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Dissection and histological analyses revealed the swimbladder of Hemiramphus far and H. robustus to comprise a matrix of discrete, gas-filled vesicles of 1-6 mm in diameter. The vesicles are not richly vascular and no discrete capillary bed organs were found. The anterior and posterior ends of the swimbladder have asymmetric projections that extend rostrad and caudad, respectively. These projections and some surrounding fatty tissue contain what we term protovesicles, which have thick walls that we infer expand to become the thin-walled vesicles of the main vesicular swimbladder. Dissection of museum specimens of other species of Hemiramphus and Oxyporhamphus convexus confirmed the presence of a vesicular swimbladder. However, examination of museum specimens of other hemiramphids, including O. micropterus, and flyingfishes revealed only a simple sac-like swimbladder. Presence of this unusual swimbladder in two genera within the same family is indicative of a strong synapomorphy that, in conjunction with recent molecular data, suggests that Hemiramphus and Oxyporhamphus convexus are closely related. [ABSTRACT FROM AUTHOR]