Filter feeding in the context of "Ecosystem engineer"

A gill (/ɡɪl/ ) is a specialized respiratory organ that many aquatic animals use for aquatic gas exchange, i.e. to extract dissolved oxygen from water and to excrete carbon dioxide. Branchia (pl.: branchiae, from Ancient Greek βράγχια) is the zoologists' academic name for gills.

The gills of some semi-aquatic clades (e.g. crabs, terrestrial hermit crabs and amphibious fishes), have also adapted to allow air respiration (breathing) on land provided the gills are kept moist. In some terrestrial chelicerates species (tetrapulmonates such as spiders and scorpions), their ancestral gills have evolved into fully air-breathing book lungs. In some planktivorous bony fish species (e.g. silver and bighead carps), the gills are also used as a filter feeding organ via comb-like projections called gill rakers.

Freshwater snails are gastropod mollusks that live in fresh water. There are many different families. They are found throughout the world in various habitats, ranging from ephemeral pools to the largest lakes, and from small seeps and springs to major rivers. The great majority of freshwater gastropods have a shell, with very few exceptions. Some groups of snails that live in freshwater respire using gills, whereas other groups need to reach the surface to breathe air. In addition, some are amphibious and have both gills and a lung (e.g. Ampullariidae). Most feed on algae, but many are detritivores and some are filter feeders.

Freshwater snails are indirectly among the deadliest animals to humans, as they carry parasitic worms that cause schistosomiasis, a disease estimated to kill between 10,000 and 200,000 people annually.

Forage fish, also called prey fish or bait fish, are small pelagic fish that feed on planktons (i.e. planktivores) and other small aquatic organisms (e.g. krill). They are in turn preyed upon by various predators including larger fish, seabirds and marine mammals, this making them keystone species in their aquatic ecosystems.

The typical ocean forage fish feed at the lower trophic level of the food chain, often by filter feeding. They include particularly fishes of the order Clupeiformes (herrings, sardines, shad, hilsa, menhaden, anchovies, and sprats), but also other small fish, including halfbeaks, silversides, smelt such as capelin and goldband fusiliers.

Snagging, also known as snag fishing, snatching, snatch fishing, jagging (Australia and New Zealand), or foul hooking, is a fishing technique for catching fish that uses sharp grappling hooks tethered to a fishing line to externally impale (i.e. "snag") into the flesh of the fish, without needing the fish to swallow any hook with its mouth like in angling. This is achieved by suddenly and vigorously pulling the line (either by handlining or with a rod) when movement is felt, causing the snag hook to "claw" into and grapple any nearby fish like a gaff. Weighted multi-hook rigs can be used to increase chances of success, and modern technologies such as underwater video camera can also be used to visually aid and time the snagging.

Some herbivorous/algaevorous fish species, such as paddlefish, are not attracted to normal angling baits or lures as they primarily filter-feed on plankton. While these fish can be caught using nets, spears or pole hooks, snagging is also used as a less strenuous and more versatile technique. There are also some species of salmon that can be legally harvested by snagging, and are targeted as they migrate upstream to spawn, making them easier to target, and at a time when the fish are at the end of their life cycle.

Aquatic feeding mechanisms face a special difficulty as compared to feeding on land, because the density of water is about the same as that of the prey, so the prey tends to be pushed away when the mouth is closed. This problem was first identified by Robert McNeill Alexander. As a result, underwater predators, especially bony fish, have evolved a number of specialized feeding mechanisms, such as filter feeding, ram feeding, suction feeding, protrusion, and pivot feeding.

Most underwater predators combine more than one of these basic principles. For example, a typical generalized predator, such as the cod, combines suction with some amount of protrusion and pivot feeding.