The high percentage of detritivorous fish, including Loricariids, seems to be relatively unique to neotropical streams. In temperate streams of the north, only a maximum of 25% of the fish in a stream feed on mostly plant material, including algae, macrophytes and leaves (Wootton and Oemke 1992). In neotropical streams, 25 to 100% of the fish in streams have a diet containing lots of plant material (Wootton and Oemke 1992), including the 684 species of Loricariid catfish (Nelson 2006). This could be the result of a lack of herbivorous invertebrates in the stream when compared with temperate streams, causing an opening of niches for fish to become herbivorous (Wootton and Oemke 1992).
Neotropical streams also contain much less plant material than temperate streams, meaning either the productivity of the land surrounding the stream or the consumption of plant matter by herbivorous fish must be higher in neotropical streams (Wootton and Oemke 1992). Armored catfish are not only one of the most diverse groups of organisms in neotropical streams. They also compose a large percentage of the fish population and play an important role in shaping the stream environment (Flecker 1996).
Armored catfish are generally detritivores or herbivores, with a large, ventral suckermouth used to feed on algae and sediment that has accumulated on rocks along the streambed (Schaefer and Lauder 1986) and some have even been shown to be able to digest wood (Nelson 2006). They are distinguished from other detritivorous fish by the presence of bony plates in the body (Schaefer and Lauder 1986), barbells or papillae on their lips, and long intestines built for the digestion of organic matter (Nelson 2006). The structure of the intestines is not the only remarkable trait in the digestive system of the armored catfish. Most armored catfish have some ability to absorb oxygen through their enlarged and highly vascular stomachs, sometimes growing to the length of the entire visceral cavity (Armbruster 1998). Since water is normally lacking oxygen during the dry season, which is also a season during which food resources are limited, Loricariids probably do not eat during the times they need to breathe (Armbruster 1998). Loricariids inhabiting very calm, frequently anoxic pools or swamps have developed diverticula, or extra organs, through which they can absorb oxygen (Armbruster 1998). Air is released through the gills and mouth, although the mechanism is not known (Armbruster 1998).
One particularly interesting feature of armored catfish anatomy is their eyes. The iris in the eye of an armored catfish can descend to cover the pupil, making the pupil almost sickle-shaped and severely limiting the amount of light entering the eye (Douglas et al. 2002). This allows the fish to adjust to a wide variety of lighting conditions while aiding in camouflage with the streambed. As a result of their highly adjustable eyes, many armored catfish are nocturnal, spending their days hidden under rocks or other cover. Armored catfish are normally mottled in color to blend with the streambed, although more colorful variations have been bred as armored catfish have increased in popularity among aquarists.
Reproductively, armored catfish are “guarders”. The male catfish builds a nest, the female enters the nest, lays the eggs, and departs, and the male guards the nest for the next four to twenty days (Fenner).
Despite their diversity and large numbers, armored catfish are not the only detritivores in neotropical streams, but are part of a large group of detritivores that function as ecosystem engineers to “sculpt” the streambed. It has been found that detritivores can comprise as much as 60% of the fish biomass in a neotropical stream (Flecker 1996). Loricariid catfish make up a large percentage of those. Alexander Flecker (1996) found that a common detritivore in a Venezuelan stream, Prochilodus mariae, functioned along with a variety of armored catfish to control the population density of stream invertebrates by competing for a common food source. A later study on Parodon apolinari, another detritivorous fish, in the same stream found that an assemblage of detritivores also plays a key role in the diversity of stream invertebrates (Flecker and Taylor 2004). When P. mariae was excluded from a study site, the total invertebrate density rose by 70 up to 400 percent (Flecker 1996). When P. apolinari was excluded the diversity of the invertebrates dropped as the population density rose (Flecker and Taylor 2004). Another observation was that when P. mariae was excluded from a portion of the streambed, the ash-free dry mass accumulating on the bottom rose at the same time as the population density rose (Flecker 1996).
An explanation for the above observations could be that detritivores, as they scrape algae and sediment from rocks, create a pattern of resources as they remove more from one area than another. This resource heterogeneity may allow invertebrates specialized to one particular resource type to flourish where, if the detritivores are not there to create those niches, they would be eclipsed by resource generalists (Flecker and Taylor 2004). Although there were no direct connections made between invertebrate diversity and resource heterogeneity, it was found that there is a correlation between the increase in sediment accumulating on the streambed and the increase in invertebrate population density with the exclusion of a detritivorous fish (Flecker and Taylor 2004).
The presence of detritivores in a stream also impacts algal populations. While the total algal biovolume rises with the reduction of one species of detritivore, the number of filamentous cyanobacteria is lower without the presence of one species of detritivore and the number of diatoms is higher without the presence of one species of detritivore (Flecker 1996). The removal of some detritivorous fish has also been observed to cause an increase in the productivity of algae growing in a particular area (Power 1990). This is because certain fish graze on the sediment overlying the algae and not the algae, keeping the algae from being suffocated by accumulating sediment. Each fish species may selectively feed on diatoms, sediment, or filamentous cyanobacteria, and when detritivorous fishes consume the sediment layer in which diatoms live, the number of diatoms is reduced while the number of cyanobacteria increases (Flecker 1996). This theory could also be expanded to explain Power’s (1990) observation regarding sediment removal and algae productivity. Each detritivorous fish plays a role in consuming a certain group of algae, detritus, or other organic matter to maintain a balance in the stream ecosystem. This has been termed a “lack of redundancy” in the food web, with each detritivore having its own very specific niche feeding on detritus, algae, or other organic matter (Flecker 1996).
Specialization seems to be a trend in the tropics, as diversity goes up in all habitats nearer to the equator (Kricher 1989). The increased productivity of the algae may, in turn, be a reason why stream invertebrate diversity rises with the removal of sediment. There is a wider range of algae and sediment to feed on when both sediment and algae are separated by detritivorous fish-constructed resource heterogeneity, allowing a wider range of invertebrate specialists to survive. The specialization of detritivorous catfishes, in particular Loricariids, seems to allow for the specialization of algae and, in turn, the specialization of stream invertebrates.
The two main predators of Loricariids are fishing birds and larger fish. After an armored catfish is larger than about five centimeters, it is no longer susceptible to piscivorous fish (Power 1990). This helps explain why larger armored catfish live at greater depths than smaller armored catfish. Fishing birds, such as herons and kingfishers, can eat a much larger fish but normally only hunt down to around 20 centimeters (Power 1984).
The equilibrium with predators also affects the armored catfishes’ influence on the density of its prey, algae and sediment, and its competition, stream invertebrates. In water deeper than 20 centimeters, larger fish are not as limited by predation by birds (Power 1984). This means that they are instead limited by the amount of algae on the bottom, and a larger percentage of the algae are consumed, making the rocks cleaner than those in shallower water (Power 1984). The small fish near the surface are limited by predation rather than the food resources around them, and so not all of the food sources are used up and more algae are visible on the bottom substrate (Power 1984). The abundance of algae will influence the abundance of certain stream invertebrates, creating a complex system of equilibrium all hinging on the population of Loricariid catfish.
Armored catfish are being fished and, in some cases, over-fished as they continue to be popular with aquarists (Fenner). Although neotropical streams, with their astonishing diversity, may be well equipped to adapt to changes in community composition, there may be a limit to a stream’s tolerance, and the removal of armored catfish may, in the future, pose a threat to the diversity of neotropical and tropical streams. Without detritivores, of which Loricariids are one of the largest groups of fishes found in neotropical streams, streams could be inundated decaying plant matter, which would smother the algae and destroy the food web from the bottom up.
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lenticular longitudinal spherical aberration and retinal topography.”
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Wootton, J.T. and M.P. Oemke. 1992. “Latitudinal differences in fish
community trophic structure, and the role of fish herbivory in a Costa
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