Anatomical findings and movements during sound production
The pectoral girdle of O. croaticus was subjected to µCT scanning and anatomical dissection to identify the various osseous structures and muscles that may be involved in sound generation. From μCT scans, three functional units were distinguished in the skeletal part of the pectoral girdle of O. croaticus : the shoulder girdle (composed of the post-temporal, the supracleithrum and the cleithrum bones) dorsally attached to the neurocranium, the shoulder plate (i.e., four large radials), and the fin plate, made up of fin rays articulated with the shoulder plate (Figure 7a-b ). On the dorsal tip of the cleithrum, anterior and posterior processes are present. The supracleithrum articulates with the post-temporal and the cleithrum, connecting with the cleithrum bone at its dorsal tip. The post-temporal is made up of a basal plate and two rostrally oriented processes (a “fork”) with dorsal and lateral attachments to the neurocranium. The rostral tip of the dorsal process is flattened and firmly attached to the epiotic bone. Putative sound producing muscles were observed during the dissection and were found originating on the neurocranium and inserting on the pectoral girdle (Figure 7a-b ). Thelevator pectoralis muscle is divided into two bundles: thepars lateralis and the pars medialis . The pars lateralis originates on the posterior part of the pterotic and inserts on the anterior dorsal process of the cleithrum. The parsmedialis is the thicker of the two muscles. It originates on the posterior part of the basioccipital and inserts on the medial part of the posterior dorsal process of the cleithrum.
Moreover, video recordings allowed to highlight characteristic fish movements during sound production, especially concerning the head region and fins. During sound production, soniferous males would usually stop swimming and would suspend the body on the fused pelvic fins. In addition, the pectoral fins were abducted, and the rays spread during sound emission. The male performed a lateral body quiver starting from the head to the tail (including dorsal fins), while the dorsal fins (both first and second) were erected prior to the production of the first pulse. Then the male would rapidly elevate the head and perform lateral head motions, while spreading the buccal and opercular cavities. The mouth was closed during the period of emission, though the anterior part of the branchial basket was slightly uplifted.