Development and organogenesis of the Patellogastropod Tectum scutum (Gastropoda)
Date
2003
Authors
Moore, Jennifer Olivia
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Abstract
The development and morphogenesis of the plate limpet, Tectura scutum, was studied from egg fertilization, through the larval phase, to 1 month post metamorphosis. T scutum is a member of the order Patellogastropoda, the most basal offshoot of the Gastropoda. Live embryos, larvae, and juveniles were observed with light microscopy, and histological sections of various larval and juvenile stages were used to study the morphogenesis of internal structures. Scanning electron microscopy was also used to observe fine detail of external structures such as the velum and protoconch. Embryos developed into trochophore larvae within 16 hours post fertilization (hpf), at l 2°C. Torsion, the ontogenetic process during which the visceropallium rotates 180° relative to the cephalopodium, occurred between 51 and 59hpf, with the entire rotation being completed in approximately 7h. Swimming, non-feeding larvae were metamorphically competent at 7dpf. During metamorphosis, the juveniles began to crawl on the substrate, the velum and operculum were lost, and teleoconch formation began. Several of the traits that were observed in larval and juvenile stages of T scutum are similar to the derived condition of higher gastropods, despite the presence of the primitive form of the trait in adults. These data suggest that traits scored as primitive for the gastropods may be secondarily derived in this basal offshoot. This emphasizes the need for analysis of developmental stages in addition to adult forms when attempting to determine evolutionary relationships or polarity of change.