Article: Feagans-Bartow, J. N. and T. T. Sutton (2014). “Ecology of the oceanic rim: pelagic eels as key ecosystem components.” Marine Ecology Progress Series 502: 257-266.
Thoughts of the deep sea conjure up images of alien-like fish and giant squid swimming through darkness; the deep sea seems like another world. To humans, the deep sea IS another world; it makes up 92% of the ocean’s volume (deep ocean defined here as >200 meters in depth) and still remains mostly a mystery. One of the biggest challenges facing scientists exploring the deep is trying to understand the transfer of energy and food web dynamics. In shallow waters, primary producers thrive using sunlight for photosynthesis, creating energy for the rest of the ecosystem. But the deep ocean lies beyond the reach of sunlight, meaning that organisms in this zone are not able to fix sunlight for energy, so they must get their energy elsewhere. Some oceanic species split their days between the deep sea and surface waters, undergoing what is called vertical migration in order to feed. Vertical migrators, as well as sinking organic matter, represent significant ways that energy and food reach the deep. Scientists have been investigating links between organisms in the continental shelf and the deep sea (Fig 1), looking for species that help transfer this energy. Recent research conducted in the Gulf of Maine has found a common pelagic eel, inhabiting the “oceanic rim,” playing an important role in bringing energy from the shelf to the deep.
On a research cruise in 2004, scientists explored the southern slope of George’s Bank in the Gulf of Maine. This area is described as an “oceanic rim ecosystem” where shelf ecosystems meet deep ocean ecosystems. Researchers trawled waters deeper than 200 meters taking record of the species found and their overall biomass. As expected, decapods (crustaceans like shrimp and crabs) and euphausiids (crustaceans like krill, Fig 2)) were found in the highest abundance. But researchers also found high abundance and biomass of fish, mostly comprised of the snipe eel (Nemichthys scolopaceus, Fig 3 and Fig 4). Due to their high abundance and their location in this rim ecosystem, snipe eels were further investigated in order to determine their role in the food web. Many of the snipe eels collected were preserved for gut content analysis and were separated by what time of day they were caught.
It was found that snipe eels mostly consumed decapods and euphausiids, the most abundant organisms in this zone. However, snipe eels are capable of consuming larger prey also found in abundance in this zone, like other species of fish and squid. This shows that the snipe eel is a selective feeder, opting for small, but abundant, crustaceans (Fig 5). Previous studies had indicated that decapods and euphausiids were an important link in the food chain between primary producers in the surface waters and pelagic fish, but surveys found that very few fish predators took advantage of this abundant resource.
Broadly stated, this research has furthered our understanding of deep sea ecosystems and food webs. Snipe eels play an important role in transferring energy from the highly productive surface waters to the deep ocean. These eels appear to be one of only a few organisms to take advantage of highly abundant crustacean prey, creating a small, but important, link in the food chain. Without these eels, decapods and euphausiids could be a dead end for energy and deep sea organisms would lose an important source of food. We may not know enough to consider snipe eels a keystone species in this ecosystem, but their role certainly seems to be valuable. We are beginning to unravel the mysteries of the deep, and studies like this are greatly helping our understanding of how these systems work and the key players involved. More research is needed in order to further our understanding of deep ocean food web dynamics, but establishing trophic relationships and potential keystone species is a major step in this process.