Allometry, design and constraint of body components and of shape in sea urchins

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Abstract
Ontogenetic allometry of body components was studied in sea urchins from the tropical and subtropical Indo-Pacific Oceans. As sea urchins grow, the body wall, lantern and gut assume relatively smaller fractions of total wet weight whereas the coelomic fluid and gonad combined become relatively larger. During growth, height (h) changes relative to diameter (d). Species that are relatively flat when small tend to become taller and if relatively tall become flatter as they grow with convergence towards h = d/2. This relationship is associated with equal volumetric increase for equal incremental changes in diameter and height at h = d/2. There is an interspecific relationship between height allometry and maximum size. If animals have a height diameter ratio less than 1:2 when they are very small, then the flatter they are initially, the larger the potential maximum size. There is a discontinuity in the relationship close to d = d/2 so if small individuals are relatively tall (h > d/2) then potential maximum size again is large. The allometric component of maximum size adds an interesting complication to analysis of size related features in life history evolution. The significance of allometric parameters for sea urchins include adaptation (α for body wall wet weight), developmental constraint (β for height: diameter allometry), and developmental plasticity (both α and β for the wet weight of Aristotle's lantern).