January 27, 2011

Technical Question of the Month: How do you distinguish a butterfly fish from an angel fish?

Angel fish – Photo by John E Randall
Butterfly fish – Photo by Robert Patzner

Previously, we discussed why we monitor butterfly fish and not angels and other notable reef species. For beginning Reef Checkers, it may be difficult to distinguish between angels and butterfly fish – particularly small angels. In general, fish taxonomists (scientists who study species differences and name new species) use skeletal characteristics such as the number of bones in a particular fin to distinguish species. For Reef Check, we need to identify live fish while they are swimming past – and we don’t have time to count fin rays – if we were lucky enough to even see a fish with a fully extended fin.

While these two fish families (Chaetodontidae and Pomacanthidae) are superficially similar – there are major and consistent differences that allow us to easily count butterfly fish without confusing them with angels:

1. Color – all butterfly fish exhibit only a limited color scheme – white, black and yellow or orange. If you see some blue, green or most other colors, then this is not likely to be a butterfly fish.

2. Mouth – all butterfly fish have a tiny mouth just a few millimeters wide — shaped like eyebrow tweezers. Angel fish by contrast, have a relative wide mouth, designed to munch on mouthfuls of sponges, algae, jellyfish and soft corals. Because of this difference in mouth shape, angels have a relatively wide “face” when viewed head on whereas butterfly fish are thin-faced.

3. Gill cover spine – All angel fish have a reverse spine at the base of the gill cover. Butterfly fish don’t have this.

4. Size – maximum size of butterfly fish is about 25 cm (10 inches) long whereas angels can grow much larger – to more than double this.

5. Behavior – butterfly fish can be found in pairs or in large schools of thousands of fish. Angel fish are typically found in pairs or individually.

Following these tips, you will be able to easily differentiate butterfly fish from angels.