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Surviving Extinction in the Abyss

Isopods

One group of isopods has been in the deep sea for 271 million years, and isopods have colonised the depths on multiple occasions over this period. These findings are not consistent with the hypothesis that the deep-sea fauna became extinct during the anoxic events of the Mesozoic.

By Luana Lins

A new study finds evidence that deep-sea creatures survived periods when the oceans contained little or no oxygen.

The deep sea is one of the largest environments on Earth. This environment is defined as the part of the ocean that is below a depth of 200 metres, where in general a break in the continental shelf marks the start of the continental slope.

Oceans cover 70% of the Earth’s surface, with the deep sea making up 90% of the volume of the oceans. Possibly because of its impressive dimensions and remoteness from terrestrial life, the deep sea has fascinated human beings for a long time. But discovering the curious creatures of the depths took a while, especially because the deep sea was long believed to hold no life. Because of the crushing pressure and the darkness of the abyss, where plants could not grow as a food source, it was generally thought that organisms could not survive. In the early 19th century, however, sea-going naturalists showed that life was indeed abundant in the depths.

The late 19th century saw the first scientific marine expeditions, of which the Challenger expedition was perhaps the most famous. This expedition sailed from England in 1872 and discovered many new animals and collected data on a wide range of ocean features, including the geology of the seafloor, before returning in December 1876.

Nowadays, the deep sea is known for its high diversity of animals, which have many intriguing body forms that are not found in the shallow waters. One of the most well-known is the angler fish, which has a projection on top of its head that contains light-producing bacteria to attract prey in the darkness of the abyss.

Although one may think that fish are the dominant group in the deep oceans, most diversity is found among small animals that are called “invertebrate benthos”. The term “benthos” refers to animals that live on, in, or near the seabed, which is also known as the benthic zone. Groups such as molluscs, polychaetes (bristle worms) and small crustaceans (amphipods, tanaids and isopods) account for the greatest number of these species.

Research on deep-sea animals requires great expense and specialised gear. For example, to get samples of tiny sea-floor animals, devices such as corers or grabs are used. The principles behind these various tools are the same: big metal machines are sent to the bottom of the ocean from a cable attached to a ship. The device collects sediments and associated animals from the bottom that will later be studied by researchers. Other instruments can be used to obtain deep-sea samples, depending on the animal of interest and the experimental design.

The technical complexities and high costs make research in the abyss much more challenging than studying the shallow waters. Consequently, many questions about this vast environment remain unsolved.

Since the early explorations, researchers have been interested in the origins of deep-sea fauna. Have animals always been in the deep sea? Or did they just colonise it recently?

Some researchers claim that animals in the deep sea could not be older than 90 million years – the age of the last major anoxic event. This was a period with little or no dissolved oxygen in the oceans. Climate data for the Mesozoic Era 245–66.5 million years ago contain evidence for many oceanic anoxic events. During this Era, the Earth had repeated periods of greenhouse climates: scientists even came up with the fanciful term “thermosphere” for the deep ocean during these times. Average surface temperatures were higher than today, and ice caps are thought to have been absent entirely. This warm Earth had a substantially different oceanic circulation from what we experience today. Dense saline but warm waters produced bottom waters with little dissolved oxygen.

In contrast, other scientists have been impressed with various “living fossils” that live only in the deep sea, such as blind lobsters (Polychelidae) and primitive single-shelled molluscs (Monoplacophora). They thought that some deep-sea animals must have survived these great changes in the oceans over the millions of years.

Determining the Age of Deep-Sea Animals

We focused on a group of crustaceans called isopods, which are related to prawns and crabs. Because isopods are highly abundant in the deep sea, they are a good model for deep-sea animals in general. The problem we faced was how to determine the age of deep-sea isopod lineages if fossils of these animals do not exist. The solution was to include species from shallow water, fresh water and terrestrial environments, and use well-dated fossils that are related to these groups.

To investigate the age of the deep-sea fauna, my colleagues and I used a technique called the molecular clock. This analysis combines the information from DNA sequences and the fossil record. For example, if we know that two species have ten mutations between their sequences of DNA and if we know the rate at which DNA mutates, then we can estimate the amount of time it would have taken for the ten mutations to occur. The fossils mark points in the phylogenetic tree to establish a known relationship with living species and to work out the speed of the molecular clock.

Ancient Deep-Sea Isopods

Our study showed that one group of isopods has been in the deep sea for 271 million years, and that isopods have colonised the depths on multiple occasions over this period. These findings are not consistent with the hypothesis that the deep-sea fauna became extinct during the anoxic events of the Mesozoic. Instead they show that the isopods colonised the deep sea during the Late Palaeozoic Era, near the boundary between the Permian and the Triassic Periods.

This boundary is known for a mass extinction event that killed 90% of marine species. During this time, the shape of the land masses on Earth was unlike today, with one supercontinent called Pangaea surrounded by the global ocean, Panthalassa. The abyss during these times was truly enormous and continuous throughout.

Dating the colonisation of the deep sea by isopods in the Permian, and their permanence in this environment until the present, shows that some animals survived the anoxic events. How exactly they survived remains unclear, but some investigators have argued that anoxia was limited to the shallower depths so the abyss was a refuge for many animals. Some animals living in the deep sea, such as the isopods, survived in these oxygenated refuges and then expanded to conquer the abyss when the oxygen-starved times were over.

Our study on the evolution of isopods in the deep waters has improved our understanding of the deep sea. By reconstructing the time frame of the isopod colonisation, we showed that the anoxic events did not kill all fauna in deep waters, reinforcing the theory that this environment was not completely depleted of oxygen. Therefore, animals other than isopods might also have survived in the deepest oceans for literally eons.

Luana Lins is a PhD candidate at the University of Sydney, and works in collaboration with the Australian Museum.