Yes, tuna is biologically a fish that possesses both fins and scales, though the latter are often difficult to see, leading to common confusion. Tuna belong to the family Scombridae, which includes mackerels and bonitos, and they are highly adapted for speed and efficiency in the open ocean. This adaptation for a hydrodynamic body is the primary reason their scales do not look like those of a typical fish. The presence of these two defining features places tuna firmly within the classification of true fish.
Anatomy of Tuna Scales and Fins
Tuna scales are present but are highly modified for their high-speed, pelagic lifestyle. They are a type of cycloid scale, meaning they have a smooth posterior edge, but they are very small and often deeply embedded in the skin. This small size and embedded nature minimizes drag, allowing the tuna to move efficiently through the water. The difficulty in detecting these tiny scales is the main source of public misunderstanding.
A distinct anatomical feature is the corselet, a patch of noticeably larger, thicker scales located on the anterior body, behind the head and around the pectoral fins. In some species, such as the Bigeye tuna, the scales in the corselet area are uniquely structured, being fat-filled and composed of cellular bone, forming a protective bony casing. The rest of the body often has scales that are minute or entirely absent, except for along the lateral line. The scales that are present are necessary for protection and maintaining the fish’s osmotic balance in saltwater.
The fins of the tuna are equally specialized for extreme swimming performance. The pectoral fins can be tucked into a dedicated slot on the body, maintaining a perfectly streamlined profile. Tuna also possess a series of small, non-retractable fins called finlets, located between the second dorsal and anal fins and the powerful caudal fin. These finlets are theorized to reduce turbulence and drag by controlling the flow of water over the tail section, significantly enhancing swimming efficiency. The massive, crescent-shaped caudal fin provides the immense thrust needed for long-distance migrations and rapid bursts of speed.
Tuna’s Classification as a Bony Fish
The presence of fins and scales is a defining characteristic rooted in tuna’s scientific classification. Tuna belongs to the superclass Osteichthyes, commonly known as the bony fishes. This group is distinguished from cartilaginous fishes, like sharks and rays, by having a skeleton composed primarily of bone.
More specifically, tuna are members of the Class Actinopterygii, or ray-finned fishes, which represents nearly 99% of all fish species. The term “ray-finned” refers to the structure of their fins, which are webs of skin supported by flexible, bony spines. Within this class, tuna falls into the order Perciformes and the family Scombridae, confirming its identity as a taxonomically standard bony fish.
All members of the bony fish lineage possess scales, even if they are drastically reduced or modified. Tuna’s highly adapted scales fit this biological requirement, confirming their systematic position. The smooth, hydrodynamic design of the tuna body is an evolutionary specialization for speed, not an absence of fundamental fish structures.
The Role of Scales in Dietary Law
The question of tuna’s scales is frequently raised in the context of specific religious dietary laws, most notably Kosher law, or Kashrut. According to these laws, a fish must possess both fins and scales to be considered permissible for consumption. A further interpretation requires that the scales must be easily removable from the fish’s skin without tearing the flesh.
Tuna’s scales, being tiny and embedded, do not easily slough off like those of a carp or salmon, which has historically created debate. However, the scientific fact that the scales exist, and are present in a visible corselet area, is sufficient for most rabbinic authorities. The accepted ruling is that tuna is a permissible fish because it technically meets the biological criteria of having both fins and scales.
Due to the difficulty in verifying the presence of scales on processed products, like skinless fillets or canned tuna, many processed tuna products require certification from a reliable supervising agency. This certification ensures that the fish came from a species, such as tuna, that is known to possess the required fins and scales. Therefore, while the anatomical presentation of the scales is unusual, the species remains accepted under the dietary guidelines.