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Paralichthys dentatusFlounder(Also: Fluke; Summer flounder)

By Ryan Brownlow

Geographic Range

Summer flounder (Paralichthys dentatus) are exclusively found in waters along the Atlantic coast, from Nova Scotia southward to Florida. They are most abundant in the Mid-Atlantic, between Massachusetts and North Carolina. ("Summer flounder, Paralichthys dentatus, life history and habitat characteristics", 1999)

Habitat

Summer flounder live in the benthic zone, often in coastal waters or estuaries. Temperature and substrate conditions play a large role in determining where summer flounder are located, and population density changes depending on the season. From late April to as early as September, summer flounder are most abundant in shallow coastal or estuarine waters at depths less than 100 m, where water temperatures range from 9 to 27 °C. From early September to early April, they are most abundant in offshore waters along the continental shelf at depths of around 150 m, where temperatures range from 4 to 13 °C.

Summer flounder are found on a variety of substrates. They are most common in substrates where there is a transition from sand to silt or clay, but they can also be found in muddy substrates and marsh beds. (Sackett, et al., 2008; "Summer flounder, Paralichthys dentatus, life history and habitat characteristics", 1999)

  • Range depth
    150 to 0 m
    492.13 to 0.00 ft
  • Average depth
    40-60 m
    ft

Physical Description

Summer flounder are adapted to lying on their right sides on the seafloor. As a result, they have a flattened body plan, their right sides lack coloration and are flatter than their left sides, and both of their eyes are on the left side of their heads. Their left sides are commonly called their ocular sides and their right sides are called their blind sides. Adults typically weigh 1 to 2 kg, although females have been reported to reach a maximum weight of 7 kg. Summer flounder males are 0.24 m long on average, whereas females are 0.32 m long on average, but are reported to reach up to 1 m in length.

Summer flounder have chromatic pores, which allow them to adapt their color pattern based on the substrate on which they are laying. This helps them effectively camouflage themselves to avoid predators and ambush prey. Their ocular sides are typically patterned with dark brown and gray colors, although they can change the color to lighter hues, such as white or pink. The coloration on their blind sides is consistently white, as they lack pigmentation and chromatic pores on this side.

In early stages of development, summer flounder more closely resemble other fish larvae, with one eye on either side of their heads. However, as they develop, their right eyes shift to the left sides of their bodies. By the time summer flounder larvae reach around 16 mm in length, both eyes are located on their ocular sides. (Burton, 2010; Morse, 1981; "Summer flounder, Paralichthys dentatus, life history and habitat characteristics", 1999)

  • Sexual Dimorphism
  • female larger
  • Range mass
    0.05 to 7 kg
    0.11 to 15.42 lb
  • Average mass
    1-2 kg
    lb
  • Range length
    0.01 to 1 m
    0.03 to 3.28 ft
  • Average length
    0.24-0.32 m
    ft

Development

Summer flounder develop in several distinct stages over a period of many months. About 85 hours after spawning, eggs hatch into larvae. The first larval stage after hatching is characterized by opening of the mouth and the bipartite gut, after which larvae begin to feed. This stage occurs around 3 to 12 days after hatching. The second stage involves flexion of notochord tips, the precursor of the central nervous system, which occurs at around 12 to 19 days after hatching. The final stage before metamorphosis is pre-transformation, which involves the deepening of the head region, and increased development of jaw musculature. This stage occurs at around 25 to 30 days after hatching.

Summer flounder metamorphosis begins with the migration of the right eye to the left side of the head, occurring roughly 3 weeks after hatching. This eye migration is unique to flatfish and contributes to the flatted shape of matured summer flounder bodies. Over a period of around 30 to 34 days after hatching, larvae grow to a length of 10.0 to 11.0 mm. At 54 to 65 days after hatching, larval summer flounder complete their metamorphosis into their adult stage and migrate inshore to coastal and estuarine areas, where they spend their first year before going back to deeper water.

Adults are considered sexually mature when they reach certain lengths. Males are considered sexually mature at 24.6 cm while females are sexually mature at 32.2 cm. For both sexes, this occurs around 2 years after hatching. (Keefe and Able, 2005; Martinez and Bolker, 2003; Schreiber and Specker, 1998; "Summer flounder, Paralichthys dentatus, life history and habitat characteristics", 1999)

Reproduction

Summer flounder spawn seasonally and are polygynandrous. They are broadcast spawners, meaning males and females release their sperm and eggs, respectively, into the water and fertilization occurs externally. Female summer flounder often release their eggs near the surface of the water and males release their sperm nearby. (Morse, 1981; "Summer flounder, Paralichthys dentatus, life history and habitat characteristics", 1999)

Male and female summer flounder reach sexual maturity at around 2 years old, after which they swim to deep waters along the continental shelf to reproduce. Because water temperature greatly affects the optimal timing of external fertilization, seasonal cooling of coastal waters plays a large role in the timing of spawning behavior. The temperature at which it is cold enough to spawn is 12 to 19 °C. Spawning first occurs farther north along the east coast of the United States in September. Spawning begins in December in areas farther south, such as the mid-Atlantic coast. The oldest, largest summer flounder are usually the first to migrate to spawning grounds.

Summer flounder breed once per year at most, but have a fairly high fecundity rate, with females releasing 1,077 to 1,265 eggs per gram of body mass. This equates to between 460,000 and 4,190,000 eggs per female in a single breeding season. Males release sperm externally and only a small percentage of the eggs that females release are successfully fertilized. Once eggs are fertilized, it takes roughly 85 hours before they hatch. Summer flounder eggs are buoyant, so the tides naturally carry them closer to shore, where hatchlings undergo development into adults. Larvae are completely independent upon hatching. (Morse, 1981; "Summer flounder, Paralichthys dentatus, life history and habitat characteristics", 1999)

  • Breeding interval
    Summer flounder breed once yearly.
  • Breeding season
    Spawning may occur from September to January, with peak breeding activity depending on local water temperature.
  • Range number of offspring
    460,000 eggs to 4,190,000 eggs
  • Average number of offspring
    1,077-1,265 eggs per gram body weight
  • Average time to hatching
    85.0 hours
  • Average time to independence
    0 minutes
  • Average age at sexual or reproductive maturity (female)
    2 years
  • Average age at sexual or reproductive maturity (male)
    2 years

Summer flounder exhibit no parental investment beyond the act of mating. Larvae are completely independent at birth. (Morse, 1981; "Summer flounder, Paralichthys dentatus, life history and habitat characteristics", 1999)

  • Parental Investment
  • no parental involvement

Lifespan/Longevity

The average lifespan of summer flounder is 6 to 7 years for both males and females. The maximum reported age for a male is around 12 years, and for a female is around 14 years. Summer flounder have an estimated 90 to 95% juvenile mortality rate, meaning they rarely survive to adulthood. Limitations to the lifespan of summer flounder in the wild include predation pressure, disease, habitat quality, and fishing practices.

Summer flounder longevity may differ depending on sex; the average natural mortality rate for males is much higher than that of females. However, size differences between sexes (females are larger) and common fishing practices may be confounding factors that affect estimates of natural mortality. (Grimes, et al., 1989; Maunder and Wong, 2011; "Summer flounder (Paralichthys dentatus) in the northwest Atlantic", 1979)

  • Range lifespan
    Status: wild
    12 to 14 years
  • Typical lifespan
    Status: wild
    6 to 7 years

Behavior

Summer flounder are solitary and are characteristically found lying in substrate on the seafloor. They rely on cryptic coloration to hide from predators and prey, and are capable of changing the coloration and patterning of their skin depending on the surrounding substrate.

Summer flounder migrate from estuaries and coastal areas to the Atlantic continental shelf, where they reproduce. After breeding season, they return to inland areas where they spend the rest of the year. Summer flounder demonstrate homing capabilities, in some cases returning to the same estuary as in previous years.

Summer flounder activity has been classified into three separate categories: resting, swimming, and eating. While resting, summer flounder either lie flat on the sand, with their heads lifted, or buried beneath sand. The resting posture they adopt is directly correlated to their degree of alertness. When swimming, summer flounder mostly remain near the seafloor, undulating their dorsal and anal fins to propel themselves. However, they may also use their body musculature to swim if needed. To feed, summer flounder rest on the sand until they visually fixate on a prey item, after which they crawl towards it and attacking it. They also opportunistically ambush prey that swim within range while they are partially buried in sediment.

Summer flounder are considered to be diurnal species, due to an observed decrease in activity levels as sunlight decreases. (Burton, 2010; Olla, et al., 1972; Sackett, et al., 2007)

Home Range

Although summer flounder are considered sedentary for most of the year, exact quantitative home ranges or territory sizes have not been reported.

Communication and Perception

Summer flounder rely on visual cues to perceive their environment. Both of their eyes are on the left side of their bodies so that they can move along the seafloor with a binocular view of their surroundings.

When summer flounder perceive a threat, they react by dropping to the sea floor and quickly burying themselves until partially or fully covered. This response is a protective reaction to evade predators. Once hidden from view, they remain covered until they no longer perceive a threat.

Summer flounder are solitary except during the breeding season, so their methods of intraspecific communication are not well studied. (Olla, et al., 1972)

Food Habits

Summer flounder are carnivorous, opportunistic feeders that consume various types of prey. Common prey items of summer flounder include decapod shrimp species (Crangon septemspinosa), mysid shrimp species (Neomysis americana), weakfish (Cynoscion regalis), Atlantic silversides (Menidia menidia), long-wristed hermit crabs (Pagurus longicarpus), and isopod species (Olencira praegustator).

During the summer, summer flounder primarily consume amphipods (order Amphipoda), because they are the most available prey. In the winter months, many food sources are harder to find and shrimp comprise the majority of their diet during this time.

Summer flounder can feed on large, quick-moving prey due to their well-developed optic lobes, large mouths with strong teeth, and stomachs with large storage capacities. These adaptations have allowed them to not only feed on prey at the sea floor, but also throughout the water column. (Powell and Schwartz, 1979; "Summer flounder, Paralichthys dentatus, life history and habitat characteristics", 1999)

  • Animal Foods
  • fish
  • mollusks
  • aquatic or marine worms
  • aquatic crustaceans
  • echinoderms
  • other marine invertebrates

Predation

Juvenile summer flounder likely serve as prey for a number of marine species, including lesser blue crabs (Callinectes similis), Chesapeake blue crabs (Callinectes sapidus), northern sea robins (Prionotus carolinus), and other larger fish that are present in estuaries and shallow waters. Larger summer flounder will also cannibalize smaller individuals. Due to a high demand from humans Homo sapiens as a seafood item, summer flounder are also victims of overfishing.

Summer flounder rely on cryptic coloration to avoid predators. They have chromatophores, which allow them to alter their coloration and patterning to blend into their surroundings. They also bury into sediment on the seafloor to remain undetected. (Burton, 2010; Francis and Bengtson, 1999; Kraus and Musick, 2001; Reichert and Van der veer, 1991)

  • Anti-predator Adaptations
  • cryptic

Ecosystem Roles

There are a number of parasites found on or in summer flounder. Parasites can be present in the blood, gills, gall bladders, intestines, mouths, rectums, skin, stomachs, and urinary bladders of summer flounder. Parasitic taxa that use summer flounder as hosts include: protists, flatworms (phylum Platyhelminthes), thorny-headed worms (phylum Acanthocephala), roundworms (phylum Nematoda), leeches (subclass Hirudinea), and copepods (subclass Copepoda). Reported protist genera and species include Amyloodinium, Cryptobia, Ichthyobodo, Hexamita, Trypanoplasma bullocki, Haemogregarina platessae, and Davisia branchiophora. Reported flatworm species include Neoheterobothrium affine, Bucephalopsis paralichthydis, Stephanostomim dentatum, Stephanostonum tenue, Lepocreadium setiferoides, Lepocreadium areolatum, Opecoeloides fimbriatus, Opecoeloides vitellosus, Parahemiurus merus, Lechithochirium synodi, Hirudinella ventricosa, Microphallus turgidus, Nybelinia bisulcata, Grillotia smarisgora, Bothriocephalus scorpii, Scolex pleuronectis type A-D, Ceratobothrium xanthocephalum, and Rhinobothrium. Reported thorny-headed worm species include Serrasentis sagittifer and Dollfusentis chandleri. Reported nematode genera and species include Capillaria, Hysterothylacium type A, Dichelyne cylindricus, and Hysterothylacium habena. A single leech species, Calliobdella vivida, was also found on summer flounder. Finally, the copepod species Argulus chesapeakensis and Acanthochondria galerita are reported to parasitize summer flounder. (Jansen and Burreson, 1990)

Commensal/Parasitic Species
  • Protists (genus Amyloodinium)
  • Protists (genus Cryptobia)
  • Protists (genus Ichthyobodo)
  • Protists (genus Hexamita)
  • Protists (Trypanoplasma bullocki)
  • Protists (Haemogregarina platessae)
  • Protists (Davisia branchiophora)
  • Flatworms (genus Rhinobothrium)
  • Flatworms (Bucephalopsis paralichthydis)
  • Flatworms (Stephanostomim dentatum)
  • Flatworms (Stephanostonum tenue)
  • Flatworms (Lepocreadium setiferoides)
  • Flatworms (Lepocreadium areolatum)
  • Flatworms (Opecoeloides fimbriatus)
  • Flatworms (Opecoeloides vitellosus)
  • Flatworms (Parahemiurus merus)
  • Flatworms (Lechithochirium synodi)
  • Flatworms (Hirudinella ventricosa)
  • Flatworms (Microphallus turgidus)
  • Flatworms (Nybelinia bisulcata)
  • Flatworms (Grillotia smarisgora)
  • Flatworms (Bothriocephalus scorpii)
  • Flatworms (Scolex pleuronectis Type A-D)
  • Flatworms (Ceratobothrium xanthocephalum)
  • Flatworms (Neoheterobothrium affine)
  • Thorny-headed worms (Serrasentis sagittifer)
  • Thorny-headed worms (Dollfusentis chandleri)
  • Nematodes (genus Capillaria)
  • Nematodes (genus Hysterothylacium type A)
  • Nematodes (Dichelyne cylindricus)
  • Nematodes (Hysterothylacium habena)
  • Leeches (Calliobdella vivida)
  • Copepods (Argulus chesapeakensis)
  • Copepod (Acanthochondria galerita)

Economic Importance for Humans: Positive

Summer flounder are caught, sold, and consumed by humans. They support both recreational and commercial fisheries along the east coast of the United States. They are one of the most common fish purchased directly from fisherman, and the price per pound in both Maryland ($2.36 per pound in 2012) and Virginia ($1.87) has been on the rise steadily since 1958.

Commercial landings of summer flounder have been on a steady decline since 1979. In Virginia, 10 million pounds of summer flounder were commercially harvested in 1979, but in 2012 only about 1.5 million pounds were harvested. The same trend exists in Maryland with 1.7 million summer flounder harvested in 1979 and only 116,031 harvested in 2012.

Recreational fishing of summer flounder also occurs, and promotes ecotourism along the east coast of the United States. There has been a similar decline in recreational summer flounder harvests since 1979, due to reduced abundance and new laws enforcing smaller bag and size limits. ("2014 Review of the Chesapeake Bay summer flounder fishery management plan", 2014)

  • Positive Impacts
  • food

Economic Importance for Humans: Negative

Summer flounder have no known adverse effects on humans.

Conservation Status

Summer flounder are listed as a species of least concern on the IUCN Red List and have no special status on other national or international conservation lists. Although they are heavily commercially fished, they show no signs of significant population decline.

There was a significant decline in summer flounder populations from the late 1970's through the early 1990's. This decline was due to commercial and recreational fishing of summer flounder, which peaked during this period. Their numbers have since rebounded and stabilized.

Several organizations such as The Summer Flounder, Scup, Black Sea Bass Fishery Management Plan (FMP), Atlantic States Marine Fisheries Commission (ASMFC), and the Mid-Atlantic Fishery Management Council (MAFMC) administer management of both commercial and recreational fishing of summer flounder. These organizations monitor 91 specific areas along the east coast of the United States to maintain appropriate population sizes and manage sufficient numbers of summer flounder. (Munroe, 2010)

Contributors

Ryan Brownlow (author), Radford University, Karen Powers (editor), Radford University, April Tingle (editor), Radford University, Emily Clark (editor), Radford University, Cari Mcgregor (editor), Radford University, Jacob Vaught (editor), Radford University, Galen Burrell (editor), Special Projects.

Glossary

Atlantic Ocean

the body of water between Africa, Europe, the southern ocean (above 60 degrees south latitude), and the western hemisphere. It is the second largest ocean in the world after the Pacific Ocean.

World Map

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

carnivore

an animal that mainly eats meat

chemical

uses smells or other chemicals to communicate

coastal

the nearshore aquatic habitats near a coast, or shoreline.

cryptic

having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.

diurnal
  1. active during the day, 2. lasting for one day.
ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

estuarine

an area where a freshwater river meets the ocean and tidal influences result in fluctuations in salinity.

external fertilization

fertilization takes place outside the female's body

fertilization

union of egg and spermatozoan

food

A substance that provides both nutrients and energy to a living thing.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

metamorphosis

A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.

migratory

makes seasonal movements between breeding and wintering grounds

molluscivore

eats mollusks, members of Phylum Mollusca

motile

having the capacity to move from one place to another.

natatorial

specialized for swimming

native range

the area in which the animal is naturally found, the region in which it is endemic.

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

piscivore

an animal that mainly eats fish

polygynandrous

the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.

seasonal breeding

breeding is confined to a particular season

sedentary

remains in the same area

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

solitary

lives alone

tactile

uses touch to communicate

visual

uses sight to communicate

References

Maryland Department of Natural Resources. 2014 Review of the Chesapeake Bay summer flounder fishery management plan. 640. Annapolis, Maryland: Fisheries Service. 2014.

Northeast Fisheries Center. Summer flounder (Paralichthys dentatus) in the northwest Atlantic. 79/31. Woods Hole, MA: National Marine Fisheries Agency. 1979.

National Oceanic and Atmospheric Administration. Summer flounder, Paralichthys dentatus, life history and habitat characteristics. 151. Highlands, New Jersey: National Marine Fisheries Service. 1999.

Burton, D. 2010. Flatfish (Pleuronectiformes) chromatic biology. Reviews in Fish Biology and Fisheries, 20: 31-46.

Francis, A., D. Bengtson. 1999. Partitioning of fish and diet selection as methods for the reduction in Paralicthys dentatus larviculture. Journal of the World Aquaculture Society, 30/3: 302-310.

Grimes, B., M. Huish, J. Kerby, D. Moran. 1989. Species profiles: Life histories and environmental requirements of coastal fishes and invertebrates (mid-Atlantic)--summer and winter flounder. Biological Report, 82/11: 1-18.

Jansen, M., E. Burreson. 1990. Parasites of summer flounder, Paralichthys dentatus, in the Chesapeake Bay. journal of the Helminthological Society of Washington, 57/1: 31-39.

Keefe, M., K. Able. 2005. Patterns of metamorphosis in summer flounder, Paralichthys dentatus. Journal of Fish Biology, 42/5: 713-728.

Kraus, R., J. Musick. 2001. A brief interpretation of summer flounder, Paralichthys dentatus, movements and stock structure with new tagging data on juveniles. Marine Fisheries Review, 63/3: 1-6.

Martinez, G., J. Bolker. 2003. Embryonic and larval staging of summer flounder (Paralichthys dentatus). Journal of Morphology, 255: 162-176.

Maunder, M., R. Wong. 2011. Approaches for estimating natural mortality: Application to summer flounder (Paralichthys dentatus) in the U.S. mid-Atlantic. Fisheries Research, 111/1-2: 92-99.

Morse, W. 1981. Reproduction of the summer flounder, Paralichthys dentatus (L.). Journal of Fish Biology, 19/2: 189-203.

Munroe, T. 2010. "Paralichthys dentatus" (On-line). The IUCN Red List of Threatened Species. Accessed March 27, 2015 at www.iucnredlist.org.

Olla, B., C. Samet, A. Studholme. 1972. Activity and feeding behavior of the summer flounder (Paralichthys denatus) under controlled laboratory conditions. Fishery Bulletin, 70/4: 1127-1136.

Powell, A., F. Schwartz. 1979. Food of Paralichthys dentatus and P. lethostigma (Pisces: Bothidae) in North Carolina estuaries. Estuaries, 2/4: 276-279.

Reichert, M., H. Van der veer. 1991. Settlement, abundance, growth and mortality of juvenile flatfish in a subtropical tidal estuary (Georgia, USA). Netherlands Journal of Sea Research, 27/3-4: 375-391.

Sackett, D., K. Able, T. Grothues. 2008. Habitat dynamics of summer flounder Paralichthys dentatus within a shallow USA estuary, based on multiple approaches using acoustic telemetry. Marine Ecology Progress Series, 364: 199-212.

Sackett, D., K. Able, T. Grothues. 2007. Dynamics of summer flounder, Paralichthys dentatus, seasonal migrations based on ultrasonic telemetry. Estuarine, Coastal and Shelf Science, 74/1-2: 119-130.

Schreiber, A., J. Specker. 1998. Metamorphosis in the summer flounder (Paralichthys dentatus): Stage-specific developmental response to altered thyroid status. General and Comparative Endocrinology, 111: 156-166.

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