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An Empire Lacking Food

Once viewed as a barren expanse, the deep seafloor is a biologically elaborate ecosystem whose fate is tied to life above, near the sea surface

Craig McClain

Adapting to Extremes

2010-11McClainF7.jpgClick to Enlarge ImageIn the deep sea, evolutionary novelties also abound: Organisms display creative solutions to deal with lower food availability. One of the best examples of defiance of the general trend of miniaturization is the giant isopod Bathynomus giganteus, a 36-centimeter-long deep-sea relative of the common pill bug. It is the largest in the order Isopoda and one of the largest known crustaceans. Baited traps placed on the deep seafloor will, within an hour, quickly attract dozens of these hungry scavengers. Greater body size translates into faster and more efficient running for mammals, and swimming for fish. Francois Bourliere of the University of Paris in 1975 stated that “the fact that a horse can move one gram of its body weight over one kilometer more cheaply than a mouse is another evolutionary advantage of a large body size.” The larger size of the giant isopod may be an adaptation that allows it to quickly and efficiently monopolize food in the deep sea. Larger size also confers a greater foraging area to an organism, whether on land or in sea—no doubt an important trait in a food-limited area.

The giant isopod can survive eight weeks between feedings. In aquaria, the gastropod Neptunea amianta, a snail the size of a tennis ball, can survive up to three months between meals. This fasting potential reflects the ability of larger organisms to hold greater lipid reserves. N. amianta also exhibits another intriguing adaptation as a possible response to food limitation: Females lay foot-high, leathery cases containing thousands of eggs. The first young to hatch crawl around the egg case devouring their unborn siblings. Although this may seem malevolent, ensuring the survival of a few young with readily available meals is essential in the deep.

Females in the fish family Ceratiidae ensure their survival in a food-limited environment with the organ responsible for the group’s common name, anglerfish. Evolutionarily derived from the spines of the dorsal fin, a lighted blue lure rests on a stalk above the forehead. Symbiotic bacteria at the lure’s tip produce light that attracts prey to the waiting mouth of the female. Siphonophore jellyfish in the genus Erenna also utilize light to attract prey. Red light, a rarity among luminescent organisms, is emitted when the jellyfish flicks its peculiarly shaped tentacles, making them closely resemble copepods, a food source for many small fish.

Bioluminescence may also help compensate for low food availability in another vital area of life—sex. A low population census resulting from a scant food supply presents a formidable challenge for creatures looking for mates. For many fishes, bioluminescence serves as a different type of lure. In the wittily named paper “Sex with the lights on?” Peter Herring of the University of Southampton in 2007 stated that for a least one family of fish there is “strong circumstantial evidence for [a bioluminescent organ’s] involvement in sexual signaling.” In anglerfish, the difficulty of finding a mate translates into more evolutionary novelty. Significantly dwarfed compared to females, male anglerfish live to find a mate and stick with her. Males seek out females using a heightened olfactory system, Upon contact, enzymes are triggered that fuse his mouth to her body, and eventually all of his organs, except for his gonads, atrophy. The parasitic male becomes a lasting source of sperm.

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