Acanthuridae: Surgeonfish, Tangs and Unicornfish

Acanthuridae is the family of surgeonfishes, tangs and unicornfishes: flat, oval reef fish that graze algae and carry a sharp, blade-like spine on each side of the tail base. FishBase lists 6 genera and 85 species found around coral reefs in all tropical oceans. The best-known member is the regal blue tang, Paracanthurus hepatus, the species of Dory in Finding Nemo (Wikipedia).

Because of the 'scalpel'. Surgeonfish have sharp, bony, blade-like plates or spines on the caudal peduncle, the narrow base of the tail (Wikipedia). With a rapid sideways sweep of the tail they can slash other fish (FishBase), and ADW notes the spine is often coated with toxin. In most species the spine folds into a groove and flicks out as the body bends; in Naso and Prionurus it is a fixed plate (ADW).

If you catch one on a line, handle it by the body with the tail pinned or use a towel; see how to handle and release fish.

• Body: deep and flattened from side to side, with the eye set high on the head (FishBase)
• Mouth: small, at the front, with a single row of close-set teeth for scraping algae (FishBase; Wikipedia)
• Fins: one long dorsal fin with 4-9 spines and 19-31 soft rays; an anal fin with 2 or 3 spines and 19-36 rays (FishBase)
• Tail base: the spine or bony plates on each side (Wikipedia)

The name 'tang' is used for the same family; a yellow tang, a regal blue tang and a surgeonfish are all acanthurids.

The count depends on which source you open:

• FishBase: 6 genera, 85 species
• Wikipedia: about 6 extant genera in 2 subfamilies, roughly 86 living species
• Animal Diversity Web (University of Michigan): 'most sources agree' on 72 species in 6-9 genera

We found no source that reconciles the three counts. For identification it matters little, because the family marks above apply to every species.

Yes. Dory, voiced by Ellen DeGeneres in Finding Nemo and the 2016 sequel Finding Dory, is a regal blue tang, Paracanthurus hepatus, a member of Acanthuridae (Wikipedia). It has more than a dozen common names, including palette surgeonfish, hippo tang and royal blue tang. Adults weigh around 600 g, and the species lives across the Indo-Pacific from East Africa to Japan and the Great Barrier Reef.

Two facts few pages mention: Wikipedia reports that online searches for blue tangs rose for two to three weeks after Finding Dory, but import data showed no significant increase; and the species was first bred in captivity in 2016 by Kevin Barden of Rising Tide Conservation after a six-year effort. Its tail spine can cause deep wounds, like that of other surgeonfish.

Some species are eaten as food (ADW), but there is a known risk: ADW reports cases of ciguatera fish poisoning from acanthurids, and Wikipedia says the regal blue tang may cause ciguatera if eaten. Follow local health advice and fishing rules about which reef fish are eaten where you are, and see a doctor promptly if you feel unwell after eating reef fish.

ADW describes grazing surgeonfish as keeping thick mats of filamentous and leafy algae from smothering corals, while plankton-eating unicornfishes add nutrients through their droppings. They spawn in large aggregations, often year-round with a late-winter-to-spring peak, and many species live for more than 30 years (ADW). Their larvae, called acronurus, are almost transparent and drift in open water before settling on the reef (Wikipedia).