Growth and mechanical correlations of calcified cartilage in Batoidea: A histomorphological study using the Raja asterias model.

This study investigates the growth and calcification of the appendicular skeleton in Raja asterias (Delaroche, 1809), a member of the Batoidea, to explore the relationship between histomorphology and the mechanics of batoid locomotion within the water column. Although much prior research has focused on the "tessellated pattern" in these fishes, the variable structure of the appendicular skeleton provides fresh insights into the understudied interplay between skeletal histomorphology and the mechanical functions of Batoidea fins. The shape and initial growth of fin cartilage are influenced by the orientation of chondrocyte mitoses prior to mineral deposition, with subsequent calcification playing a pivotal role in shaping skeletal architecture. This study documents two distinct growth patterns: "crustal" and "catenated." The crustal pattern is predominantly observed in larger skeletal elements, such as the central body structures (skull, rostrum, and jaws), girdles, pterygia, and compound radials, whereas fin radials follow the catenated growth pattern. Notably, early-stage chondrichthyan cartilage shares similarities with mammalian metaphyseal growth plate cartilage, though in chondrichthyans, the calcified matrix is not resorbed or replaced by bone. Additionally, a previously unrecognized calcification pattern is identified in the pelvic-fin radials of R. asterias, indicating that the mechanical demands of locomotion in the water column may have driven the evolution of variable fin flexibility in Batoidea. This flexibility is achieved through joint mobility (diarthroses and amphiarthroses), specialized fin structures, and the distinct calcification patterns of the pectoral and pelvic fins.