Morphological development of the juvenile through to the adult in the freshwater pearl mussel, Hyriopsis (Limnoscapha) myersiana, under artificial culture

Abstract

Summary The morphological development of the freshwater mussel, Hyriopsis (Limnoscapha) myersiana (Lea, 1856), was observed using light and scanning electron microscopes, from the newly transformed (0 days old) juvenile to the onset of the adult stage (360 days old). As in the glochidium, the early juvenile has a semi-oval and equivalve shell with an equilateral valve. After day 1 the shell develops a larger anterior than posterior region until day 40, after which the posterior region grows larger than the anterior region. The form of the juvenile at 260-day-old resembles that of a fully grown adult. The shell microstructure of 0–20-day-old juveniles shows two differentiated layers, the periostracum and the prismatic layer. By day 30 the prismatic layer lies under a clear columnar structure that has formed a third layer, the nacreous layer. The mantle develops incurrent and excurrent siphons when juveniles are 60 days old. The development of juvenile gills initiates from a pair of gill bars at 0 days old, and formation of the inner demibranch starts from 10 days old and the outer demibranch from 90 days old. From this stage, numerous cilia form the latero-frontal cirri of the inner demibranchs. Additionally, longitudinal and transversal interfilamentous junctions of the inner and outer demibranchs begin to develop when juveniles are 200 and 240 days old and are complete at 230 and 260 days of age, respectively. Interlamellar septa join the inner surface of descending and ascending gill filaments to form water chambers when juveniles are 250 and 280 days old, respectively, and the development of inner and outer demibranchs is complete.