Flower:

Flowers are usually single in the leaf axils all along the stem and branches, with separate male and female flowers on the same plant (monoecious); occasionally there may be 2 or 3 flowers in an axil. Male flowers are on the upper part of stems and branches, oval to egg-shaped, 1.5 to 2 mm (less than 1/8 inch) long, have a single stamen with a 1-chambered anther. Female flowers are all along the stems and branches, narrowly elliptic with a long beak, .5 to 2.7 mm long with a slightly off-center, 2-parted style.

Leaves and stems:

Leaves are all submersed, opposite to whorled, flexible and thread-like, 1 to 4 cm (to 1½ inches) long, .1 to .4 mm wide, pointed at the tip, with minute, widely spaced, spine-like teeth around the edges, 12 to 20 teeth per side and 2 or 3 teeth at the very tip. Prickles along the midrib are absent.

The leaf base has a thin sheath wrapping the stem, generally rectangular, more or less straight across the top edge usually with 4 to 7 small lobes, each with a minute, spine-like tooth at the tip. Stems are round, slender, usually much branched, green, and lack prickles.

Fruit:

Fruit is a narrowly elliptic seed (achene), 2 to 3.2 mm long, widest near the middle, yellowish to light brown, dull and pitted, the pits regularly arranged in about 40 vertical rows.

Notes:

Slender Waternymph is occasional to common in north-central Minnesota, where it reaches the western edge of its range. It is typically found in clear, soft water lakes and ponds, usually those with sandy bottoms, and most often in shallow water. According to the DNR, even though it's been found in about 200 lakes, it may be more susceptible to deteriorating water quality than other aquatics, and is likely why it has declined throughout a large portion of its North American range. It was listed as a Special Concern species in 1996 and is currently Special Concern in Wisconsin.

It is distinguished from other Najas by the flexible, thread-like leaves (rarely more than .3 mm wide) with minute, widely spaced, spine-like teeth; sheaths that are more or less rectangular with a few small, spine-tipped lobes across the top edge; 1 to 3 flowers in leaf axils; dull fruits covered in pits arranged in about 40 vertical rows. Magnification is required to see some (or most) of these traits. Several references note that the style on flowers and fruits is off-center but we think it's not dramatically so.

Most similar are Nodding Waternymph (Najas flexilis) and Southern Waternymph (Najas guadalupensis), both of which tend to have wider, toothier leaves and neither of which have lobes along sheath edge. N. flexilis leaves have a long, gradual taper to the leaf tip, and fruit is shiny and smooth or obscurely pitted at best. N. guadalupensis sheaths tend to be more tapered (more triangular when laid out flat) or rounded, tends to be less leafy (leaves usually opposite or whorled in 3s) with shorter leaves, and stems are commonly brown or reddish. Brittle Waternymph (Najas minor), an invasive species only recorded once in Minnesota, is noted as similar to N. gracillima when young, but more mature plants have visibly toothed leaves about 1 mm wide and the achene has a ladder-like arrangement of pits on the surface, the pits wider than long.