gills

Air-breathing

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A mudskipper is an amphibious, air-breathing fish (photo credit: National Aquarium).

Guest post: Gretchen Stokes, University of Florida

Air-breathing fishes are those that respire using aerial gas exchange instead of, or in addition to, that of the water.  Air-breathing is an ancient vertebrate specialization that evolved from aquatic hypoxia (e.g., areas of seasonally flooded wetlands, under ice).  Air-breathing fishes are equipped with one or more modified respiratory structure to breathe, such as modified air bladders, lungs, skin or gills.  Some air-breathing fishes (facultative air-breathers) rely mainly on gills with supplemental gas exchange from aerial breathing when conditions are poor or it is too costly to obtain oxygen from water.  Other air-breathers (obligate) require continuous, regular surface breathing.  Yet another group of air-breathing fishes are amphibious, venturing to land either in an active or inactive state.

Air-breathing occurs in an estimated 450 species and 49 families of fishes, most of which are found in the tropics.  In North America, common air-breathing species include gar, tarpon and bowfin.  Aquarium enthusiasts may be familiar with the air-breathing habits of well-known betas.  Globally the list becomes expansive with many unique adaptations, such as lungfishes, mudskippers, snakeheads or air-breathing catfishes.  Whether a gulp of air at the water’s surface, crawling on land to feed, or maintaining a high tolerance to air exposure with specialized skin, air-breathing allows for unique adaptations and survival in extreme environments.

Gill rakers

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Gill rakers (above gill arch) from an American Shad Alosa sapidissima are not to be confused with gill filaments (below); they are particularly important for filter-feeding fish (photo credit: NOAA).

While gill rakers have no role in gas exchange, the predominant function of gills,  they do perform an equally important function for filter-feeding fish – food acquisition.  These bony projections serve as a sieve to trap food particles.  They vary greatly in length and number and these characteristic differences have evolved with different feeding strategies for fish.  A plankton feeder, for example, such as American Shad Alosa sapidissima, has very tightly-packed, comb-like, gill rakers to efficiently filter their food from the water column. An omnivore or piscivore, on the other hand, has shorter, more widely-spaced gill rakers, better for larger prey items.  In fact, the morphology of gill rakers is so diverse that they are often used as a taxonomic tool to identify and classify fish species (e.g., gill raker counts can differentiate species on a dichotomous key).