It's not entirely easy to tell
Ancistrus and
Hypancistrus apart (with one single characteristic feature).
Here's my amateur view:
Hypancistrus is defined, amongst other things, based on it's "few teeth" - all species of
Hypancistrus have relatively few teeth in both upper and lower jaws, but more so in the lower jaw, where
Ancistrus have quite a number of (smaller) teeth.
There are some other fairly obvious differences in mature males of Ancistrus - namely the "bristles" or "tentacles" on the snout.
Ancistrus are also, to all known species so far, dark brown, dark grey or black (possibly with a little hint of blue), and most Ancistrus have fairly fine spots in some light colour (yellow, light brown, light grey or white). Some
Hypancistrus are dark body colouration with spots, but several others have lines on the body pattern, a few species have spots and stripes.
Jon Armbruster is one of the leading ichtyologists in the field of Loricariidae, and has a website dedicated to these fish. His definition for identifying differens species, a "key", is
here.
The key leads to
Ancistrus as follows:
1. Body behind head completely plated dorsally and laterally (plates may be absent ventrally).
3. Ventral surface of the pectoral girdle covered in skin or plates mesial to the coracoid strut (coracoid strut may be exposed; plates may cover the pectoral girdle, but the odontodes are supported by the plates and not the girdle.
5. Edge of snout naked.
6. Cheek plates evertible to 75 degrees or greater. Nuptial male with no hypertrophied odontodes along snout (Fig. 6a-c) and no thick flesh present on posterior process of cleithrum.
7. Fleshy tentacles on snout (best developed in males, but females possess some small tentacles at edge of snout.
From the same source,
Hypancistrus differs at:
5. Edge of snout covered with plates.
then takes this path for identification:
9. Postdorsal ridge absent or restricted to usually 1 (occasionally 0 or 2) median, unpaired preadipose plates
12. Seven branched dorsal fin rays.
22. Dorsal fin with membrane of last ray not expanded posteriorly, not attaching to preadipose plate or the adipose spine.
25. Teeth viliform (threadlike) or large with long stalk.
27. Cheek plates evertible (loosely embedded in connective tissue, not firmly attached to suspensorium), usually with hypertrophied odontodes (VIEW - B). Evertible cheek plates and odontodes can be held almost at right angle to head.
45. Median row of odontodes on each lateral plate not forming sharp keels.
47. Fewer than 10 teeth per dentary. Dentaries short, forming acute angle at union.
48. Lower lip oval. Body strongly dorsoventrally flattened.
49. Dentary teeth moderately long; crowns consisting of two nearly equally sized lobes, not spoon-shaped. Small adult size <100 mm.
(Bold is marking the point where
Hypancistrus and
Ancistrus "split").
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Mats