Meeting Abstract
Many thousands of fish species possess mechanically dynamic feeding mechanisms that employ protrusion of the upper jaw. This motion is driven by the downward rotation of the mandible during mouth opening. Such functional integration between the upper and lower jaws requires the morphological integration of these structures. An immense number of fishes metamorphose from a distinct larval stage into a juvenile stage. It is after this metamorphosis that jaw protrusion typically appears. Since metamorphosis is strongly associated with a sharp increase in thyroid hormone levels, we sought to investigate the role of the thyroid hormone thyroxine (T4) in establishing jaw integration in the zebrafish (Danio rerio). To this end we examined two genetically modified lines: transgenic fish in which the thyroid gland can be ablated at a chosen stage; and the mutant line opallus , which has elevated levels of T4 (hyperthyroid treatment). We compared the cranial morphology and feeding kinematics of hyperthyroid fish with transgenic fish in which the thyroid gland had been ablated (hypothyroid treatment), non-ablated transgenic fish (control) and two strains of wild-type zebrafish (AB and TL). We found that manipulating T4 levels had a strong effect on both craniofacial morphology and feeding kinematics. Upper jaws were highly reduced in hypothyroid fish and they exhibited little to no protrusion. They were forced to rely heavily on hyoid driven suction feeding. Hyperthyroid fish tended to have enlarged lower jaws, but exhibited less pronounced changes in feeding kinematics. Feeding by hypothyroid fish more closely resembles that of pre-metamorphic zebrafish. This preliminary data suggests that T4 may play an important role in establishing the functional integration of fish jaws.
