Ultrastructural evidence of the ion-transporting role of the adult and larval neck organ of the marine gymnomeran Cladocera (Crustacea, Branchiopoda).

The neck organ of adult and immature forms of four marine gymnomeran Cladocera species (Podonidae) has been characterized morphologically by light-scanning- and electron microscopy. Although displaying some special features, a comparison with organs assuming a comparable regulating function in other organisms indicates that the neck organ of the adult Podonidae exhibits ultrastructural evidence of involvement in ion-transporting mechanisms. The extensive ventral plasma membrane infoldings closely connected with the chondriom and the peculiar organization of the cuticular site associated with the subjacent apical cell limits of the organ are among the chief structural specializations. The fact that similar structures are already present in the embryonic neck organ confirms previous indirect evidence that podonid embryos can hypo-osmoregulate as soon as the ion concentration of the brood chamber rises to that of the outside medium.