The swordfish, one of the world’s fastest swimmers, is often taken for a vicious fighter because of its unique proboscis which it uses to slash at its prey.
Now a new study has revealed that at the base of a swordfish’s bill lies a very tender organ which emits oil across its shiny head – possibly for faster, more streamlined swimming.
To reach this hypothesis, John Videler from Groningen University in the Netherlands was inspired by a paper released last year by Maria Habegger and colleagues.
“They were surprised to find that there is a very weak spot just at the end of the sword, where it enters the head,” Videler explains.
To investigate this point of fragility, Videler and colleagues decided to revisit MRI scans of swordfish dating back to the 1990s, along with skin and anatomy analyses.
The team’s findings, published in the Journal of Experimental Biology, point to an oil gland that sits on the fish’s jawline, right where the sword meets the head.
“I saw this gland,” Videler remembers. “It was so big there was hardly any room for bony structure and the bone around it was very thin.”
Using micro-imaging, the team also identified a network of capillaries in the skin, connecting the oil gland to a series of tiny pores spreading across the swordfish’s head.
When the researchers heated the organ, they saw oil excreted from these pores.
“During dissection of the head, the oil was observed to flow out of the pores as soon as the oil gland was heated,” the researchers write, adding that another organ inside the head of swordfish can act as a localised heater for the brain, eyes and, probably, this newly discovered oil gland.
The oil being secreted from the head-pores was found to match the natural fatty acids found on the fish’s skin, with similar beneficial properties including hydrodynamics: they likely increase slip and reduce drag when the fish moves through water, allowing the fish’s extreme speeds of up to 100 kilometres per hour.
There may also be temperature-based benefits. “The oil solidifies below approximately 8 °C, making it persistent on the skin in cold seawater,” the paper explains. “In the Western North Atlantic, tagged swordfish were shown to hunt in deep water with temperatures down to 8 °C during the day.”
Although swordfish can’t be kept in captivity, Videler is keen to see this new-found skill studied using modelling of swordfish anatomy, adding to our knowledge of this unique, fast-moving sea-dweller.
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