Challenges of the deep

Eric Anderson

The deep oceans contain marine species that can easily be mistaken for aliens.  Fish, cephalopods, coral, worms, and crustaceans have evolved extraordinary adaptations to live in extreme conditions.  The region of the ocean these organisms live in is known as the bathyal (1000-4000 meters below the ocean surface) and abyssal zone (greater than 4000 meters).  Immense pressure, low oxygen, extreme cold, and limited light and food are several of the challenges these organisms face in these extreme environments.  They are also what make these species so allusive to researchers.  However, the recent advances in technology of remotely operated vehicles had allowed for scientists to  discover new species and observe their behaviors in their natural environment.  Their unique adaptations and behaviors are what motivate researchers to devote many hours and resources to the study of these peculiar creatures.

The Blob Sculpin (Psychrolutes phrictus)
 is a good example of a "globular" body shape

The pressure that fish and invertebrates experience at such depths are so great, that terrestrial and shallow-water organisms would implode.  In order to avoid being crushed, the pressure inside the bodies of these organisms must be consistent with their surroundings.  This is one of the reasons deep-sea fish look vastly different from their shallower relatives.  One common characteristic abyssal fish is their "globular" body shape.  A reduced muscular structure and water-filled body cavity  is one method of functioning at great pressures.  However, this restricts the speed at which these fish can swim, so most have evolved to become sit-and-wait predators and have come up with alternative methods of movement, such as "walking" using their pectoral fins.

The increased pressure at which these animals live also affects them at at the cellular level.  Pressure decreases the fluidity of membranes as the molecules are crammed together.  Proteins that are functional at normal pressures are denatured and not functional at high pressures.  Therefore, deep-sea organisms have a greater proportion of unsaturated fatty acids, which increases membrane fluidity, than shallow-water organisms and have proteins that are adapted to function at extremely high pressures.

Another challenge bathyal or abyssal animals face is the scarcity of food and oxygen.  Most animals have adapted to function at extremely low metabolic rates.  Some have also evolved unique characteristics to increase their efficiency in capturing prey.  Most predators use the sit-and-wait technique and have an upward-turned mouth containing long teeth.

The Fangtooth (Anoplogaster cornuta) has the largest teeth proportional to body size than any fish in the ocean

Furthermore, deep sea anglerfish have evolved a unique structure known as an esca connected to a modified dorsal fin ray (or icillium), which it dangles over its upward-turned mouth in order to attract prey.  When the prey item gets close enough to the lure, the angler fish will quickly close its mouth, trapping the helpless animal in its long and sharp teeth.

An additional adaptation the angler fish has evolved aids with reproduction.  Because the bathyal and abyssal zones are so vast, finding a mate can be extremely difficult.  Female anglerfish are much larger and more developed compared to males.  When a male encounters a female, it will bite onto the underbelly of the female and remain attached for life (talk about a clinger).  Eventually the male will fuse with the female.  They will share a common circulatory system and his sensory organs will degenerate, until he is nothing more than a sack of sperm.  This allows the female anglerfish to reproduce whenever she wants, without having to find a mate.

A female anglerfish with two males attached to her underbelly

Both the bathyal and abyssal zones constitute what is know as the aphotic zone, meaning little or no light penetrates to these depths.  Most species have reduced eyes and rely on their other senses to find food and mates.  However, there are examples of fish that have adaptations in order to make use of the sense of sight.  Flashlight fish and lantern fish have bioluminescent organs beneath their eyes.  These organs contain symbiotic bioluminescent bacteria and provide light for their field of vision.  Other species of fish and invertebrates utilize bioluminescence to attract prey, confuse predators, or to change their silhouette in order to hide from predators.

A notable adaptation to improve vision in the darkness of the ocean is the tubular eyes of the barreleye fish.  Its tubular eyes are extremely efficient for collecting any available light and are contained within the fishes transparent head.  Usually its eyes are maintained in an upward facing position so it can look for food to steal from jellies, where its transparent head is useful for protecting these sensitive organs from being stung by tentacles.  However, recently recearchers have discovered that the barreleye has the ability to rotate its eyes facing forward, allowing its moth to be in its field of vision.

The Barreleye (Macropinna microstoma) contains tubular eyes (green spheres) inside its transparent head.  The dark circles above its mouth, which resemble eyes, are actually its olfactory organs

Deep sea gigantism is a phenomenon often seen in the deep ocean environment.  Scientists hypothesize that it may be due decreased predation and temperature.  The Japanese spider crab, giant oarfish, and giant squid are examples of deep sea giants.  Researchers have recently captured footage of the elusive giant squid in its natural environment.  Discover Channel is set to air an episode on this amazing giant on January 27th, so be on the look out!

The Giant Squid (Architeuthis sp.) caught by researchers off the Ogasawara Islands, Japan

The environments of the bathyal and abyssal zones of the ocean are so harsh that marine scientists once thought them to be completely void of life.  But with recent technological advances, new species have been discovered almost every observational study.  However, they are still poorly understood, as keeping them in captivity in order to perform experiments is nearly impossible.  So for now, we will have to be satisfied with images and videos of these bizarre creatures, leaving many hypotheses about their life history and behaviors up to the imagination of curious observers.

REFERENCES

Cowles, D.L. and J.J. Childress.  1995.  Aerobic metabolism of the anglerfish Melanocetus johnsoni, a deep-pelagic marine sit-and-wait predator.  Deep-Sea Research Part I 42(9): 1631-1638.

Lundsten, L., S.B. Johnson, G.M. Cailliet, A.P. DeVogelaere, and D. A. Clague.  2012.  Morphological, molecular, and in situ behavioral observations of the rare deep-sea anglerfish Chaunacops coloratus (Garman, 1899), order Lophiiformes, in the eastern North Pacific.  Deep-Sea Research Part I 68: 46-53.

McClain, C.R., A.G. Boyer, and G. Rosenberg.  2006.  The island rule and the evolution of body size in the deep sea.  Journal of Biogeography  33: 1578-1584.

Robison, B.H. and K.R. Reisenbichler.  2008.  Macropinna microstoma and the paradox of its tubular eyes.  Copeia 4: 780-784.

Wharton D.A.  2004.  Life at the Limits Organisms in Extreme Environments.  Cambridge University Press.  Cambridge, United Kindom.