The broadbill swordfish (Xiphias gladius) is recognized globally for its speed and its distinctive, elongated bill. While adult swordfish appear scaleless, their skin structure is nuanced, linking their early development to their mature, highly adapted skin. They do not possess the large, overlapping plates seen on many other fish species. Instead, they have structures representing a modified form of scale embedded within their skin, which allows the swordfish to be one of the fastest predators in the open ocean.
Scales in Juvenile Swordfish
Swordfish begin life with distinct scales that differ from the smooth appearance of the adult fish. Larvae and young juveniles possess two types of specialized scales: large, multi-spined scales and smaller, single-spined scales. These structures are rough and prickly, covering the body of the young fish. These early scales are rough, bony tubercules, not overlapping like those of a carp or bass. As the swordfish grows, reaching about one meter in length, these scales do not simply fall off. They become progressively less visible as the fish matures into its adult form.
The Structure of Adult Swordfish Skin
The apparent “scalelessness” of the adult swordfish is due to a transformation in its skin structure as it increases in size. As the fish grows, the dermis, the inner layer of the skin, thickens considerably. This thickening causes the original juvenile scales to become deeply embedded within the skin tissue. The scales do not dissolve; rather, they are buried by the surrounding skin. In the adult fish, only the pointed tips of the scale spines may protrude faintly from the surface, resulting in a texture often described as smooth to the touch, or sandpaper-like in certain areas. The adult skin is also covered by a thick layer of mucus secreted from an extensive network of mucous canals within the epidermis. This mucus layer protects the skin and contributes to the fish’s overall streamlined profile.
Hydrodynamic Function of Scaleless Skin
The unique skin structure of the adult swordfish is directly linked to its exceptional speed in the water. The loss of large, overlapping scales and the presence of a smooth, mucous-coated surface are adaptations for minimizing drag. This hydrodynamic streamlining allows the swordfish to reach burst speeds approaching 60 miles per hour. The minute, residual scale tips on the adult skin may function similarly to the dermal denticles found on sharks, which manipulate the flow of water. These microscopic structures help manage the boundary layer of water flowing over the body, maintaining an attached flow and reducing skin friction drag. While a perfectly smooth surface might seem ideal for speed, the rough texture provided by the embedded structures can aid in minimizing overall drag in turbulent flow conditions. The swordfish’s body shape and skin work together to ensure that the water flows efficiently over its surface.