In many gastropod larvae, an extracardiac structure termed the larval heart appears during the veliger stage. The adult heart develops later and there is a transitional phase during which both hearts beat at the same time before the larval heart degenerates. Detailed information about the function and ontogeny of the two structures is lacking. Here, I used descriptive and experimental approaches to investigate the ontogeny of cardiovascular function and control in Littorina obtusata, an intertidal gastropod with direct development. Based on an earlier hypothesis that the function of the larval heart is directly linked to that of the velum, I examined the relative timing of appearance and disappearance of the two structures. Additionally, physiological (larval and adult heart rate) and behavioural (spinning) responses to declining levels of dissolved oxygen were examined in different developmental stages. There was a considerable mismatch between the appearance of the velum and the appearance of the larval heart: The velum started developing on day 4, but the larval heart did not begin to beat until day 12. This mismatch is in contrast to that observed in planktonic larvae where the appearance of the two structures is much more attuned. Physiological and behavioural responses to hypoxia differed remarkably between developmental stages. Larval and adult hearts were insensitive to hypoxia during the first 3-4 days of their appearance but after this time showed a bradycardic response. Once the adult heart began to beat, the larval heart again became insensitive to hypoxia. Embryos of all examined developmental stages increased time spent spinning under hypoxia, although the increase in time spent spinning generally decreased with progressing development. Together, these findings indicate that circulation through the larval heart alone sufficed in early larval stages, where embryos were small in size and had a high ability to spin. With progressive development, consequent increasing metabolic demands and a decreasing ability to spin, oxygen supply became critical under hypoxic conditions, expressed as a bradycardia of the larval heart at that stage. The additional support provided by the adult heart assisted in meeting the increased energetic demands, even under hypoxic conditions.Faculty of Science & Technolog
