Vertebral Column

The fish vertebral column is a marvel of biomechanics. Where a mammal walks on stiff limbs, a fish undulates - each vertebra bending against its neighbour to bow the body left, then right, driving a wave of thrust down the length of the fish and out through the tail. The number of vertebrae in a species is often diagnostic: a herring has 55-60, a tuna about 40, an eel over 100. Anglers use vertebral counts as one of the most reliable meristic characters for species identification.

Each vertebra consists of a centrum (main body), a neural arch enclosing the spinal cord, and (in the trunk) a haemal arch enclosing major blood vessels. Vertebrae are connected by intervertebral joints filled with remnants of the ancestral notochord. Cartilaginous fish (sharks, rays) retain more notochord-derived material than bony fish.

Muscle attachment (myotomes attach to each vertebra), spinal cord protection, and rigidity for undulatory swimming. Trunk vertebrae bear ribs and haemal arches; caudal vertebrae bear haemal spines for tail-fin muscle attachment.

Eels and elongate fish have 100+ vertebrae. Boxfish and pufferfish have reduced counts. Sharks show partial calcification. Some deep-sea fish reduce vertebral ossification to save calcium and reduce weight.

Scoliosis (lateral curvature) from mycobacteriosis, whirling disease, nutritional deficiency, or genetic causes. Kyphosis (humpback) from selective breeding in some ornamentals.

A tuna vertebral column bends less than a herrings - because tuna generate thrust mostly from the peduncle and tail, not full-body undulation. The relative stiffness of the spine is directly matched to swimming style.