Knife Livebearer Identification Guide

Key identification features

• Slender, strongly laterally compressed body
• Sharp, knife-edge keeled belly that gives the species its common name
• Silvery body color with a distinct dark mid-lateral stripe running from head to tail
• Small, upturned mouth adapted for feeding near the surface
• Compact size, reaching only about 7 cm

Common look-alikes

• Pike livebearer: much larger overall with elongated predatory jaws, unlike the knife livebearer's small upturned mouth.
• Mollies: have a rounder belly profile without the sharp keel and a blunter, non-upturned mouth.
• Hatchetfish: share a keeled belly silhouette, but hatchetfish have a much deeper, more exaggerated chest and are unrelated egg-layers.

Where you'll see one

Knife livebearers are native to fast-flowing streams and rivers on the Atlantic slope of Costa Rica, Nicaragua, and Panama, where they hold position near the surface in current, feeding on insects and other small prey that fall or land on the water. They tend to school in open runs rather than hiding among dense plant cover, making the keeled belly and stripe easy to spot from above.