Mechanisms for Increasing Respiratory Capacity through Ontogeny in the Blastoid Genus Pentremites

This study was conducted to determine how the hydrospires in blastoids (the respiratory channels through which blastoids respire) change in shape and capacity during ontogeny. As the volume of a blastoid increases ontogenetically, the respiratory capacity of the hydrospires must increase to match the additional respiratory requirements. Ontogenetically, volume increases at a cubic rate, therefore the surface area of the respiratory structures should increase at a similar rate. Using transverse cross sections of the theca through an ontogenetic series in two species of the blastoid Pentremites, the surface area and volume of the hydrospires was quantified. The data demonstrated that the hydrospires increased surface area with increasing volume to maintain respiratory capacity and that this was accomplished using different mechanisms depending upon the species. In the species Pentremites godoni, increased hydrospire surface area was developed through increased length of the hydrospires through ontogeny. In the species Pentremites pyriformis, increased surface area of the hydrospires was accomplished by increasing the number of hydrospire folds within the body through ontogeny.