Ontogenetic Torsion and Protoconch Form in the Archaeogastropod Haliotis kamtschatkana: Evolutionary Implications

Abstract

AbstractOntogenetic torsion in archaeogastropods has strongly influenced theories about early gastropod evolution, but the seminal studies by Crofts (1937, 1955) remain the major source of information about tissue movements during this developmental process. Computer‐generated reconstructions of histological sections indicate that the cephalopodium of Haliotis kamtschatkanaJonas, 1845 rotates by a full 180° relative to the shell and visceral lobe during the first quarter of pre‐metamorphic development. However, a portion of pallial epithelium, including some of the shell field, accompanies the rotating cephalopodium; a process facilitated by detachment of the pallium from the apertural rim of the protoconch. Transmission electron microscopy indicates that a tract of the larval retractor muscle, which Crofts (1955) implicated in the mechanism of torsion, inserts on both pedal andpallial cells. A deep invagination of shell field epithelium is a major focus of rotational torque. As a result of pallial deformation during cephalopodial rotation, the anus and gill rudiment are restricted to the right half of the larval body for 2 days after cephalopodial rotation by 180° has been completed. Scanning and transmission electron microscopy indicate that grooves in the lateral flanks of the protoconch correspond to the deep invagination of shell field epithelium. The grooves are not created by a coiling type of accrelionary shell growth or by flexion of the protoconch. A calcareous shelf is secondarily added to the periostracal template of the protoconch along its visceral apertural rim. Morphogenetic movements during ontogenetic torsion in this species are more complex than a simple rotation between cephalopodium and visceropallium and the protoconch shows no evidence of exogastric coiling. © 1997 Published by Elsevier Science Ltd on behalf of The Royal Swedish Academy of Sciences.