Ultrastructural Characterization of Daphnia Heart Muscle

The hear ts of vertebrates are myogenic whereas those of most ar thropods are neurogenic. However, the hear t of Daphnia is an exception, as indicated by Bekker and Kri jgsman (1) whose studies led to the conclusion that Daphnia hear t is myogenic with apparen t inhibi t ion by extracardiac cholinergic nerves. Since Daphnia have hearts similar to ver tebra te hearts and are transpa ren t and easy to handle, this species is a useful one for the study of cardiac response to drugs. Viehoever and Cohen (2), and Sollman and W e b b (3) have utilized Daphnia for this purpose. Al though cardiac function studies have been conducted for Daphnia, the lack of knowledge of the histology or ul t rastructure of its hear t has precluded any correlat ion of structure and function for this tissue. Cardiac musculature is reported to be striated in Ar th ropoda (4) bu t has not been character ized for Daphnia. Ultras t ructura l studies on the hear t have not been carried out for any of the arthropods. Therefore, we set out to determine the ul t rastructure of Daphnia pulex hear t and specifically to determine whether it is striated.