The Adhesion System of Remora Fish Under Study

Researchers Examine Remora Fish Adhesion System When a shark is spotted in the ocean, humans and marine animals typically flee in similar fashion. The remora fish, however, is an exception—it swims toward a shark and uses a suction disc on its head to attach itself to the predatory shark. While we understand why remoras attach to larger marine animals for transportation, protection and food, the question of how they attach and detach without harming the creatures they bond with has remained unanswered. A new study conducted by researchers at the Georgia Tech Research Institute (GTRI) provides details on the structure and tissue characteristics of the remora's unique adhesion system. The researchers plan to use this information to create a remora-inspired, reversible adhesive that could be used to develop painless bandages that leave no residue, attach sensors to objects in aquatic or military reconnaissance environments, replace surgical clamps, and assist robots in climbing. Jason Nadler, senior research engineer at GTRI: "While other organisms with unique adhesive properties, such as geckos, tree frogs and insects, have inspired laboratory-produced adhesives, the remora has been overlooked until now. The remora's attachment mechanism differs significantly from other suction-based systems—from fasteners or adhesives that can only stick to flat surfaces or cannot be detached without harming the host." The research, supported by the Georgia Research Alliance and GTRI, was presented at the Materials Research Society Fall Meeting. The remora's suction disc is a greatly developed dorsal fin that sits atop the fish's body. The fin flattens into a disc-like pad and is surrounded by a thick, fleshy lip of connective tissue that creates a seal between the remora and its host. The lip encircles platelike structures called lamellae, from which rows of vertical, tooth-like structures called spinules project. The complex skeletal structure enables efficient attachment to surfaces including sharks, sea turtles, whales and even boats. Nadler and Allison Mercer, research scientist at GTRI, partnered with researchers from Georgia Tech School of Biology and Woodruff School of Mechanical Engineering to better understand how remoras attach to a host by investigating and quantitatively analyzing the structure and hierarchical nature of the remora adhesion system. Remoras attach to larger marine animals for three reasons: transportation (a free ride that allows the remora to save energy), protection (being attached to a shark makes it unlikely to be attacked) and food, because sharks are very messy eaters and often leave plenty of tasty morsels floating around for the remora to consume. However, it was unclear whether this attachment was active or passive. Results from the GTRI study indicate that remoras use a passive adhesion mechanism, meaning the fish do not have to expend additional energy to maintain their attachment. The researchers are evaluating the possibility that drag forces generated as the host marine animal swims increase the strength of the adhesion. Dissection experiments showed that a remora's attachment to or release from an animal could be controlled by muscles that raise or lower the lamellae. Dissection also revealed light-colored muscle tissue surrounding the suction disc, indicating low myoglobin levels. For the remora to maintain active muscle control while attached to a marine animal over long distances, the muscle tissue would need to show myoglobin in high concentrations, as seen only in much darker swimming muscles. Mercer: "Discovering that the adhesion is passive excited us greatly. We can use and develop some of the fish's adhesive properties to produce a synthetic material." The researchers also developed a technique that allowed them to collect thousands of measurements from three remora specimens, providing new information about the shape, arrangement and spacing of their features. Initially, they imaged remoras in both attached and detached states using microcomputed tomography, optical microscopy and scanning electron microscopy. The researchers digitally reconstructed each specimen, measuring structural similarities between samples and measuring important size differences. Detailed microcomputed tomography-based surface treatments of the lamellae showed one row of spinules placed at shorter, more regular intervals and densely packed, and another row of spinules placed at longer, less densely spaced intervals. Quantitative analysis revealed similarities in the suction disc structure according to lamellae and spinule size and position, despite significant specimen size differences. One fish's disc was twice as long as the others', but the researchers observed that each specimen's adhesion disc length-to-width ratio fell within 16 percent of the average. Through additional experiments, the researchers demonstrated that the spacing between spinules on remoras is quite similar to the spacing between scales on mako sharks. Mercer: "The complementary gap between features on the remora and between a shark likely contributes to the greater adhesive force observed when remoras adhere to shark skin compared to smoother surfaces." The researchers plan to conduct further tests to better understand the roles of various suction disc structural elements and their interactions, in order to create a successful attachment and release system in the laboratory. Nadler: "We are not trying to copy the complete remora adhesion structure that occurs in nature. We want to identify, characterize and use the critical properties to design and test adhesion systems that provide these unique adhesive functions. Ultimately, we want to optimize a nature-inspired adhesive for a wide range of applications with capabilities and performance advantages over adhesives or fasteners currently available today."