Ray-finned fishes are essential components of most ecosystems in which they occur. While many ray-finned fishes prey on each other, they can also have significant impacts on nearly all other animals in their habitats. Zooplanktivorous fishes, for instance, select for specific types and sizes of zooplankton when they feed, thus influencing the type and quantity of zooplankton, and, by extension, phytoplankton present in surface waters (zooplankton consume algae; together they are simply termed plankton). When non-native species invade new habitats (usually through human intervention), the fragility of this balance is dramatically illustrated. For instance, when alewives (family Clupeidae) invaded Lake Michigan, they decimated two larger species of zooplankton and dramatically reduced two midsize species, resulting in the increase of ten smaller species and higher algal content. Later, Pacific salmon (genus Oncorhynchus) were introduced into the lake and dramatically reduced alewife populations and the larger zooplankton species recovered. Because the larger species grazed on algae more efficiently, phytoplankton density decreased dramatically and the lake cleared. This is an example of a trophic cascade, and although the ecosystem achieved relative balance in this example, this is not always the case. For instance, the introduction of Nile perch , a voracious predator, into Lake Victoria (Africa) caused a precipitous decline of many small, planktivorous cichlids. These cichlid species exerted considerable predation pressure on zooplankton, and after they were eliminated the zooplankton community changed drastically, to the point that a new and very large cladoceran species appeared in the lake, Daphnia magna. Unfortunately, this introduction resulted in one of the largest mass extinctions of endemic species in modern times, and the repercussions did not stop with the perch introduction. Many local people consumed the smaller cichlid species and hung them in the sun to dry and preserve them. When Nile perch began to impact local cichlid fisheries, locals started to consume Nile perch, but this fish required firewood for drying and preservation because it is much larger. Consequently, deforestation started to occur around Lake Victoria, leading to increased runoff and siltation during rainy periods, and consequently, decreasing water quality. Decreasing water quality further endangered endemic cichlids, resulting in even more extinctions. The latter example illustrates the complexity of ecological interactions and the fact that ecological interactions are not confined to aquatic organisms. Because ray-finned fishes are often important food source to terrestrial organisms (see below), including humans (see Economic Importance and Conservation), changes in ray-finned fish communities can have significant ecological implications.
A variety of terrestrial vertebrates, such as mammals , amphibians , reptiles , and many marine and freshwater birds depend on ray-finned fishes as a primary source of food. Piscivorous ray-finned fishes compete with many of the organisms above and in some cases are involved in symbiotic relationships with them. A simultaneous competitive and commensal (one benefits and the other is unaffected) relationship is found between bluefish and common terns. These two species interact at a critical period of the terns’ feeding cycle, just after mating when there are chicks to feed. At this time, bluefish migrate to feed on anchovies, concentrating and driving them up in the water column, where terns can catch sight of the anchovies (commensalism). However, bluefish reduce anchovies’ populations considerably, and terns that breed after the bluefish migration are usually unsuccessful (competition). There are numerous other examples of symbiosis, mutualism, commensalism and parasitism between ray-finned fishes and other groups. For example, gobies share burrows with several shrimp-like crustaceans (mutualism) or live among sponges and corals (commensalism). Cardinalfishes and pearlfishes live inside large gastropods and mollusks , respectively (inquilism-sheltering inside living invertebrates). Recently, researchers have begun to appreciate the importance of fish in linking terrestrial and aquatic ecosystems. This is especially true of anadromous species, which grow primarily in the sea but return to aquatic areas before they, spreading nutrients from the ocean up and down rivers. During rainy periods in tropical watersheds, ray-finned fish forage in flooded areas, consuming seeds and dispersing them throughout the floodplain.
Several groups of invertebrates (mostly marine), such as cone shells , crabs , anemones , squids and siphonophores (colonies of organisms, e.g. man-o-war), also regularly consume various ray-finned fish. There is even some unlikely predators like dinoflagellates , that can cause large fish kills, known as “red tides”. Some dinoflagellates consume the scales of the dead fish as they sink. Ray-finned fishes also have significant impacts on a variety of plant species. The trophic cascade example (above) illustrated an indirect connection between microscopic plants (phytoplankton) and fish, but fish also excrete soluble nutrients into the water, such as phosphorus. Phosphorus is essential for phytoplankton growth, and fish secretions may provide significant amounts of nutrients in some lakes. A more direct connection is simply the consumption of numerous plant species (see Food Habits). Finally, fish may significantly alter the geological dynamics of their habitats. Many ray-finned fish build nests or burrows (e.g. several minnows , trout and salmon and tilefishes), while others break down substrates, such as dead coral, into sand (e.g. parrotfishes , wrasses , surgeonfishes , triggerfishes and pufferfishes).
Ecosystem Impact: disperses seeds; creates habitat; biodegradation ; keystone species ; parasite
Species Used as Host:
- other fish
- Jonna, R., J. Lehman. 2002. The Invasion of Lake Victoria by the Large Bodied Herbivorous Cladoceran Daphnia magna . Pp. 321-333 in E Odada, D Olago, eds. The East African Great Lakes: Limnology, Paleolimnology and Biodiversity. Boston: Kluwer Academic Publishers.
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