Some Wonders of the Ocean

In the last frontier on Earth, Dr Anjani Ganase discusses some anomalies and discoveries in a warming ocean.

Storm-chasing petrels

In tracking the Desertas petrels, scientists were amazed to find them following hurricanes for thousands of miles across the Atlantic Ocean. Desertas petrels nest on Bugio Island, Portugal off the west coast of Africa where hurricanes are born. Scientists found a strong correlation between the storm winds and the foraging behaviour of these birds. Petrels typically forage for fauna that occur hundreds of metres below the ocean surface. How do they do this? They wait for the creatures to surface at night and feed in the nutrient rich waters. However, researchers have found them foraging in areas where winds generated large wave action (up to 8m in height) resulting in the mixing of the water column and the upward movement of the deeper cooler and richer water from the deep to the surface. As a result, deeper dwelling organisms come up to feed close to the surface and, at the same time, allows the petrels to feed on squid, fish and other organisms attracted to the ocean’s surface after a storm has passed.

 

A school of hammerhead sharks, Mikimoto, Japan. Credit: Masayuki Agawa / Ocean Image Bank

Warming oceans and the rise of roaming sharks

It’s hard to believe but the proportion of modern sharks that roam the open ocean today only makes up 13 % of the subclass. Yet most of our understanding is largely based on these few, in particular the largest of the predators - the great white, bull, and tiger sharks. Most modern-day sharks live along the seafloor as part of the benthic habitat, feeding on the life close to the bottom. Scientists from the University of California looked into the evolutionary history of sharks to determine at what point sharks started roaming the open oceans. To do this, the researcher meticulously measured the ratio of body length to fin length for over 500 specimens of fossil and living shark species. They recognised that a larger fin to body ratio meant more efficient swimming for long distance and faster speeds; it is a trait that could be tracked back through time. Sharks have been around for about 400 million years with modern sharks appearing around 200 million years ago. During a major global warming period in the early cretaceous period (122 million years ago) with high volcanic activity and deoxygenation of the ocean, scientists found the first of many branches adapted towards open ocean. A species from that time was named the ginsu shark (Cretoxyrhina mantelli) similar to the modern-day great white shark.  Rising ocean temperatures then were much warmer than today’s ocean temperatures; global average sea surface temperatures then was 28 C, compared to 17 C today. This warming event during the mid-cretaceous period resulted in warming conditions and an anoxic ocean. As a result, it was advantageous to be able to swimming farther, faster and more efficiently in warmer oceans. The muscle associated with swimming is temperature sensitive as is known in modern sharks. While this study provides insights into the potential response of shark adaptation under warmer conditions, the cretaceous warming event also resulted in the extinction of many species. The rate of warming experienced today is much faster, happening over centuries rather than over millenia. Rapid changes in ocean temperatures may not facilitate natural adaptation and instead are likely to be associated with shark species extinction.

 

Coral reef in acid oceans

Papua New Guinea along with many countries occur along the edge of the Pacific Ring of Fire (where a number of undersea volcanoes occur). These volcanoes have significant influence on the island and coral atoll formation in the Indo-Pacific. The proximity of volcanoes and coral reefs can result in carbon dioxide seeps from volcanic vents along shallow coral reefs. These coral reefs are therefore naturally exposed to more acidic ocean conditions; here’s what coral reefs may look like with climate-related ocean acidification. The absorption of carbon dioxide into the ocean results in lowering of the ocean’s pH. This means less calcium carbonate available to crustaceans – snails, corals, shellfish - to build their shells or skeletons. Scientists from the University of Adelaide found that the coral communities which survive under conditions of greater acidification are structurally simpler, predominantly composed of massive and brain coral species, as opposed to complex branching species at sites farther away. The shift in the coral species composition and morphology had consequential impacts on the fish community present and their behaviour when compared to reefs under normal conditions (500 m or further away from the seep).

 

Two views of a deep ocean brine pool – hot tub of despair – Photos by the Ocean Exploration Trust

                                                                                                          

Hot tub of despair                                                                                

In 2015, the deep-sea explorer, Nautilus, discovered a brine pool a thousand metres below the surface in the Gulf of Mexico. Brine pools can form on the bottom of the ocean along cold seeps – areas where oil and gas may seep out of the earth’s crust with high concentrations of methane and other hydrocarbons. This particular brine pool, referred to as the “hot tub of despair” was 30.5 m in circumference and nearly 4 m in diameter with steep walls all around. The brine solution in the pool was at least four times saltier than the surrounding ocean with temperature of around 19 C. In contrast, the average temperature of the ocean at that depth is ~ 4C. Any organism that finds its way into the pool meets an untimely death and is embalmed for eternity. Only a specific species of mussels appeared to tolerate the toxic brine conditions.

 

References

Priest, Jamie, et al. "Out of shape: Ocean acidification simplifies coral reef architecture and reshuffles fish assemblages." Journal of Animal Ecology (2024).

Sternes, Phillip C., Lars Schmitz, and Timothy E. Higham. "The rise of pelagic sharks and adaptive evolution of pectoral fin morphology during the Cretaceous." Current Biology (2024).

Ventura, Francesco, et al. "Oceanic seabirds chase tropical cyclones." Current Biology (2024).

Parre, S.  Hot Tub of Despair: The deadly ocean pool that traps and pickles creatures that fall in. (2024) https://www.livescience.com/planet-earth/rivers-oceans/hot-tub-of-despair-the-deadly-ocean-pool-that-traps-and-pickles-creatures-that-fall-in