VENTILATORY AND VASCULAR ROUTES IN A SAND-BURYING SWIMMING CRAB, OVALIPES CATHARUS (WHITE, 1843) (BRACHYURA: PORTUNIDAE)

Abstract

Ovalipes catharus is a burrowing portunid which utilizes both forward and reverse ventilation. The proportions of time spent in each ventilatory mode depend upon burial state and activity. In forward ventilation, water enters the hypobranchial space of the branchial chambers at the bases of the legs, flows between the gill lamellae to the epibranchial space, and is exhaled anteriorly via the prebranchial apertures lateral to the mouthparts. This flow is countercurrent to the general hemolymph flow in the gills. In reverse-ventilating crabs, water enters anteriorly into the epibranchial space. Most of this water does not ventilate the lamellae, but passes between the epibranchial surfaces of the gills and the branchiostegites to be exhaled posteriorly between the fourth and fifth pereiopods. A variable (usually minor) fraction of the water exits from the Milne Edwards apertures, presumably taking a concurrent, epibranchial to hypobranchial, route. The arterial and venous circulations are described from vascular casts. From these the potential for the branchiostegal circulation to function as an alternative site for respiratory gas exchange was examined. Each branchiostegite is extensively vascularized with a sheet of interconnecting sinuses and lacunae. Hemolymph enters anteriorly from the hepatic sinuses and is returned to the pericardium laterally and posteriorly, thus forming a vascular shunt parallel to the branchial circulation. It is unclear whether the branchiostegite is important in gas exchange in this crab. Vascular casts of a number of other significant structures, including the heart, cor frontale, and antennal glands are described.