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Countershading

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Guest post by Emily Argo

Countershading, originally described in the late 1800s, is when one side of an animal is dark and the other is light, serving as a form of camouflage. In fish, such as the Atlantic Bluefin Tuna (Thunnus thynnus) pictured, this typically means the ventral side (bottom) is light and the dorsal side (top) is dark. This is useful for fish because the dark dorsal side helps them blend in the with substrate or deeper water below if they are being viewed from above. Then, the lighter dorsal side helps fish blend in with the water (and light backdrop of the sky) above them if you are looking at the from below. Countershading is seen in fish species in coastal and open ocean habitats.

Atlantic Bluefin Tuna are one of many pelagic species that exhibit countershading.

However, some species, such as the Nile catfish (Synodontis batensoda), exhibit reverse countershading where the ventral side is dark and the dorsal side is light. The Nile catfish feeds while swimming upside down in the water column and the reverse countershading helps it camouflage.

While evidence of the mechanisms that drive countershading are lacking, studies suggest that there is an adaptive advantage to countershading in aquatic habitats where the scattering of light through the water column remains relatively uniform throughout the day compared to terrestrial environments (Ruxton et al. 2004).

Can you think of other animals that exhibit countershading?

Reference:

Ruxton, Graeme D., Michael P. Speed, David J. Kelly, What, if anything, is the adaptive function of countershading?, Animal Behaviour, Volume 68, Issue 3, September 2004, Pages 445-451, ISSN 0003-3472, http://dx.doi.org/10.1016/j.anbehav.2003.12.009.
(http://www.sciencedirect.com/science/article/pii/S0003347204001794)

Chondrichthyes

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Chimaeras are a predominately deep sea branch of elasmobranchs with most found deeper than 200m (NOAA).

Chimaeras are a predominately deep sea branch of chondrichthyes with most found deeper than 200m (NOAA).

Chondrichthyes are a taxonomic class of cartilaginous fishes that encompass sharks and rays (elasmobranchs) and chimaera.  Though there are exceptions, in general, Chondrichthyes have exposed gills, no swim bladder, internal fertilization, and placoid denticles.  These characteristics differentiate them from the more evolutionarily derived branch of fishes, bony fish (Osteichthyes).

Catadromous

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Eels have a unique larval form, leptocephali, which can utilize marine currents to travel long distances.

Eels have a unique larval form, leptocephali, which can utilize marine currents to travel long distances.

A catadromous fish hatches and spawns in salt water but spends most of its life in fresh water.  Catadromous life cycles are much less common than anadromous life cycles.  American and European Eels (Anguilla rostrate and A. anguilla, respectively) are among the more famous fish that utilize this unusual migration pattern.  They have a highly specialized larvae, leptocephali, which resemble a transparent leaf.  Their specialized shape allows leptocephali to ride marine currents to the continental shelf using relatively low amounts of energy.  In coastal waters, they metamorphose into glass eels.  As they continue to grow and start developing pigmentation, they enter the yellow eel phase.  Yellow eels migrate into estuaries and onto fresh water where they will remain until they reach sexual maturity as a silver eel and return out to sea to spawn.

For more information, check out the “CAN YOU SAY ANADROMOUS, CATADROMOUS, AMPHIDROMOUS, OCEANODROMOUS, OR POTAMODROMOUS?” post on The Fisheries Blog!

Caudal peduncle

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The caudal peduncle is the narrow region of a fish's body anterior of the caudal fin.

The caudal peduncle is the narrow region of a fish’s body anterior of the caudal fin

The caudal peduncle is the tapered region behind the dorsal and anal fins where the caudal fin attaches to the body.  The depth of the caudal peduncle, which is measured at its narrowest point, gives some indication of the power of a fish and the speed at which it can swim.  For example, ambush predators, like barracudas or gars, have a caudal peduncle that is not much narrower than their torpedo-shaped bodies.  They can lie-in-wait and then give a few powerful thrusts of their tail to surprise a prey fish.  Other very fast swimming, powerful fish, like tunas and mackerels, have a very narrow caudal peduncle.  They can even have keels, like those on a boat, to help support and stabilize the caudal fin and make swimming far and fast more efficient.

A narrow caudal peduncle is common in fast, far-ranging fish

A narrow caudal peduncle is common in fast, far-ranging fish

Ctenoid scales

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Ctenoid scales are characterized by small teeth on their posterior margin

Ctenoid scales are characterized by small teeth on their posterior margin

Ctenoid scales are scales with comb-like edge found in higher order teleost fishes, such as perch and sunfish.  Cteni are the tiny teeth on the posterior margin of the scale.  Similar to cycloid scales, they are overlapping which allows for greater flexibility in movement than other types of scales such as ganoid scales.  The surface layer of the scale is comprised of calcium-based salts and the inner layer is predominately collagen.  As a fish grows, its scales grow, adding concentric layers, similar to tree rings.  For certain species, these rings can be counted to estimate the age of a fish.

Cycloid scales

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Cycloid scales are characterized by having a smooth margin

Cycloid scales are characterized by having a smooth outer margin

Cycloid scales are smooth-edged scales predominately found in lower order teleost fishes, such as salmon, carp and other soft fin rayed fish.  Similar to ctenoid scales, they are overlapping which allow for greater flexibility in movement than other types of scales such as ganoid scales.  The surface layer of the scale is comprised of calcium-based salts and the inner layer is predominately collagen.  As a fish grows, its scales grow, adding concentric layers, similar to tree rings.  For certain species, these rings can be counted to estimate the age of a fish.

Caudal fin

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A caudal fin graced the Summer 2012 cover of Eddies magazine (fws.gov/eddies)

A caudal fin graced the Summer 2012 cover of Eddies magazine (fws.gov/eddies)

The caudal fin, or tail, of a fish is the only fin to be connected to the vertebral column.  It is the primary means of locomotion for most fish.  Unlike many marine mammals with tails that use an up-and-down motion, fish generally use a side-to-side thrust of their caudal fin for propulsion.  The shape of the caudal fin can be indicative of the style of motion for a fish.  For example, very fast swimming fish like tunas have lunate caudal fins for more efficient swimming while lie-in-wait predators like grouper have a strong, wide base of the tail, the caudal peduncle, for faster acceleration.

Caudal fin come in many shapes and sizes; form often follows function

Caudal fins come in many shapes and sizes; form often follows function