Functional morphology and significance of the circulatory system of Ostracoda, exemplified by Vargula hilgendorfii (Myodocopida)

Abstract

Combined high-resolution video-recordings of live specimens, SEM and serial microtome sections were used to document the morphofunctional aspects of blood (hemolymph) circulation in Ostracoda, exemplified by the nektobenthic myodocopid Vargula hilgendorfii from Japan. The circulatory system is comprised of a single-chambered dorsal heart (pericardium, myocardium with two ostia), efferent vessels (aorta and secondary arteries) and an integumental afferent network of sinuses radiating from the adductor muscle area to a peripheral channel leading to heart. The heartbeat and the linear velocity of hemolymph in sinuses range from 0.5 to 6 times s-1 and 200 to 1000 μm s-1, respectively. Hemocytes (10 to 17 μm) of irregular shapes occur within the circulating hemolymph. This typical open circulatory system is found in most myodocope ostracodes and other crustaceans. It is totally absent in small (mostly <2 mm) ostracodes such as Podocopa. We conclude that the fluid convection of hemolymph augments the diffusion process of O2 in larger animals. Oxygen uptake is assumed to occur preferentially through the inner (posterior) surface of the carapace where hemolymph sinuses are best developed and in close contact with sea water. Hemocytes may be involved in coagulation processes as in other crustaceans (e.g. Decapoda). Integumental circulation, preserved as anastomosing features in fossil Ostracoda, is known from the early Palaeozoic through to the Recent.