Morphology and postlarval development of the ligament of thracia phaseolina (bivalvia: Thraciidae), with a discussion of model choice in allometric studies

Abstract

The ligamental apparatus of Thracia comprises external and internal resilient components, as well as a calcified ossicle termed the lithodesma. All these elements have hitherto been considered portions of a single unit and their allometry and functional morphology interpreted as such. However, analyses of the fine structure of the hinge of a growth series of Thracia phaseolina using conventional light, confocal and scanning electron microscopy revealed that, instead, external and internal components comprise distinct, independent layers of fibrous ligament, each formed at different ontogenetic stages. The internal ligament, with a lithodesma along its mid-sagittal sector, comprises the sole early juvenile ligament of the species and its main function is to force the shell valves open, not align them as had been previously suggested. The external (parivincular) ligament only appears at a shell length of c. 2.5 mm, but grows faster than the internal ligament to become the main component of the hinge of adults. Two- and three-parameter power functions (simple and full allometric equations) were fitted to measurements of the different ligament parts, and the performance and appropriateness of these models in describing the allometry of each of the studied traits are evaluated. Use of the full allometric model is recommended whenever the body size (or other standard) at which a trait of interest arises lies within or close to the limits of the sampled range.