Wayne Leibel, 2003
Cichlid Room Companion

Of Pickled Fish and Ichthyological Sleuthing: A Primer For the Advanced Aquarist.. Part II: Identifying Your Fish

By , 1985. image

Classification: Taxonomy and phylogeny.

characters Figure 3: Definition of characters used in describing and keying cichlids. Photo by Wayne S. Leibel.

Assembling A Library

Where do I start in the name game? To identify aquarium fishes from the original descriptions one must conduct his/her ichthyological sleuthing as the professionals do. The first step for any serious study of a genus or larger assemblage of related fishes is to accumulate a working library of original descriptions. This task is not nearly as formidable as it seems. First, retrieve your previously-maligned Sterba (1966), or Hoedemann (1975), from the trash (Axelrod [1980] is considerably less useful), or your specialty text (e.g., Cichlids of the World, Rivulins, Characoids of the World, Rasboras, etc.). Page first to the section containing the fishes that you are interested in, the neotropical cichlid genus Aequidens, for example, and note that the Latin name of each species is followed by a name and date. This is the surname of the original author (describer) of that species and the year of publication of that species description. A quick scan of the Aequidens listings in Sterba (1966) indicates that Heckel (1840), Steindachner (1878), and Hensel (1870) among others, were responsible for the description of many of the species of the genus Aequidens. There are two ways to proceed next to the primary literature. Hoedemann (1975) provides the original sources (journal and volume) for many of these in the bibliography at the back of his book, and Sterba (1966) offers a selected bibliography of species by geographical distribution. Several of these suggested sources, with regard to our example Aequidens above, would provide a useful starting point for the assembling of an Aequidens library. One of these on Sterba's list, Eigenmann and Allen (1942: The Fishes of Western South America, see citation page 857 in Sterba) contains a list including descriptions of members of the family Cichlidae from this region including a key to the genus Aequidens and a run-down of several nominal species with complete bibliographic citation of the original descriptions for each. In fact, taken together and used as a basis for flushing out primary descriptions of the genus Aequidens, Sterba's (1966) suggested Eigenmann reviews produce, with some leg work, a complete list of pre-1942 systematic literature concerning the genus Aequidens.

For more recent literature pertaining to members of this genus, there is the Zoological Record, a yearbook listing all publications appearing in that year organized according to species, geographic distribution, and author. A quick check of the entry for the genus Aequidens under Pisces in the systematic section produces a list, by author, of relevant publications for that year. A discussion of, and a sample page from the Zoological Record, appear in Hoedemann (1975) pages 203-204, and serves as a sufficient orientation to this useful tool. The Zoological Record is also useful in tracking down an original description if only the author's name and the year of description, and not the complete citation, are given. For example, using only the entry Aequidens itanyi Puyo 1943 and the Zoological Record, the original citation can be obtained by searching Puyo in the author index. Because of the obscurity of the journal in which the description was published, the Puyo citation appears in the 1947 and not 1943 volume, however, a little persistence usually pays off. One drawback of this powerful tool: most public libraries do not have it, and many smaller colleges likewise cannot afford a subscription. More on accessing such valuable esoterica later.

One extremely valuable source of bibliographic material is the aforementioned specialty books. We, in the cichlid hobby, are very fortunate to have Goldstein's two cichlid books (1970, 1973) and the scholarly articles by Paul Loiselle which appear almost monthly in FAMA as a source for bibliographic material. Many of the important papers pertain to cichlids in general and Aequidens, as our example, are listed completely with title in Goldstein's books. Loiselle has recently reviewed the genus Aequidens (FAMA, Aug., Sept., Oct., 1983) and bibliographies accompanying these and his other articles (Geophagus, for example) are quite complete and usable.

Generating such a bibliography is the easy part of the process; actually laying your hands on the materials can be a source of major disappointment and frustration. Many larger universities have complete collections of early ichthyological literature and many of these, Yale University for example, have an open stack policy that allows anyone access to these materials. A free Saturday, a pocketful of change for photocopying, a complete bibliography, and the assertiveness to approach library staff for help when all else fails, are all that are required for a successful expedition. Call ahead to find out if the major university in your area is similarly community-friendly: many are not. Some, like Harvard University, for example, charge a membership fee for the use of the library collections. Although somewhat steep ($50 per annum), this sum allows the serious aquarist access to much of what is needed. In the case of Harvard's Museum of Comparative Zoology library, fifty dollars buys in to a complete shelving of Louis Agassiz' ichthyological collection including obscure papers from his contemporaries (1800's) throughout the world. It is well worth the price of admission. University libraries will retrieve and photocopy material if complete bibliographic citations are supplied. Generally these services are not provided for individuals, but rather for individuals working through other libraries. My friend, FAMA contributing editor Lee Finley, then of Bridgeport, Connecticut, was able to retrieve needed papers on Synodontis from Harvard's MCZ library through the Bridgeport Public Library. He found it took some pressure to get the (public library) staff to help him, but if he preconfirmed by phone the availability of a particular journal at the MCZ library, and, if he provided complete citations (author, year, title, journal, volume, and pages to be copied), the interlibrary loan program would respond with the desired papers in about three to four weeks for photocopying costs and a small service charge. The joys of rural living need not preclude access to primary fish literature.

Making Sense of the Primary Literature

Aequidens dorsigerus Figure 1: Aequidens dorsigerus. A dwarf Acara from the La Plata drainage. Typical non-spawning coloration. See ADI 28:11 for spectacular breeding trim. Photo by Wayne S. Leibel.

So you've managed finally to procure that seminal revision of the genus Aequidens so that you'll be able to nail down the ID of that gorgeous-finned beast in your tank. But the information in the description bears little relationship to any living Aequidens that you've ever seen, and there are no pictures! What now? Your feelings of disappointment are warranted, my friend. As I cautioned earlier in this article, the descriptions deal in meristics and morphometrics, and when included, descriptions of coloration are usually restricted to dead, pickled specimens. A sample entry, Aequidens dorsigerus, the commonly available dwarf Acara, Ae. curviceps look-alike with the bright red breast (Fig. 1), (good color photo of breeding trim in Aquarium Digest International 28:11), from Regan (1905) appears in Figure 2.

Note first the citation of the original description: (Heckel 1840), and the subsequent list of publications in which this fish has been discussed (Gunther 1862, Steindachner 1875, Pellegrin 1903). The parentheses indicate that, although Heckel was the original author of this species, the original name, usually the genus, has been changed. (Aequidens dorsigerus was originally Acara dorsiger until the genus Acara was sunk by Eigenmann and Bray in 1894 and replaced with Aequidens.) Regan then proceeds to describe the fish in terms of meristics and morphometrics. The actual characters measured vary from family to family, but for cichlids the important ones are indicated in Figure 3. A good beginning ichthyology text will further explain each of these indicated characters (e.g., Lagler et al. 1977, pages 403-405, or Goldstein 1970, pages 8-16).

14. Acara dorsigera

Acara dorsigera, Heck, Ann. Mus. Wien, II. 1840, p-348; Gunth. Cat. Fish. IV. p. 280 (1862); Steind. Sitzb. Ak. Wien, lxxi. 1875, p. 76; Pellegrin Mem. Soc. Zool. France, xvi. 1903, p 174 (1904).

Depth of body 2-1/5 in the length, length of head 2-1/2-2-4/5. Head very similar to that of A. tetramerus, but with 2 series of scales on the cheek and 1 series on the praeoperculumn; 6 gill-rakers on the lower part of anterior arch. Scales 23-24 2-1/2/9. Dorsal XIII-XIV 7-10. Anal III 8. Dorsal and anal fins scaleless. Pectoral as long as the head. Caudal rounded. Caudal peduncle 1/2 as long as deep. Olivaceous, with 7 or 8 more or less distinct dsrker crossbars; a dark blotch below the ninth to eleventh scales of the lateral line; a dark longitudinal band from eye to lateral blotch; a dark spot on the spinous dorsal between the eighth and tenth spines; soft dorsal, anal, and caudal with series of alternate light and dark spots. R. Amazon; R. Paraguay. The examples described by Heckel and Steindachner measure up to 53 mm in total length.

Figure 2: Description of Aequidens dorsigerus from C.T. Regan, 1905 (Ann. Mus. Nat. Hist. (7)15:329). See text.

In the early literature, major characters are expressed as ratios relative to standard length, in the modern literature as percent standard length. Often dimensions of the head are compared to eye diameter (e.g., "diameter of eye 3 in the length of the head"). Longitudinal scale counts are made along the lateral line. In cases where the lateral line is discontinuous; divided into upper and lower portions, longitudinal counts are made along upper and lower lines and expressed as a compound measurement (e.g., 17 + 13). Transverse scale counts (vertical) are made on the row just in front of the anterior dorsal fin insertion down to the lateral line, and from the lateral line down to the insertion of the anal fin.

These scale counts are expressed as a ratio: the numerator expressing the number of scales above the lateral line, the denominator expressing scale rows below the lateral line. Thus in our example, "scales 23-24 2½/9" translates to: undivided lateral line with 23-24 scales, positioned 2½ scale rows below the dorsal fin with an additional 9 rows of scales from lateral line ventrally to the anal fin insertion. Fin ray counts for cichlids, with both hard and soft rays, are expressed as a combination of Roman (hard ray) and Arabic (soft ray) numerals. "Dorsal XIII-XIV, 7-10" translates to 13-14 spiny rays and 7-10 soft rays in the dorsal fin. Compare Regan's (1905) verbal description of the coloration of Aequidens dorsigerus to the splendid photos of this animal in ADI, or to the living beast in your own aquarium. Quite clearly this is a description of a dead, pickled animal, and not the magnificently pastel living animal. Obviously much (guess) work will be needed to identify the living fish from these scientific descriptions, but don't hang it up quite yet. The end result, a reasonably accurate identification, is quite worth the effort.

Preparing the Specimen for Study

There are several alternatives available for attempting the necessary measurements on your mystery fish. If you have good eyes and a fast brain, you may attempt fin ray counts on your living specimen when it poses with fins fully spread. In practice this is difficult (at least for me). A second alternative is to take a good close-up photo of the fish and to work from a print of that photo. If you are a crackerjack lens person it may be possible to catch a full lateral shot whose fine focus allows counting and measurement of the various diagnostic characters. A third alternative is to anesthetize the fish with a tricaine drug or carbon dioxide bubbled through the water (Alka Seltzer™) and attempt quick counts on your frequently-moistened comatose specimen. You do run the (real) risk of killing the fish either by anesthetic overdose or out-of-water exposure, and this alternative is recommended only for the very impatient. The fourth alternative is to kill the fish (an alternative I am not yet able to do myself) or to wait for its eventual demise and to pickle the corpse for further study. Given the stress associated with collecting and importing, many wild fish do die, and it is not unreasonable to buy of this mortality. Purposely buying an obviously dying specimen is a good way to obtain the pickled specimen you require for identification and, often, when apprised of your interest and intent, a dealer may gift you with the soon-to-be corpse. The body does no good rotting in the dealer's tank.

If your obsession breeds heartlessness, fish can be humanely euthanized in a variety of ways: anesthetic overdose (including Alka Seltzer CO2) provides intact corpses. Assuming a dead body, premeditated or otherwise, rapid preservation is a must if the corpse is to be at all useful in the ID game. Formalin (10%) in a tightly stoppered bottle is the fixative of choice, but it is difficult to obtain. It is a controlled chemical and sold only to schools or labs, and not (to my knowledge) private individuals. Your friendly high school or college teacher is the best source for formalin. Please exercise care in its use: formalin is poisonous, corrosive, get at it. Always wear gloves when working with it, preferably outdoors, and make transfers quickly and neatly with tongs. Note: some adults, myself included, develop hypersensitivity to formalin in their later years even if no allergy was previously present. A brief whiff of the stuff leaves the afflicted dizzy and dulled, and direct skin contact is to be avoided. The advantage of formalin is its quick fixative properties. Not only does it preserve tissue, but it stops tissue decay immediately upon contact with little or no tissue distortion. This is important if further work, for instance bone and cartilage staining, is to be attempted. For larger specimens, the abdomen should be slit with a razor blade to allow quick penetration of the fixative into the viscera. To cut down on tissue shrinkage, formalin, which is acidic, may be neutralized by the addition of sodium bicarbonate (1 teaspoon per half-gallon: Lagler et al. 1977) or a few dolomite chips (Hartel, personal communication).

Alcohol, while lacking the fixative properties of formalin, is a good preservative and sufficient for our purposes. Either ethanol, or the more commonly available isopropyl alcohol (rubbing alcohol) will do with 50% more than adequate (1 part 100% alcohol to 1 part water). Again, slitting the belly of larger specimens is helpful. In a pinch, I suppose vodka would do, but what a waste of fine alcohol. If a fish dies and you are without formalin or alcohol, simply wrap and freeze the body until you can secure preservatives. Animals initially fixed in formalin should be transferred to alcohol for long term storage and for easy and safety of handling (below). again, bottles of pure preservative or pickled specimens should be well-sealed and kept out of the reach of young children to avoid poisoning.

Counting and Measuring: Obtaining the Necessary Data

Specimens should be soaked in preservative for several days and then transferred to alcohol before examining them. During this time period you will begin to see what Regan's color description of Aequidens dorsigerus is all about. All life coloration is reduced to shades of brown which only hint at the life colors of the animal, and these become increasingly dull over time. (You will, of course, have wisely accumulated decent color slides of the animal before its demise so that these can be extrapolated to the pickled specimen once identification is made.)

image Figure 3: An arsenal of essential sleuthing equipment can be assembled inexpensively. Photo by Wayne S. Leibel.
image Figure 4: A shallow wax-filled pan and latex surgical gloves complete the recommended equipment: All we need is a body! Photo by Wayne S. Leibel.
image Figure 5: Altough somewhat tattered and discolored by preservative, this corpse will provide the necessary meristics and morphometrics to pin down its true identity in consultation with the scientific literature. Photo by Wayne S. Leibel.

At this stage, data collection can be attempted. There are several tools that make this job easy. Vernier calipers are one such tool. Research grade calipers fabricated in metal and accurate to the nearest .001 inch are nice, but I find the $40 price tag a little steep. For $3.50 (approximately), Carolina Biological Supply Company (phone: 1-800 334-5551) sells a plastic metric caliper that is accurate to 0.5 mm and expands to 13 cm (Catalog #70-2648). For our purposes, this tool is quite adequate. Couple this with a small plastic centimeter ruler and a sewing needle inserted blunt end into a pencil eraser (a budget dissecting needle) for fin ray counts, and you're all set (Figure 4). A shallow pan underneath the specimen will catch the dripping preservative (Figure 5). A good desk lamp does wonders too. You may wish to wear thin latex gloves to protect your hands from formalin, or if you are inherently squeamish. Then begin your counts: standard length, head length, body and head depths, eye diameter, pre and post-orbitals, lateral line counts, dorsal and anal fin ray counts, etc. (Figure 6). This raw data can be simply massaged with a cheap calculator to produce percent standard length, or eye/character ratios as dictated by the descriptions you are working from.

Aequidens mysterioso

Often your attempts to match your living (now pickled) fish with original descriptions are less than successful. There are a number of reasons why even the best stabs at identifying fish fail. First, consider the inherent variability of a given species over its entire geographic range and be aware that some valid, recognized species are based on single collected specimens. It would be a miracle if your specimen matched that type specimen exactly even if they are the same species. Secondly, there is considerable overlap of meristics and morphometrics between described species. According to Eigenmann's (1942) data, Aequidens tetramerus and Aequidens potaroensis overlap almost completely with dorsigerus except for the presence of an additional vertical bar through the eye in the latter and differences in maximal total length! And yet living examples of either of these two animals are completely different from each other and from dorsigerus.

You do have an ace in the hole: geographic distribution. It is often possible to learn from wholesalers (dealers are less able and less interested) where the particular shipment that contained your fish originated from. If this information is available and accurate, it can help narrow down the species you should consider. Often noting which fish (of known identity and distribution) your oddball came in with will provide the final piece to complete your puzzle.

All in all, if there is some doubt as to what the critter is but you are leaning towards one ID, it is only fair to indicate this ambiguity by attaching the prefix species affinis (sp. aff.) or cf. immediately preceding the specific nomen when listing the fish for trade or in an article. This designation means that your fish strongly resembles the nominal species, but there are important differences which make this judgment somewhat ambiguous. It is, of course, possible that the fish you have measured which does not correspond to any of the descriptions is, in fact, an unnamed species. If you believe this to be the case, contact a professional ichthyologist who you know to be involved with that group of fishes (a recent Zoological Record would help here) and determine with him/her whether shipment of the carcass for further study is warranted. You might have a fish named after you for your efforts, as did Lee Finley: Chromidotilapia finleyi Trevawas 1974!

Finally, if you don't already own a camera and know how to photograph fishes, invest the time and money and learn how. Perhaps the best basic article is that by Dan Fromm (FAMA April and May 1983). The point of all of this is not just to properly identify fish for yourself, but to share your discoveries with the hobby at large. What the hobby needs is a library of accurate photographic IDs established by actual taxonomic work-up of observed and photographed live captive specimens.

Conclusions and Further Encouragement

Although an obsession with proper identification is a decidedly esoteric enterprise geared more to the intellectual than the aesthetic pursuit of the hobby, it has all the unexpected excitement of a good detective novel. The progress is slow, difficult, and very often exact identifications can not be made, but the personal satisfaction and the real contribution to the hobby that results when such attempts are successful serve as sufficient carrots. I like knowing who my fish really are, and I prefer my hobby publications accurate. There is considerable room for the serious aquarist to make real inroads in the resolution of longstanding discrepancies in the published identification of living aquarium fishes. Here's hoping you'll take up the challenge.

Postscript to Part Two.

Whereas these guidelines and hints will, I hope, prove helpful for the interested hobbyist, there is nothing like concrete example to put them all together. Perhaps the best sleuthing job in print is Paul Loiselle's vintage The Identity of the Red Hump Geophagus (1974, Buntbarsche Bulletin 40: 9-19) which details his assignment of the name Geophagus steindachneri to the (then) newly-imported Red Hump Eartheater. The article is a joy in demonstrating how one methodically assigns an ID to an unknown fish based on the scientific literature, close scrutiny of living animals, and just good common sense. Equally instructive is Gary Grant's account of determining the true identity of Apistogramma borelli, another "problem" fish (Eighty-Six Years of Confusion, 1981, Buntbarsche Bulletin 87: 13-18). I modestly, but proudly, offer my own maiden effort, On the Identity of Geophagus acuticeps, as a third example of the genre (1983, Buntbarsche Bulletin 94: 2-10, 95: 11-19).

For the serious cichlid hobbyist in search of more directly applicable ichthyological description of the characters used in describing and keying cichlids, I recommend the two Barel (et al. 1977) papers that appear in the bibliography. A first draft of this article appeared in The Sifter, Journal of the Geophagus/Aequidens Study Group, Volume 1 (1):20. Special thanks to Ian Sellick, Anthony Terceira, and Gene Aldridge, Jr. for use of their slides.


  • Fromm, D.W., 1983. Photography Primer for Aquarists (Two Parts), F.A.M.A. Part 1, April; Part 2, May.
  • Goldstein, , 1970. Cichlids, TFH Publ., Neptune, New Jersey.
  • Hoedemann, J.J., 1975. Naturalist's Guide to Freshwater Aquarium Fish, Sterling Publ. Co., New York.
  • Lagler, K.F., J.E. Bardach, R.R. Miller, and D.R. May Passino, 1977. Ichthyology, Second Edition, John Wiley & Sons, Chapter 13.

(This article was originally published in Freshwater and Marine Aquarium Magazine, Feb-85 pp. 48-54. It is here reproduced with the permission of author Dr. Wayne S. Leibel.).

References (3):


Leibel, Wayne. (Nov 16, 1997). "Of Pickled Fish and Ichthyological Sleuthing: A Primer For the Advanced Aquarist.. Part II: Identifying Your Fish". Cichlid Room Companion. Retrieved on Dec 01, 2023, from: