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)

Phenology

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Phenological events triggers Sockeye Salmon migrations (photo credit: USFWS).

Phenology is the study of seasonal or periodic cycles in ecosystems.  It is, essentially, “nature’s calendar.” For fish, phenology is often linked to timing of important life events such as spawning, migrations, and hatching.  These events are triggered by non-biological factors like day length, temperature, and precipitation.  If the timing of the main “cue” shifts, the alteration can have significant implications for the fish populations that have evolved to optimize the timing of their life events with that factor.

Many fall–spawning Pacific Salmon populations (Oncorhynchus spp.), for example, are beginning their freshwater migrations earlier than in the past (e.g. Kovach et al. 2015).  This consistent trend across species and populations strongly suggests that a shared environmental driver (i.e., climate change) is responsible.  Unfortunately, these altered behaviors can be maladaptive – Cooke et al. (2004) found that increased pre-spawn mortality rates – sometimes in excess of 90% – for several stocks of Fraser River sockeye salmon (Oncorhynchus nerka).

 

References

Cooke, S.J., Hinch, S.G., Farrell, A.P., F., L.M., Jones, S.R.M., Macdonald, J.S., Patterson, D.A., Healey, M.C., Van Der Kraak, G., 2004. Abnormal Migration Timing and High en route Mortality of Sockeye Salmon in the Fraser River, British Columbia. Fisheries 29, 22–33. doi:10.1577/1548-8446(2004)29

Kovach, R.P., Ellison, S.C., Pyare, S., Tallmon, D.A., 2015. Temporal patterns in adult salmon migration timing across southeast Alaska. Glob. Chang. Biol. 21, 1821–1833. doi:10.1111/gcb.12829

Panmictic

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Panmictic refers to a random mating strategy, frequently employed by fish, where breeding is just as likely to occur between any two individuals in a population as between any two others.  Mating in this way is not influenced by any any environmental (e.g., geographic proximity), hereditary (e.g., timing of spawning), or social interaction (e.g., polygamous mating systems).

American Eels have a panmictic population.

In a population genetics context, if a species is a panmictic population, there is no genetic evidence of population structure throughout its range.  For example, American Eels (Anguilla rostrata) are catadromous fish found from the Caribbean to Greenland.  However, they all migrate to breed in the same location in the Sargasso Sea so genetic samples from throughout their North American distribution range show a complete lack of genetic differentiation, or complete panmixia.

 

T & E species

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Atlantic Salmon are an endangered species protected by USFWS and NOAA in the U.S.

T & E is an acronym for threatened and endangered.

These are both status metrics for risk of extinction.  Endangered species are considered on the “brink of extinction” throughout their range and threatened species are likely to become endangered in the near future.

The species that are on this list in the United States receive certain protections under the Endangered Species Act of 1973 which recognizes the “esthetic, ecological, educational, historical, recreational, and scientific value [of these species] to the Nation and its people.”  The U.S. Fish and Wildlife Service (USFWS) and the National Oceanic and Atmospheric Administration (NOAA) are mandated to protect the T & E fish species, such as Atlantic Salmon (Salmo salar), among other aquatic and terrestrial species that fall under their jurisdictions.

Endemic species

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Devil's Hole Pupfish is endemic to Devil's Hole, Nevada.

Devil’s Hole Pupfish is endemic to Devil’s Hole, Nevada.

A fish species is considered endemic to a location if that is the only place it occurs naturally.  It may have evolved in that region or over time has become so adapted to that specific environment that it cannot survive elsewhere.  For example, Devil’s Hole Pupfish (Cyprinodon diabolis) is only found in Devil’s Hole, Nevada.  If a species is naturally located in multiple locations, it is considered a native species, but is not an endemic one.  Many endemic species are T&E species, threatened with extinction due to loss or modification of their habitat by agriculture, urbanization, or other human land uses.

 

Invasive species

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Lionfish are non-native and invasive to the Caribbean Sea.

Lionfish are non-native and invasive to the Caribbean Sea.

A fish species is considered invasive if its position in an ecosystem negatively impacts other species.  Generally, these are non-native species which have been introduced into a region by humans (either intentionally or unintentionally) with detrimental consequence to resident fish and other aquatic organisms.  However, some argue that a native species can also be considered invasive if humans have altered its natural ecosystem to the point where the species can throw the whole system into disequilibrium.

For further information, please check out a related post on The Fisheries Blog – Blurred lines: Can climate change-induced range expansion qualify a species as invasive?

Non-native species

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Sea lamprey are non-native to the Laurentian Great Lakes.

Sea lamprey are non-native to the Laurentian Great Lakes (photo credit: Great Lakes Fishery Commission).

A fish species is considered non-native to a location if it does not occur naturally there and only is present as a result of direct or unintentional human introduction.  This does not necessarily mean that these fish cannot thrive in their non-native habitat.  In fact, some fish, such as Sea Lamprey (Petromyzon marinus), can be in decline in their native range but can be considered invasive and even imperil native species in their non-native locations.

Native species

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Lake Trout are a native species in the Great Lakes but not lakes in the western U.S.

Lake Trout are a native species in the Great Lakes but not lakes in the western U.S.

A fish species is considered native to a location if it occurs naturally there.  It may have evolved in that region or dispersed and become established there without human assistance.  For example, Lake Trout (Salvelinus namaycush) is native to some locations in North America but is considered non-native in other locations where it has been introduced by people.

Holomictic

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Dimictic lakes commonly have turnover in spring and fall with stratification in summer and winter (Nature Times).

Dimictic lakes commonly have turnover in spring and fall with stratification in summer and winter (Nature Times).

Holomictic references the most common type of lake which turns over at least once per year (as opposed to meromictic lakes which are constantly stratified).  This mixing is an important process for maintaining fish and aquatic communities by distributing nutrients and oxygen throughout the lake before stratification occurs again.  There are four categories of holomictic lakes:

  • Oligomictic: mixing is irregular,
  • Monomictic: turnover occurs once a year (most common in polar areas)
  • Dimictic: turnover occurs twice a year (most common in temperate areas), and
  • Polymictic: frequent turnover (most common in tropical areas).

Meromictic

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The Black Sea is the largest meromictic lake (NASA).

The Black Sea is the largest meromictic lake (NASA).

Meromictic references a type of lake which is constantly stratified.  The surface and bottom waters do not ever mix.  In most cases, the bottom layer has very low oxygen levels, where few fish and other organisms can live, restricting them to the surface layer.  Meromictic lakes are uncommon (most lakes are holomictic and turnover at least once per year) and may be formed because they are:

  1. small and unusually deep, or
  2. the lower layer is denser and more saline than the surface layer.