Deepocean Benthic Organisms Most Often Feed on

Deep-Sea Benthos

New Caledonia

Claude E. Payri , ... Sarah Samadi , in World Seas: an Environmental Evaluation (Second Edition), 2019

27.6.5 Deep-Sea Benthos and Seamounts

For almost 40 years, through the Tropical Deep-Sea Benthos program (TDSB), the MNHN and the IRD have explored the deep-sea benthos of the New Caledonia EEZ, with many expeditions held around New Caledonia and in the Coral Sea.

The deep-sea habitats of the upper bathyal zone (200–2000   m deep) are characterized by the dominance of hard bottoms, notably on seamounts. Exploration of these led to the discovery of many organisms belonging to lineages that were only (or mostly) known from fossil records (Fig. 27.10). New Caledonia deep-sea habitats are recognized as a center of diversity for many taxa (e.g., Cairns, 2015). The high level of diversity and apparent endemism of the deep-sea fauna was first evidenced from data gathered on seamounts. However, further research has shown that there is no clear difference in the level of diversity and the rate of endemism between the seamounts and the reef slopes, although ongoing research on other taxa points out the sampling gaps for both habitat types (cf. Pante, France, Gey, Cruaud, & Samadi, 2015 and reference therein). The seamounts and banks are separated by deep basins, and form a very fragmented habitat. The fauna of deep basins is mostly unknown. Several studies (e.g., Castelin et al., 2010) demonstrated that distribution and connectivity between habitats are determined not only by the dispersal abilities of the organisms but also by the availability of suitable habitats.

Fig. 27.10. Example of organisms discovered in New Caledonia and belonging to lineages that previously were known only from fossil records.

(Courtesy S. Samadi, MNHN.)

Seamounts have also been recognized as foraging posts and meeting points where highly migratory pelagic species aggregate. Satellite tracking reveals the importance of New Caledonian shallow seamounts for an endangered South Pacific humpback whale population (Garrigue, Clapham, Geyer, Kennedy, & Zerbini, 2015).

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The Coral Sea

Daniela M. Ceccarelli , in World Seas: an Environmental Evaluation (Second Edition), 2019

31.3.3 The Deep Sea

The Coral Sea has benefited from extensive research programs targeting deep-sea species, such as the MUSORSTOM Tropical Deep-Sea Benthos Program and CIDARIS expeditions, which have described >  2000 new species from bathyal habitats (Richer de Forges, 1998). Some features, such as the Gloria Knolls in ~   1000   m off the GBR shelf, were mapped and sampled in 2007/2008, revealing rich cold-water coral communities with similarities to those found at equivalent depths at higher latitudes in the western Pacific (Beaman & Webster, 2008). The New Caledonian bathyal environment, including the Loyalty Basin and its slopes, the Norfolk Ridge, and the Lord Howe Rise, is one of the best known in the Pacific (Richer de Forges, Hoffschir, Chauvin, & Berthault, 2005; Roux, 1994). Deep-sea benthic communities are primarily structured along depth gradients and are correlated with geomorphological structures (Anderson et al., 2011; Beaman & Harris, 2007). Generally, species richness declines with depth, especially below 800   m (Castelin et al., 2011), but abundance or density can increase in some cases. Slopes of the Loyalty Basin and the Norfolk Ridge revealed "living fossils", including stalked crinoids, sponges, and pterobranchs previously thought to have been extinct since the Jurassic (Roux, 1994).

Deep-water fishes in the Coral Sea are known from expeditions targeting specific areas and often linked to fisheries; many species remain to be properly described. The expedition HALIPRO 2 sampled deep ichthyofauna between 230 and 1860   m in the southeastern Coral Sea, and 40% of the collected species were new to science (Grandperrin et al., 1997). Exploratory deep-water fishing in the Solomon Islands, Vanuatu, and New Caledonia discovered >   200 species of deep-water demersal fishes belonging to 93 genera, although many more were not identified (Dalzell & Preston, 1992). Last et al. (2005) found that of the ~   1500 fish species from Australian continental slopes, 21% did not have current names, and many were likely to be new to science.

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Importance and Usefulness of Trace Fossils and Bioturbation in Paleoceanography

Ludvig Löwemark , in Trace Fossils, 2007

Biology and Actuoichnology

Recent interdisciplinary studies combining oceanography, sedimentology, and biology have investigated tiered benthic communities under different environmental conditions, for instance in the oxygen minimum zone in the Arabian Sea (Gage et al., 2000) and at the eastern North Atlantic margin (van Weering and McCave, 2002 ). The findings from studies of this kind are of fundamental importance to our understanding of the trace fossil record, and more studies are needed from a range of environments to improve our understanding of how different environmental conditions are reflected by the trace fossils. One limitation, however, is that biological studies of deep-sea benthos tend to focus on the relatively densely populated uppermost layer of the sediment, whereas the traces actually preserved in the fossil record belong to the much sparser fauna of deeply penetrating organisms. This chasm between deep-sea benthos biologists and ichnologists could be easily closed through cooperative, interdisciplinary sampling strategies of box cores and core top samples. In studies by Meadows et al. (2000) and Smith et al. (2000), benthic fauna and the type and intensity of bioturbation were studied in relation to changes in geochemistry and substrate parameters across the oxygen minimum zone in the Arabian Sea. Important results from their studies show that a decrease in bottom water oxygen levels corresponded with decreased mixed layer thickness and decreased burrow diameters, they also found a strong correlation between burrow diversity and species diversity, suggesting that trace fossil diversity could be used to estimate benthic faunal diversity. The most important observation, however, was that neither maximum burrow size nor maximum penetration depth of the burrows were correlated to bottom water oxygenation levels. Here ichnologists should take special notice, because the deeper and larger burrows have a considerably larger chance of being preserved in the fossil record, but these well-preserved burrows might actually mask the 'true' response to oxygen variations and thus bias the environmental interpretation. Another example is a study comparing the benthic megafaunas of the Iberian and Celtic continental margins (Lavaleye et al., 2002). This study showed a relationship between high energy environments and the prevalence of filter feeders, and low energy environments and the occurrence of deposit feeders.

Continued interdisciplinary studies are necessary to follow up on these important studies. One central aspect that deserves more attention is whether the benthic system observed is actually in steady state or not. As pointed out by Werner and Wetzel (1982, p. 280), with a penetration depth of up to more than 50 cm for the deepest tiers, the development of an ichnofabric reflecting steady state conditions would require steady conditions over more than 5000 years. A requirement seldom met, as can be deduced from the stable isotope record of climate change. It is therefore necessary to confirm whether the benthic fauna (and the related trace fossils) of the deeper tiers are associated with present conditions or if they represent responses to past conditions.

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Free-Living Protozoa

Genoveva F. Esteban , ... Alan Warren , in Thorp and Covich's Freshwater Invertebrates (Fourth Edition), 2015

Functional Roles of Free-Living Protozoa

The functional roles of free-living protozoa derive from their small size. There are three major "body plans" in the protozoa regardless of any taxonomic affiliations: the ameboid, the flagellated, and the ciliated protozoa. The smallest flagellates are 2–4   μm, and most are <20   μm, whereas most amebae are <50   μm and most ciliates are <200 μm. Some exceptions are certain ameboid protozoa, such as the marine radiolarians and the agglutinated foraminiferans of the deep-sea benthos, which may reach 2  mm or more, especially if they have spiny extensions. Because protozoa are so small, most prey items are smaller microbes. Protozoa are the principal consumers of bacteria, and because they have population growth rates that are similar to those of the microbes on which they feed (doubling times in the order of one to several days), they can usually control microbial abundance. Flagellated protozoa can probably consume all bacterial production in the plankton. In the benthos, protozoa overlap in their niche requirements with nematodes, rotifers, tardigrades, turbellarians, and gastrotrichs; but because of their great abundance, protozoa are indeed quantitatively the most important grazers in the freshwater and marine (including deep-sea) benthos. Also, just as microbes achieve astronomical abundance on a global scale, so too are the protozoa that graze on them represented by species populations with proportionately smaller but still unimaginably large global abundances.

Protozoan grazing on microorganisms stimulates activity of the whole microbial community in both oxic and anoxic environments (Esteban et al., 2012). The process involved is not fully understood, although it may operate by increasing the rate of turnover of essential nutrients that would otherwise remain locked up in bacterial biomass. The net effect is that grazing by protozoa stimulates the rate of decomposition of organic matter (Fenchel, 1987; Finlay and Esteban, 1998).

The variety of shapes, sizes, and relative abundances of microbial food items has driven the evolution of a comprehensive suite of methods to capture them and a considerable diversification of protozoan morphologies. In general, the size of a protozoon relative to its prey dictates the most efficient food-capturing mechanism. Where the predator is typically much larger than its prey, filter-feeding prevails; and where the size difference is less, the protozoon is more likely to be a raptorial feeder—one that seeks out relatively large individual food items. Thus, the planktonic choanoflagellate feeding on a dilute suspension of bacteria 20 times smaller than itself does so with a very fine filter; and the ciliate feeding on dinoflagellates half its size seeks out and captures each one individually. There are, of course, many exceptions, such as the marine heterotrophic dinoflagellates that use a feeding veil to trap and digest diatoms much larger than themselves.

The third main feeding type in protozoa is termed "diffusion feeding." This is particularly common in planktonic ameboid protozoa (radiolarians, foraminiferans, and heliozoans) and in the suctorian ciliates (i.e., ciliates that have tentacles to catch prey and suck out the cell content Figure 7.1). Diffusion feeding works when prey items collide with the sticky spines, tentacles, or axopods that radiate from the protozoon. Unlike the other two main modes of feeding, the protozoon simply waits for the arrival of its prey, much as a spider waits for an insect to be snared in its web.

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Marine Life

M.G. Reuscher , ... S.K. Sturdivant , in Encyclopedia of Ocean Sciences (Third Edition), 2019

Introduction

About two-thirds of the Earth is covered by ocean. Most of the ocean floor is covered by sediments, making the coastal and marine benthos one of the largest single habitats on Earth. Yet, our main sediment sampling tools, such as corers or grabs, are limited to small points of limited area. If we added up the total area of the Earth's sediment habitat that has been sampled, it would be an infinitesimally small percentage of the total habitat area because the sampling devices are so small. For example, in the early part of the 21^st century, an extensive sampling program was undertaken to characterize the deep-sea benthos of the northern Gulf of Mexico. A total of 569 box core samples (area  =   0.2   m² each) were collected from 53 stations over 3 years, which means a total area of about 114   m² was sampled in a body of water that is about 7   ×   10¹¹  m², meaning the entire area was characterized by sampling only about 10^{−   10}% of the area (but only 10^{−   11}% if just stations are counted). These kinds of calculations have been made many times, and have led to the conclusion that we know more about the surface of the moon than we do about the surface of deep-sea sediments.

Curiosity about life in the deep-sea will continue to drive our sampling efforts regardless of the difficulties of the task. In fact, in the 19th century, the common thought was that life could not exist below 549   m (1800   ft) because there was no light and the pressures were too high. Testing this hypothesis was an important task of one of the first marine science studies performed during the Challenger expedition between 1872 and 1876. Deep-sea sediments were sampled with trawls, and living organisms were found in every sample. Thus began the study of deep-sea biology.

Today, there is tension between those who believe that there is still much to learn about deep-sea sediments and those who believe the era of exploration is over and descriptive studies should be abandoned as relicts of the past. As pointed out above, sampling the deep is such a challenge that there is still much to discover, not only about biodiversity, but also about mineral resources, genetic resources, chemicals that could lead to new pharmaceuticals, and ecological processes that sustain life in the deep. The deep-sea is part of the Earth's biosphere, yet so much of it remains unknown primarily because we have seen or sampled so little of it. Sampling issues will not fade away, and they are more important than ever because exploitation of natural resources often depends upon environmental assessments, which make it a necessary to know what is there so that risks can be quantified. Indeed, much of what is known about deep-sea sediments and biology has been discovered because of environmental assessments required for exploitation of fisheries, minerals, and hydrocarbons.

Today, the tools for sampling the deep-sea are varied but fall into three categories: point samplers (such as grabs and corers), drag samplers (such as dredges and trawls), and reconnaissance samplers (such as cameras). Cameras can deployed by mooring, point devices, and ROVs (remotely operated vehicles). Here is a survey of the most common sampling techniques (excluding ROVs), sample processing methods, and issues related to sampling programs.

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Benthic Shelf and Slope Habitats

Barry Wilson , in The Biogeography of the Australian North West Shelf, 2013

7.2 Continental Slope (Bathyal)

The slope, terraces, and plateaux from depths of 200 to 2000   m are referred to as the bathyal or deep-sea zone. The bathyal zone and the abyssal and hadal zones below it are referred to as the deep sea. In much of the hydrocarbon resource area of the North West Shelf, the sea bed lies in this depth zone.

Sediments of the North West Shelf slope are carbonates, generally silty sands composed of skeletal remains of pelagic foraminifera. Ripple marks indicate bottom currents, but apart from these, the topography of the seabed is regular and featureless except for the high biohermic banks such as the Rowley Shoals and Scott and Seringapatam Reefs on the slope terraces. Typically, the physical conditions of these benthic bathyal habitats are relatively uniform and constant compared to the benthic shelf. However, reviews of the world's deep-sea benthos ¹⁷ show that this ecosystem is patchy but with surprisingly high biomass in places and with high levels of species-richness that surpass that of many terrestrial and other marine systems that are commonly regarded as supporting high biodiversity.

The most abundant and species-rich component of the bathyal fauna is small infaunal invertebrates, predominantly polychaetes, nematodes, foraminifers, crustaceans, and bivalved molluscs. ¹⁸ These may be in dense communities, feeding on detrital organic material sinking to the sea floor from the water column. There are also infaunal pogonophoran and sipunculid worms and many epifaunal species including echinoderms, sponges, and coelenterates. Supported by this trophic base of secondary producers are assemblages of predatory crustaceans, gastropod molluscs, and fishes.

The deep-sea benthic bathyal fauna of Australia, especially that of the North West Shelf, is little known. ¹⁹ A review of key ecological features in the Northwest Marine Region ¹² provides a summary of knowledge of the deep-sea environment and identified databases of relevant information and specimens from the region. Two systematic accounts of benthic slope invertebrates in the region deal with ophiuroids ²⁰ and azooxanthellate corals ²¹ and include references to species that inhabit this depth zone.

There is some information on demersal fishes of the continental slope including the results of trawling conducted by CSIRO. In the 1970s, a scampi trawling industry operated on the Rowley Terrace. There have been several inspections of the sea floor, using ROV technology, and some grab sampling on behalf of the petroleum and gas industry operating in the region. For the most part, the reports of these surveys are not publicly accessible. Nevertheless, some fish and invertebrate specimens from these surveys and from the commercial scampi trawlers have been deposited in the collections of the Western Australian Museum. In particular, there are samples of benthic molluscs, notably from the scampi grounds in the vicinity of the Rowley Shoals at depths from 300 to 450   m and from grab samples taken at the Pluto gas field off the Montebello Islands. ²²

Of particular note are specimens of gastropods (e.g., the pleurotomarid Perotrochus westralis and unidentified species of the trochid genus Calliostoma) that feed on organic detritus gathered from the sediment surface, deposit-feeding bivalves of the genera Verticordia, Poromya, and Amygdalum, and many species of large predatory gastropods of the families Olividae, Volutidae, Muricidae, Conidae, and Turridae. These gastropods are mostly vermivores. Their variety and numbers indicate the presence in these bathyal habitats of a dense and diverse infaunal community of small detrital-feeding invertebrates. A selection of the shells of these predatory gastropods is illustrated in Figure 7.2.

Figure 7.2. A selection of large predatory gastropods of the bathyal zone trawled at depths of 300-660 m on the Rowley Terrace, on the continental slope of the North West Shelf. (A) Mipus vicdani (Muricidae), (B) Comitas sp. (Turridae), (C) Pinguigemmula philippensis (Turridae), (D) Conus teramachii (Conidae); (E) Thatcheria mirabilis (Turridae), (F) Teramachia dalli (Volutidae), (G) Conus ichinoseana (Conidae), (H) Bathytoma atractoides (Turridae), (I) Teramachia johnsoni (Volutidae), (J) Gemmula unedo (Turridae).

From the biogeographic perspective, these molluscs appear to be representative of taxa that are widespread bathyal species in the Indo-West Pacific realm, although this may be a simplistic assumption based on little information. Some of the large gastropod species are described from comparable habitats in the Western Pacific. They have no biogeographic affinity with the benthic molluscan fauna of the adjacent shelf and represent a deep-sea fauna that probably has had a quite different evolutionary history to that of the shelf fauna. They are likely to respond to disturbance in different ways. The bathyal fauna along the margins of the North West Shelf is a significant element of the Australian marine biota that appears likely to be rich in species and patchily dense in biomass but which is virtually undescribed.

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Protozoa

Bland J. Finlay , Genoveva F. Esteban , in Encyclopedia of Biodiversity (Second Edition), 2013

Functional Roles

All of the important functional roles of free-living protozoa derive from their small size. The smallest flagellates are 2–4   μm and most are <20   μm, most amebae are <50   μm, and most ciliates are <200 μm. Exceptionally, some ameboid protozoa, such as the radiolarians and foraminiferans, and the agglutinated foraminiferans of the deep-sea benthos, may reach 2  mm or more, especially if they have spiny extensions. Because protozoa are so small, most suitable prey items are other, smaller microbes. Protozoa are the principal consumers of the immense natural resource of bacteria and other microorganisms, and because they have population growth rates that are similar to those of the microbes on which they feed (doubling times on the order of one to several days), they are usually able to control microbial abundance. Flagellated protozoa can probably consume all bacterial production in the plankton. In the benthos, protozoa overlap in their niche requirements with nematodes, rotifers, tardigrades, turbellarians, and gastrotrichs, but because of their great abundance protozoa are indeed quantitatively the most important grazers in the freshwater and marine (including deep-sea) benthos. Also, just as microbes achieve astronomical abundance on a global scale, so too are the protozoa that graze on them represented by species populations with proportionately smaller but still unimaginably large global abundances.

Protozoan grazing on microorganisms also stimulates activity of the whole microbial community, in both oxic and anoxic environments. The process involved is not fully understood, although it may operate by increasing the rate of turnover of essential nutrients that would otherwise remain locked up in the bacterial biomass. The net effect is that grazing by protozoa stimulates the rate of decomposition of organic matter (Fenchel, 1985; Finlay et al., 1997).

The variety of shapes, sizes, and relative abundances of microbial food items has driven the evolution of a comprehensive suite of methods to capture them and a considerable diversification of protozoan morphologies. In general, the size of a protozoon relative to its prey dictates the most efficient food-capturing mechanism. Where the predator is typically much larger than its prey, filter-feeding prevails, and where the size difference is less the protozoon is more likely to be a raptorial feeder – one that seeks out relatively large individual food items. Thus, the planktonic choanoflagellate feeding on a dilute suspension of tiny bacteria 20 times smaller than itself does so with a very fine filter, and the ciliate feeding on dinoflagellates half of its size seeks out and captures each one individually. There are, of course, many exceptions, such as the marine heterotrophic dinoflagellates that use a feeding veil to trap and digest diatoms much larger than themselves.

The third main feeding type in protozoa is termed "diffusion feeding." This is particularly common in planktonic ameboid protozoa (radiolarians, foraminiferans, and heliozoans) and in the suctorian ciliates. It works when prey items collide with the sticky spines, tentacles, or axopods that radiate from the protozoon. Unlike the other two main modes of feeding, the protozoon simply waits for the arrival of its prey, much as a spider waits for an insect to be snared in its web.

Thus, there is a close link between protozoan morphology (especially of the food-capturing organelles) and the way in which a protozoon functions as a grazer. Therefore, when the authors classify the free-living protozoa into broad morphological groups, they are simultaneously allocated to broad functional groups. The three broadest morphological–functional groups are the ameboid, the flagellated, and the ciliated protozoa, and each has its own strengths as a phagotroph. Representatives of all three may feed on the same type of microbes in the same place (e.g., in an aquatic sediment), but they will differ in the mechanics and efficiency of capture of any particular food particle. A filter-feeding flagellate will have a relatively large filter area, a high volume-specific clearance and competitive superiority over filter-feeding ciliates when grazing on planktonic bacteria. A helioflagellate and a suctorian ciliate will both practice diffusion feeding, but the former will be adapted for snaring bacteria, whereas a diffusion-feeding suctorian will specialize in trapping flagellates and ciliates.

Many protozoa are microaerophilic: They seek out habitats with a low level of dissolved oxygen that is just sufficient to drive their aerobic respiration and low enough to exclude metazoan competitors and predators. Microaerobic habitats are common in aquatic sediments and in oceanic oxygen minimum zones. These are zones in which the raw materials for microbial growth arrive from opposite directions (e.g., where oxygen and light arriving from above meet carbon dioxide and sulfide from below) and in which there is therefore an elevated abundance of microbial food. Therefore, microaerophily is an adaptive behavior: It brings protozoa into contact with high abundances of microbial food. It also stimulates the growth of nutritional symbionts such as sulfide-oxidizing bacteria and endosymbiotic algae. Many microaerophilic protozoa are also temporary anaerobes, but unlike the true anaerobes that live permanently in the absence of oxygen, their metabolism is fundamentally aerobic. The true anaerobes – those that complete their entire life cycle in the absence of oxygen – live principally in aquatic sediments. There are many species, but none is ever abundant. Most use hydrogen-evolving fermentations for energy generation, and the hydrogen is used by anaerobic bacteria, especially endosymbiotic methanogens. Thus, methane is released from these protozoan consortia. The anaerobic protozoa are probably the only phagotrophic organisms capable of living permanently in the absence of dissolved oxygen (Fenchel and Finlay, 1995).

The real diversity of symbiotic associations involving protozoa is poorly known. In some cases, complex interactive behaviors have evolved between the partners. In the marine, sand-dwelling ciliate Kentrophoros, the entire dorsal surface of the ciliate is a coat of sulfide-oxidizing bacteria that can grow only in the narrow layer within the sediment where oxygen and sulfide overlap. The ciliate host's innate microaerobic behavior enables it to seek out the habitat that the symbiotic bacteria need for growth. The ciliate then invaginates its dorsal surface and digests the bacteria because it does not have a mouth, and this is its only source of nutrition.

It is clear that many of these symbiotic consortia involving protozoa represent tightly integrated functional units; indeed, the symbionts may be almost as deeply embedded functionally in the consortium as the protozoon's other organelles. Two points must be noted. First, it is the combined phenotype of the consortium, rather than that of any individual consortium partners, on which natural selection will operate, and there are examples of how the fitness of a protozoon in a particular habitat can be improved by the acquisition of endosymbionts (e.g., the algal symbionts of ciliates living in the metalimnia of freshwater lakes). Second, the biodiversity of protozoa, when quantified simply in terms of protozoan species richness, will fail to take account of the large supplementary microbial diversity with which the protozoa are necessarily associated.

Therefore, the diversity of free-living protozoa may be classified into broad morphological–functional groups: ameboid, flagellated, and ciliated protozoa. Almost any free-living protozoon can be placed without difficulty in one of these groups. It must be stressed, however, that these groups are not concordant with any system of classification of protozoa published in recent years; nor are they in most cases aligned with the independent lineages that are emerging in the molecular phylogenies (e.g., those based on sequence variation in ribosomal RNAs) which reflect the main episodes in the history of eukaryotic evolution. Heterotrophic flagellate groups, such as the diplomonads and trichomonads, appear in the early emerging lineages, but other flagellates (e.g., choanoflagellates, chrysomonads, dinoflagellates, and haptomonads) are classified within recently diverging lineages. The ameboid protozoa too are scattered across many lineages. The naked amebae without mitochondria (the pelobionts) diverge early and close to the diplomonads, whereas the vahlkampfiids, slime molds, and various other groups of naked and testate amebae appear in other independent lineages that are evolutionarily quite distant from each other. The morphological–functional group of the ciliates is the only one which remains intact, as a monophyletic group, in current molecular phylogenies.

The process of distilling the vast quantity of molecular information generated in recent years has generated some entirely new phyletic assemblages, including the stramenopiles, a group containing organisms as morphologically and functionally dissimilar as chrysomonad flagellates and diatoms, and the alveolates – a group that embraces the dinoflagellates, the ciliates, and a large group of exclusively intracellular parasites (the apicomplexans) (Adl et al., 2005). In the next section, the authors focus on the broad morphological–functional groups of free-living protozoa, define what is meant by species, and quantify the species within these groups.

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Crustacea

Luis M. Mejía-Ortíz , in Encyclopedia of Caves (Third Edition), 2019

Superorder Peracarida

This is the superorder with greater representation in the cave environment as it groups more than 1900 species in eight orders; they are present in marine environments, anchialine, freshwater, and terrestrial. These organisms tend to reduce their carapace and they have a maxilliped (rarely two), gills are in the thoracic or abdominal appendages, an antenna typically with a propodus of three segments, the females have a brood pouch usually formed by medial lamellar. The pleopods have no internal appendice and the telson is without caudal rami.

A single family represents the order Spelaeogriphacea, with four species inhabiting in South Africa, South America, and Western Australia. They are small blind and depigmented stygobitic that have a short carapace fused with the first thoracic appendages and that in its anterior projection produces a rostrum in the form of a triangle. They have a pair of maxillipeds, pereiopods from 1 to 7 are simple or biramous and shorter than exopods. The first exopods are modified to produce currents that help them move and the last are modified as gills. Also, their pleopods are modified to swim.

The order Thermosbaenacea comprised of four families (Halosbaenidae, Mondellidae, Thermosbaenidae, and Tulumellida) where Monodellidae is best represented with more than 20 species. These organisms are small 2–5   mm with a short shell fused with the first thoracic somites. In females the carapace provides a brood pouch, with a single pair of maxillipeds. Pereopods are biramous, simple and without epipods. These organisms are known from anchialine environments, caves, cenotes, interstitial environments; and springs of thermal or cold waters (Wagner, 1994) (Fig. 8).

Fig. 8. Class Malacostraca; Thermobanaecea Tulumella sp.

(Photo by L.M. Mejía-Ortíz.)

The Mysida Order is composed of three families (Fig. 6). The Mysidae is the best represented in subterranean habitats with more than 25 species. There are freshwater species and anchihaline environments that show that they have colonized these environments due to marine regressions that have occurred in different regions of the world. There are also stygophilic or stygoxenous species in those subterranean environments with a clear connection to the coast, but those exclusively stygobitics present a reduction in pigments, the loss or diminution of eyes but not the eyestalks because they contain the endocrine organs. A carapace that covers almost the entire thorax generally characterizes this group. The compound eyes have an eyestalk, they have a pair of maxillipeds not associated with the cephalic appendages, the abdomen is elongated; the pereopods are biramous although modified and the statocysito is usually located in each uropod endopod.

The order Mictacea is represented in subterranean environments by the family Mictorcarididae. This order is completely stygobitic and is monotypic with a single species described for marine caves of Bermuda. They are characterized by having a body lacking in pigmentation and without any visual element in the reduced eyestalk. The head is narrow previously forming a triangular rostrum, which is later merged with the first thoracomere. The gills are absent, the pereopods are simple, and the pleopods are reduced and uniramous. The uropods are biramous with two to five segmented rami (Fig. 9).

Fig. 9. Peracarids stygobitic species to each family.

The order Bochusacea, originally grouped in the order Mictacea, was recently erected to separate three species from the deep-sea benthos, but which now has two stygobitic species of anchialine and marine caves of the Bahamas. Although these organisms are similar to the mictaceans, they have clear differences in the conformation of the pereopods among other characteristics, it has also been reported that they are filtering and the pereopod 1 is adapted to feed and not to move ( Fig. 9).

The order Amphipoda is characterized by the absence of a fused carapace. It is clearly divided into head, thorax, and abdomen, each of these parts with their respective appendices; for example, the head contains seven pairs of pereopods uniramous with the first, second modified with a chela or subchela and the gills are thoracic. This group of crustaceans is usually compressed laterally and although some dorsoventrally compressed species occur infrequently. They exist in diverse aquatic bodies around the world, and they have been able to colonize freshwater, brackish, and marine environments. They have more than 900 species described so far, grouped into 29 families (Fig. 10), with the Niphargidae family being the one with the highest number of specimens described with 220 species.

Fig. 10. Peracarid Amphipods stygobitic species to each families.

The order Isopoda is a successful group in its plasticity to inhabit different environments and conditions, because we can find them in free life or as parasites of other species. It is also possible to find them in the deep sea, the shallow and coastal marine waters, as well as the brackish waters of the estuaries, the freshwater environments, and in the terrestrial environment. They have a great diversity of more than 10,343 species recognized so far, which occupy diverse trophic niches as omnivores, herbivores, detritivores, predators, parasites or scavengers. This group is characterized by being dorsoventrally compressed, they lack fused shell and have a head with the first fused thoracomere. The pereopods are attached to the thorax and are modified as ambulatory appendages, swimming or prehensile, the pleopods are biramous and modified for swimming activities as well as for the exchange of gases; while the telson is merged with the pleonites from 1 to 6. The stygobitic members of this order are grouped into seven suborders that in turn have 36 families. The suborder Phreatocidea has four families reported for freshwater from caves in South Africa, Australia, India, and New Zealand. The suborder Anthuridea, which is generally marine has among its stygobitic member's organisms of two families that can be found in the sediment of anchialine environments, or freshwater caves in the Canary Islands, Indonesia, Mexico, New Zealand, and South America. The suborder Microcerberidae with two families one of them monotypic (Atlantasellidae) and another one with a good representation of organisms (Microcerberidae) in the interstices of the caves and marine coasts as well as wells of Africa, Asia, the Caribbean, the islands of the Indian Ocean, Japan, the Mediterranean, and the West of North America. The Flabellifera suborder is well represented by two families on one side Cirolanidae which is a predominantly marine family with more than 90 species reported for many of the freshwater caves and anchialine of Mexico (Fig. 11); while the Sphaeromatidae family is represented by species with distribution in the South of Europe, the Southeast of the United States, and the north central portion of Mexico. The Asellota Suborder with the Asellidae family spearheading the diversity of these aquatic organisms and which is also comprised of seven other families. This suborder is represented in the caves of many regions of the world virtually on all continents and in the coastal, anchialine, and freshwater underground environments of the planet. The monotypic suborder Calabozoidea is known only from a well in northern Venezuela. This transparent and blind isopod is apparently restricted to groundwater. The subleader Oniscidea is usually made up of terrestrial species grouped into 16 families. Although the majority of this group has adaptations to terrestrial environments such as the presence of a pseudo trachea in the pleopods, there are still stygobitic species of this group (Fig. 12). However, the right numbers of species is not reported here because there is a great diversity around the world but also not always is specified is they have cave adaptations.

Fig. 11. Class Malacostraca; Order Peracarida Metacirolana mayana.

(Photo by L.M. Mejía-Ortíz.)

Fig. 12. Peracarids Isopod stygobitic species to each families.

The order Tanaidacea is known from marine benthic habitats around the world. Its carapace is fused with the first two thorax segments, the first and second thoracopods are maxilliped, with the second containing a chela while the rest are simple and serve to move. It has 12 families grouped in two suborders, inhabiting anchialine and marine caves in different regions of the world such as Bermuda and several islands of the South Pacific (Fig. 13).

Fig. 13. Peracarids stygobitic species to each families

The order Cumacea represented by three families has only been reported for Bermuda, Bahamas and Jamaica, mainly for underwater caves. This group in general has a worldwide distribution with more than 800 recognized species, but there is still a need to dedicate efforts to increase their knowledge of stygobitic species (Fig. 13).

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Marine Life

Brian J. Bett , in Encyclopedia of Ocean Sciences (Third Edition), 2019

Ecology

Standing Stock

The abundance (numerical density) of megafaunal assemblages is typically a factor of 10⁶ less than that of the meiofauna, and a factor of 10⁴ less than that of the macrofauna (Fig. 1), consistent with corresponding orders of magnitude differences in the typical body sizes of each group (e.g., 100   ng, 10   μg, 400   mg wet weight). Megafaunal abundance varies considerably (×   10⁴) across the marine environment primarily driven by the availability of food and the in situ temperature, and modified locally by the occurrence of inimical conditions such as hypoxia and various human impacts (demersal fishing, pollution etc.). The approximate match between abundance and the inverse of body mass indicates that there is relatively modest variation (<   ×   10¹) in the biomass of the three body size groups (Fig. 1).

Fig. 1. Variation in the standing stocks of marine benthos with water depth (200–6000   m), extracted from the dataset compiled by Wei et al. (2010). Data summarized as analyses of covariance by body size group: meio- (red), macro- (blue), and megafauna (invertebrates and fish, green). For density the prediction R ²  =   90%, the slope   =   −   0.664, and the intercepts   =   3.43   ×   10⁷ (meio-), 1.88   ×   10⁵ (macro-), and 4.98   ×   10⁰ (megafauna). For biomass the prediction R ²  =   39%, the slope   =   −   1.074, and the intercepts =   9.17   ×   10⁴ (meio-), 2.31   ×   10⁵ (macro-), and 3.13   ×   10⁴ (megafauna).

There is an apparent difference in the exponents of the power relationships between standing stocks and water depth (Fig. 1): density   =   −   0.66 (−   0.70, −   0.62 95% CI), biomass   =   −   1.07 (−   1.13, − 1.02 95% CI). This has led some authors to suggest the occurrence of "dwarfism" in the deep-sea benthos, that is, if biomass declines more rapidly than abundance then the animals present must be smaller in deep water. This is a problematic conclusion as estimated biomass is typically a sample-size-dependent parameter, for example, typically, the greater the number of specimens measured/weighed the greater the average specimen mass recorded—this is the inevitable consequence of the power law distribution of body masses ( Edwards et al., 2017). Fig. 2 illustrates this effect in megafaunal biomass data, showing the asymptotic response of the biomass estimate to increasing numbers of specimens sampled.

Fig. 2. Sample size dependence in estimates of megafaunal biomass illustrated with megafaunal data extracted from the dataset compiled by Wei et al. (2010). Field data are shown as open symbols, simulated mean values (solid line) and corresponding 95% CI (gray shade) are shown for 10,000 random draws of selected numbers of specimens from a power law distribution with an exponent of −   1.8 and bounded in the megafaunal body mass range 2.42   μgC to 303   mgC.

As noted above, the standing stock of megafauna is typically driven by the availability of food and the in situ temperature, this is illustrated in Fig. 3. The rate of food supply to the seafloor (particulate organic carbon flux) is thought to exhibit a power relationship with water depth having an exponent of c. −   0.9, temperature at the seafloor also varies with water depth, declining rapidly in the upper ocean, before slowing, and reaching a local minimum of c. 1.5°C in the abyssal realm (3500–5500   m water depth). Other things being equal, temperature determines the rate at which food is consumed (metabolic rate) and thereby the standing stock of megafauna that can be supported at a given food supply rate. Combining these two factors mathematically, suggests that the stock of megafauna should decline with water depth at a lower rate than the decline in food supply rate, with the best fit power relationship having an exponent of −   0.65 comparable to that seen in field data (Fig. 1 density exponent   =   −   0.66).

Fig. 3. Variations in "average ocean" temperature (World Ocean Atlas 2013 v2; Locarnini et al., 2013), and particulate organic carbon (POC) flux (Martin et al., 1987) with water depth, plotted with the estimated standing stock supported by that flux when corrected for the influence of temperature on metabolism (Gillooly et al., 2001). Note the change in slope in the power relationships between flux and depth (−   0.90) and stock and depth (−   0.65).

Biological Diversity

Studies of the megafauna/large macrofauna have played a key role in the development of theories concerning the controls on marine biological diversity. In the 1960s, Robert Hessler and Howard Sanders of the Woods Hole Oceanographic Institution designed an epibenthic sled fitted with a 1   mm net mesh to assess the diversity of epifaunal organisms in the deep sea. Then conceived a means of making fair comparisons of diversity from collections comprising highly variable numbers of specimens—the rarefaction methodology. Then produced one of the first general theories of marine biological diversity—the stability time hypothesis. Although that hypothesis was not maintained for long, the sled and rarefaction methodology remain in use today, as does the use of megafauna in the development of new theories.

Recently, Woolley et al. (2016) have attempted a global synthesis of marine biological diversity based on brittle stars (Ophiuroidea), often a dominant component of the megafauna. In common with standing stocks, diversity appears to be primarily controlled by the availability of food and the in situ temperature. In shelf seas and on the upper continental slope (0–2000   m water depth), diversity is predominantly driven by temperature. In the deep sea, ophiuroid species richness appears to be governed by particulate organic carbon flux and distance from the continental margin, both indicators of food supply rate. Again, as in the case of standing stocks, megafaunal diversity is depressed in hypoxic environments, and can generally be expected to decline in response to various human impacts (demersal fishing, pollution etc.).

Biogeography

The major patterns in megafaunal standing stock and biological diversity can, to a first approximation, be equated with variations in latitude and water depth, through their connections to corresponding variations in food availability and water temperature (see e.g., Fig. 3). Dividing the totality of the marine environment into smaller units—bioregionalization—provides a means of dealing with this great complexity in a practical manner. The book "Ecological Geography of the Sea" by Alan Longhurst, first published in 1998, is a seminal example of such effort in the pelagic realm. A global classification of the coastal and shelf sea benthic realm was subsequently produced in the form of the Marine Ecoregions of the World (MEOW) system (Spalding et al., 2007). These regions are defined as areas of relatively homogeneous species composition, distinct from adjacent systems, with species composition determined by a set of oceanographic or topographic conditions. Key factors controlling this biogeography include geographic isolation, upwelling and nutrient supply, freshwater input, temperature and ice regimes, wave exposure and current speeds, sediment type, and bathymetric or coastal complexity.

Extending the MEOW system, or similar, to the full extent of the global ocean would be a very challenging task, given the low spatial resolution of biological knowledge over most of the ocean floor. Zonation of megafaunal assemblages by water depth has been long observed and mapped, not least by Edward Forbes in the mid-1850s in his "Map of the distribution of marine life, illustrated chiefly by fishes, molluscs and radiata; showing also the extent and limits of the homoiozoic belts." The three-dimensional biogeography of the ocean floor imagined by Forbes, encompassing 2-d biogeographies, such as Longhurst provinces and MEOWs, and water depth zones (bathomes) is being developed (see e.g., Table 1). In poorly known regions, these bioregionalization schemes can operate via physical characteristics of the environment, such as water depth, major topographic features, and simple seabed type classifications, with the aim of identifying the specific biotopes or species groups that are required to form the basis of practical environmental management schemes.

Table 1. An hierarchical framework for the bioregionalization of megafaunal assemblages

Level	Nomenclature	Characterization
1	Biogeographic provinces	Evolutionary-scale barriers
2	Bathomes	Depth-related variation
3	Geomorphological	Physiographic features
4	Primary biotopes	Gross seabed type
5	Secondary biotopes	Finer seabed type
6	Biological facies	Specific assemblages/indicator species

Adapted from Last, P. R., Lyne, V. D., Williams, A., Davies, C. R., Butler, A. J. and Yearsley, G. K. (2010). A hierarchical framework for classifying seabed biodiversity with application to planning and managing Australia's marine biological resources. Biological Conservation 143, 1675–1686.

Threats

The need for sustainable environmental management has become increasing evident as the human population, and its rate of use of natural resources, has continued to grow at an alarming rate. Megafauna are a key target of that exploitation and a key indicator of its successful or unsuccessful management. Fish and various invertebrate megafauna (e.g., molluscs, crustaceans, and echinoderms) are subject to extensive commercial harvesting, with many of the fishing methods used having substantial impacts on non-target megafaunal species and their physical habitats. Within national jurisdictions, the management and conservation of target stocks, and that of non-target species and habitats, are typically subject to various forms of control. In 2006, the United Nations General Assembly began to tackle these issues in the 64% of the global ocean that lies beyond national jurisdiction. This UN action seeks to protect vulnerable marine ecosystems (VMEs) from the impacts of bottom trawling.

The definition of VMEs has focussed on megafaunal species, including examples such as (a) reef-and forest-forming corals and hydroids (Scleractinia, Octocorallia, Antipatharia, Stylasteridae), (b) sponge aggregations (Demospongiae, Hexactinellida), (c) other aggregated epifaunal taxa that form important structural habitats (Xenophyophoroidea, Hydrozoa, Bryozoa), and (d) the highly localized faunas specifically associated with cold seep and hydrothermal vent systems. Similar lists of threatened taxa and/or habitats are typically included in designations by national or regional jurisdictions. For example, the multinational OSPAR Commission, that aims to protect and conserve the NE Atlantic and its resources, list includes: coral gardens, deep-sea sponge aggregations, Mytilus/Modiolus/Ostrea beds, Lophelia/Sabellaria reefs, hydrothermal vents, sea-pen and burrowing megafauna communities.

Prospectus

The threats to megafauna throughout the global ocean are very substantial, not least from demersal fishing, but also via rapid climate change, and emerging issues such as deep-sea mining. In part, our capacity to deal with these challenges depends on the ability to map and monitor these organisms over large spatial scales and over significant periods of time. The coming UN Decade of Ocean Science for Sustainable Development (2021   −   30) calls for a Clean, Healthy, Predicted, Safe, Sustainable, and Accessible ocean for all nations, stakeholders and citizens. Its strategic objectives include (i) an expanded knowledge of the ocean's biodiversity and seafloor, and (ii) an enhanced ocean observing network. The methods and technologies to realistically achieve these objectives are developing rapidly. Long-term and long-range autonomous systems are starting to have a major impact on the conduct of ocean science, currently best exemplified by the Argo Program (www.argo.ucsd.edu). Similar approaches are envisaged for fixed-point observatories in the ocean and for autonomous vehicles capable of missions of ocean basin-scale extent.

In terms of mapping and monitoring seafloor biodiversity using autonomous systems, the megafauna are likely to have a central role to play—being both amenable to such study and often the key target of policy objectives. Simply by their physical size, the megafauna can be assessed visually through mass photography of the seafloor environment, in both time and space. Fixed-point time-lapse photography already has a 40   + year history in deep-sea research, assisting in the discovery of seasonality in food supply and the importance of scavenging by the mega-hyperbenthos (Bett, 2003). Large-area mapping by photography—at scales of 10s of hectares via 100,000   s images – can now be accomplished at modest cost in shiptime via deep-ocean autonomous underwater vehicles (Morris et al., 2016). Use of such techniques in research and monitoring applications is set to increase rapidly in the marine environment, as it has with aerial drones in coastal and terrestrial settings. Further development of machine learning, artificial intelligence, in the generation of data from mass imagery will provide an additional boost to this approach.

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High resolution temporal and spatial study of the benthic biology and geochemistry of a North-Eastern Atlantic abyssal locality (BENGAL)

D.F Eardly , ... J.W Patching , in Progress in Oceanography, 2001

There have been few long-term measurements of deep sea benthos. BENGAL (High-resolution temporal and spatial study of the BENthic biology and Geochemistry of a north-eastern Atlantic abyssal Locality), was an EU-funded international experiment that sought to gain more information about seasonality in the deep sea. During the programme intensive studies of the Porcupine Abyssal Plain (PAP site) in the North-east Atlantic were conducted over a period of two years. Several research cruises visited the experimental site at 48°50′N, 16°30′W where the water depth was ~4850 m at different times of year. A multidisciplinary approach, harnessed expertise from 17 European laboratories to study biological and geochemical processes in deep sea benthos, with particular emphasis on establishing the responses shown by the benthic biota to the seasonal deposition of phytodetritus (Billett & Rice, 2001). To study the activity of the microbial community in the sediment, rates of microbial DNA and protein synthesis were measured using the incorporation of tritiated thymidine (DNA) and leucine (protein). Bacterial abundances were determined by counting under epifluorescence microscopy. In this paper, we present the results of such observations conducted during the BANGAL programme at the eutrophic site on PAP in the Eastern North Atlantic, where the sediments receive an annual deposition of phytodetritus (Rice, Thurston, & Bett, 1994), and also from an earlier EU programme (DEEPSEAS) during which observations were carried both at PAP and an oligotrophic site (EUMELI) in the subtropical East Atlantic, where no large phytodetritus depositions occur (Jacques, 1993).

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Source: https://www.sciencedirect.com/topics/earth-and-planetary-sciences/deep-sea-benthos

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