Future Unwritten: Antarctic Sea Pens’ Secrets to Success

A sponge, Haliclonissa verrucosa, filter feeds in water off of Spume Island (Source: Chuck Amsler).

On the seafloor, beneath the cold, dark waters surrounding Antarctica, life blooms. Sea stars make their glacially-slow journeys along rocks, sponges rhythmically pulse water through their internal cavities, and one particular coral, the delicate sea pen Malacobelemnon daytoni, flourishes.

Sea pens are colonial, meaning that many individuals work together as a whole, each conducting a specialized task necessary for survival. The resulting shape resembles a quill pen, earning them their creative common name.

M. daytoni is one of the most abundant species in Potter Cove, located off the southwest side of King George Island in the South Shetland Islands. The environment of Potter Cove is heavily influenced by local glacial retreat, which discharges increasing quantities of sediment into the ocean. Researchers know little about benthic (defined as the lowest layer in a body of water) ecology in this region, challenging as it is to conduct scientific research in the cove’s remote, frigid waters. A recent paper in Marine Environmental Research by an international team from the Instituto Antártico Argentino, the National Scientific and Technical Research Council, Universidad Nacional de Córdoba, and the Alfred Wegener Institute analyzed the biochemistry of M. daytoni to understand its ecological success. They found that the key is a flexible, omnivorous diet and strategic reproductive techniques.

A sea pen in Potter Cove (Source: Ricardo Sahade).

Natalia Servetto, lead author on the study, is part of a group that has been studying the Potter Cove benthos since 1994. These efforts have revealed what Servetto called an “unexpected and marked shift in the system… linked to ongoing climate change processes” and to glacial retreat, which has increased sedimentation rates, affecting the benthic fauna. In the last few years, Servetto says, the abundance and distribution of M. daytoni has increased significantly, while many other Potter Cove invertebrates are becoming less abundant.

Why should the sea pen thrive while its neighbors perish? To answer this question, a team based out of the Argentine Carlini Station scuba dove every month for one year, sometimes through holes in the sea ice, to depths of 15 meters to take tissue samples from sea pens in Potter Cove.

This in itself is a feat. Chuck Amsler, a biology professor at University of Alabama at Birmingham who studies macroalgae and invertebrates in the Western Antarctic Peninsula, told GlacierHub that diving to study Antarctic life is a challenge because, “It’s damn cold!” However, Amsler added, “The scientific reward is that you have the opportunity to observe your study system directly. There is no substitute for the kind of insights one can get from that.”

Carlini Station provides access to the remote Potter Cove (Source: Natalia Servetto).

Many such insights came to fruition back in the lab, where the researchers analyzed the carbohydrate content, stable isotope ratios, and fatty acids in the coral tissues, looking for chemical clues to what the pens eat through the year. Just as a savvy New Englander might buy groceries with the seasons, eating peaches in summer, apples in autumn, root vegetables in winter, and fresh maple syrup in spring, the researchers found that the sea pen’s diet changes seasonally. In summer, M. daytoni feasts on copepods (a type of small invertebrate), phytoplankton, and a bit of macroalgae detritus. In autumn, the menu features more phytoplankton and microalgae, and in winter, macroalgae detritus and sediment are the sea pens’ bread and butter. In spring, their diet becomes fresher again when phytoplankton and microplankton return to the table.

This omnivorous, opportunistic feeding strategy allows the sea pens to eat whatever is available in a given season, reducing pressure on the species during times of food depletion. Such depletion peaks in winter and autumn, forcing M. daytoni to scavenge for organic sediment and detritus that become re-suspended from the seafloor. Sea pens have another major advantage over their neighbors: inorganic sediment from melting glaciers can clog the respiration and feeding mechanisms of filter-feeding invertebrates, while M. daytoni continues to chow down, undisturbed.

Amsler and his team begin a dive to study benthic macroalgae and invertebrates (Source: Maggie Amsler/Antarctic Photo Library).

Not only do they feed resourcefully, but the sea pens also optimize the energy they obtain. Servetto’s team found that lipid content in their tissues, associated with reproduction, increased in rapid bursts that were seasonally linked with higher food availability. This pattern suggests that sea pens can take the energetic resources offered by the environment at a given time and shunt them into reproduction, the most important process for any organism.

Beyond any single year, and beyond the bounds of the Potter Cove ecosystem, the opportunistic feeding and reproductive strategies of M. daytoni will help this species thrive. Nearly 90 percent of glaciers are retreating along the Antarctic Peninsula, causing environmental shifts that threaten many species, but could create an opportunity for sea pens to actually expand their range, Servetto says. As glacial retreat creates new ice-free areas, colonization may occur, according to Servetto, “at a previously unimagined speed.”

However, it’s not yet clear how the Antarctic coastal system will evolve as glaciers melt. Amsler says that decreasing sea ice cover will likely favor macroalgae and their associated communities of small, mobile invertebrates like amphipods and gastropods, and threaten shallow-water sessile invertebrates like the sea pens, probably pushing them into deeper water. Decrease in sea ice cover may also have broader climate impacts: the Antarctic benthos is a net carbon sink (a natural process that stores carbon), and as ice cover decreases, Amsler expects that benthic primary production will increase, removing even more carbon from the atmosphere.

Divers in Antarctica face difficult conditions, including brash ice (Source: Chuck Amsler).

No matter the outcome, the impacts of climate change on benthic Antarctic invertebrates will be manifold. Other forces, like ocean warming and acidification, will also affect M. daytoni and reshape benthic invertebrate communities, though Amsler says more work is needed to understand how. “I don’t know what the community will look like in 100 years, but I’m confident that it will be different from what we see today,” he predicted. As climate changes and glaciers melt, the flexible diet and efficient reproductive strategy of M. daytoni will give it an advantage in changing Antarctic coastal ecosystems.

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