Catfish Corner: Papers of Note

Author: Lee Finley

Greetings all. I’m back from a one month’s unintended vacation, my thanks to the editors and readers of this magazine for allowing me this little blip in my regular column.

Catfish Findings

In my last column (TFH August 2007) we looked at water changing, which is one of those necessities of aquarium maintenance—whether you keep catfishes or not. For this month I’d like to present some results from a couple of important scientific papers dealing with catfishes. One provides a revision of a genus of large South American catfishes and the other explores the topic of eating habits, breathing, and holding in place (suction) of a loricariid catfish.

Tiger Notes

Anyone who regularly reads this column knows that I do not promote—in fact, I discourage—the aquarium keeping of catfishes that reach a very large adult size. But I am a realist and know that the keeping of such catfishes has its own fan base. Consequently, for those who do enjoy such fishes, it is fitting to provide some information based on a recent revision of the South American genus Pseudoplatystoma (family Pimelodidae). These fishes are commonly known in the aquarium trade as tiger shovelmouth (or shovelnose) catfish.

For the most part, recent literature has treated the genus as consisting of three species (Pseudoplatystoma fasciatum, P. tigrinum, and P. corruscans). The new revision recognizes an additional five species. Three of these are new species (P. orinocoense, P. metaense,and P. magdaleniatum), and the remaining two are previously described species that have been treated as synonyms, but are now revived.

One interesting point is that the true P. fasciatum is currently, due to its range, very limited to non-existent in the hobby. Most likely to be seen in the hobby are those Amazonian River Basin species from Brazil, Peru, and southern Colombia.

It is beyond the scope of this piece to get into the differentiation of the species, so I will direct you to the paper for descriptive material and illustrations. Do beware, though, that the provided material is generally only good for adult specimens. With juveniles of the various species little information is available. The authors do note seeing “tiny young” of Pseudoplatystoma in the aquarium trade and state that slight differences in pattern and shape may indicate the presence of more than one species. So to figure out the species of a particular imported small individual, you may have to raise it up a bit… and therein lies the rub.

But I must concede here that the raising up in aquaria of such small Pseudoplatystoma species may potentially provide a benefit to science. In that the juvenile color patterns of the various species are for the most part unknown, the photo documentation of the growth of any specimens of Pseudoplatystomais desirable. Such photos could then be shared with the interested scientific community on a one-on-one basis or published in a magazine such as this, thereby becoming part of the public record.

Loricariid Catfish Mouths

Loricariid catfishes are quite interesting in the use of their mouths. Three main non-reproductive functions of the mouth include holding the catfish in place (by suction), eating, and the taking in of water for the breathing process. Each of these is interesting in its own right, but the situation is even more so in that all three functions occur more or less together. Work has been done on this topic in the past, but a new paper (Geerinckx et al., 2007) reports new results utilizing multiple techniques that take the study well beyond that of previous works.

Recent Study

The catfish used in the study is designated as Ancistrus cf. triradiatus. In that Ancistrus specialist Sonia Fisch-Muller is thanked for advice on the identification used, it is probably as close as one could hope to get with this confusing genus.

For this project the authors undertook an in-depth study of the bones, muscles, and ligaments of the head of the fish under study. The methods used included:

·         Clearing and staining: this technique allows the visualization of the bones and cartilage of a prepared fish.

·         Dissection for the observation of both hard and soft tissue.

·         Histological sections: this is the use of ultra-thin sections of tissue that are stained and looked at with a microscope. This is basically like a biopsy that a human might have. These were done on a large scale (so-called serial, or multiple, sections on a whole fish head) and smaller scale (tissue samples taken from the lips, oral valve, and its connecting tissues to the upper jaws and the cartilage connector between the two lower jaws).

·         In addition, observation and experimentation on live fishes were also made. These were greatly enhanced by limited (but important) use of high-speed digital video (200 frames per second!).

Additional Details from the Paper

I will note that a large amount of the material presented in the paper is of an anatomical nature (and its biomechanical implications) and highly technical in its presentation. A great amount of such material is beyond me—but I’m working on it. But what I would like to offer below is a small extraction of some information from the paper.

Tandem Catfish Processes

As noted, the processes of feeding, breathing, and suction all occur more or less in tandem. While the suction is constant, the feeding and respiration with the species studied occurs in cycles (albeit very fast cycles). Within a two-second period it was recorded that there were three respiratory cycles that were followed by three scraping (removing food) cycles. At these speeds it is no wonder that both processes appear to be a continuous one, even though there is a regular alternating of each. During a feeding period the respiratory cycles were noted to increase in speed.

The above is all managed by an overall buccal pump system. This in part allows the inflow of both water (for breathing) and food while helping to maintain the suction, which keeps the fish adhered to the substrate. The amount of water that flows in is relatively small and it is taken in a thin stream at a controllable elevated opening of the upper lip behind the maxillary barbel(s). The authors note that it was often observed that water only enters on one side. This side shows some lip motion while the other side is still. How could they see water entering such a fine opening (and in such a small amount)? By the use of milk. A small amount of diluted milk was released from a pipette in the vicinity of the head and its path was then visually tracked.

The lower jaws of A. cf. triradiatus are so constructed as to allow them to move independently of each other, while the upper jaws act as one. The lower jaw is held to the substrate via suction. The upper jaw apparently also benefits from some suction to stay in place, but it is also held to the substrate in part by downward movement of the head. When the fish is feeding the jaws move, but not just in a straight scraping pattern (as it might appear). Both posterior and anterior scraping takes place. In addition, there is a large degree of rotation of both the upper and lower jaws. This would provide extra motion and energy to help dislodge food from the substrate. Also of potential use in the feeding process is the lower lip. This has numerous keratinized projections called unculi and it is felt that these may also assist in dislodging food. As the fish goes through the above actions there is generally also a steady movement forward (or to the side) to constantly exploit new areas of potential food.

The above only touches on some of the aspects of the paper but will hopefully give you an idea of some of the things involved when an Ancistrus species goes about its day. Very complicated anatomical structures and their interactions are involved in the described processes. I can do no better to sum this up than to quote from the last sentence of the paper: “…the loricariid head is one of the most impressive examples of structural diversification and refinement shaped by evolution.” After that it will be difficult to look at an Ancistrus (or any other loricariid for that matter) again without feeling a little more respect for it.

See the full article on TFH Digital http://www.tfhdigital.com/tfh/200710/#pg60