The fish community from a Zostera capricorni habitat near Dunwich, southeast Queensland, was sampled by seine netting over a period of one year. The composition of this seagrass fish community was similar in structure to that reported of other Australian seagrass habitats where similar sampling gear was used. Hemiramphids were found to form an important, recurrent component of the samples, and to dominate herbivore biomass.
Gut analysis data from two hemiramphids, Hyporhamphus regularis ardelio and Hyporhamphus quoyi, captured from Dunwich, indicated that they both undergo an ontogenetic trophic shift from carnivory to herbivory at standard lengths of 80 - 110 mm. Seagrass was the major dietary substrate of larger fish, and was consumed mainly during the day. A dietary analysis of Arrhamphus sclerolepis krefftii sampled from Wivenhoe Dam, revealed an ontogenetic trophic shift at 40 mm standard length, from a diet of small insects and plankton to a diet of filamentous green algae. Intraspecific and interspecific differences in standard length at ontogenetic trophic shift were detected. A. sclerolepis krefftii were observed to feed on detached, floating fragments of seagrass and Valisnaria, and a series of experiments confirmed that in the absence of such material this species is able to feed on submerged, attached vegetation. Analysis of a population of A. sclerolepis krefftii from Wivenhoe Dam showed that their pre-ontogenetic carnivorous phase is short; ending between 2 and 4 months of hatching. After this period, and under favourable conditions, they are almost exclusively herbivorous.
Ingestive and digestive structures which were presumed to be important in this hemiramphid' s ability to feed on plant material were examined for evidence of a relationship between the ontogenetic development of such structures and the onset of the ontogenetic trophic shift. No clear relationship was detected.
The peculiarity of A. sclerolepis krefftii's pharyngeal jaw apparatus (PJA), among other alimentary specializations, indicated the special role that this structure might play in their herbivory. A functional morphological study of this species' PJ A, allied with the simultaneous video, X -ray fluoroscopic and hydrophonic analysis of movements made, and sounds produced, by their PJA, resulted in a hypothetical model of the PJA's involvement in food treatment.
Acidic glycoprotein (AGP), produced by the abundant mucogenic tissue in the pharyngeal and oesophageal mucosa of A . sclerolepis krefftii, was conjectured to be involved in a nutrient extraction process that could be pivotal to hemiramphid herbivory.
The study of the PIA of A. sclerolepis krefftii revealed features and hypothetical models that have not hitherto been reported for teleosts. A survey of pharyngeal anatomy of the component genera of the Hemiramphidae and representatives of the Belonidae and Exocoetidae, was performed. Pharyngeal character states identified from these data were used in conjunction with non-pharyngeal characters in a phylogenetic analysis of the Exocoetoidea. The resultant cladistic model, based on pharyngeal and non-pharyngeal characters, showed the Hemiramphidae to be polyphyletic. Accordingly, the taxonomy of the Exocoetoidea was provisionally revised. Thus the functional models and general biology revealed by the current study is applicable to only a restricted set of "hemiramphid" genera belonging to a new sub-family, the Hemiramphinae.