That octopuses can survive in the extreme, sunless environments around deep hydrothermal vents is surprising enough. But comparing octopuses that make their homes there has led to some even more interesting discoveries about animal development.

The rarely seen Muusoctopus hydrothermalis live some 2,495 to 2,620 meters below the surface, along the East Pacific Rise. There, they crawl along the ocean floor—at a safe distance from the superheated hydrothermal vents nearby. The vents support life sans photosynthesis by supplying nutrients that feed microorganisms. These small creatures are the foundation for a robust food web that can sustain even voracious octopuses. The M. hydrothermalis have been seen flocking to feed on amphipods near the vents.

But not all of these pale white M. hydrothermalis specimens were the same. Those collected at similar locations showed some striking physical differences, including varying quantities of suckers. Sucker counts have been assumed to be constant within single species and are often used to tell one octopus type from another (especially for poorly studied, less-than-colorful deep-sea species). So to have evidence that these qualities can, indeed range within a species is important for taxonomists and others attempting to classify different octopuses—especially those that have been preserved in museum collections, a process that often alters subtler morphological features.

Janet Voight, a curator at the Field Museum in Chicago, found that the M. hydrothermalis specimens from 9 degrees (N) latitude had some 12 to 23 percent more suckers on each arm than did those collected closer to 13 degrees (N) latitude—say 150 suckers on an arm versus 130. This pattern follows a principle known as "Jordan's Rule," named for ichthyologist David Starr Jordan. He had noted in 1891 that fish that developed in cooler temperatures had more features than those of the same species that lived in warmer waters. For example, they sometimes have more vertebrae or more gills.

Voight's discovery, published last year in the Journal of the Marine Biological Association of the United Kingdom, marks the first time this phenomenon has been described outside of fish. And its appearance in a wholly different phylum (the mollusks), suggests it might be more widely spread throughout the animal kingdom.

The find is especially curious because earlier molecular tests of different populations in this species did not detect any underlying genetic differences that would lead to these alterations.

"Why these differences exist among conspecifics remains unknown," Voight noted in her paper. But she hypothesizes that the temperature difference during development might come not from the latitude itself but the chemical composition of the hydrothermal vents. The vents closer to the 13 degrees site expelled more dissolved metals, which can bind with the poisonous sulfide ions also in the vent fluids. This might have allowed these octopuses to brood their eggs closer to the heat source and thus in warmer waters than the octopuses seeking greater distance from the sulfide and retreating to cooler waters.

The 13 M. hydrothermalis octopuses used in the study were collected by dives with the deep submergence vehicle Alvin.

Illustration courtesy of Ivan Phillipsen