The egg masses of lotic macroinvertebrates: proximate cues for oviposition site selection and implications for larval abundance and distribution

As the abundance and distribution of biological populations may be regulated at various life history stages during an organism's lifetime, it is imperative that information relating to all life history stages be considered when explanations for perceived patterns are sought. In the case of many lotic invertebrates, the life cycle is complex comprising a relatively long-lived aquatic larval stage followed by a shorter terrestrial period during which adults are reproductive and eggs are deposited back into the stream. These two latter phases of the life cycle however, are rarely incorporated into explanations for the distribution and/or abundance of larvae. This research addresses this obvious gap in our knowledge by documenting the nature and distribution of egg masses representing a range of lotic taxa and ultimately, by exploring relationships between egg mass and larval distributions. Initially, an extensive survey of the egg mass fauna at two Victorian upland streams was conducted. Previously undescribed egg masses belonging to a range of lotic taxa were characterised. In addition, the abundance and distribution of egg masses at multiple spatial and temporal scales were quantified. At both streams, the egg mass fauna belonged mostly to species of Trichoptera and Diptera. The families Hydrobiosidae, Hydropsychidae, Hydroptilidae and Chironomidae were numerically dominant. Of these common families, the number of egg masses found was remarkably consistent both between sampling times and between different sampling locations on each river. In terms of oviposition site selection, egg masses of most species were found almost exclusively on larger rocks that protruded above the water's surface. Multivariate analyses determined that substrate size and current speed were important factors in determining the distribution of eggs masses. In particular, numbers of egg masses from two different groups of Hydrobiosidae (Taschorema complex and Ulmerochorema sp.) responded positively to increased measurements of both these physical variables. Egg masses belonging to a third group within this family (Apsilochorema sp.) were found in greater numbers on larger substrates, but did not appear to be influenced by current speed. An experiment testing the effects of current speed on egg mass abundance, found both Taschorema and Ulmerochorema in greater numbers on substrates located in fast (>0.6 ms'') flow. Interestingly, significantly more egg masses belonging to Apsilochorema were found on substrates characterised by slow (