First record of complex release strategies and morphometry of glochidia in sympatric Echyridella species (Bivalvia: Unionida: Hyriidae)

Larvae (glochidia) of the freshwater mussel order Unionida undergo a brief parasitic phase by attaching to and metamorphosing on suitable host fish. Here, novel observations of complex glochidia release strategies and glochidia morphometry are reported and compared in two sympatric New Zealand hyriid species, Echyridella menziesii (Gray, 1843) and Echyridella aucklandica (Gray, 1843). Echyridella menziesii produced glochidia averaging 277 ± 0.7 (SE) µm in diameter which were broadcast individually and bound to mucus threads into the water column. In contrast, the sympatric E. aucklandica produced miniature glochidia (99 ± 0.3 µm SE) embedded in functional conglutinates, thought to facilitate host fish attraction. To our knowledge, this is the first Unionida species, outside of North America, reported to be using functional conglutinates to mimic host diet as an infection strategy. The production of miniature glochidia that were morphologically distinguishable from those of E. menziesii , coupled with contrasting release strategies, highlights the potential for partitioning of host resources through contrasting attachment strategies and infection times. Additionally, these findings provide the basis for distinguishing glochidia of the two Echyridella species in field studies of host fish infection and highlight the need to develop novel methods for captive propagation of E. aucklandica to support restoration of declining populations.