Phyllidiella pustulosa vs Phyllidiopsis fissurata: How to Tell Them Apart

Two of the most commonly photographed Phyllidiidae nudibranchs in the Indo-Pacific look almost identical at first glance: Phyllidiella pustulosa and Phyllidiopsis fissurata. Same body shape, similar size, similar color palette of pale pink-cream tubercles on a dark background. They sit in two different genera, but you wouldn't know it from a casual photo.

Phyllidiella pustulosa

Phyllidiopsis fissurata

The arrangement of dorsal tubercles is sometimes cited as a difference, but both species show enough individual variation that you can't reliably tell them apart from the back.

There's a much easier and more consistent feature to check: the rhinophores.

Phyllidiella pustulosa — rhinophore close-up

Phyllidiopsis fissurata — rhinophore close-up

Phyllidiella pustulosa has rhinophores that are uniformly black all the way down. Phyllidiopsis fissurata has rhinophores where only the upper-rear section is black, with the lower part lighter. This pattern is consistent across individuals and gives you a reliable ID.

When you photograph one of these animals, make sure both rhinophores are fully extended and in focus. That single frame is usually enough to settle the ID.

One caveat: a couple of related Phyllidiopsis species — Phyllidiopsis krempfi and Phyllidiopsis pipeki — have similarly patterned rhinophores (only the upper-rear section dark). So "upper-rear black only" rules out P. pustulosa, but you'll still need to look at the dorsal pattern to distinguish among the Phyllidiopsis.

Quick reference:
• Phyllidiella pustulosa
• Phyllidiopsis fissurata
• Phyllidiopsis krempfi
• Phyllidiopsis pipeki