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We investigated the function of the m. hyohyoideus superior (MHS) and inferior (MHI) in the head of three carp (Cyprinus carpio L., 29.7 ± 2.1 cm FL ) during three movements (normal breathing, stressed movements and food uptake). Both muscle parts are located ventrally at the inner side of the operculum and branchiostegal rays and consist of red (mainly in MHI) and white (mainly in MHS) fibres. Contrasting views exist about the functional role of these muscles during ventilation and food uptake. Therefore, we analysed muscle activities of the MHS and MHI using electromyography (EMG) and measured the strain pattern of the MHS using sonomicrometry. Carp were also filmed from a ventral viewpoint using high-speed video at 250 frames s[sup -1]. EMG and sonomicrometry data showed an increase in muscle strain amplitudes, cycle frequency and (relative) stimulus duration while stimulus on- and off-times occurred earlier in the strain cycle from normal breathing to stressed movements to food uptake. The MHS and MHI were always simultaneously active. We concluded that: 1) the MHI is not responsible for high frequency movements (i.e. stressed movements and food uptake); 2) both muscle parts assist in the onset of opercular closing, and 3) the MHS and MHI do not act as antagonists in carp. [ABSTRACT FROM AUTHOR]