FRESHWATER MOLLUSCS First record of complex release strategies and morphometry of glochidia in sympatric Echyridella species (Bivalvia: Unionida: Hyriidae) Michele Melchior . Kevin J. Collier . Susan J. Clearwater Received: 2 March 2019 / Revised: 16 May 2019 / Accepted: 11 June 2019 Ó Springer Nature Switzerland AG 2019 Abstract Larvae (glochidia) of the freshwater mus- sel 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) lm in diameter which were broadcast individu- ally and bound to mucus threads into the water column. In contrast, the sympatric E. aucklandica produced miniature glochidia (99 ± 0.3 lm 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 distin- guishable 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. Addition- ally, 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. Keywords Freshwater mussel Á Conglutinate Á Reproduction Á Mimicry Á Host fish Á New Zealand Introduction Specialised larvae (glochidia) of the freshwater mus- sel order Unionida undergo a critical period during which they must parasitise suitable host fish to complete their life cycle (Kat, 1984). During repro- duction, sperm aggregations (spermatozeugmata) are broadcast by unionid males into the water column and are carried by currents to nearby females, allowing for the uptake of sperm through their inhalant apertures Guest editors: Manuel P. M. Lopes-Lima, Nicoletta Riccardi, Maria Urbanska & Ronaldo G. Sousa / Biology and Conservation of Freshwater Molluscs Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10750-019-03995-3) con- tains supplementary material, which is available to authorized users. M. Melchior (&) Á K. J. Collier Environmental Research Institute, The University of Waikato, Hamilton, New Zealand e-mail: michele.melchior@niwa.co.nz M. Melchior Á S. J. Clearwater National Institute of Water & Atmospheric Research, Hamilton, New Zealand 123 Hydrobiologia https://doi.org/10.1007/s10750-019-03995-3