As the deepest regions of the ocean, hadal zones are among the most hostile environments on Earth for the high hydrostatic pressure, darkness, limited food resources, low temperatures, and hypoxia. However, they host active and diverse biological communities.  

Liparid snailfishes is the most common hadal vertebrate species with habitats ranging from intertidal to depths exceeding 8,100 m. The researchers have found a series of adaptive characteristics on morphology and genome in a Mariana snailfish.

In a study published in Genes, the researchers from Institute of Hydrobiology (IHB) of the Chinese Academy of Sciences reported the research progress in the olfactory evolution of the Mariana snailfish, which provides new insights into the evolutionary adaptation to the hadal environment.  

Through comparative analysis of two main olfactory receptor family, olfactory receptor (OR) and trace amine-associated receptor (TAAR) with eight fish species, the researchers revealed the evolution dynamic of olfactory receptor gene repertoire in the Mariana snailfish for the first time.  

The results showed that the number of OR gene family in the Mariana snailfish does not increase to compensate the visual degradation, but decrease dramatically with a large proportion of pseudogenes. In contrast, the results showed that the number of TAAR gene family is comparable with its shallow-sea relative.

The massive loss in ORs but not in TAARs suggested decreased dependence on OR gene function, while its reliance on TAAR may still be important. This is consistent with its specific feeding in the simple chemical environment and the ability to fast respond to amines released from biological decay.  

Besides, the researchers found that distinct subfamilies of OR and TAAR gene family evolve independently. The OR gene subfamilies showed overall contraction while the number of TAAR gene subfamilies were reduced.

Moreover, they detected strongly relaxed selection in OR but observed slightly enhanced selective strength in TAAR, which is consistent with natural selection as the main force to shape the gene family evolution.

Two Mariana snailfish (Pseudoliparis swirei) feeding on amphipods attracted by the bait. (Image by IHB)