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

Ovalipes catharus is a burrowing portunid which utilizes both forward and reverse ventilation. The proportions of time spent in each ventila tory mode depend upon burial state and activity. In forward ventilatio n, water enters the hypobranchial space of the branchial chambers at t he bases of the legs, hows between the gill lamellae to the epibranchi al space, and is exhaled anteriorly via the prebranchial apertures lat eral to the mouthparts. This flow is countercurrent to the general hem olymph how in the gills. In reverse-ventilating crabs, water enters an teriorly into the epibranchial space. Most of this water does not vent ilate the lamellae, but passes between the epibranchial surfaces of th e gills and the branchiostegites to be exhaled posteriorly between the fourth and fifth pereiopods. A variable (usually minor) fraction of t he water exits from the Milne Edwards apertures, presumably taking a c oncurrent, epibranchial to hypobranchial, route. The arterial and veno us circulations are described from vascular casts. From these the pote ntial 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 an d 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 w hether 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.