Spine skeleton morphogenesis during regeneration in clypeasteroid and camarodont sea urchins

The process of skeleton morphogenesis is described for broken and totally removed spines in clypeasteroid (hollow spine) and camarodont (solid spine) sea urchins. Spine regeneration after total spine removal is completed in 40–45 days in clypeasteroids and in 60–70 days in camarodont sea urchins. Along with common stages of formation of longitudinal ribs in both hollow and solid spines, fundamental differences were found between the initial stages of reparative growth of the spine shaft. The spine shaft is formed from a single median process in clypeasteroids and from many simultaneously growing processes in camarodont sea urchins. Reparative morphogenesis of totally removed and partly broken spines in clypeasteroid sea urchins and totally removed spines in camarodont sea urchins leads to the formation of a skeletal structure identical to the intact spine. However, during the regeneration of broken camarodont spines, lateral growth is markedly retarded. As a result, the regenerated part of the spine shaft has a smaller diameter when the initial spine length is achieved. A hypothesis is proposed on a paedomorphic origin of spines in the clypeasteroid sea urchins on the basis of the juvenile stage of definitive spines in the camarodont sea urchins.