The development of the musculature in the limpet Patella with implications on its role in the process of ontogenetic torsion

Abstract

Summary Scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM) following application of a fluorescent dye for filamenteous actin revealed four distinct muscle systems in the pretorsional larvae of Patella, i.e., the velum ring, the pedal plexus and the main and accessory larval retractors. After torsion, two adult (i.e., left and right) shell muscles arise independently from all larval muscles. In addition, the tentacular as well as the adult mantle and buccal musculature are formed during subsequent development. Both larval retractors and the velum ring are lost during or shortly after metamorphosis, while the pedal plexus, left and right (adult) shell muscles, tentacular, mantle and the buccal musculature remain functional in the adult animal. These findings, together with observations of living larvae, strongly support the hypothesis that muscular and hydraulic activity are primarily responsible for the process of ontogenetic torsion. Shell formation in Patella caerulea includes an unsculptured, symmetrical embryonic shell (protoconch I) as well as the successively mineralized juvenile/adult limpet-shaped teleoconch. Considering the Patellogastropoda as the earliest offshoot of the class Gastropoda (see below), we regard the following three conditions as basal for gastropods: (1) the adult shell muscles arise independently from the larval shell musculature, (2) ontogenetic torsion is primarily a larval process, (3) the embryonic shell is symmetrically shaped—asymmetrical/helicoid conditions of the adult shell in higher gastropods are independent from the ontogenetic torsion process.