The Japanese sea cucumber is not only a prized Asian seafood, but also an unusual and fascinating echinoderm which has a particularly disgusting method of repelling predators: when threatened, it can eject its internal organs. A new study in PLOS Biology examined the genes that might enable it to regenerate the viscera rapidly, as well as the animal’s evolutionary pathway.
To find out more, I interviewed Dr. Lina Sun from the Institute of Oceanology at the Chinese Academy of Sciences via email.
Why did you decide to study the genetics of marine species, and specifically of sea cucumbers?
LS: The sea cucumber Apostichopus japonicus is being cultivated commercially on a large scale and is one of the most valuable sea foods worldwide, due to its potent nutritional and medicinal properties. In China alone, around 200,000 tons of sea cucumbers are produced every year. A. japonicus exhibits many fascinating biological characteristics which have attracted many scientists, making it one of the most studied echinoderms.
Sea cucumbers might be named after a vegetable, but are animals in the echinoderm taxon. What distinguishes them from other echinoderms such as starfish and sea urchins?
LS: The sea cucumber body is soft and cylindrical, and somewhat resembles a cucumber. This appearance is very different from the hard calcified exoskeleton and five-sided symmetry of other echinoderms.
Tell us about their clever, if disgusting, mechanism of repelling predators.
LS: Sea cucumbers can abandon their viscera or body wall when threatened. They eject certain internal organs, including their gut, through their anus to repel potential predators. The organs can then rapidly regenerate so that the sea cucumber survives. It is very amazing.
How did you obtain your sea cucumber samples?
LS: We have already extensively studied certain interesting characteristics of sea cucumbers, including their regeneration and their response to adverse circumstances. We have therefore been cultivating the animal, and have bred new varieties including White, Purple, Thorny, Fast-growing and High Temperature-tolerant sea cucumbers. Of course, it is also possible to harvest sea cucumbers from the sea floor.
What did you find out about the genes behind sea cucumber regeneration?
LS: We discovered a number of sea cucumber-specific genes, PSP94-like genes. The cluster is located on scaffold889, with 11 tandem duplications. We observed that expression of the cluster was upregulated more than ten thousand-fold in the sea cucumber intestine during the early stages of regeneration. These genes were not expressed in any other stages, so may have key roles in visceral regeneration.
What did you learn about the sea cucumber’s likely evolutionary history?
LS: We found evidence to date the divergence of echinoderms from their sister taxon, hemichordates, to about 533 million years ago, around the time of the Cambrian Explosion. Our phylogenomic analysis supports the theory that sea cucumbers, as sister group of sea urchins and sea lilies, formed the basal taxon of echinoderms. Sea cucumbers were estimated to have diverged from sea urchins and sea lilies approximately 479 million years ago, indicating a long evolutionary history for the echinoderms.
What are the next steps for your research?
LS: The sea cucumber is a particularly promising model animal for regenerative medicine. In the future, we will continue to research its special behaviors, as well as its geographical distribution and adaptive evolution. We also hope that our work with selective breeding and aquaculture of sea cucumbers and sea urchins might help to ensure the long term sustainability of these animals.
Research Article: Zhang X, Sun L, Yuan J, Sun Y, Gao Y, Zhang L, et al. (2017) The sea cucumber genome provides insights into morphological evolution and visceral regeneration. PLoS Biol 15(10): e2003790. https://doi.org/10.1371/journal.pbio.2003790
Images Credits: Qiang Xu; Lina Sun