THE EFFECT OF ONTOGENY ON HYDROSPIRE RESPIRATORY CAPACITY IN BLASTOIDS

Blastoids are common Paleozoic stalked echinoderms that are often used as a model for function in brachiole-bearing blastozoans. Nearly all blastozoans have some type of respiratory structure often in the form of thin porous internal canals through which seawater is circulated for gas exchange. This study investigated the efficiency of these respiratory structures, which in blastoids are called hydrospires. How the hydrospires change in shape and capacity through ontogeny was determined by calculating their surface area and volume. Because diffusion occurs over the surface area of hydrospires, respiratory capacity can only increase at a squared rate relative to the height of the theca unless the hydrospires increase in number, depth, or shape. If there is non-linear relationship between the hydrospire surface area and the visceral volume, other factors must be involved to influence the uptake of oxygen, such as alternate respiratory structures, changes in efficiency with maturity, or greatly exceeding the metabolic requirements in the juvenile stage. It is currently unknown whether the brachioles had a water vascular system running down their length. If so, the brachioles may transport additional oxygen into the theca, increasing the metabolic intake of the individual without increasing hydrospire surface area. A growth series of the blastoid Pentremites godoni was collected and serially sectioned. The volume of the whole animal was determined using the mass of the theca and the known density of Mg-calcite and the number of brachiolar facets was counted. The visceral cavity and hydrospire folds were digitally photographed in each of the serial sections. Using the lengths of the hydrospire folds, the area enclosed by these folds and the known thicknesses of each section, the surface area and volume of the internal structures was modeled and calculated. The only modern analogs to blastoids are stalked crinoids that are generally found in deep water and respire through podia in the feeding arms. Consequently, little comparison can be made between the respiration of modern crinoids and blastoids. Since the hydrospires in blastoids represent a general model for respiration common among many groups of stalked echinoderms, this study has greater implications on a now extinct mode of echinoderm respiration.