Echyridella aucklandica

Echyridella aucklandica was first described by John Edward Gray in 1843 and given the name Unio aucklandicus.[2] It was described again by American malacologist Charles Torrey Simpson in 1902, who used the name Diplodon websteri.[3] Simpson's original text (the type description) reads as follows:

Diplodon websteri Simpson.

Shell long, rhomboid, compressed or subcompressed, inequilateral; beaks subcompressed, pointed, their sculpture apparently a few irregular lachrymose nodules arranged in a somewhat radial pattern; surface with uneven growth lines and impressed rest marks, sculptured throughout with lachrymose nodules which are often V-shaped, those along the upper part of the low posterior ridge slightly knobbed; epidermis dark olive green, clouded with lighter green, rather dull; pseudo-cardinals small, subcompressed, granulose, two in each valve ; laterals straight, two in the left valve, one in the right; muscle scars small, shallow and irregular; nacre bluish,

lurid purple near and in the beak cavities, thicker in front.

The length of the shell is 62–81 mm. The height of the shell is 32–39 mm. The width of the shell is 14–20 mm.[3][4]

Echyridella aucklandica is found in two separate locations, Southland and the lower North Island. This broken distribution may have come about due to transportation by Māori.[5] It inhabits lakes and streams.[4]

The larval stage of Echyridella aucklandica attaches itself exclusively to the New Zealand smelt, unlike the more common Echyridella menziesii, which generalises to many different host species.[6] It is theorised that the species may have used the New Zealand grayling (upokororo), an extinct species, as its primary host in the past.[5]