The aim of this paper is to provide a real time monitoring of the performances of microbial fuel cells (MFCs) employing two different anode configurations with a mixed consortia coming from seawater: a planar structure, constituted by carbon felt, and an innovative 3D-packed structure, constituted by graphitized Berl saddles. A detailed exam of the dynamical behavior of the two cells is presented in order to analyze the differences between planar and 3D-packed structures. Both the bacteria communities composition and MFCs electrical properties have been monitored over 31 days. The effects on the cell performances of the start-up phase, of the feeding operation and of an external applied resistance are discussed. The energy losses inside the MFCs along time, before and after refill of chemical solutions have been obtained by means of Electrochemical Impedance Spectroscopy. Results show that after 10 days of operations the total internal resistances decreased of about 30% and 50% for carbon felt and graphitized saddles anodes, respectively. The reduction of internal resistances is in agreement with improved performance in terms of power density. Moreover, for both MFCs the refill operation leads to a reduction of the impedances, in particular the anodic resistances decreases while the ohmic and the cathodic ones are quite unaffected. In addition, the energy production of the two devices was studied applying resistive loads for 10 days. The saddle-MFC presents more stable voltage values if compared to the other cell, implying a larger energy production over time. Finally, Quantitative real-time Polymerase Chain Reaction analysis, performed over the whole period of investigation on planktonic phase, reveals the presence of two typical electrogens bacteria, such as Geobacter and Shewanella. (C) 2015 Elsevier B.V. All rights reserved.