The Crustacean Open Circulatory System: A Reexamination

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Abstract
Open circulatory systems have been portrayed aspoorly designed systems with poor performance characteristics, lacking in adequate tissue perfusion or fine control mechanisms. Recent studies cast doubt on these assumptions. Open circulatory systems of at least the higher Malacostraca have elaborate capillary beds in many tissues. Cardiac outputs are generally higher than those of the closed systems of poikilothermic vertebrates of equivalent size and activity potential. Although arterial pressures and flows are often lower than those characteristic of poikilothermic vertebrates, the crustacean arterial system is adapted to deliver equivalent flows under these conditions. The control systems of the crustacean neurogenic heart appear capable offine graded control over cardiac output. In addition, although crustaceans lack the arteriolar smooth muscle upon which much of theperipheral circulatory control of vertebrate closed systems depends, cardioarterial valvular mechanisms under neural and neurohormonal control appear to be capable ofselective distribution of this output between the several separate arterial systems. Thus, although open circulatory systems differ greatly in anatomyfrom closed blood systems, they are, nonetheless, functionally equivalent.

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