Videler, JJ, Haydar, D, Snoek, R, Hoving, HJT, Szabo, B, (2016). Lubricating the swordfish head. Journal of Experimental Biology. 219: 1953-1956. doi:10.1242/jeb.139634
Swordfish are one of the fastest swimmers on the planet, clocking in at speeds over 100km/hr (62 miles/hr). Their impressive swimming prowess is a result of an evolutionary arms race with their prey: the humble, but agile squid. To achieve their need for speed, swordfish have evolved several physical adaptations from the tip of their bills to the tip of their tails. For example, their famous swords or bills that can make up 40-45% of body length not only slice through water as the envy of every knight in Camelot, but also boast a rough surface generating micro turbulence, thus decreasing drag. Swordfish bodies also hit the sweet spot of thickness-to-length ratio per largest volume for optimal drag reduction, and their stiff, crescent-shaped tails help shave seconds off their swim time. Scientists have recently discovered yet another adaptation that allows swordfish to swim fast: they lubricate their heads.
This greasy discovery was sparked when a mysterious soft spot near the base of the bill was identified using CT scanning in a separate study looking at the bill potential to slash prey. By taking a closer look at the area with MRI scans with additional observations made with high-powered (50x – 500x) microscopes and dissections (all specimens were obtained from commercial fishers in 1996 near the French island, Corsica), Videler et al., found a previously undescribed oil gland at the base of the bill, in front of the swordfish eyes–Fig 1.
This oil gland, dubbed glandula oleofera by the authors has three components: a lower component that branches into two left and right lobes–see Fig 1. From the gland, a network of capillaries named rete lubricans connect to pores on the skin of the swordfish head. The pores are surrounded by small tooth-like projections called denticles and are highly distributed on the skin covering the oil gland, and near the edges of the gill cover. To confirm that oil from the gland was actually being secreted on the swordfish head, several lines of evidence were used: ink solution was injected into the capillaries to visually trace the pathway of oil from the gland to pores, heat and airflow applied by a hairdryer initiated secretion of oil through the pores, and lastly, the chemical composition of oil in the gland was tested and found identical to the oil on the surface of the head.
What is the advantage of having an oil gland lubricate a swordfish head? An oily, water repellant head would decrease friction and drag, giving the swordfish an extra edge over their fast prey. Swordfish have been known to hunt in water temperatures approaching 8°C (46°F), which is the congealing point of the oil. The authors suggest that the oil within the oil gland receives some warmth to remain runny internally since it is situated in front of a swordfish’s heated and insulated eyes. Furthermore, swordfish rarely swim in water colder than 8°C but stay at higher temperatures where the oil remains fluid. The oil is then sucked out of the pores by the pressure that results from the the shape of the head and is probably not wasted too quickly due to the presence of the denticles.
Swordfish are heavily fished recreationally and commercially, but their behaviour is largely unknown due to the difficulty of studying organisms in the open ocean. In the meantime, this lab-based study has revealed yet another impressive adaptation to an arsenal of hacks that allows the swordfish to be one of the fastest swimming fish on Earth.