Larval Development in Two Genera of the Indo-Pacific Gobioid Fish Family Xenisthmidae: Allomicrodesmus and Xenisthmus

Larval development in the family Xenisthmidae is described for the first time. Ten and five larvae, respectively, of Allomicrodesmus and Xenisthmus (2.1-7.6 mm) form the basis for the description. All were captured in plankton hauls in the northern Coral Sea. Identification was established from cleared-and-stained specimens, based on counts of vertebrae and fin rays, the caudal complex, and osteology of the snout region. Identification was corroborated by the presence of the larval pigment pattern in a newly settled Xenisthmus clarus. Larvae are characterized by externally expanded, bladelike, cartilaginous dorsal- and analfin pterygiophores; spination on the branchiostegal rays, ventrally on the cleithra and at the angle of the lower jaw; gut morphology; a large, expanded gas bladder with ontogenetic posterior movement; and pigment pattern. All, except pigment pattern and ontogenetic movement of the gas bladder, are unique among gobioid larvae and both exceptions are unusual. Pterygiophore "blades" are otherwise found only in some larval blenniids. Spination on the cleithra ventral to the pectorals and spines on the branchiostegals are unique among perciform fishes. Assuming the other xenisthmid genera possess these synapomorphies, the larvae provide strong evidence of the monophyly of the family.