Sympatric species of freshwater mussels in the order Unionida may need to partition resources to enable coexistence due to their relatively sedentary life-style and complex symbiotic life-phase dependent on host-fish. Although new data on their reproductive biology is being increasingly documented, particularly in the Northern Hemisphere, major gaps remain in New Zealand where two threatened Hyriidae, Echyridella aucklandica and E. menziesii, can co-occur. This thesis combines multiple approaches to elucidate the reproductive ecology of the previously unstudied E. aucklandica compared to the more widespread E. menziesii, and to examine mechanisms enabling their successful coexistence. I use field and laboratory investigations to compare reproductive niche parameters along three major resource use dimensions: host-fish species, reproductive phenology (time), and habitat use (space), and show that these two sympatric congeneric species have evolved sharply contrasting reproductive strategies. Complex adaptations were identified among the two Echyridella species through contrasting use of larval (glochidia) host-fish infestation strategies and contrasting glochidia morphometry. Female E. aucklandica were found to produce conglutinates, mucus packages containing miniature glochidia thought to lure specific fish to them by resembling fish prey. This is thought to be one of the first Unionida species outside of North America reported to be using functional conglutinates to mimic host diet as an infestation strategy. Miniature glochidia produced by E. aucklandica were around three times smaller than those of E. menziesii, which along with other features such as shape and buoyancy, were consistent with morphological features found in host-specific unionid species elsewhere. For the first time, the brooding phenology of E. aucklandica is reported, filling an important data gap on the basic biology of this poorly known and threatened species. Though E. aucklandica began brooding earlier and remained gravid for longer than E. menziesii, the brooding onset for both species generally occurred in winter (E. aucklandica in May–July, E. menziesii in August), reaching peak brooding (and thus glochidia release) in late austral spring to austral summer (November and December). High temporal overlap in glochidia development of these two species was observed, particularly during peak brooding when mature glochidia are expected to be released. Intra- and interspecific differences in reproductive timing identified key thermal cues (particularly accumulated degree days) associated with brooding onset and glochidia maturation in both species. The importance of water temperature suggests that changes in climatic conditions have the potential to cause negative effects on both species by causing mismatches between mussel reproduction and host phenology. Field and laboratory studies of glochidia attachment and development on fish confirmed host specificity for E. aucklandica and host species partitioning between the two Echyridella species. Echyridella menziesii, was found to infest a wide range of fish species, being particularly prevalent on benthic Gobiomorphus species and Anguilla dieffenbachii, in contrast to E. aucklandica which produced viable juveniles only on the pelagic Retropinna retropinna. However, this Retropinnidae species was found only in low numbers across both study sites investigated and found to be infested with consistently low numbers of E. aucklandica glochidia, suggesting it may be a secondary rather than the primary host. Laboratory investigations confirmed that E. aucklandica’s miniature glochidia encysted exclusively on the gills of R. retropinna, in comparison to E. menziesii which attached not only on the gills but also externally on the fins and skin of its host. Miniaturised glochidia of E. aucklandica grew nearly five times their original size on R. retropinna before maturing as juveniles, compared to the larger E. menziesii glochidia which stayed the same size throughout metamorphosis. As a result, metamorphosis duration for E. aucklandica glochidia was significantly longer than for the larger E. menziesii, but only by two to three weeks. Passive integrator transponder (PIT) tagging was used in a coastal Waikato stream in combination with electrofishing to track movements of both mussel species and determine host fish species locations within mapped habitats, to better understand spatial and temporal movement patterns of both sexes in relation to species-specific timing of fertilisation, glochidia release and host fish infestation. During the glochidia release season, results showed evidence of relatively high net horizontal movement rates and active bank-ward cluster formation in tagged individuals of both species. Spatial overlap between mussel species and their respective host fish was partially observed for E. menziesii, but could not be confirmed for the host-specific E. aucklandica due to only one R. retropinna being captured. Furthermore, vertical positions of mussels varied throughout the onset brooding period for both species, but generally, proportions of female mussels increased at the sediment surface during respective reproductive onsets. A moderate, bed-moving flood event that occurred during this experiment was associated with downstream displacement of both mussel species, with potential flood-flow refugia associated with riparian vegetation and debris in bank habitats apparently providing resistance to dislodgement. This research provides critical information on the reproductive biology and partitioning of reproductive niche dimensions associated with the coexistence of sympatric E. aucklandica and E. menziesii. The findings will assist in the development of conservation strategies and stream management interventions to enhance freshwater mussel recruitment and survival in northern New Zealand streams.