“Southern Ocean mesopelagic fish comply with Bergmann’s rule”
Ryan A. Saunders and Geraint A. Tarling (Mar 2018)
Lanternfish reveal how ocean warming impacts the ocean’s twilight zone
A new study from the British Antarctic Survey shows how lanternfish, small bioluminescent fish from the ocean’s twilight zone, are likely to respond to the warming of the Southern Ocean. These fish are one of the most abundant groups of organisms in the oceans and inhabit the twilight zone where they feed on small crustaceans. Their large collective biomass feeds a multitude of Southern Ocean predators, including penguins and seals. Changes in their distribution can have a devastating impact on these predators.
The team studied patterns in body size of lanternfish in relation to temperature and latitude across the Scotia-Weddell sector of the Southern Ocean. By examining net samples and in situ temperature measurements from recent research surveys (2006-2009), it was found that lanternfish body size increases with decreasing temperature and increasing latitude. Furthermore, the team revealed that attaining a greater body size is vital for these organisms to survive in colder regions further south.
If ocean warming trends continue, many smaller sub-Antarctic species will also be able to reach the far south, possibly displacing the larger Antarctic species presently there. Such small fish will be less energy rich than their larger counterparts, which will have ramifications for the penguins and seals that depend on them.
Lead author Dr. Ryan Saunders says, “Understanding how lanternfish are governed by their environment is an important step to being able to predict how the Southern Ocean ecosystem will respond to future change.” The work was carried out as part of the Ecosystems program at the British Antarctic Survey, which examines the operation of Southern Ocean food-webs and their sensitivity to climatic variability and change. Lanternfish remain relatively understudied in the Southern Ocean, but are increasingly being recognized as an important alternative food source to Antarctic krill for many Southern Ocean predators. At present there is no developed fishery for lanternfish although they are receiving increasing international interest as a potential source of fishmeal.
The applicability of macroecological rules to patterns in body size varies between taxa. One of the most examined is Bergmann’s rule, which states that body size increases with decreasing temperature and increasing latitude, although the rule is not universal and the proposed mechanisms underpinning it are multifarious and lack congruence. This study considers the degree to which Bergmann’s rule applies to the Southern Ocean mesopelagic fish community. We studied patterns in body size, temperature and latitude across a 12° latitudinal gradient within the Scotia-Weddell sector. Intra-specific Bergmann’s rule was found to apply to 8 out of the 11 biomass-dominant species in the family Myctophidae. The rule was also apparent at an inter-specific level. Our study suggests that greater attainable body size in this community is a necessary attribute to reach colder regions further south. The adherence of these taxa to Bergmann’s rule enables such species to act as sentinels for identifying the drivers and consequences of ocean warming on the Southern Ocean ecosystem.