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Bone structure helps explain how marlin live

By BY DONNELL TATE/Harbor Report - | Nov 13, 2020

Bones invariably form the framework for anatomical descriptions. Quite apart from their own intrinsic fascination and beauty, their structure can tell us a great deal about how animals live. They dictate body form, provide protection for vital structures and transform the power from the swimming muscles into locomotion.

For example, if we know that a particular bone grows by yearly bursts, we can estimate the age of the animal in the absence of a birth date. By examining the way they fit together, we can understand the range of movements at the joints and get a clearer idea of how animals behave.

Bones are relatively easily collected and stored — much easier than soft tissues — and so their study can be relaxed, unhurried by the prospect of losing material to the forces of putrefaction There are over 400 bones in a marlin, and most are in the fins or fin supports. About 120 or so are in the head while the rest make up the backbone and ribs.

The bill is the most distinctive bony feature of the head. This rostral extension of the skull is characteristic of the group and has been assigned many possible functions, including feeding and hydrodynamic streamlining. It is made of very hard bone and is primarily an extension of an upper jaw bone, the premaxilla. Its high density and strength suggest that it is subjected to strong forces when a prey item is hit or that the water pressure is very great when a marlin swims at speed.

Some healthy marlin are found with broken bills, which indicates that it is not absolutely essential for normal existence.

The ear bones or otoliths, commonly used for aging other fish species, are tiny in marlin, only a few millimeters long. Although they show growth ridges, their small size and the difficulty of finding them makes techniques that use the spines of the dorsal or anal fins much more attractive for aging purposes.

Marlin gills are covered in a lattice of fine bony material. This sieve over the surface of the gills is common to all billfishes and is also seen in large tunas. It provides mechanical support to the gills, which in other fisher are rather floppy, and appears better developed in large tunas and in larger billfish. It modifies water flow through the gill plates, especially in fast swimming fish, and is thought to protect the delicate gas exchange tissue of the gills from physical damage and parasites. Marlin have no gill rakers, whereas they are well-developed on the first gill arch of tunas.

The backbone is made up of 24 interlocking vertebrae, about half the number present in most other fish. The backbone (or axial skeleton) consists of 12 precaudal (with ribs) and 12 caudal vertebrae (without ribs) in striped marlin and others of the genus Tetrapturus. Blue and black marlin have 11 precaudal and 13 caudal vertebrae, a total of 24 in all marlin. The way they snugly fit together allows for almost no sideways movement at the intervertebral joints when we try to bend them, yet photographs show marlin with quite tight bends in their bodies.

All of the fins have undergone evolutionary adaptations to the rather specific requirements for fast swimming. Suffice to say here that the long, thin shapes of the tail and paired fins (pectoral and pelvic) are reminiscent of the lines of long thin dagger boards of racing sail boats that can be raised or lowered as needed. These are examples of man-made structures under similar constraints for optimum performance in the same medium.