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Figure 4.7: Veiled anglemouth, Cyclothone microdon, a mesopelagic (to abyssopelatic) bristlemouth of the family Gonostomatidae. Size 10 cm.
Figure 4.8: From top: Lanternfish Myctophum punctatum (10 cm); Atlantic silver hatchetfish Argyropelecus aculeatus (7 cm); Slender Lightfish Vinciguerria attenuata (4 cm); an anglerfish Bufocer-atias wedli (10 cm). Note extreme eye size, easily larger than the brain itself.
their ventral side.
Food is often limited and patchy in the mesopelagic, lead-ing to dietary adaptations. Common adaptations fish may have include sensitive eyes and huge jaws for enhanced and opportunistic feeding. Fish are also generally small to reduce the energy requirement for growth and muscle formation.
Other feeding adaptations include jaws that can unhinge, elas-tic throats, and massive, long teeth. Some predators develop bioluminescent lures, such as the tasselled anglerfish, which can attract prey, while others respond to pressure or chemical cues instead of relying on vision.
4.1.3 Deep Sea
In the deep ocean, the waters extend far below the epipelagic zone (Fig. 4.1), and support very different types of pelagic life forms adapted to living in these deeper zones. Some deep-sea pelagic groups, such as the lanternfish, ridgehead, marine hatchetfish, and lightfish families (Fig. 4.8) are sometimes termed “pseudoceanic”, because rather than having an even distribution in open water, they occur in significantly higher abundances around structural oases, notably seamounts and over continental slopes. The phenomenon is explained by the likewise abundance of prey species which are also attracted to the structures.
The fish in the different pelagic and deep water benthic zones are physically structured, and behave in ways that differ markedly from each other. Groups of coexisting species within each zone all seem to operate in similar ways, such as the small mesopelagic vertically migrating plankton-feeders, the bathypelagic anglerfishes, and the deep water benthic grandiers.
Figure 4.9: Common brittlestar Ophiura ophiura, a typical species of the benthos. Size 15 cm.
4Which is still an order of magnitude higher than in the water column: Ritzau, W. (1996).
Microbial activity in the benthic boundary layer:
Small-scale distribution and its relationship to the hydrodynamic regime. Journal of Sea Research, 36(3–4):171–180
5This is the reason why there are no “shark falls”
or “tuna falls”: their density is marginally lower than that of water.
from 1 mm to 1 cm every 1000 years.
“Benthos” (from Greek β ´eνθoς = “the depth”) is the com-munity of organisms which live on, in, or near the seabed.
This community lives in or near marine sedimentary envi-ronments, from tidal pools along the foreshore, out to the continental shelf, and then down to the abyssal depths. The benthic zone is the ecological region on, in and immediately above the seabed, including the sediment surface and some sub-surface layers. Benthos generally live in close relationship with the substrate bottom, and many such organisms are permanently attached to the bottom.
In the euphotic zone, phytoplankton and algae contribute much of the energy input for benthic organisms. In the dark, food sources are any form of organic material, e.g. marine snow or detritus. Filter feeders, such as sponges and bivalves, dominate hard, sandy bottoms. Deposit feeders, such as polychaetes, populate softer bottoms. Fish as well as sea and brittle stars (Fig. 4.9), snails, cephalopods, and crustaceans are important predators and scavengers.
Due to the extremely low supply of the dark benthos with en-ergy,4even invertebrate scavengers (many echinoderms, crus-taceans but also snails and clams) have developed a fine sense of smell and aggregate on food sources from many kilometers. A par-ticularly spectacular case is the succession of species scavenging on whale carcasses.
Whale falls
A whale fall occurs when the carcass of a whale has fallen onto the ocean floor at a depth greater than 1,000 m, in the bathyal or abyssal zones. On the sea floor, these carcasses can create complex localized ecosystems that supply suste-nance to deep-sea organisms for decades. This is unlike in shallower waters, where a whale carcass will be consumed by scavengers over a relatively short period of time.
The bodies of most great whales (which includes sperm whales and many species of baleen whale) are slightly denser than the surrounding seawater, and only become positively buoyant when the lungs are filled with air.5When the lungs deflate, the whale carcasses can reach the sea floor quickly and relatively intact due to a lack of significant whale-fall scavengers in the water column. Once in the deep-sea, cold temperatures slow decomposition rates, and high hydrostatic pressures increase gas solubility, allowing whale falls to remain intact and sink to ever greater depths.
Deep-sea whale falls are thought to be hotspots of adap-tive radiation for specialized fauna. Organisms that have been observed at deep-sea whale fall sites include giant iso-pods, squat lobsters, polychaetes, prawns, shrimp, lobsters, hagfish, the boneworm Osedax (an annelid, p. 93), crabs, sea cucumbers, and sleeper sharks (Somniosus spp.). New
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Figure 4.10: A chemoautotrophic whale-fall com-munity in the Santa Cruz basin off southern California at a depth of 1,674 m, including bac-teria mats, vesicomyid clams in the sediments, galatheid crabs, polynoids, and a variety of other invertebrates.
6Have a look at a Natural World Facts video (8 min) on the topic.
Figure 4.11: Black smokers were first discovered in 1979on the East Pacific Rise at 21°north latitude at a depth below 2500 m.
species have been discovered, including some potentially specializing in whale falls.6Researchers estimate that 690,000 carcasses/skeletons of the nine largest whale species are in one of the four stages of decomposition at any one time. This estimate implies an average spacing of 12 km and as little as 5 km along migration routes. They hypothesize that this distance is short enough to allow decomposers’ larvae to disperse/migrate from one to another.
Chemosynthesis - hydrothermal vents and cold seeps
A hydrothermal vent is a fissure on the sea floor from which geothermally heated water discharges. Hydrothermal vents are commonly found near volcanically active places, areas where tectonic plates are moving apart at spreading centres, ocean basins, and hotspots. Relative to the majority of the deep sea, the areas around submarine hydrothermal vents are biologically more productive, often hosting complex com-munities fuelled by the chemicals dissolved in the vent fluids.
Chemosynthetic (rather than photosynthetic or organotrophic) bacteria and archaea form the base of the food chain, support-ing diverse organisms, includsupport-ing giant tube worms, clams, limpets and shrimp.
The most spectacular, and most common, hydrothermal vent is the Black Smoker (Fig. 4.11). They appear as black, chimney-like structures that emit a cloud of black material.
Black smokers are formed in fields hundreds of meters wide when superheated water from below Earth’s crust comes through the ocean floor (water may attain temperatures above 400°C). This water is rich in dissolved minerals from the crust, most notably sulfides.
The hydrothermal vents are recognized as a type of chemo-synthetic-based ecosystems where primary productivity is
7Instead of releasing oxygen gas while fixing carbon dioxide as in photosynthesis, hydrogen sulfide chemosynthesis produces solid globules of sulfur in the process:
18H2S + 6 CO2+ 3 O2→ C6H12O6+ 12 H2O + 18 S
Figure 4.12: Large concentrations of tubeworm Riftia pachyptila, with anemones and mussels colonizing in close proximity in the Galapagos Rift.
Figure 4.13: An azimuthal projection showing (top) the Arctic Ocean and the North Pole, and (bottom) the South Geographic Pole (1), South Magnetic Pole (2), South Geomagnetic Pole (3; not of our concern) and South Pole of Inaccessibility (4; ditto).
Outermost blue lines are 60°.
8Schofield, O., Ducklow, H. W., Martinson, D. G., Meredith, M. P., Moline, M. A., and Fraser, W. R.
(2010). How do polar marine ecosystems respond to rapid climate change? Science, 328(5985):1520–3
fuelled by chemical compounds as energy sources instead of light (chemoautotrophy).7The chemosynthetic bacteria grow into a thick mat which attracts other organisms, such as amphipods and copepods, which graze upon the bacteria directly. Larger organisms, such as snails, shrimp, crabs, tube worms, fish (especially eelpout, cutthroat eel, ophidiiforms and Symphurus thermophilus), and octopuses (notably Vulcanoc-topus hydrothermalis), form a food chain of predator and prey relationships above the primary consumers. The main fami-lies of organisms found around seafloor vents are annelida, tubeworms, gastropods, and crustaceans, with large bivalves and “eyeless” shrimp making up the bulk of non-microbial organisms.
Tube worms (Siboglinidae), which may grow to over 2 m tall in the largest species, often form an important part of the community around a hydrothermal vent (Fig. 4.12). They have no mouth or digestive tract, and like parasitic worms, absorb nutrients produced by the bacteria in their tissues.
About 10 billion bacteria are found per g of tubeworm tissue.
Tubeworms have red plumes which contain hemoglobin.
Hemoglobin combines with hydrogen sulfide and transfers it to the bacteria living inside the worm. In return, the bacteria nourish the worm with carbon compounds.