Investigation of a Subcutaneous Gelatinous Tissue in the Hadal Snailfish Proximate Chemical Composition, Comparative Video Analysis, and Robotic Modelling

Abstract

Deep-sea fishes are known to have muscles that are higher in water content than their shallow-living relatives. In addition to this watery muscle, some deep-dwelling fishes also have a gelatinous layer either directly below the skin or around the spine. This study investigated the composition and implications of this mysterious gelatinous tissue in one of the planet’s deepest-living fishes. Gel tissues from eight deep water species were analyzed for water content, ionic composition, and osmolality. Bulk protein, lipid, and carbohydrate assays were also conducted. These analyses do not support the hypotheses that this tissue plays a role in nutrient storage or buoyancy. The gelatinous layer is most obvious in the hadal snailfish Notoliparis kermadecensis, making it an appropriate model organism to investigate gel function. The authors propose that the gelatinous tissue that surrounds the fish’s muscle may act as an energetically inexpensive method of increasing swimming efficiency. To test this hypothesis, swimming performance in the gelatinous hadal snailfish was compared to swimming performance in the tidal snailfish, Liparis florae, which have similar morphology, but with no subcutaneous gel. Video analyses show that Liparis florae swam more body lengths per second than their hadal counterparts. A robotic snailfish model was also used to analyze the impacts of the gelatinous layer on locomotory performance. The robot swam trials with tails of varying water volume. The model showed higher swimming performance with a water-filled tail than with an empty one. Data from these three analyses suggest that the gelatinous layer may aid hadal snailfish locomotion.