Southern Flounder
(Paralichthys lethostigma)
James T. Hooker
If you have ever been waist deep wading in water and seen a shadow dart off, it probably wasn't a southern flounder. These ray finned fish are highly adapted in their camouflage and specialization to live on the silty bottoms of estuaries and coastal waters. They aren't aesthetically pleasing, but as far as getting the job done these fish can. Unfortunately with all their adaptations to hunt and prevent them from being hunted, the one thing that does threaten them is humans. commercial and recreational fishing over the last two decades have reduced populations to a point of concern (Sneed, 2015), but enough with the bad news let me tell you more about this amazing fish.
The southern flounder is a left-eye (sinistral) flat fish. This means that the right eye migrates to the left side as it matures. A few species of flat fish follow this process, while other flounder species and flat fish have similar, but opposite adaptations (Zimmer, 2008).
Common names: flounder, fluke
Taxonomy
Kingdom: Animalia
Phylum: Chordata
Class: Actinopterygii
Order: Pleuronectiformes
Family: Paralichthyidae
Genus: Paralichthys
Species: Paralichthys lethostigma
Below is a Phylogenic tree highlighting the Pleuronectiformes clade to which the southern flounder belongs, and a diagram of orbital migration in common ancestors.
Image of flounder in sand retrieved from: https://gcrl.usm.edu/public/fish/images/flounders/flounder.buried.jpg
Evolutionary History
The most obvious of adaptations for this fish is the migrating eye. The Southern flounder is born with both eyes on opposite sides of the head with bilateral symmetry. As a larvae matures to about 30-40 days the right eye will drift to the opposite side of the head. The entire orbital and skull bones shift giving the nostril, and jaw a tilt as well. Once the metamorphosis is complete after 2-3 weeks the fish will also change its swimming pattern and rests on the ocean or estuary bottom (Daniels, 2000 ).
Another interesting adaptation is the coloring. Flounders will develop dark pigmentation on the upper surface, or in the Southern flounders case the left side to camouflage it to the bottom. the under side of the fish remains white and lacks pigment. Having two eyes on the lookout, while blending in with surroundings has made this fish an outstanding ambush predator, as well as a vigilant defender against predators.
The highlighted areas are the Southern flounders known distribution. Notice, it is curiously split by southern Florida. It is possible that these two groups may undergo speciation in time.
Niche and Habitat
The Southern flounder lives most of its five years after larvae stage on the silty bottoms of estuaries, wetlands, and low salinity marine environments. Individuals can tolerate a rage of temperatures from 5-35 degrees Celsius, and salinity from 0-60%. They are sexually dimorphic and females are almost twice as large as males which are ~40cm. Upon maturation after the second year, and from there they spawn in offshore water on the Atlantic continental shelf, and gulf of Mexico. from there the Larvae begin to mature and head back to the low salinity and safety of inland waters (Munroe, 2015).
Environmental Impacts
Habitat fragmentation and wetland loss can contribute to the loss of viable living space. This species is estuarine dependent and development in these areas has a negative impact. These areas are also biodiversity hot spots and considered of "Major importance" (Munroe, 2015). The diminishing habitat with the fact that Southern flounders typically do not migrate far distances, means once the habitat is lost they will perish.
https://www.iucnredlist.org/species/202632/46958684#assessment-information
Conservation
The major factor in this species of fish becoming threatened is because of fishing. Flounder are managed by respective states and in North Carolina Southern flounder is "among the highest of any finfish in North Carolina" caught (NCDEQ, 2020). There are management practices in place such as gill netting restrictions, seasonal closures, gear specifications and size requirements. This will help limit the spawning futures, but the last estimates show Southern flounder remaining a declining species, soon to be threatened (Munroe, 2015)
Web Resources
Here is an interesting aquaculture Sea Grant given to NC State. The Southern flounder shows promise to being farmed which would help with the stress on current wild spawning populations.
If you would like to learn how to catch flounder, here are some helpful tips to lead you in the right direction.
https://www.saltwatersportsman.com/how-to-catch-more-flounder/
Here is Wade O. Watanabe, PH. D. a resident expert at our UNCW Center for Marine Science. He too is working on aquaculture, and an all around guru on finfish.
https://uncw.edu/aquaculture/people/bios-cvs/watanabe.cv.html
Here is a website with some flounder jokes to brighten your day nerds. Here is one I found funny.
"I don't think the Flounder I caught today was very happy with me. He was looking at me kinda sideways."
Friends or Foes?
Because of the lifestyle and adaptation of the flounder there are few predators for this fish, but dolphins and sharks are known to feed on adult flounders (GCRL, 2020). Also, while there is some overlap in the territories of Southern flounder and other species, they are not often seen together. One other worry not as common to the flounder is Vibrio. This bacteria can cause lesions and tail rot in some cases (Overstreet, 2017).
Resources
Daniels, H. (2000) Species Profile Southern Flounder. Southern Regional Aquatic Center. https://appliedecology.cals.ncsu.edu/wp-content/uploads/726_flounder.pdf
Gulf Coast Research Labratory (GCRL). (2020). https://gcrl.usm.edu/public/fish/flounder.php
Munroe, T. (2015). Paralichthys lethostigma. The IUCN Red List of Threatened Species 2015: https://www.iucnredlist.org/species/202632/46958684#assessment-information
North Carolina Department of Environmental Quality (NCDEQ). (2020). http://portal.ncdenr.org/web/mf/southern-flounder
Overstreet R.M., Hawkins W.E. (2017) Diseases and Mortalities of Fishes and Other Animals in The Gulf of Mexico. In: Ward C. (eds) Habitats and Biota of the Gulf of Mexico: Before the Deepwater Horizon Oil Spill. Springer, New York, NY https://link.springer.com/chapter/10.1007/978-1-4939-3456-0_6
Sneed, D. (2015) The dark side to North Carolina's fishing heritage. The News & Observer. https://www.newsobserver.com/opinion/op-ed/article25614817.html
Zimmer, C. (2008) The Evolution of Extraordinary Eyes: The Cases of Flatfishes and Stalk-eyed Flies. Evo Edu Outreach 1, 487–492. https://link.springer.com/article/10.1007/s12052-008-0089-9