlantic. The highest values recorded with the nephelometer exceeded all previous measurements worldwide. Rapid changes indicated a high degree of activity near the sea floor, and some increases may have been related to atmospheric storms.

title: Directly measured mid-depth circulation in the northeastern North Atlantic Ocean
abstract: The circulation of water masses in the northeastern North Atlantic Ocean has a strong influence on global climate owing to the northward transport of warm subtropical water to high latitudes. But the ocean circulation at depths below the reach of satellite observations is difficult to measure, and only recently have comprehensive, direct observations of whole ocean basins been possible. Here we present quantitative maps of the absolute velocities at two levels in the northeastern North Atlantic as obtained from acoustically tracked floats. We find that most of the mean flow transported northward by the Gulf Stream system at the thermocline level (about 600 m depth) remains within the subpolar region, and only relatively little enters the Rockall trough or the Nordic seas. Contrary to previous work, our data indicate that warm, saline water from the Mediterranean Sea reaches the high latitudes through a combination of narrow slope currents and mixing processes. At both depths under investigation, currents cross the Mid-Atlantic Ridge preferentially over deep gaps in the ridge, demonstrating that sea-floor topography can constrain even upper-ocean circulation patterns.

title: A riot of species in an environmental calm: the paradox of the species-rich deep-sea floor
abstract: Deep-sea ecosystems are the most extensive and remote ecosystems on Earth. Perception of the deep-sea benthic environment has changed dramatically in the last century from one of an azoic, or at least species-poor habitat to one that is rich in species. The early misconception was created, in part, by evidence of vast, monotonous expanses of cold, dark sediment plains with little obvious spatial or temporal heterogeneity. Given that many species-rich ecosystems on Earth are obviously heterogeneous, it is surprising that some estimates of species numbers in the deep sea (e.g. ∼10 7 macrofaunal species) rival those for tropical rainforests. Although other estimates are more conservative (e.g. 5 x 10 5 macrofaunal species), it is clear that deep-sea benthic habitats contain many species. The paradox of high deep-sea diversity has generated a number of explanatory hypotheses, including some that are currently difficult to test and others that are the focus of ongoing study. Approaches include analyses of local, regional, and global patterns, and experimental manipulations within habitats. Mechanistic generalisations are difficult to make because experimentation and sampling coverage are spatially and temporally limited, but evidence to date suggests that small-scale habitat variability and patchy disturbance, as well as global and regional variability, may play roles in maintaining deep-sea diversity. The importance of small-scale habitat variability and patchy disturbance has been demonstrated for only a small subset of species, many of which are opportunists. Broad inferences from global and regional patterns of species diversity are debatable because many areas remain poorly sampled and causes of patterns are ambiguous. Nonetheless, our understanding of diversity patterns in the deep-sea benthos has increased dramatically in the last three decades. If the approaching decades hold even a portion of the surprises seen in the recent past, then science can expect very exciting discoveries from the deep ocean in the near future.

title: Offspring in the Sea. (Book Reviews: The Bio-Physics of Marine Larval Dispersal.; Reproduction, Larval Biology, and Recruitment of the Deep-Sea Benthos.)
abstract: This study explores the most fundamental biological processes in the largest and least understood habitat on earth- the deep sea. The fifteen chapters assemble an impressive list of contributers, including prominent reproductive biologists, oceanographers, and larval ecologists, to illuminate the many levels of biological integration from the cell to the community, and covering most major phyla of marine invertebrates.

title: Some common features in the drift of pelagic larvae and juvenile stages of bottom invertebrates with marine currents in temperate regions
abstract: Using data from the Norwegian and Oyashio Currents some common features of the dispersion of pelagic larvae and juveniles of bottom invertebrates from the continental shelf are discussed. Special attention is paid to changes in the vertical and horizontal distribution of the larvae as they are carried away from the continental shelf. The chances of metamorphosis while still in the plankton and of settlement of the juveniles in the bathyal and abyssal zones are also discussed on the basis of records of post-larval ophiuroids in the 3,000–4,000 m and 2,000–4,000 m layers and of juvenile spionids in the 4,000–7,000 m and 6,000–7,000 m layers beneath the Oyashio Current.

title: The Mediterranean deep-sea fauna: pseudopopulations of Atlantic species?
abstract: The present deep-sea fauna of the Mediterranean is characterized by a very low degree of endemism and a low diversity compared to the fauna of the Northeast Atlantic. The Gibraltar sill, 280 m deep, has historically been regarded as the physical barrier to the potential colonization of the Mediterranean from the richer Atlantic fauna. We suggest instead that the larval ecology of individual species is the single most important factor governing the composition of the deep Mediterranean benthos. A number of deep-water species have planktotrophic larvae that undergo vertical ontogenetic migrations to surface layers and are thus potential colonizers of the Mediterranean via the inflowing surface current across the Gibraltar sill. The hydrological nature of the Mediterranean bottom water (high temperature, high salinity) is thought to act as a barrier to the successful establishment of species with such a dispersal capacity, firstly at the time of metamorphosis then at the time of reproduction. We also suggest that much of this Mediterranean deep-sea fauna consist of reproductively sterile pseudopopulations that are constantly derived through larval inflow from Atlantic mother populations. The same model applies, mutatus mutandis, to the deep benthos of other threshold basins, among which the Red Sea is probably the most remarkable example.

title: The inflationary effects of environmental fluctuations in source–sink systems
abstract: Ecological communities are open to the immigration of individuals and are variable through time. In open habitats immigration may permit populations of a species to persist locally even though local biotic and abiotic processes tend to exclude such “sink” populations. A general model for a sink population reveals that autocorrelated environmental variation can dramatically inflate local abundance and that such populations display a characteristic “outbreak” pattern. An experimental protist microcosm exhibits these predicted effects. Because the many ecological and environmental processes that set the rate of exclusion are typically autocorrelated, these theoretical and empirical results have broad implications for our understanding of community structure and highlight a previously unsuspected potential effect of anthropogenic climate change.

title: Long-term discrepancy between food supply and demand in the deep eastern north pacific
abstract: A 7-year study of food supply [sinking particulate organic carbon (POC)] and food demand [sediment community oxygen consumption (SCOC)] in the abyssal eastern North Pacific revealed a long-term deficit in food supply. The POC:SCOC ratio decreased by 52 to 59 percent between 1989 and 1996. A possible explanation for this trend is the documented sea surface temperature increase and concomitant plankton biomass decrease in the eastern North Pacific, resulting in an apparent reduction in POC export from surface waters to the deep ocean. Continuation of this trend could profoundly impact geochemical cycling as well as the structure and dynamics of deep-sea communities.

title: Polychaete species diversity in the central Pacific abyss: Local and regional patterns and relationships with productivity
abstract: We investigated the relationship between productivity and local species diversity, and the degree of species turnover, at 8 sites on the central equatorial Pacific abyssal plain. The 8 sites span
a 4-fold difference in seafloor particulate organic carbon (POC) flux and, hence, community productivity. The sites are similar in water depth (4300 to 5100 m), degree of isolation from terrigenous influences, and hydrodynamic regime. Three sites lie under the influence of equatorial upwelling, and are subject to enhanced deep POC flux derived from high overlying primary production. The remaining sites lie beneath the oligotrophic north Pacific gyre. The number of polychaete species collected at a
single site ranged from 14 to 113, with at least 90% apparently being new to science. We found no evidence for the purported unimodal relationship between productivity and diversity seen in other ecosystems, including deep-sea slopes, and found only weak evidence of a monotonic increase in diversity with productivity. Rates of species turnover were low over scales of ~200 to 3000 km for the dominant polychaete species in the communities, and all sites were dominated by a core group of
biogeographically widespread, locally abundant species. In contrast, there was little between-site similarity in the long list of rare species found at each site, implying either a high turnover of rare species at 200 to 3000 km scales, or incomplete sampling of the rare species list at each site. More
intensive sampling studies using both morphological and molecular methods are needed to resolve the distribution patterns of rare species in the Pacific abyss. Local polychaete species diversity beneath equatorial Pacific upwelling (measured by rarefaction) appears to be unusually high for the deep sea, exceeding by at least 10 to 20% that measured in abyssal sites in the Atlantic and Pacific, and on the continental slopes of the North Atlantic, North Pacific, and Indian Oceans.

title: Reproductive biology of the abyssal holothurian Amperima rosea: an opportunistic response to variable flux of surface derived organic matter?
abstract: A radical change in the abundance of invertebrate megafauna has occurred over a vast area of the Porcupine Abyssal Plain, north-east Atlantic, in recent years. In particular, the holothurian Amperima rosea has increased in abundance by three orders of magnitude. The sudden increase in abundance of A. rosea appears to be the result of environmental forcing rather than localized stochastic population variations. Amperima rosea produces small eggs ([les ]200 μm) indicating (1) planktotrophic larval development or (2) lecithotrophic larval development with an abbreviated larval stage. It also reaches maturity at a very small size. The reproductive biology of A. rosea indicates that it can increase rapidly in population size and can colonize large areas quickly. It has high fecundity, as predicted by gonad indices and observed in histological studies. Fecundity was greater in winter than in summer months, but there was no clear evidence of seasonal or episodic reproductive events. All males, irrespective of sample season or year, were mature with spermatozoa. Most oocytes were of an intermediate size (70 to 120 μm), either at the late previtellogenic (70–90 μm) or early vitellogenic (100–120 μm) stage of development. It is postulated that development of full vitellogenesis, leading to episodic spawning, might be dependent on certain environmental stimuli. The most likely stimulus is food supply. Amperima rosea has been shown to feed preferentially on phytodetritus and to have a requirement for certain sterols in its diet. Qualitative changes in the flux of organic matter to the sea-floor may control vitellogenesis and fecundity, and hence have an effect on the population dynamics of the species.

title: Deep-Sea species diversity : does it have a characteristic scale?
abstract: Dispersion patterns and species diversities of deep-sea macrobenthos were examined for evi- dence that diversity-controlling processes operate predominantly on any one of several spatial scales . Identification of such scales, if any, would aid in the identification of the diversity-regu- lating processes themselves . The specific hypothesis that species diversity is independent of scale and location within the deep sea was tested with replicated, partitioned box cores taken at one station in the Santa Catalina Basin (1130 m) and one station in the San Diego Trough (1230 m) of the Southern California continental borderland . Attention was focused on within- community scales . Bathyal rather than abyssal sampling areas were selected to provide ade- quate animal densities for quantitative treatment . The hypothesis was discredited for some taxa at all sampling scales

title: Deep-Sea Species Richness: Regional and Local Diversity Estimates from Quantitative Bottom Samples
abstract: The deep-sea communities of the continental slope and rise off the eastern coast of the United States have a remarkably high diversity-measured regionally or locally either as species richness or as the evenness of relative abundance among species. In a 1,500-2,500-m depth range off New Jersey and Delaware, 233 30 x 30-cm samples contained 798 species in 171 families and 14 phyla. Addition of stations from sites to the north and south approximately doubled the number of samples and doubled the number of species to 1,597. Species-area curves do not level off within stations or when stations are added together. Moreover, the proportion of species represented by single individuals is high at all scales of sampling, which indicates that much more sampling is needed to adequately represent the species richness either locally or regionally. Diversity changes very little through time at a single site or with distance along a 180-km transect at a depth of 2,100 m. Diversity is maintained by a combination of biogenic microhabitat heterogeneity in a system with few barriers to dispersal, disturbance created by feeding activities of larger animals, and food resources divided into patches of a few square meters to square centimeters initiated by specific temporally separated events.

title: The Desmosomatidae (Isopoda, Asellota) of the Gay Head-Bermuda Transect
abstract: Author(s): Hessler, Robert R |  This study treats asellote isopods of the family Desmosomatidae from benthic samples takengon, or in the vicinity of, a transect between Woods Hole, Massachusetts, and the islands of Bermuda. Samples came from a depth range of 20-5,100 meters and represent the continental shelf, slope, and rise, the abyssal plain beneath the Sargasso Sea, and the Bermuda Rise.Thirty-nine species have been found in this region, and all but ten of them are new. Nearly the full range of morphologies known to exist in the family is represented. Study of this material revealed the necessity for a complete revision of the family. Fifteen genera belonging to two subfamilies are recognized. This classification reflects a basic evolutionary pattern within the family. The Desmosomatinae n. subf. includes the most primitive genera and those genera in which the first pereopod has become reduced and attenuated. The Eugerdellatinae n. subf. contains genera whose first pereopod has become robust and raptorial or chelate.The large number of individuals in single species from some of the samples has allowed close examination of development, sexual dimorphism, and individual variation. Eugerda tetarta n. sp. is described in detail from these points of view. In the female there are three manca and three juvenile stages. These are followed by two or three cycles of reproductive maturity in which each brooding stage is preceded by a preparatory one. Molting into the preparatory stage is accompanied by growth, but none is exhibited as the animal achieves brooding condition.Males are first distinguishable at the third manca stage. This stage is followed by two juvenile and one preparatory stage in which the male looks like the female except for the usual differences in the second antenna and pleopods I and II. Profound metamorphosis accompanies the ecdysis into the copulatory stage, and as a result the mature male bears little resemblance to the female. The adaptive meaning of these changes is discussed.Analysis of several specimens from one station reveals little individual variation. Differences in the shape of body and limb segments are usually too subtle to be detected in visual inspection. Variation in setal count is slight and tends to be allometric.Our knowledge of geographic distribution must be regarded as fragmentary because adequate samples are lacking from most oceans. Present information indicates that nearly all the genera are cosmopolitan or will prove to be so. Species, on the other hand, have limited distributions. Only seven have been found in more than one ocean or sea. The limited distribution of species is undoubtedly related to the absence of dispersive stages in development.The Desmosomatidae is primarily a cold-water group. Typically its members occur in shallow water only at high latitudes. In those rare cases where species have been found in shallow warm water, the annual range of temperature change is slight. On the transect, the distribution of desmosomatids in water shoaler than 200 meters is limited to the outer portion of the shelf, thus avoiding the main effects of the pronounced temperature fluctuations along this part of the coast. All species show some degree of zonation with depth. Only two display depth ranges extending beyond 2,500 meters. A few species exhibit polar emergence.All species found on the transect are fully described and illustrated. The descriptions of previously known species are based on transect material, not on the original types.

title: The Deep-Sea Benthic Fish Fauna of the Northeastern Pacific Ocean on Cascadia and Tufts Abyssal Plains and Adjoining Continental Slopes
abstract: AbstractThe faunal composition of deep-sea fishes is described from 579 beam and otter trawl collections from the continental slope and abyssal plains off Oregon, at depths from 400 to 5,180 m. Ninety of the 104 species known to inhabit depths greater than 400 m in this area of the northeastern Pacific were captured. Sixty-one species were found on the continental slope, 28 on Cascadia Abyssal Plain, nine on eastern Tufts Abyssal Plain, and two on western Tufts Plain.Families represented by the most species were Scorpaenidae, Liparididae, and Zoarcidae on the slope, and Liparididae, Zoarcidae, and Macrouridae on the abyssal plains. Scorpaenids were numerically dominant on the upper slope, and macrourids on the mid-lower slope and abyssal plains. The macrourid Coryphaenoides armatus was caught at all abyssal stations and was consistently the most abundant fish at depths greater than 2,500 m. Coryphaenoides filifer and C. leptolepis were consistently the second and third most numerous species on Cascadia.Th...

title: Summary atlas of deep-living demersal fishes in the North Atlantic Basin
abstract: Data from 9 major trawling surveys made around the North Atlantic Basin at slope, rise and abyssal depths provide directly comparable information on deep demersal fish distribution and faunal composition. 338 species in 57 families and 189 genera are recorded, and semi-diagrammatic maps showing relative abundance both horizontally and vertically for 67 of these are presented. Alepocephalidae, Gadidae, Macrouridae, Moridae and Ophidiidae comprise nearly 2/3 of all species taken. 296 species were taken at slope depths (200–2250 m), 99 at rise depths (2250–4500 m), and 32 at abyssal depths (>4500 m). Diversity and the rate of endemism were highest on the slope. The Norwegian, Caribbean and Mediterranean Basins appear to harbor impoverished versions of the adjacent Atlantic Basin fauna. Each species distribution appeared to be rather exceptional and no repeatable pattern could be found amongst the maps.

title: Depth-related distribution and abundance of seastars (Echinodermata: Asteroidea) in the Porcupine Seabight and Porcupine Abyssal Plain, N.E. Atlantic
abstract: The depth-related distribution of seastar (Echinodermata: Asteroidea) species between 150 and 4950 m in the Porcupine Seabight and Porcupine Abyssal Plain is described. 47 species of asteroid were identified from ~14,000 individuals collected. The bathymetric range of each species is recorded. What are considered quantitative data, from an acoustically monitored epibenthic sledge and supplementary data from otter trawls, are used to display the relative abundance of individuals within their bathymetric range. Asteroid species are found to have very narrow centres of distribution in which they are abundant, despite much wider total adult depth ranges. Centres of distribution may be skewed. This might result from competition for resources or be related to the occurrence of favourable habitats at particular depths. The bathymetric distributions of the juveniles of some species extend outside the adult depth ranges. There is a distinct pattern of zonation with two major regions of faunal change and six distinct zones. An upper slope zone ranges from 150 to ~700 m depth, an upper bathyal zone between 700 and 1100 m, a mid-bathyal zone from 1100 to1700 m and a lower bathyal zone between 1700 and 2500 m. Below 2500 m the lower continental slope and continental rise have a characteristic asteroid fauna. The abyssal zone starts at about 2800 m. Regions of major faunal change are identified at the boundaries of both upper and mid-bathyal zones and at the transition of bathyal to abyssal fauna. Diversity is greatest at ~1800 m, decreasing with depth to ~2600 m before increasing again to high levels at ~4700 m.

title: DNA sequencing of formalin-fixed crustaceans from archival research collections.
abstract: Marine invertebrate collections have historically been maintained in ethanol following fixation in formalin. These collections may represent rare or extinct species or populations, provide detailed time-series samples, or come from presently inaccessible or difficult-to-sample localities. We tested the viability of obtaining DNA sequence data from formalin-fixed, ethanol-preserved (FFEP) deep-sea crustaceans, and found that nucleotide sequences for mitochondrial 16S rRNA and COI genes can be recovered from FFEP collections of varying age, and that these sequences are unmodified compared with those derived from frozen specimens. These results were repeatable among multiple specimens and collections for several species. Our results indicate that in the absence of fresh or frozen tissues, archived FFEP specimens may prove a useful source of material for analysis of gene sequence data by polymerase chain reaction (PCR) and direct sequencing.

title: Bathymetric patterns of genetic variation in a deep-sea protobranch bivalve, Deminucula atacellana
abstract: The origin of the deep-sea benthic fauna is poorly understood and represents an enormous gap in our understanding of basic evolutionary phenomena. One obstacle to studying evolutionary patterns in the deep sea has been the technical difficulty of measuring genetic variation in species that are typically minute, rare, and must be recovered from extreme depths. We used molecular genetic techniques to quantify variation in the 16S rRNA mitochondrial gene within and among populations of the common protobranch bivalve Deminucula atacellana (Schenck, 1939). We analyzed 89 individuals from nine samples collected in the 1960s along a depth gradient from 1100 to 3800 m in the western North Atlantic. Genetic variability within populations is much lower than between populations, and peak haplotype numbers occur near the center of its depth distribution. Continental slope (<2500 m) and rise (>2500 m) populations were genetically distinct despite the lack of any obvious topographic or oceanographic features that would impede gene flow. These findings indicate that the deep-sea macrofauna can have strong population structure over small (134 km) spatial scales, similar to that observed in shallow-water and terrestrial organisms. This surprisingly high biodiversity at the genetic level affords the potential for adaptation and evolutionary diversification, the ultimate historical causes of high species diversity in the deep-sea benthos.

title: A genetic dimension to deep-sea biodiversity
abstract: Our knowledge of deep-sea biodiversity is based almost entirely on morphological distinctions at the species level. Here, we use haplotype variations in the mitochondrial 16S ribosomal gene to assess biodiversity at the genome level in four deep-sea molluscan morphospecies. Genetic divergence levels among populations of the morphospecies fall within the range of interspecific divergence found in coastal marine and aquatic molluscan genera. Results indicate a rich population structure at the genetic level in deep-sea mollusks, and suggest the possibility that some seemingly coherent morphospecies are composed of cryptic species.

title: Nested clade analyses of phylogeographic data: testing hypotheses about gene flow and population history
abstract: Since the 1920s, population geneticists have had measures that describe how genetic variation is distributed spatially within a species’ geographical range. Modern genetic survey techniques frequently yield information on the evolutionary relationships among the alleles or haplotypes as well as information on allele frequencies and their spatial distributions. This evolutionary information is often expressed in the form of an estimated haplotype or allele tree. Traditional statistics of population structure, such as F statistics, do not make use of evolutionary genealogical information, so it is necessary to develop new statistical estimators and tests that explicitly incorporate information from the haplotype tree. One such technique is to use the haplotype tree to define a nested series of branches (clades), thereby allowing an evolutionary nested analysis of the spatial distribution of genetic variation. Such a nested analysis can be performed regarding the geographical sampling locations either as categorical or continuous variables (i.e. some measure of spatial distance). It is shown that such nested phylogeographical analyses have more power to detect geographical associations than traditional, nonhistorical analyses and, as a consequence, allow a broader range of gene‐flow parameters to be estimated in a precise fashion. More importantly, such nested analyses can discriminate between phylogeographical associations due to recurrent but restricted gene flow vs. historical events operating at the population level (e.g. past fragmentation, colonization, or range expansion events). Restricted gene flow and historical events can be intertwined, and the cladistic analyses can reconstruct their temporal juxtapositions, thereby yielding great insight into both the evolutionary history and population structure of the species. Examples are given that illustrate these properties, concentrating on the detection of range expansion events.

title: Maximum likelihood estimation of a migration matrix and effective population sizes in n subpopulations by using a coalescent approach
abstract: A maximum likelihood estimator based on the coalescent for unequal migration rates and different subpopulation sizes is developed. The method uses a Markov chain Monte Carlo approach to investigate possible genealogies with branch lengths and with migration events. Properties of the new method are shown by using simulated data from a four-population n-island model and a source–sink population model. Our estimation method as coded in migrate is tested against genetree; both programs deliver a very similar likelihood surface. The algorithm converges to the estimates fairly quickly, even when the Markov chain is started from unfavorable parameters. The method was used to estimate gene flow in the Nile valley by using mtDNA data from three human populations.

title: A COMPARATIVE STUDY OF ASYMMETRIC MIGRATION EVENTS ACROSS A MARINE BIOGEOGRAPHIC BOUNDARY
abstract: In many nonclonal, benthic marine species, geographic distribution is mediated by the dispersal of their larvae. The dispersal and recruitment of marine larvae may be limited by temperature gradients that can affect mortality or by ocean currents that can directly affect the movements of pelagic larvae. We focus on Point Conception, a well‐known biogeographic boundary between the Californian and Oregonian biogeographic provinces, to investigate whether ocean currents affect patterns of gene flow in intertidal marine invertebrates. The predominance of pelagically dispersing species with northern range limits at Point Conception suggests that ocean currents can affect species distributions by erecting barriers to the dispersal of planktonic larvae. In this paper, we investigate whether the predominantly southward currents have left a recognizable genetic signature in species with pelagically dispersing larvae whose ranges span Point Conception. We use patterns of genetic diversity and a new method for inferring cladistic migration events to test the hypothesis that southward currents increase southward gene flow for species with pelagically dispersing larvae. We collected mitochondrial DNA (mtDNA) sequence data for the barnacles Balanus glandula and Chthamalus fissus and also reanalyzed a previously published mtDNA dataset (Strongylocentrotus purpuratus, Edmands et al. 1996). For all three species, our cladistic approach identified an excess of southward migration events across Point Conception. In data from a fourth species with nondispersing larvae (Nucella emarginata, Marko 1998), our method suggests that ocean currents have not played a role in generating genetic structure.

title: The deep-sea floor ecosystem: current status and prospects of anthropogenic change by the year 2025
abstract: The goal of this paper is to review current impacts of human activities on the deep-sea floor ecosystem, and to predict anthropogenic changes to this ecosystem by the year 2025. The deep-sea floor ecosystem is one of the largest on the planet, covering roughly 60% of the Earth's solid surface. Despite this vast size, our knowledge of the deep sea is poor relative to other marine ecosystems, and future human threats are difficult to predict. Low productivity, low physical energy, low biological rates, and the vastness of the soft-sediment deep sea create an unusual suite of conservation challenges relative to shallow water. The numerous, but widely spaced, island habitats of the deep ocean (for example seamounts, hydrothermal vents and submarine canyons) differ from typical deep-sea soft sediments in substrate type (hard) and levels of productivity (often high); these habitats will respond differently to anthropogenic impacts and climate change. The principal human threats to the deep sea are the disposal of wastes (structures, radioactive wastes, munitions and carbon dioxide), deep-sea fishing, oil and gas extraction, marine mineral extraction, and climate change. Current international regulations prohibit deep-sea dumping of structures, radioactive waste and munitions. Future disposal activities that could be significant by 2025 include deep-sea carbon-dioxide sequestration, sewage-sludge emplacement and dredge-spoil disposal. As fish stocks dwindle in the upper ocean, deep-sea fisheries are increasingly targeted. Most (perhaps all) of these deep-sea fisheries are not sustainable in the long term given current management practices; deep-sea fish are long-lived, slow growing and very slow to recruit in the face of sustained fishing pressure. Oil and gas exploitation has begun, and will continue, in deep water, creating significant localized impacts resulting mainly from accumulation of contaminated drill cuttings. Marine mineral extraction, in particular manganese nodule mining, represents one of the most significant conservation challenges in the deep sea. The vast spatial scales planned for nodule mining dwarf other potential direct human impacts. Nodule-mining disturbance will likely affect tens to hundreds of thousands of square kilometres with ecosystem recovery requiring many decades to millions of years (for nodule regrowth). Limited knowledge of the taxonomy, species structure, biogeography and basic natural history of deep-sea animals prevents accurate assessment of the risk of species extinctions from large-scale mining. While there are close linkages between benthic, pelagic and climatic processes, it is difficult to predict the impact of climate change on deep-sea benthic ecosystems; it is certain, however, that changes in primary production in surface waters will alter the standing stocks in the food-limited, deep-sea benthic. Long time-series studies from the abyssal North Pacific and North Atlantic suggest that even seemingly stable deep-sea ecosystems may exhibit change in key ecological parameters on decadal time scales. The causes of these decadal changes remain enigmatic. Compared to the rest of the planet, the bulk of the deep sea will probably remain relatively unimpacted by human activities and climate change in the year 2025. However, increased pressure on terrestrial resources will certainly lead to an expansion of direct human activities in the deep sea, and to direct and indirect environmental impacts. Because so little is known about this remote environment, the deep-sea ecosystem may well be substantially modified before its natural state is fully understood.