Swimbladder structure suggests relationship between halfbeaks

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The discovery of a highly unusual swimbladder structure in two different groups of halfbeak fishes has provided evidence to suggest that the genera are closely related.

The swimbladder, an organ used by fishes to control buoyancy, is typically a gas-filled sac, and when present, lies above the internal organs beneath the spine.

However, previous studies have shown that the swimbladder of the beloniform halfbeak genus Hemirhamphus is rather different.

The Hemirhamphus swimbladder is composed of a multitude of tiny gas-filled vesicles about 1-6mm in diameter, rather than a single large sac.

Ian Tibbetts, Bruce Collette, Robert Isaac and Philip Kreiter examined a range of specimens from the Beloniformes Order to assess how widespread this structure (known as meristocystis) was between members of the three beloniform families, Hemirhamphidae, Exocoetidae and Zenarchopteridae.

The results showed that species of Hemirhamphus and Oxyporhamphus convexus had the vesicular swimbladder, while other hemirhamphids, including O. micropterus and flyingfishes had the normal 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 Hemirhamphus and Oxyporhamphus convexus are closely related," the authors wrote.

"The vesicular swimbladder seemingly provides evidence of a close link between an animal that looks like a flyingfish but is widely considered a halfbeak, and a halfbeak. However, and intriguingly, recent genetic evidence suggests that the genus Hemiramphus is sister genus to Oxyporhamphus, which together may be more closely related to flying fishes than to other halfbeaks (Lovejoy et al., 2004).

"Thus Hemiramphus becomes the oddity, looking like a halfbeak while having closer evolutionary affinities with the flyingfishes."

For more information, see the paper: Tibbetts IR, Collette BB, Isaac R and P Kreiter (2007) - Functional and phylogenetic implications of the vesicular swimbladder of Hemirhamphus and Oxyporhamphus convexus (Beloniformes: Teleostei). Copeia, 2007(4), pp. 808-817.