Arthropod hemocyanins (Hcs) are a family of large, high molecular mass, extracellular oxygen transport proteins. They form oligomeric quaternary structures based on different arrangements of a basic 6 x 75 kDa hexameric unit. Their complex quaternary structures present binding sites for allosteric effectors and regulate the oxygen binding process in a cooperative manner. In order to describe the functional regulation of arthropod Hcs, a detailed description of their quaternary structure is necessary. We have utilized small angle X-ray scattering to characterize the structure of three arthropod Hcs in unperturbed conditions. Two different levels of complexity are evaluated: for the 2 x 6-meric case, we analyzed the Hcs of the portunid crab Carcinus aestuarii and stomatopod Squilla mantis, while in the case of 4 x 6-meric structures, we studied the Hc of the thalassinid shrimp Upogebia pusilla. While C aestuarii Hc presented a structure comparable to other 2 x 6-meric crustacean Hcs, S. mantis Hc shows a peculiar and quite unique arrangement of its building blocks, resembling a substructure of giant Hcs found among cheliceratans. For U. pusilla, the arrangement of its subunits is described as tetrahedral, in contrast to the more common square planar 4 x 6-meric structure found in other arthropod Hcs. (C) 2010 Elsevier Inc. All rights reserved.