Extreme Earth Life: Flatworm Cocoons Discovered In Abyssal Depths Of Over 6,000 Meters

Editor’s note: when we begin to robotically explore the ice-covered oceans of Enceladus, Europa, Ganymede, Mimas etc. we’ll need to make the most of the visits. Learning how to best operate robotics systems in a deep ocean environment searching for unknown and unexpected life forms is something we can perfect – now – on Earth. Finding life forms on our home world that can tolerate extreme conditions expands the envelope of what is possible for life to do on other worlds. Studying adaptations on Earth helps inform future explorers what to look for – and where to look – on the worlds we’ll one day explore.

While knowledge of early ontogeny in abyssal animals is highly limited in general, it was completely lacking for abyssal, free-living platyhelminths.

We discovered flatworm egg capsules (or ‘cocoons’) on rocks collected at depths of 6176-6200 m on the abyssal slope of the Kuril-Kamchatka Trench, northwestern Pacific.

The egg capsules were black and spherical, around 3 mm in diameter, and contained three to seven individuals (n = 4) at the same developmental stage, either the spherical (putative early embryo) or vermiform (putative late embryo) stages.

A molecular phylogenetic analysis based on 18S and 28S rRNA sequences revealed that the flatworms belong in suborder Maricola in Tricladida and suggested that they may have colonized from shallow to deep waters. This study provides the deepest record for free-living flatworms and the first information on their early life stages in the abyssal zone, which were very similar to those in shallow-water forms.

This similarity in development between the relatively benign shallow-water and the extreme abyssal environments suggests that triclads adapting to the latter faced primarily physiological and/or ecological adaptive challenges rather than developmental ones.

Flatworm cocoons in the abyss: same plan under pressure, Biology Letters via PubMed
Astrobiology,