Powerful predators

The enormous mouth of Petenia splendida is very protrusible. Photo by Ad Konings.

Imagine the scenario faced by a small fish when confronted by a predatory cichlid. Movies like "Jaws" and "Jurassic Park" have prepared us - or at least forewarned us - for what it must be like to face a daunting set of teeth. The jaws of a Tyrannosaurus rex or the toothy grin of a great white shark are terrifying. But as is so often the case, the real world is even more awe-inspiring, particularly for a prey item faced with a predatory cichlid. Not only does the prey face an oncoming set of teeth and a powerful set of jaws, but there is something that aquatic predators use that the movies seldom capture: cichlids suck. That is to say, cichlids create powerful suction when they feed. This suction pulls the prey forcibly into the mouth of the predator before the jaws close.

Early studies discovered and explored the basic mechanism by which cichlids create this suction (Liem, 1991). The primary force producing the suction is rapid expansion of the buccal (mouth) cavity. This is accomplished by the complex interaction of the many muscles and bones that make up the jaw region of a typical cichlid. Humans have a relatively simple jaw structure with a lower mandible that pivots at the rear. Not so for most fishes, and cichlids in particular. They have many bones, cross-connected with various muscles, that allow the fish to not only drop the lower jaw like we can, but also to pull out the sides of the mouth at the same time (you can see this in action if you watch a cichlid "yawn", which they do from time to time). This sudden increase in the size of the mouth cavity creates strong suction through the mouth opening, pulling in whatever is just in front of it. Another series of muscles then rapidly contract the walls of the mouth and raise the floor, literally forcing the food back down the throat of the fish.

If you maintain predatory cichlids, you can actually watch this happen. Drop brine shrimp into the tank and watch carefully what happens. Alternatively, if you feed your cichlids live feeder fish, such as goldfish or guppies, the results will be even more dramatic. [Personally, I am very cautious about feeding live feeder fish to my pike cichlids. Feeder goldfish often carry diseases, in particular, ich (Ichthyophthirius), which can readily infect your cichlids.]

If you should ever be so unfortunate as to have a cichlid die, particularly a large one, take the opportunity to look at the many bones and muscles in the jaw area. While we know the basic story of how these work, understanding the exact details is rather tricky because the expansion and contraction phases of attacking a prey item all occur in a second or two. Even careful observers can only postulate about what happened and in what order.

Fortunately, technology has been developed that makes it possible to understand much more precisely how the cichlid jaw works and how the jaws of different cichlids vary. Many earlier studies focused on the use of electromyography. Electromyography records the electrical impulses given off by muscles as they are contracting. By carefully recording which muscles are firing, and when, it is possible to reconstruct how the many components of the cichlid mouth fit together to perform a task like eating a small fish.

Another technique for understanding feeding is the use of high-speed film, shooting hundreds of frames per second. A researcher would film a few seconds of a cichlid feeding, often on X-ray film, then spend an enormous amount of time analyzing each frame by hand, noting the position of each of the bones of the jaw. Remember: 10 seconds at 100 frames per second is 1000 frames to analyze!

More recently, high-speed video has greatly assisted in these investigations. Video of feeding cichlids can now be fed directly into a computer analysis system which can automate many of the measurements. Modern laboratory video systems record at rates of 500 frames per second, providing tremendous detail and insight into the ordering of events.

Cichlids use another means of ensuring that potential prey do not escape, namely closing the distance between the predator's mouth and the prey. In some cases, this is obvious: the predator swims rapidly towards the prey. But some cichlids also use a far more insidious way of decreasing the predator-prey distance: they rapidly extend their jaws forward at the prey. One species of cichlid, Petenia splendida, is particularly well-known for this ability. We call the idea of getting closer to the prey "ram feeding" and the idea of pulling in the prey "suction feeding".

Recently there has been much interest in the relative roles of ram and suction feeding in fishes. Peter Wainwright and his collaborators at the University of California (Davis) took advantage of the new video technology to examine the relative roles of ram and suction in a selection of seven cichlid species. These seven species are found in diverse habitats and while all are predatory to some extent, they likely feed in different ways and on different things.

The seven species examined were Petenia splendida (the bay snook), a lake-dwelling piscivore from Central America; Cichla ocellaris (the peacock bass) a large-mouthed piscivore from South America; Herichthys minckleyi, largely a plant-eater from northern México; Astronotus ocellatus (the oscar), a fish that seldom meets another fish it doesn't like (to eat); Crenicichla geayi, a pike cichlid from South America; Heros severus (the severum), which typically feeds on small invertebrates and plants, and Cyprichromis leptosoma, an open-water zooplanktivore from Lake Tanganyika.

With its protrusible mouth Cyprichromis sp. "Leptosoma Jumbo" sucks zooplankton from the water column. Photo by Ad Konings.

Wainwright and colleagues videoed individuals of each of these species feeding on adult brine shrimp and on live guppies. The former are mobile but don't swim very fast, while the latter are both larger and capable of avoiding a predator by swimming away rapidly.

The results were both fascinating and surprising. I had the good fortune to view some of the videos produced from these trials and to say the least, they are impressive. I am glad I am not a guppy. Typically, as the predator approaches the guppy, there is very little time for the guppy to respond. At first the cichlid starts to open its mouth slowly, then in a split second the mouth rapidly opens up, and you can see the guppy literally vacuumed into the approaching jaws.

What was surprising was the fact that the various species showed little variation in the distance at which they were able to pull prey in. Previous work, both theoretical and experimental, had suggested that fish with a smaller mouth opening should be able to generate greater suction, assuming equally-sized mouth cavities. This greater suction should translate into being able to pull in prey from greater distances. Wainwright et al. did not find much support for that hypothesis. The experimenters suggest two possible explanations for their results: perhaps the different species can actually generate very different suction pressures but just didn't use those during these experiments. A second explanation, and the one they favor, is that the mechanics of sucking water through a mouth may actually be a whole lot more complicated than was originally thought.

Real biological processes are often far trickier than theoretical constructs on a piece of paper. In this case, they suspect that while different species may in fact be able to produce different intake pressures and velocities at the mouth opening, the relative magnitude of these differences may be small when it comes down to the bottom line of how far away a prey can be and still get inhaled.

In contrast to the suction distance results, the ram distance varied greatly for the different species and varied for the type of prey. The species employed different strategies for minimizing the distance to the prey before striking. Some, like Petenia splendida, Cichla ocellaris and Crenicichla geayi swam rapidly at the prey and protruded their jaws. Others like the severum used a more stealthy approach, but then protruded the jaws dramatically in the final attack.

Wainwright et al. summarize their views as follows. While previous researchers have thought of ram and suction as two ends of a continuum, i. e., any particular fish might use a lot of one and a little of the other or vice versa, that is not what they found. They suggest that the various species use ram, either by swimming at the prey and/or by extending the jaws, to get those jaws close enough to then apply a relatively fixed suction force.

These results show how essential it is to actually do carefully controlled experiments to test any idea, no matter how reasonable that idea sounds. I think this is an excellent lesson for cichlid aquarists: things that appear to make sense at first thought may not actually be that important in the end or may play a small role relative to other larger factors. Careful observations and the willingness to throw out a favorite idea in the face of strong new evidence can benefit everyone.

Cichla ocellaris is a large predator from South American waters. Photo by Ad Konings.

References

• Liem, K. F.; 1991; Functional morphology. Pp. 129-150 in Cichlid Fishes: Behaviour, Ecology and Evolution (M. H. A. Keenleyside, ed.), Chapman & Hall, New York.

(This article was originally published in Cichlid News Magazine, Jan-02, It is reproduced here with the permission of author Ron Coleman and Aquatic promotions).