The supergiant bathynomid is a deep-sea isopod famous for surviving more than five years without food. Despite residing in an extremely low-nutrient habitat, these organisms exhibit pronounced body gigantism, a trait that requires substantial energy. This raises an energy paradox: How do these apparently energy-hungry isopods sustain their enormous size given the sporadic availability of food in the deep sea?
To answer this question, a research team from the Institute of Oceanology of the Chinese Academy of Sciences (IOCAS) has uncovered the mechanisms enabling these deep-sea supergiants to thrive under extreme nutritional constraints.
By combining multi-omics analyses with functional assays, the researchers discovered that deep-sea isopods have a dual survival strategy to cope with nutrient-poor (oligotrophic) conditions: an enlarged stomach that can store large amounts of food and an extremely low basal metabolic rate (BMR).
The findings were published in Cell on June 5.
For their study, the researchers analyzed two isopod species from different depths: Bathynomus jamesi from approximately 898 meters and Bathynomus doederleini from around 300 meters. By integrating comparative genomics with morphological, physiological, behavioral, and metagenomic analyses, they revealed a strategy of "increasing revenue and reducing expenditure" for coping with food-limited conditions.
The stomach of deep-sea isopods occupies about two-thirds of their entire body, far larger than that of their shallower-water or intertidal relatives. When fully filled with food, the stomach contains a finely ground, extensively digested, mud-like mixture that involves a relatively low proportion of digestive bacteria such as Firmicutes. Instead, it is enriched with Chlamydiae, which are associated with lipid storage. These features suggest that deep-sea isopods may consume large amounts of food when feeding opportunities arise and then drastically reduce their BMR, allowing these food reserves to be digested and utilized over extended periods.
The researchers also identified a horizontally transferred gene, ND1, that originated from an exogenous symbiotic bacterium and was subsequently integrated into the isopod genome. This gene, which is homologous to a component of Complex I in the electron transport chain, is hypothesized to play a critical role in energy metabolism. As an exogenously acquired gene, ND1 appears to overcome certain limitations of horizontal gene transfer by being able to undergo post-transfer duplication and achieve ultra-high expression.
Moreover, the researchers discovered a gene expression regulation mechanism in deep-sea isopods that achieves "high efficiency, energy conservation, and precise control" via epigenetic modification of histones. The ultra-high expression of ND1 is specifically regulated by histone acetylation.
To test the gene's function, the researchers introduced ND1 into zebrafish, nematodes, and human 293T cells. The results showed that ND1 accelerated energy metabolism at normal temperatures, making the organisms less tolerant of starvation. However, under low-temperature conditions (which simulate the deep-sea environment), ND1 knock-in effectively suppressed energy metabolism and reduced mitochondrial activity. This increased starvation tolerance in zebrafish by 37%.
These results indicate that ND1 modulates the mitochondrial metabolic network by fine-tuning the degree of metabolic depression, thereby resolving the fundamental trade-off between the high energy demand of gigantism and the need for metabolic suppression in extreme environments.
This study reveals for the first time a novel evolutionary strategy in which deep‑sea megafauna reprogram their energy allocation through a combination of horizontal gene transfer and epigenetic optimization.
"Our work not only deciphers the mystery of ultra-long starvation tolerance in deep-sea isopods," said YUAN Jianbo, first author of the study, "but also provides an important paradigm for understanding how life balances growth and survival in extreme environments."
