PhD student Michele Melchior is using field and laboratory techniques to study kākahi – host fish interactions to improve conservation efforts.
New Zealand has two freshwater mussel species (kākahi) that occur in northern Waikato streams. The widely distributed Echyridella menziesii and the threatened Echyridella aucklandica. Until recently, little information existed on the basic biology of these species which can live for more than 50 years. Both species undergo an obligate symbiotic phase on fish during their early life stage as larvae (termed glochidia).
Previous research suggests that E. menziesii glochidia are host fish generalists, able to attach on a range of native fish taxa. By comparison, host-relationships for E. aucklandica have remained unknown. However, recent evidence from Michele’s PhD of contrasting glochidia release and infection behaviour of the two species (e.g broadcast release in E. menziesii and conglutinate release in E. aucklandica i.e. mucus packages containing glochidia that mimic fish food) has highlighted the potential for host – resource partitioning and that E. aucklandica may be a host fish specialist (able to attach only to few closely related species) rather than E. menziesii which is a host generalist.
To assess glochidia – host relationships, field analyses were undertaken via electrofishing and gee-minnow trapping in Waikato streams known to contain large populations of both Echyridella species. Results from the field were validated in the laboratory via host-fish infection trials, to further understand metamorphosis success and infection intensity on fish in E. aucklandica compared to E. menziesii.
From field studies E. aucklandica was found to only attach to common smelt (Retropinna retropinna), while E. menziesii attached to a range of fish taxa, but not to common smelt. This confirms that host-fish partitioning is occurring between kākahi species and this is the first observation of host specific attachment by E. aucklandica glochidia. Subsequent laboratory investigations confirmed common smelt as a host for E. aucklandica, with high prevalence of infection (80%) on fish samples, but low mean infection intensities in all infected individuals (x̄ =1.6 glochidia per infected fish). Suggesting that common smelt are not the preferred primary host species, but further research is being conducted. Furthermore, E. aucklandica and E. menziesii exhibited contrasting reproductive modes once attached on compatible host fish species, requiring contrasting attachment sites and developmental modes (metamorphosis durations and growth on the host).
Because kākahi are entirely dependent on hosts to complete their life-cycle, a lack of suitable hosts (particularly for specialists with a small host range) may lead to a lack of juvenile recruitment, and increasing vulnerability to co-declines, and potential extinctions should the host-glochidia relationship be disrupted. Knowledge of fish hosts is essential in developing conservation management strategies such as restoration of appropriate habitats for threatened unionid populations and facilitating upstream passage for migratory host fish at critical times of the year (i.e glochidia release).
This research is part of NIWA's Cultural Keystone Species research programme, which is funded by the Ministry of Business, Innovation and Employment.
Research lead: Kevin Collier
Developing E. aucklandica encapsulated on gill filaments of common smelt (top) and ventral view of head region (bottom) in common smelt with encapsulated E. aucklandica (white circle) on the left side (under operculum) on ventral gill segment (glochidia length is 432 µm).
One month old Echyridella aucklandica transformed on common smelt gill
Life-cycle of E. aucklandica which releases glochidia enwrapped in conglutinates that infest on fish which then metamorphose and drop off as benthic juvenile mussels.