The Circulatory System in Malacostraca – Evaluating Character Evolution on the Basis of Differing Phylogenetic Hypotheses

> Abstract The circulatory system is one of the major organ systems in Malacostraca. As no consensus on malacostracan phylogeny exists it becomes necessary to evaluate different phylogenetic hypotheses for their impact on circulatory system evolution. Four prevailing hypotheses were therefore used to map different circulatory system characters. The comparison shows that the length of the heart, i.e. the position of the anterior and posterior borders of the heart, changed extensively within malacostracan taxa. However, a general back shift of at least the anterior border from more anterior positions is feasible. A pattern of ostial evolution is not discernable as both an increase and a decrease in the number of pairs of ostia took place. A segmental arrangement is seen as the plesiomorphic condition. Cardiac artery patterns, i.e. the spatial arrangement of cardiac arteries along the heart, changed within malacostracans from a pattern of segmentally arranged arteries to localized patterns. A descending artery, a mostly unpaired artery in the posterior thorax connecting the dorsal and the ventral vessel, evolved only once within Malacostraca. In a second step the general plausibility of the four alternative hypotheses in the explanation of circulatory system characters was evaluated through the calculation of homoplasy indices (HI). A comparison of HI values challenges the only molecular phylogeny compared, while all other morphology-based hypotheses perform almost equally well.