Date of Award
Doctor of Philosophy (PhD)
J. Michael Fitzsimons
The endemic goby Sicyopterus stimpsoni is unique among five species of Hawaiian freshwater fishes because it undergoes rapid metamorphosis of cranial structures during postlarval development. These fishes are amphidromous and return after a prolonged stay in the ocean to streams where they are confined to estuaries until completion of cranial restructuring. Within 48 hours after entering fresh water, head width increases significantly, while total length and head length remains unchanged. Weight decreases by approximately 15% during this period. The upper lip enlarges greatly, and the mouth position shifts from terminal to nearly ventral. Shape analysis of radiographs taken from a sequence of fish preserved at two-hour intervals after entering fresh water revealed drastic reallocation of the premaxilla-maxilla complex and dentary. The cranium and most dorsal and caudal structures of the skull remained unchanged. Microscopic observations showed development of a buccal velum, a ridge on the lower lip, and a greatly enlarged upper lip during metamorphosis. In the second half of metamorphosis, tooth buds develop, and a gland in the upper lip becomes prominent. After completion of metamorphosis, Sicyopterus stimproni is able to climb waterfalls by alternating use of the pelvic sucking-disk and mouth, with which it also is able to scrape diatoms from rock surfaces by rapid rostrocaudal movement of the upper-jaw complex. Anatomical analysis of adult fish suggested a four-bar-linkage model for feeding and climbing; it consists of the (1) cranium, (2a) hyomandibular-quadrate-triangle, (b) lower jaw complex, (3) upper jaw complex, and (4) palatine-ectopterygoid axis. The premaxilla-ethmoidal ligament and interopercular-articular ligament provide elasticity to the model. Non-linear, highly dynamic cranial development with extensive reallocation of cranial function has not been described previously for fishes, and the analysis provides a unique example for the theory of terminal addition during ontogeny of an organism. A relatively small change in structure at the end of larval development has enormous implications for the entire ontogeny of the species. The metamorphosis of Sicyopterus stimpsoni constitutes a departure from typical linear development in the transition from larva to adult, and can be used to hypothesize evolutionary mechanisms guiding the phylogeny of a taxon.
Schoenfuss, Heiko Lars, "Metamorphosis of the Hawaiian Stream Goby Sicyopterus Stimpsoni: A Structural, Functional, and Behavioral Analysis." (1997). LSU Historical Dissertations and Theses. 6595.