Big cleft? Little cleft?

Stumbled across some leftover photos from our July Hawaii trip:

They’re beautiful, weird, and ubiquitous on tropical Pacific reefs. We see Longnose Butterflyfish wandering nonchalantly over the reefs on pretty much every one of our snorkel and dive outings. There are two species that go by the name Longnose Butterfly: the common Forcipiger flavissimus and the somewhat uncommon F. longirostris. John Hoover (hawaiisfishes.com) calls flavissimus Common Longnose Butterflyfish and refers to longirostris as Big Longnose Butterflyfish. Other authorities, such as Keoki Stender (marinelifephotography.com), refer to the respective species as Forcepsfish and Longnose Butterflyfish. Flavissimus is also sometimes called Yellow Longnose Butterflyfish and longirostris is sometimes called Big Forcepsfish. Confusing to you? Me too. Do you care? Well, I do.

Besides being inconsistently named, the two species can be difficult to tell apart. Longirostris tends to be a bit bigger, with a longer snout, but there’s some overlap in size. Two features are generally used to distinguish the species in the field. The first is coloration: I usually look for tiny black dots on the fish’s chest—longirostris has them, flavissimus doesn’t. Trouble is, you have to be close and have good eyesight (and a non-fogged mask) to see this. The second is mouth shape: Flavissimus is supposed to have a deeper oral cleft than longirostris, but I can’t tell the difference, at least not on a living, moving fish. Photos make it a little easier. Like these:

This is F. flavissimus. No chest spots and a slightly deeper oral cleft.

F. longirostris. Tiny black spots on the chest, and the oral cleft is a bit shorter than in the previous photo.

These two very similar species share the same habitat throughout much of the Indopacific, but they don’t directly compete. Flavissimus is a dietary generalist, eating all sorts of small invertebrates, and, rather oddly, often plucking off the tube feet of echinoderms such as urchins and sea stars. (The echinoderms have plenty of feet to spare, but it still sounds painful.) Longirostris on the other hand specializes in picking tiny shrimp from deep inside coral heads.

Anyway, there’s your fish minutiae for the day.

Subtle and obvious distinctions

One of the nice things about fishwatching on the Hawaiian reefs is that most of the fish species you encounter have clear field identification markings.  See a large, blueish surgeonfish with yellowish fins?  If it’s got white scalpels (little but obvious sharp things near the tail) then it’s an eyestripe (aka Dussemier’s) surgeon.  If the scalpels are dark it’s a yellowfin surgeon.  Easy.  Most Hawaiian reef fish are pretty easy to identify even if you just get a quick look, provided you know what to look for.

Most, but not all.  Two species that are fairly difficult—at least for me—to distinguish in the water are the the common longnose butterflyfish and the big longnose butterflyfish. (Even the common names for these fish are kind of confusing.  The common longnose is sometimes called forcepsfish and the big longnose is sometimes called just plain longnose butterflyfish. Sheesh.)  As the name suggests, the big longnose gets a little bigger than the common and it also has a longer snout than the common.  But these relative differences can be hard to pick out underwater.  The mouth opening on the big longnose is smaller that that of the common longnose, but this is hard to see unless you get a really long, close look at the fish.  For me, the easiest distinguishing feature is the area of small black dots just behind the pectoral fins on the big longnose.  These are absent on the common longnose.  But you have to be pretty close and have good vision to make the dots out.

Anyway, common longnoses are very common at Mahukona, but big longnoses are relatively rare, so I always keep an eye out for the latter.  I saw a solitary big longnose a couple of weeks ago and managed to get a photo that shows the black chest dots.

Big longnose butterflyfish at Mahukona.  The black chest dots and tiny mouth are visible here.

Common longnose butterflyfish at Mahukona. No chest dots and a somewhat larger mouth.

Common longnose from a different angle

Big longnose butterflies are much more common further south around Puako and Makaiwa Bay.  Earlier this week we went for a two-tank scuba dive in this area with our good friends at Kohala Divers.  Not only were big longnose butterflyfish common down there, but there were a number of the interesting dark variant of the bignoses.  The dark variant, starkly different from the normal yellow morph, leaves no doubt as to which species you’re looking at because it only occurs among the big longnoses.

A normal colored and a dark morph big longnose butterfly at about forty feet in Makaiwa Bay.