SCALING AND EVOLUTION OF THE LOPHOPHORE IN THECIDEIDE BRACHIOPODS

The lophophore is the feeding and respiratory organ in brachiopods, and the surface area of the lophophore scales with both body size and metabolism. In order to pack a large surface into a small volume, brachiopods can fold their lophophore into a ptycholophe, coil into a spirolophe, or loop into a plectolophe. Thecideide brachidia are ridges that form as outgrowths of the dorsal valve, and indicate the configuration of the lophophore. Species-level phylogenetic analyses indicate that thecideide brachiopods have iteratively evolved a multilobed, ptycholophe lophophore configuration from a simple bilobed or schizolophe lophophore. Several developmental pathways utilized by thecideides can produce a multilobed adult. Gould suggested that thecideides add lobes in development and evolution to accommodate increasing body size, and that the relationship between lobe number and body size would be allometric. I measured the length of the brachidium of >500 individuals from 29 different species from several different thecideide subclades. Using the length of the lophophore, which would be proportional to lophophore area, as a proxy for body size, I found that the relationship between body size and lobe number is strongly linear (R^2=.81) for all analyzed thecideide species and developmental stages. These data suggest that thecideides do add lobes as a consequence of increasing size, but that lobes are not added allometrically. The primary functional implication is that lobe size is limited by some other factor, likely the fluid dynamics of the lophophore system, indicating that there should be a maximum size that a brachiopod with a schizolophe could attain. Comparison with more distantly related brachiopods with ptycholophe lophophores will provide a test of whether these lobe size constraints are universal within Brachiopoda, and if not, provide an indication why.