Record keeping capacity among insects found

Left: A seal louse has a firm grip on a seal’s hair. Right: Detailed view of the underwater parasite’s “carabiner hook”. © Anika Preuss, University of Kiel

Amazingly stubborn tiny creatures: Seal lice now hold the record for clinging ability among insects, researchers report. The enormous performance of the underwater parasites is based on “carabiner”-like structures with which they attach themselves to the hair of their victims. This enabled the seal lice to adapt to the strong current pressure and slippery conditions in the water. According to the scientists, the discovery is also interesting from a technical point of view: the natural patent could inspire the development of underwater grabs.

Insects have produced the greatest diversity of species in the animal kingdom and have been able to conquer very different habitats through many sophisticated adaptations. However, the sea apparently caused problems for these creatures: only 13 species are able to survive in the open sea for long periods of time. These are special representatives of lice. They developed the corresponding adaptations because they followed their hosts into the water in the course of evolution: The marine lice emerged from ancestors that already plagued the land-dwelling ancestors of seals and co. While marine mammals developed fins and other adaptations over the course of their evolutionary history, their parasites also had to adapt to the special challenges in the sea.

The clamping mechanism in sight

The focus of the study by researchers led by Anika Preuss from the University of Kiel was the seal louse (Echinophthirius horridus). It lives on the blood of seals and harbor seals in the North and Baltic Seas. It has to withstand the extreme conditions to which it is exposed on its hosts. Apparently, these insects developed an amazing tolerance to the salt water, lack of oxygen and intense fluctuations in temperature and pressure. They also withstand another challenge that their terrestrial relatives do not have to face: strong current pressure in slippery conditions. Seal lice can also anchor themselves firmly in the fur of seals as they move at speeds of up to 20 kilometers per hour. Preuss and her colleagues have now investigated the question of how the underwater parasites manage to do this.

The team carried out the investigations on seal lice, which come from harbor seals and gray seals from the North and Baltic Seas. The researchers examined the body structure of the insects, which are around two millimeters in size, using scanning electron microscopy and laser scanning technology. They also carried out measurements of the holding power of the tiny insects and compared their results with the performance of the clamping systems of other parasitic and non-parasitic insects. As the team reports, the results of the force measurements were impressive: only when the researchers pulled on the lice with a force equivalent to 18,000 times their weight did their holding system detach from the trapped seal hair. “This means that seal lice have the highest holding forces that, to our knowledge, have ever been measured in insects,” says Preuss.

“Carabiner hook” with braking function

It turned out that this performance is based on a biomechanical system that differs from that of land-dwelling lice. Instead of a simple claw structure, the seal lice's six gripping organs have a special structure that is similar to a carabiner. In addition, a special element prevents the hair from slipping in the water. “In addition to the pronounced claws, the louse also has soft, cushion-like structures on the inside that function like a kind of rubber ring,” says Preuss. “When the louse closes its claw, these soft pads come into direct contact with the seal hair, increasing friction so that the louse cannot easily slip off the seal’s fur. A system that works quite efficiently under water and also with very variable hair diameters,” explains the scientist.

The study results now shed light on the adaptations of these amazing insect representatives to the marine lifestyle of their hosts. “The example of the seal louse shows many challenges associated with structure-function relationships in marine wild animals,” says senior author Stanislav Gorb from the University of Kiel. As the team concludes, the significance of the results goes beyond biology: According to them, the natural patent could also inspire technical developments. Gorb says: “The clamping mechanism could give us clues for innovations in the area of ​​underwater or universal grippers: the highly specialized mechanisms and structures of the parasites could serve as models for us,” says the scientist.

Source: Christian Albrechts University of Kiel, specialist article: Nature, Communications Biology, doi: 10.1038/s42003-023-05722-0

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