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Food For Thought

Por , 2000. print format
Publicado
Ron Coleman, 2000

Traductor Antón David Pérez Rodrígez (20-jul-2004)
Versiones English
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" The enormous number of cichlid species equally show much variated diets, Dr. Coleman explains an interesting experiment carried on shrimp feeding cichlids from lake Tanganyika, which produced fascinating results "

(This article was originally published in Cichlid News Magazine, Jul-00 pp. 32-34, It is reproduced here with the permission of author Ron Coleman and Aquatic promotions).

Feeding is an important part of every organism's life. Cichlids are no exception. Every aquarist would like to be sure that they are giving their cichlids proper types and amounts of food, but what is the proper food for a given cichlid?

Neolamprologus leleupi

Neolamprologus leleupi, a cichlid that relies almost exclusively on feeding shrimp in lake Tanganyika. Photo by Don Danko.

As with so many things about cichlids, the answer is never simple. The enormous number of cichlid species suggests an equally great diversity of cichlid feeding biology. Indeed many researchers have argued that it is the diversity of feeding styles that has been instrumental in generating the diversity of cichlids. The relative roles of feeding diversity versus, for example, sexual selection and other diversity-generating forces, are still debated, but there is no doubt that the particular arrangement of bones and biting surfaces in the mouth and throat of cichlids has facilitated rampant speciation. The moveable pharyngeal jaw apparatus in the back of the "throat" allows cichlids to process (chew; grind) food there and utilize the main part of the mouth for other purposes, including mouthbrooding in some species, and highly specialized food acquisition in others (see Coleman, 1997). Keep in mind that without any hands to grab things, fish have to capture, hold, and process food with the mouth. This is not a trivial matter if the food is inclined to escape.

Cichlids come in many sizes and shapes and live in numerous different environments, so we expect diversity in feeding modes. For example, Yamaoka (1991) describes how the fishes of Lake Tanganyika, Malawi and Victoria have very distinctive and specialized food-gathering behaviors. For example, there are algae grazers (defined as those that pick up large quantities of substrate while scraping or rasping at an algae-covered surface); algae browsers which are more selective and seldom ingest substrate; algae scrapers which make audible scraping sounds as they rake the substrate with specialized teeth; algae tappers which pull out algae by entangling the threads on tooth edges; and algae suckers which consume mainly loose algae. There are also piscivores (fish-eaters) ranging from those that consume whole fish prey to those that specialize on only parts of another fish, such as the scale eaters and paedophages. Paedophages are cichlids which ram the mouth of a mouthbrooder to dislodge the contents and then rapidly consume the eggs or young. Finally, there are cichlids which specialize on eating insects or other aquatic invertebrates such as shrimp.

These broad categories help us to get a general picture of what is going on, but the more we investigate feeding, the more we discover a network of interrelated questions, few of which have been answered. For example, what exactly do cichlids eat in the wild? What would they prefer to eat? Does what they eat affect them in different ways, and, finally, what should we, as aquarists, feed our cichlids? There are many researchers currently working on aspects of each of these topics. For now, I'll focus on the first.

Cichlids typically eat different things when they are juveniles than when they are adults.

What do cichlids eat in the wild? There are really two ways to answer this. You can observe a fish to see what it eats, or you can catch one, cut open its stomach, and identify what is in there. Direct observation of feeding is valuable but often extremely difficult in the wild. Furthermore it isn't always obvious what is going on. A fish may appear to be eating bits of algae, but in fact, it may be eating the tiny invertebrates living on the algae. Direct observation may miss this fine distinction. Methods which evaluate the abundance of food items in the environment may also be misleading (as we will see below): just because a food item is present doesn't mean it gets eaten. Gut-contents analysis tells you what actually made it into the stomach of a cichlid and is often regarded as an accurate reflection of the diet of a wild fish. Even here, however, we must proceed with caution. Certain food items, like eggs or larvae, can be digested very rapidly - so rapidly that these items may not reside in the gut long enough to be detected by a typical gut analysis.

The other big problem with analyzing diets is that there may be substantial variation in the diet of different individuals or in the diet of a given individual through time. In particular, cichlids typically eat different things when they are juveniles than when they are adults. Many cichlids are also opportunists, ready to take advantage of easy-to-get food should it appear in the form of unguarded eggs or larvae of other cichlids. So in our haste to categorize cichlids as say "algae eaters" versus "piscivores," we may miss a lot of interesting and important biology.

Neolamprologus tretocephalus

Neolamprologus tretocephalus eats shrimp very rarely, preferring molluscs and insect nymphs. Photo by Peter A. Lewis.

This raises the question of why some cichlids are so specialized. Two possible explanations come to mind: dietary requirements versus niche partitioning. It is possible that certain species have highly specialized dietary requirements such that a given species of cichlid requires the nutrients found in a certain type of prey. This may exist to the extent that if deprived of that prey, the fish will not do well or may even die. Although a common belief among some cichlid hobbyists, there is little evidence to support (or refute) this hypothesis. Niche partitioning, on the other hand, suggests that different cichlid species have evolved to eat different foods and to do so in different ways to avoid competition with each other. For example, one species may have evolved a particular tooth structure to allow it to effectively browse algae from smooth rocks (e.g., Tropheus moori), while another has evolved a body and mouth shape to snatch unwary invertebrates from the substrate (e.g., Altolamprologus compressiceps). The two species could live close to each other in the wild but not be in direct competition because of their different diets. Of course through time, subsequent evolution may alter the digestive systems of each of these species such that they do in fact have different dietary requirements. Or, foods which are palatable to one species may become unpalatable to another, leading to digestive problems. This has been suggested as an explanation for the occurrence of "bloat" in some algae eating African cichlids, though again, we have little hard evidence to actually verify this explanation.

A recent study by Masahide Yuma and co-workers (Kyoto University) exemplifies how complex the relationships can be among a group of predators and prey - in this case fourteen species of shrimp-eating cichlids (many of which are common cichlid aquarium inhabitants) and thirteen species of atyid shrimp in Lake Tanganyika. All these many species of both cichlids and shrimp are found at the same location in the lake. Indeed, many of the different cichlids forage in mixed-species foraging groups. How do so many cichlids co-exist on this rather specialized resource? Yuma et al. (1998) sampled shrimp abundance in the lake, both day and night. They also captured cichlids and examined their stomach contents. Fortunately for the researchers, these cichlids tend to consume shrimp whole, making it possible to identify the species of shrimp found in the cichlid stomachs.

Yuma et al. revealed complex relationships among the shrimp and the cichlids. Cichlid species varied in how much of their diet was composed of shrimp, which species of shrimp they ate, and how they captured shrimp.

Neolamprologus lemairii

Neolamprologus lemairii specializes in darting after shrimp that tries to escape from other shrimp eaters. Photo by Peter A. Lewis.

Five species of cichlids (Gnathochromis pfefferi, Lamprologus callipterus, Altolamprologus compressiceps, Neolamprologus toae, and N. leleupi) along with small individuals of N. fasciatus fed almost exclusively on shrimp - shrimp forming 80% of their diets. The other cichlids (L. labiatus, L. lemairii, N. savoryi, N. furcifer, N. mondabu, L. elongatus, and L. profundicola) ate shrimp as well as other prey items, such as diptera larvae, epheme-roptera nymphs, small fish, and fish eggs. Neolamprologus tretocephalus ate shrimp very rarely, preferring molluscs and insect nymphs.

The thirteen shrimp species can be categorized into short, medium, and long species. Sampling revealed that most of these shrimp were active at night, but one species, a long shrimp, Limnocaridina latipes, was active on the surfaces of rocks during the day. The other shrimps tended to hide under rocks or stones during the day. This is important because in general cichlids only feed during the day. So even though some of the short species were very abundant (at night), most cichlids didn't eat them. In fact, the long shrimp Limnocaridina latipes was the main shrimp eaten by all the species that ate shrimp.

The size of the fish was important in how it captured its food. The small cichlids (Gnathochromis pfefferi, Lamprologus callipterus and Neolamprologus leleupi) snatched stationary shrimp from rock surfaces during the daytime. Slightly larger cichlids, including A. compressiceps, L. labiatus and small individuals of L.lemairii and L. elongatus, specialized in darting after shrimp that were trying to escape from other shrimp eaters. Because larger shrimp were better able to escape the small predators than smaller shrimp, the larger cichlids tended to eat larger shrimp.

A few species of cichlids, such as L. callipterus, L. labiatus and Neolamprologus mondabu, were able to extract hiding shrimp from around stones and among debris and were thereby able to eat shrimp species that were not active during the day.

Neolamprologus callipterus

Neolamprologus callipterus are able to extract nocturnal shrimps during the day from around stones and among debris. Photo by Peter A. Lewis.

Together these results show that while these fourteen species of cichlids all eat shrimp to some extent, there is substantial variation in how they do it. Now consider that all-in-all these cichlids are fairly similar to each other, at least in comparison with many other cichlids, such as pike cichlids or earth-eaters. Understanding the full complexity of feeding behaviors in the cichlid famile as a whole is going to take a lot more research and provide much food for thought.

References

Cita:

Coleman, Ron. (marzo 10, 2003). "Food For Thought". El Cichlid Room Companion. Consultado en abril 19, 2014, desde: http://www.cichlidae.com/article.php?id=182&lang=es.