Oxygen is an essential gas needed by most living organisms on Earth, as it plays an important role in their growth, reproduction, and food production. Even deep sea creatures have adapted to extract oxygen directly from the water. Although breathing oxygen is considered a fundamental characteristic of multicellular animals, at least one group of marine invertebrates called Loriciferans has lost this ability.
Sediment-Dwelling Animals
Phylum Loricifera refers to a group of microscopes, sediment-dwelling marine invertebrates distributed worldwide across an extensive range of depths. They are so tiny, so rare, and so recently discovered that they have yet to receive a common name.
Its members are characterized by a hard, corset-shaped protective shell called lorica. Inside this shell is a head covered in more than 200 elongated spines called scalids. They are tiny, measuring less than half a millimeter long, making them invisible to the naked eye.
Loriciferans typically thrived in the space between sand and mud particles in the ocean. This is where they obtain their food, which consists of microalgae, bacteria, and organic particles, which are too small for most other organisms.
Since the phylum was discovered in 1983 near Roscoff, France, by R.M. Kristensen, just under 90 species have been identified by experts. Despite their recent discovery, the discovery of a preserved loriciferan from Cambrian mudstones in Canada showed that they are an ancient group that has scarcely altered their body plan for 500 million years.
First Oxygen-Free Animals
Deep on the floor of the Mediterranean Sea, a team of scientists led by Roberto Danovaro from Marche Polytechnic University in Ancona, Italy, discovered three new species from the Loricifera group. One species has been officially named Spinoloricus Cinzia, while the other are currently designated Rugiloricus and Pliciloricus.
The creatures reside deep in the L'Atalante basin, one of the harshest environments on Earth. This place contains salt brine so dense that it does not mix with oxygen-containing waters above.
In this study, the research team concluded that the newly discovered species completely lack mitochondria, the cell organelles that burn oxygen to produce energy for the body. However, these researchers did not sequence the mitochondrial DNA of the creatures to confirm their claim.
In a separate study entitled "A cnidarian parasite of salmon (Myxozoa: Henneguya) lacks a mitochondrial genome," another team led by Dayana Yahalomi of Tel Aviv University focused on Henneguya salminicola. This parasite afflicts certain species of salmon. The experts used deep sequencing approaches to confirm the lack of mitochondrial DNA in H. salminicola.
Because of the lack of this important biomolecule, the animal lost its ability to perform aerobic cellular respiration. It does not only indicate that Henneguya salminicola does not rely on oxygen to survive, but it also suggests that some major eukaryotic properties are not ubiquitous among animals.
According to the scientist, these species are examples of animals that evolved to become less complex over time instead of being more complex to survive in their environment inside the muscles of salmon.
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