Ruppia is a small genus of aquatic monocotyledons in their own family, Ruppiaceae. Vegetatively they're nothing unusual, with slender leaves like so many others. Their flowers are clustered and hermaphrodite, but after flowering the ovary stalks elongate (they're called podogynes, but it's not clear to me why they're not simply called carpophores) so each fruit is separated from the others of its flower cluster.
Ruppia megacarpa fruit and top of podogyne (scale 1mm)
Ruppia megacarpa was first described from New Zealand specimens, by the late Ruth Mason. The fruits look like little birds' heads when they're ripe. Early on, they have a fleshy outer layer; what's left after that's gone is endocarp (equivalent to the outer part of a peach stone) and a single seed inside. According to Flora of New Zealand Volume 2 (Moore & Edgar 1970), R. megacarpa is found in both North and South Islands in brackish and saline lagoons and ponds.