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Hadal trenches: the ecology of thedeepest places on EarthAlan J. Jamieson, Toyonobu Fujii, Daniel J. Mayor, Martin Solan andImants G. Priede

Oceanlab, University of Aberdeen, Newburgh, AB41 6AA, UK

Review

Glossary

Adiabatic: a process in which, when a fluid is compressed, its pressure

increases and its temperature rises without the gain or loss of any heat.

Allochthonous: an external source [of food].

Autochthonous: an internal source [of food].

Benthic: organisms living on or in the seabed.

Biogeographical province: a biological subdivision of the surface of the Earth

incorporating both faunal and floral characteristics.

Biozone: biological depth zones: littoral (0–1000 m), bathyal (1000–3000 m),

abyssal (3000–6000 m), hadal (6000–11 000 m).

Carrion (food) falls: the deposit of dead or decaying flesh on the seafloor (e.g.

fish, jellyfish or cetacean carcasses).

Deposit feeding: a feeding strategy whereby organisms acquire food by

ingesting large volumes of sediment and extract nutrients from the small

organic fraction of the ingested sediment.

Eurybathy: the ability to occupy a wide range of depths.

Eurythermic: the tolerating of a wide temperature range.

Heterotrophic: requiring complex organic compounds of nitrogen and carbon

for metabolic synthesis.

Meiofauna: organisms passing through a 250–500-mm-sieve and retained on a

41–63-mm-sieve.

Necrophagy: feeding on carcasses.

Ocean acidification: the ongoing decrease in the pH of the Earth’s oceans,

caused by uptake of carbon dioxide from the atmosphere.

Ossified: hardened.

Particulate Organic Matter (POM): particles of organic solids >0.4 mm

suspended within the water column.

Phytopigments: a pigment that undergoes a physical or chemical change upon

exposure to light.

Primary production: the production of organic compounds from atmospheric

Hadal trenches account for the deepest 45% of theoceanic depth range and host active and diversebiological communities. Advances in our understandingof hadal community structure and function have, untilrecently, relied on technologies that were unable todocument ecological information. Renewed inter-national interest in exploring the deepest marineenvironment on Earth provides impetus to re-evaluatehadal community ecology. We review the abiotic andbiotic characteristics of trenches and offer a contempor-ary perspective of trench ecology. The application ofexisting, rather than the generation of novel, ecologicaltheory offers the best prospect of understanding deepocean ecology.

The hadal environmentThe deepest areas of the ocean, commonly referred to as thehadal zone (6000–11 000 m [1]), represent �1–2% of theglobal benthic area (see Glossary), but they constitute thedeepest 45% of the vertical depth gradient. They are almostexclusively comprised of trenches representing spatiallydisjunct environments separated by shallower areas(Figure 1, Box 1). Hadal trenches remain one of the leastunderstood habitats on Earth.

Marine biozones that are based on observed faunaltransitions with depth [2] have been used as a convenientmeans to divide the ocean into a series of realms. Indeed,species composition, density, biomass and diversity ofhadal zones often contrast to that of the surroundingabyssal area. This nomenclature ignores that depth iscontinuous and that hadal trenches are intrinsically linkedto shallower ecosystems. Topography, geographical iso-lation and spatio-temporal variation in food supply, aswell as elevated hydrostatic pressure and low temperatureare all factors that might have encouraged speciation and,thus, shaped present faunal assemblage structure.

The first major trench sampling campaigns were con-ducted during the early 1950s on the Danish Galathea andRussian Vitjaz global expeditions. Using trawl and grabmethods, the diversity, abundance and biomass of benthicepifaunal and infaunal invertebrates were described. Ofthe 300 metazoan species documented in this relativelysparse data set [1], 58%were thought to be endemic, a levelcomparable to neighbouring abyssal environments. Sinceall subsequent hadal reviews [1,3,4] have primarily beenbased on these two data sets, the current perception of

Corresponding author: Jamieson, A.J. (a.jamieson@abdn.ac.uk).

0169-5347/$ – see front matter � 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.tree.2009.

hadal trench ecosystems lacks an up-to-date ecologicalinterpretation. Research efforts have continued over thelast 30 years although they have been sporadic anduncoordinated. Recent advances in technological capacity[e.g. 5,6] provide impetus for a renewed wave of hadalexploration. Here, as a first step towards synthesisingand integrating available knowledge, we provide a con-temporary perspective on hadal trench environmentsand argue that the separation of environments by depthzonation alone is likely to hamper our understanding ofdeep ocean ecology.

Hydrographic and physical characteristics of hadaltrenchesIt is now known that the bottom water of the hadaltrenches is not stagnant and that deep currents flowthrough and ventilate the trenches [7]. For example, thedeep water flowing through the West Pacific Trenchesoriginates from the Southern Ocean. There are two majorwater masses present in the deep Pacific (>1000 m); theLower Circum-Polar Water (LCPW) and the North PacificDeepWater (NPDW) [8]. The LCPWenters the Pacific from

or aquatic carbon dioxide, principally through the process of photosynthesis.

Stenobathy: confined or restricted to a small depth range.

09.009 Available online xxxxxx 1

Figure 1. Major hadal trenches of the World. (a) Sunda (Java) Trench, 7450 m, (b)

Philippine Trench, 10 540 m, (c) Marianas Trench, 10 989 m, (d) Izu-Bonin (Izu-

Ogasawara) Trench, 9810 m, (e) Japan Trench, 8412 m, (f) Kurile-Kamchatka

Trench, 10 542 m, (g) Aleutian Trench, 7820 m, (h) Tonga Trench, 10 800 m, (i)

Kermadec Trench, 10 047 m, (j) Middle America Trench, 6662 m, (k) Cayman

Trench, 7093 m, (l) Puerto Rico Trench, 8385 m, (m) Peru–Chile Trench, 8055 m,

and (n) South Sandwich Trench, 8428 m.

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the south and flows northward [9,10], in a clockwise direc-tion, passing through the trenches on thewest of the Pacific(i.e. the Kermadec Trench and the Tonga Trench [7]).Through the Samoan Passage, it flows northwest acrossthe equator to the east Mariana Basin and into a north andwestward flow. The westward branch flows through thewest Mariana Basin [8], and through the Izu-Ogasawara,Japan and Kuril-Kamchatka Trenches before headingsouthwards around the Emperor Seamounts towardsthe Aleutian Trench. As for the NPDW, trench currentsthen flow westwards back around the Aleutian and

Box 1. The hadal zone: origins and characteristics

Early literature refers to the hadal zone as ‘ultra-abyssal’ [11]. The

term hadal is derived from the ancient Greek ‘Hades’, in reference to

the ancient Greek underworld, and the abode of Hades. It was

coined in the 1950s to avoid confusion between abyssal, lower-

abyssal and ultra-abyssal, and in accordance with the terms littoral,

bathyal and abyssal [88].

The hadal zone consists of deep trenches that can plunge from

6000 m to as deep as �11 000 m where hydrostatic pressures reach

1000 bar. Trenches are formed as the tectonic plates of the Earth’s

crust move away from mid-oceanic ridges, causing neighbouring

plates to collide [89]. During this collision, the heavier oceanic plates

are forced down towards the mantle, whereas the lighter continental

plates rise upwards, resulting in narrow plate boundary zones, or

subduction zones, resulting in the formation of a trench [89]. As

newly formed magma rises from the mid-oceanic ridges and ages

with distance, the deep trenches represent the oldest seafloor [11].

The modern trenches were formed during the Cenozoic period when

the continents moved into their current positions and might have

existed for as long as 107 years [1]. Trenches are typically long and

narrow (few are more than 2000 km long) and run parallel to, and

near, extensive island-arc systems or continental landmasses.

Physically, trenches are typified by a V-shape cross-section with

an average steepness of 5–158, reaching on occasion 458. Most

trench floors have narrow, flat, sedimentary bottoms, typically 2–

5 km wide. Similar to the mid-oceanic ridges, the trenches are

seismically active, resulting in frequent earthquakes and volcanic

eruptions, resulting in occasional gravity-driven sediment slides

[67,68].

Of the 37 known hadal trenches and troughs, five are in the

Atlantic Ocean, four in the Indian Ocean and 28 (�75%) are around

the Pacific Ocean rim, where the nine deepest trenches in the world

are found in the western region. Although nearly 75% of the ocean

floor is between 2000 and 6000 m deep, only 4.5 � 104 km2 of the

seafloor reaches depths >6000 m, accounting for just 1%.

2

Kuril-Kamchatka Trenches and southwards throughthe Japan and Izu-Ogasawara Trenches. This flow ofwater brings sufficient dissolved oxygen (mean con-centration = 3.43 mL L�1) to support aerobic organisms[11].

Temperature is often a major environmental driver forspecies distribution, varying vertically in the water columnand with latitude [12]. Small temperature changes canmean the success or failure of species over time [12] or caninhibit vertical or horizontal migration [13]. The tempera-ture range beyond 6000 m is typically 1.0–2.5 8C. The insitu bottom water of the Western Pacific Trenches warmsby �0.5 8C as the water masses flow from the Southern toNorthern Hemisphere (Box 2). Just as increases in bottomtemperature from south to north might affect the variationin community structure between trenches, changes intemperature with depth are trench specific. Temperaturegenerally decreases with increasing depth, but this trendreverses below about 4000 m due to adiabatic heating withincreasing pressure (Box 2). South Pacific Trench tempera-ture increases from 1.16 to 1.91 8C (�40%) between 6000and 10 000 m. In the North Pacific Trenches, over the samedepth range, temperature rises from 1.67 to 2.40 8C(�30%). These in situ temperatures are thus comparableto those of the continental margins (�3000 m). The salinityof water within the trenches (salinity = 34–35 ppt) remainssimilar to typical abyssal plain values and is unaffected bypressure [11]. Bottom currents in the Marianas Trench, atdepths between 6000 and 10 890 m, are <1.5 cm s�1 for22.9–63.8% of the time [14]. However, at 10 890 m, thedeepest point on Earth, current velocities reach a maxi-mum of 8.1 cm s�1. These currents exhibit tidal cycles withsemi-lunar and lunar periodicity, comparable to thoseobserved on abyssal plains [15]. Thus, with the exceptionof hydrostatic pressure, the physical characteristics of thetrenches are not exceptional and reflect those found atshallower depths.

Life under high pressureThere is a general decrease in the abundance and biomassof organisms with increasing depth [16,17]. Nonetheless,sampling campaigns in hadal trenches have revealed adiverse array of metazoan organisms [1,11] consistingprimarily of benthic fauna, such as fish, holothurians,polychaetes, bivalves, isopods, actinians, amphipods andgastropods (Figure 2). The richness of trench communities,thought to originate from the abyssal plains [3,11,18], alsodeclines with depth [1], although the relative role ofincreased pressure versus other environmental correlatesremains unresolved. Nevertheless, adaptations to highhydrostatic pressures and low temperatures are common[19–21]. Conspicuous examples include the use of intra-cellular protein-stabilising osmolytes, such as trimethyla-mine N-oxide (TMAO) [22], which act to maintain enzymefunction by increasing cell volume to counteract the effectsof pressure, and the increased use of unsaturated fattyacids in cell membrane phospholipids to maintain theirfluidity and, hence, cellular function [19]. Linear relation-ships between such adaptations and the depth of capture inmarine fish, from shallow to >4500 m, have been inter-preted as causal evidence for pressure adaptations [22,23].

Box 2. In situ versus potential temperature

Temperature per se does not ultimately define species zonation but is

certainly one of the pivotal abiotic factors [12,90]. Small temperature

changes can influence the vertical or horizontal distribution of species

[13]. Unlike pressure, temperature is not linear with depth and can

vary between trenches at equivalent depths. The in situ bottom water

temperature within a trench warms with increasing hydrostatic

pressure (i.e. depth), since a compressibility effect occurs whereby

water molecules under increasing pressure warm in an adiabatic

process without exchanging (gaining) heat from their environment.

Oceanographers generally remove this pressure-influenced tempera-

ture increase (which has no dynamical effect) by conversion to

potential temperature, therefore enabling comparison of water

masses [91]. Potential temperature, derived from the laws of thermo-

dynamics, is the temperature that a water parcel would have if it were

brought from its in situ depth to the sea surface without exchanging

heat or salt with its surroundings. These comparisons show a rise in

potential temperature of approximately 0.5 8C between the South

Pacific (Tonga and Kermadec Trenches) to the North Pacific (Marianas

and Japan Trenches) of 0.6–1 8C (Figure I), an increase of >30%.

Organisms inhabiting these depths only experience the in situ

temperature. Within the trench, the in situ bottom water temperature

rises by �1 8C between 6000 and 11 000 m. Similarly, by examining

surface to full ocean depth temperature, natural adiabatic heating can

be detected in the water column [92]. A steady decrease in

temperature occurs from the surface to �4000 m where upon it

begins to rise (Figure II). The in situ temperature at 10 000 m for

example, is therefore equivalent to that of shallower depths

(�3000 m).

Figure I. In situ bottom temperature (closed symbols) and potential

temperature (open symbols) for pooled data from the Southern (blue) and

Northern (red) Pacific Trenches. The in situ bottom temperature increases with

depth by �1 8C. Although the trend remains the same, a temperature rise of

�0.5 8C occurs between the southern and northern trenches. By conversion to

potential temperature (eliminating the effects of pressure), this south to north

rise is readily detected.

Figure II. Water column temperature profiles from surface to seafloor and a

magnified inset of the deep-water temperature (inset), in this example for the

Tonga Trench, SW Pacific Ocean. The adiabatic temperature rise can be seen

beyond 4000 m resulting in the temperature at 10 000 m equalling that of

3000 m (indicated by arrow).

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Nonetheless, distinguishing between the contributingeffects of temperature and hydrostatic pressure is complexbecause these variables are inversely related Figure 3.

Mobile pelagic fauna, such as decapod shrimps and fish,show a well-defined reduction in metabolic rate withincreasing depth, irrespective of temperature [24]. How-ever, the possibility of hydrostatic pressure as a key controlon the physiological characteristics of pelagic deep-seaanimals has been rejected because there is no consistentrelationship between pressure and metabolic rate acrosstaxa [25,26]. This suggests that pressure effects do notnecessarily influence energy generation for locomotoryactivities, and thus do not inhibit the colonisation oftrenches by active animals.

The distribution of many deep-sea fauna are, never-theless, constrained within species-specific defined depthlimits [18,27]. As this range might be influenced by

ontogenic stage, pressure might significantly influencelarval colonisation potential [27–29]. The relative changein pressure experienced by shallow-water fauna, however,exceeds that experienced by deep sea species; an organismdescending from the surface to 10 m undergoes a 10-foldchange in pressure (1–10 bar), whilst a descent from 6000to 11 000 mwill experience less than a doubling in pressure(600–1100 bar). Thus, trenches are accessible to someeurybathic abyssal fauna, including grenadier fishes(Macrouridae) and natantian prawns (Benthesicymidae)[30,31], although these are largely confined within 6000–

7000 m. Conversely, many species that inhabit the flattopography of the abyssal plains never experience substan-tial variations in pressure (i.e. extreme stenobathy) and,therefore, might be incapable of utilising adjacent steeptrench habitat. As evolutionary processes operate overgeological time-scales, however, stenobathic fauna may

3

Figure 2. Examples of trench fauna. Recent observations and collections of animals from the deepest parts of the ocean. All these images were taken either by baited

camera or collected by baited traps. (a) Aggregation of endemic snailfish (Liparidae) Pseudoliparis amblystomopsis on the trench floor at 7703 m in the Japan Trench and

(b) a specimen caught simultaneously in a baited trap. (c) A natantian decapod Benthescymnus crenatus filmed feeding on small crustaceans at 6100 m in the Kermadec

Trench. (d) Soft-shelled gastropods (unidentified) from 7703 m in the Japan Trench. (e) Thousands of endemic amphipods (Hirondellea dubia) feeding at bait at 10 000 m in

the Tonga Trench. (f) Two large scavenging amphipods (unidentified) from 7703 m in the Japan Trench. (g) Thousands of amphipods being emptied from a baited trap after

just 8 h on the seafloor at 9316 m in the Izu-Ogasawara Trench. (h) Large unidentified amphipods from 7703 m in the Japan Trench. (i) Small scavenging amphipods from

8100 m in the Izu-Ogasawara Trench. (j) Unidentified Cumacea from 7100 m in the Japan Trench. Scales bars are 100 mm (thick line), 20 mm (medium line) and 5 mm (thin

line). All images reproduced with permission from the HADEEP project, Universities of Aberdeen (UK) and Tokyo (Japan).

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have sufficient time to adapt to higher pressures as thebottom descends, as has been speculated for snailfish(Liparidae) [30]. In turn, this might explain why highlevels of intra-trench endemism at species level is foundalongside inter-trench similarities at higher taxonomiclevels within the same zoogeographic province, despitecommon, shallower water ancestry [11,18].

The ‘carbonate compensation depth’ (CCD), the depth atwhich calcium carbonate (calcite and aragonite) supplyequals the rate of solvation, has also been proposed as aphysiological barrier to deep ocean colonisation [4].Calcium carbonate is widely used as a structural com-ponent by foraminiferans, corals, crustaceans and mol-luscs. The CCD range is 4000–5000 m in the PacificOcean, but tends to occur at shallower depths towardshigher latitudes [32]. As carbonate solubility increaseswith increasing hydrostatic pressure, ossification becomes

4

more difficult [33], explaining why ossified groups (e.g.ophiuroids and echinoids) tend to be replaced by soft-bodied organisms (e.g. holothurians, and soft and organicwalled foraminifera) with increasing depth [34–36]. Ofcontemporary importance, the physiological adaptationsthat have permitted deep ocean colonisation to take placebeyond the CCDmay yield important insights with respectto the effects of ocean acidification on calcifiers in thefuture upper ocean [37].

Food supply in trench environmentsChemosynthetic bacterial communities occur in trenches[38], providing localised resources for a host of specialisedorganisms. Few have been found to date, but their closeassociation with subduction zones and other geologicalfeatures [39,40] suggest that more will be discovered assampling effort increases. Surface-derived particulate

Figure 3. Trench ecosystem. Each trench system can be characterised by extrinsic factors such as: (a) geological age, which is likely to affect the degree of endemism; (b)

pattern of primary productivity influencing overall food supply; (c) global hydrodynamics controlling oxygen supply and regional water temperature; (d) proximity to land

mass affecting sediment influx; (e) seismic activity, which could operate as one of the driving forces for sediment flux or catastrophic disturbance; and (f) topography, which

determines area, steepness or habitat heterogeneity. Within a trench, local ecological community (diversity and functional groups) can be structured by: (g) physiological

adaptation of individual species coping with various physical stresses; (h) local depth, which reflects hydrostatic pressure and local temperature; (i) predation and

competition for food; (j) local hydrodynamics, which can be utilised to locate food, obtain organic matter supply or disperse; (k) quality and quantity of food resources,

which appear to vary over time and space; (l) substratum can affect type of organisms settling; (m) life history (e.g. reproductive strategy or ontogenetic migration); and (n)

chemosynthetic community, which can provide local increases in food supply.

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organic matter (POM) and carrion falls, such as the car-casses of mammals, fish and large invertebrates, evidentlyplay a role in the supply of food to trench organisms [41–

43]. The gravitational flux of POM into the deep sea variesin space [44,45] and time [46], depending largely on bio-geographical province [47], proximity to continental landmasses and variability in surface ocean and climatic pro-cesses. The proximity of each trench to these factors,therefore results in temporal and spatial variation inthe quantity and quality of POM reaching the trench floor.Such resource pulses are a widespread phenomenon inboth terrestrial and aquatic environments [48,49], andcan trigger biological responses, including increased repro-ductive activities [50], and large-scale changes in theabundance, size distributions and compositions of deep-sea benthic communities [51–53].

Most sinking POM is intercepted and either solubilisedor mineralised by heterotrophic bacteria and zooplanktonbefore it reaches deep waters [54,55]. Deep-sea commu-nities are thus typically considered to be energy (organiccarbon)-limited systems [56]. Nonetheless, there is a grow-ing appreciation that qualitative aspects of the POM, suchas proteins, essential fatty acids (EFAs) and phytopig-ments, also play a significant role in the ecology of deep-sea communities [57–59]. Pelagic heterotrophs selectivelyremove these highly labile compounds from POM duringits passage into the deep, reducing both the quantity andquality of the POM that reaches bathyal depths andbeyond [60,61].Much of the ‘food’ input to deep-sea systemsis therefore nutrient poor, consisting largely of refractorycompounds. Large carrion falls [62,63], which arrive at theseabed relatively quickly compared to POM, potentially

represent an exception. For example, pelagic fish areknown for their high EFA content [64], and could representan energy- and nutrient-rich resource for deep-dwellingcommunities. Another exception is the occurrence of short-term pulses of relatively ‘fresh’ phytoplankton aggregatesto the seabed [65]. These ‘phytodetrital’ pulses contributesubstantially to the export of both organic carbon andnutritious compounds into the ocean interior [46,54,66].The presence of large quantities of labile, phytoplankton-derived compounds in trench sediments confirms thatpulses of fresh POM are received occasionally, at leastat certain locations [58].

Patterns of food supply are also affected by the physicaltopography of hadal environments. The steep slope oftrenches create a downward transport and subsequentaccumulation of POM along the trench axis [58,45,67,68],making the supply of resources to trench systems funda-mentally different to that on the flat neighbouring abyssalplains. This accumulation of organic matter is evident incontinental-shelf submarine canyons that have similartopography to trenches [69] and the increase in depositfeeders (e.g. holothurians) on the trench floor act as anindicator for increased food supply [11]. The availabilityof food along the trenchaxis, or the ‘trench resource accumu-lation depth’ (TRAD), is occasionally influenced by masstransport of sediment (slides) following seismic activity[67,68,70]. Such events would result in the quantity of foodon the trench axis and slopes being respectively higher andlower thanwhatwould have otherwise fallen onflat ground.The food impoverished slopes above the TRAD mightserve as biological barriers, impeding exploitation of theaccumulated resources by downward migrating fauna.

5

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Observations of high numbers of deposit feeders (holothur-ians) and facultative scavengers (amphipods) at the deepestparts of the trenches, regardless of depth [1,11,71,72], pro-vide anecdotal support for resource accumulation.However,conclusive evidence is lacking, and the relative importanceof autochthonous and allochthonous production in trenchcommunities remains to be established.

Ecological interactions at hadal depthsScavenging amphipods represent a particularly conspicu-ous and ubiquitous component of trench fauna. Fourspecies of lyssianassoid amphipods have been collectedfrom the Tonga and Kermadec Trench, each occupying adistinct vertical zone of �3.5 km [71,73]. Ontogenetic ver-tical partitioning has been proposed to explain the occur-rence of juvenile stages towards the upper limit of thedepth range of an individual species [71]. This may resultin higher juvenile growth rates by relieving hydrostaticallyinduced metabolic suppression and/or allowing access to amore nutritious food resource [71], although supportingdata are currently lacking.

Amphipods occurring at depths >6000 m have beenassumed to be obligate necrophages, and reports on therapid interception and consumption of bait by amphipodsin the trench confirm that these animals consume carrion[30,31]. Nonetheless, cannibalism, carnivory and detritiv-ory are also reported, with species-specific vertical andontogenetic variation in the apparent dominance of anyparticular feeding mode [74]. The prevalence of sedimentin the guts of juvenile individuals [74] presumably providesa source of both energy and nutrition, as hadal sedimentbacteria are known to synthesise large quantities of EFAs[75,76]. The digestion of refractory organic compounds ispotentially enhanced by the presence of gut bacteria[74,77,78], an adaptation apparent in shallower-watercounterparts [79,80].

Distinct differences among the morphologies and lifehistories of the lyssianassoids enable them to be separatedinto two guilds [43,81]. The relatively large benthopelagicamphipods have shearing mandibles and capacious guts,which are thought to enable them to take advantage ofsporadic food falls. They process food as discrete batches,and are adapted for bursts of feeding activity followed bylengthy periods of digestion and fasting. This lifestyle issupported by the presence of wax esters in their tissues[82], which serve as energy reserves in crustaceans thatencounter prolonged periods of food deprivation [83]. Theseare also hypothesised to reduce the energetic costs ofswimming by helping to maintain neutral buoyancy [81].By contrast, the smaller demersal species have triturative(grinding) mandibles and a smaller gut, enabling them tofeed and process food continuously while retaining theability to brood young at the same time. Their relativelyquiescent life style and feeding mode possibly negate thenecessity for large lipid reserves, although this has yet tobe confirmed.

Seasonal and geographical changes in the quantity andquality of POM reaching trench sediments have beeninvoked to explain the high inter-trench variability inmeiofaunal biomass, which is reported to range from44 � 10 [84] to 6378 � 3061 individuals per 10 cm�2 [57].

6

The assemblages of benthic nematodes, harpacticoid cope-pods, kinorhynchs, polychaetes and gastrotrichs in theAtacama Trench are approximately one third smaller thantheir bathyal relatives, although the selective pressure(s)driving this response remain unclear [57]. Meiofaunaldwarfism contrasts starkly with the gigantism noted forsome trench-dwelling crustaceans, including amphipods,tanaids, mysids and almost all isopods [1]. These speciesare larger than any other representative of the genus, andtheir unusually large size might be a response to ephem-eral food resources, competition or predation [43,85]. Forexample, the overlap of amphipod depth zones, and theresulting ontogenetic partitioning of food resources,suggests that competition may be an important structuralfeature in food-limited environments [72] where predationis reduced or absent. Recent observations in severaltrenches using baited cameras at 6000–8000 m, however,have revealed that larger crustaceans (decapods) and fish(liparids) preferentially consume mid-sized (�1.5 cm)amphipods, presumably exploiting the high numbers ofprey that congregate at food falls [30,31]. Similarly,tanaids also appear to prey on smaller individuals at thedeeper parts of the trenches (>8000 m). Whilst predationprovides a mechanistic explanation of why smaller sizedindividuals of certain taxa may be absent [86], predationalong the deepest trench axes (�10 000 m), where largeramphipods are more abundant, has not been documented.Thus, the relative abundance and size of amphipods ismost likely to be related to food supply and perhaps also topredation risk, a pattern contrary to previous consensussuggesting that invertebrate abundance declines only as afunction of depth [16,17,87].

ConclusionAn immediate challenge in understanding the ecology ofhadal trenches is to distinguish trench-specific communitystructuring factors from those which are typically ‘hadal’.This will require consideration of the effects of latitude,overlying productivity and seasonality. There are no apriori reasons to exclude the application of existing eco-logical theory to explain the diversity of trench commu-nities; their generic environmental characteristics (e.g.temperature, salinity and oxygen) are known to be com-parable to those at shallower depths, and differences inhydrostatic pressure are not overwhelming. Although it isintuitive that some abrupt changes, such as formation ofthe CCD, may form a physical barrier for some species [35],there are numerous examples of adaptations that over-come this potential limitation. Exposure to high pressureand difficulty in forming hard exoskeletons are not exclu-sively challenges faced by trench-dwelling organisms. Theability to tolerate food deprivation and rapidly interceptand capitalise on ephemeral food falls provides anadditional adaptation by which organisms can penetratebeyond the impoverished upper trench slopes. We contendthat many features of the ‘hadal zone’ are merely exten-sions of those found at shallower depths. Nonetheless, it isapparent that each trench system has unique character-istics owing to their geographical isolation. A naıve hypoth-esis is that inter-trench variation in species composition islikely to be primarily driven by the interaction of the

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overlying biogeographical province (POM quantity andquality) and trench topography, rather than specific adap-tations to hydrostatic pressure, temperature and/or anyother correlate of depth. However, the role of chemosyn-thetic production will need to be incorporated into thishypothesis as knowledge of its distribution and importanceaccumulates.

Trenches are poorly sampled and our knowledge of theecological structure and functioning of this environmentremains rudimentary. A current difficulty is that exist-ing data are not sufficient to confidently apply overarch-ing ecological theory. Indeed, it is not yet possible toreliably distinguish between taxa of non-viable vagrantsfrom shallower populations and those which are trenchendemics. Considering all trenches to be a single habitatis likely confound the interpretation of their ecology.The collection of multidisciplinary observational andexperimental data, replicated across trenches, is a pre-requisite for testing the generality of existing hypothe-ses. It will be essential to apply a broad spectrum oftechniques to examine phylogenetic relationships, phys-iological adaptations, diet, levels of biodiversity andevolutionary traits of the inhabitants. Although a for-midable task, technological advances, such as the Japa-nese remotely operated vehicle (ROV) Kaiko II, UK–

Japan HADEEP lander vehicles [6] and the US HybridROV Nereus vehicle [5], already exist and are oper-ational. These present the opportunity for an interna-tionally coordinated research campaign that considershow ecological processes operate across the full span ofocean depth.

AcknowledgementsThis research, part of the HADEEP project (including T.F.), wassupported jointly by the Natural Environmental Research Council (UK)and the Nippon Foundation (Japan) with additional support from theUniversity of Aberdeen, Scotland. D.J.M. is currently funded by theLeverhulme Trust. We thank Dr. Henry Ruhl and Prof. Paul Tyler andone other anonymous reviewer for their comments.

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