The myocardial ultrastructure in three amphipod species from Lake Baikal.

The heart ultrastructure of several crustacean species has previously been studied. Special interest has been paid to the development of the membrane systems of the cardiac myofiber relative to the different sarcomeric levels. This system seems to give phylogenetic information, but there still remains some uncertainty concerning its reliability. Is it affected by the size of the species, or is there any geographical variation within the different crustacean orders? The aim of this study is to compare the heart ultrastructure of some large amphipids from Lake Baikal(Acanthogammarus albus, Parapallasea puzylli and Poekilogammarus sukaczewi) with smaller amphipods from Europe and Northern America. It is shown that species size and geographical isolation do not seem to affect the membrane systems qualitatively. The isolation and adaptive radiation of the amphipods in Lake Baikal have not brought about any significant changes in the heart ultrastructure which separates them from other amphipod species. These findings support the use of cardiac membrane systems as a phylogenetic tool in the classification of Crustacea.