Burrowing fishes: Kinematics, morphology and phylogeny of sand-diving wrasses (Labridae).

Burrowing through the substrate is a common behaviour in many organisms, both invertebrate and vertebrate. Sand-diving, a burrowing behaviour in the fish family Labridae, consists of a quick and forceful headfirst plunge into the sediment followed by undulatory axial body movements until the fish is completely concealed beneath the surface. This study determined that sand-diving of the slippery dick wrasse Halichoeres bivittatus is composed of two distinct phases of undulatory axial body movements. In the first phase, body undulations occur at high frequencies and wave speeds and low amplitudes, while in the second phase, frequencies and wave speeds decrease while amplitude increases. Furthermore, this study examined several morphological features of sand-diving labrids, including narrow, elongated bodies and lengthened neural spines that overlap with the dorsal pterygiophores, that may be anatomical traits that contribute to burrowing ability. Finally, ancestral state reconstruction showed that sand-diving occurs exclusively in the upper half of the labrid phylogenetic tree with an evolutionary history indicating that sand-diving may have evolved once and then been lost three to five times or may have evolved independently at least three times in family Labridae.