Flower:

Flowers are single in the leaf axils all along the stem and branches, with male and female flowers on separate plants (dioecious). Male flowers are green, oblong-elliptic, 1.7 to 3 mm (to 1/8 inch) long, have a single stamen with a 4-chambered anther. Female flowers are reddish, oval-elliptic, 2.5 to 5.7 mm long with a 3-parted style.

Leaves and stems:

Leaves are all submersed, opposite to whorled, firm, bright green turning brownish later in the season, linear in outline, .5 to 4 cm (to 1½ inches) long, .4 to 4.5 mm (to 1/6 inch) wide, pointed at the tip, sheathed at the base, and with large, pointed, spine-tipped teeth or lobes around the edges, 8 to 13 teeth per side on larger leaves, and usually a few small prickles along the midrib on the underside.

Stems are round, much branched, green to brownish, with scattered large spine-tipped prickles.

Fruit:

Fruit is an oval seed (achene), 2.2 to 4.5 mm long, irregularly pitted on the surface, reddish-brown at maturity.

Notes:

Spiny Naiad has a world-wide distribution but a scattered one in North America, considered a rare species in Minnesota and introduced in Wisconsin, even potentially invasive in some states. According to the DNR, in Minnesota it is limited to hard water prairie lakes in the western part of the state, which are subject to pollution primarily from agriculture. It has only been recorded here 35 times and was listed as a Special Concern species in 1996.

It is the easiest Najas species to identify from the visibly toothed leaves with scattered prickles along the midrib and on the stems. It is also the only dioecious Najas, where the others are monoecious, having separate male and female flowers on the same plant. Most similar is Brittle Water Nymph (Najas minor), an invasive species only recorded once in Minnesota, which also has visibly toothed leaves, but its leaves are narrower (max 1.2 mm wide), it lacks prickles on stems and the leaf midrib, and the achene has a regular, ladder-like arrangement of pits on the surface.