Scophthalmidae


The Scophthalmidae are a family of flatfish found in the North Atlantic Ocean, Baltic Sea, Mediterranean Sea, and Black Sea. Fish of this family are known commonly as turbots, though this name can refer specifically to Scophthalmus maximus, as well. Some common names found in species of this family are turbots, windowpanes, and brills.
Cladistic analysis reveals that this family is a monophyletic group.
Of all the scophthalmids, the largest species reaches approximately one meter in length. Some species in the family have been historically fished commercially.

Taxonomy

The Scophthalmidae family is composed of two main clades, four genera, and eight species. The four genera are Zeugopterus, Lepidorhombus, Phrynorhombus, and Scophthalmus, all sharing synapomorphies yet also subtle distinctions. One fossil genus, †Oligoscophthalmus Sakamoto, Uyeno & Micklich, 2003 is known from the Early Oligocene of Germany.

Discovery

The first known fossil was dated to the Cenozoic era during the Oligocene epoch. This specific fossil is believed to be from the upper Tertiary and is the oldest specimen of the family.

Taxonomic history

  • 1900: The features of the family were first recorded and compared with other taxa. The relationship between scophthalmids and other flatfish became apparent.
  • 1910: This distinction was then further specified when scophthalmids were paired with other left-eyed species of flatfish. This subfamily is called Bothidae, and are unique in comparison to other fish in their sinistral nature.
  • 1934: Again, taking a step further the fish are now categorized into a subfamily of Bothidae, and are named Scophthalmidae. To make said distinction, various species of similar traits were combined, primarily from the Paralichthyidae and Bothidae families of flatfish. These traits can be localized to three main requirements: "i) anterior extension of the two pelvic fins; ii) eyes present on the left-side; iii) presence of vomerine teeth."
  • 1993: After some confusion, it was concluded that the Scophthalmidae family is a sister clade to other subfamilies of flatfish and need to be revisited. Chapleau, from The Interrelationships of Scophthalmid Fishes, redefined the subfamily's traits to two more distinct features: " anterior extension of the two pelvic fins; an elongated supraoccipital process in contact with the dorsal margin of the blind side frontal." Again, these traits are also not perfect as defining a specific monophyly for any group is rather complex.
  • 1998–1999: Both traits proposed in 1993 were confirmed synapomorphies of Scophthalmidae. In addition to these synapomorphies, a third was added, namely the "presence of asymmetrical transverse apophyses."
  • 2003: The Interrelationships of Scophthalmid Fishes is released by Bruno Chanet, recognizing two main subfamilies of scophthalmids.

    Distribution and habitat

Turbots are benthic animals and can be found in both Marine and Brackish environments, but not in freshwater. While certain flatfish can be found in the other regions, Scophthalmidae are native to the North Atlantic, as well as the Baltic, Mediterranean, and Black seas. According to Fishes of the Word and excluding one species located in the Western Atlantic, all other scophthalmids reside in the general Northeastern Atlantic region. However, mapping out a specific organism's changes in habitat over their lifespan would be rather complex. Environmental events can displace fish or incline them to leave their home ranges. All fishes, including scophthalmids, also face predation risks and prey scarcity, as well as the complexities of spawning behaviors. While true, movements of the species can be tracked for instances of displacement as well as seasonal cycles of migration–both of which "appear to be triggered by changes in water temperature, light, current velocity, and irregular episodic events."
The movement of water is crucial to the species as it is a primary form of transportation. All stages of the flatfish use the elliptic and cyclic nature of Earth's tides in locomotion. The rising tide carries individuals shoreward and during the ebb tide they can exit the water column and sink to the bottom. This is crucial to energy expenditure, especially in migratory events between feeding and spawning grounds that take place every year. During migrations tidal cycles are primary in determining the vertical displacement of flatfish in the water column, while on feeding and spawning grounds these behaviors are more influenced by the diel cycle. The location of turbots is cryptic and multivariable, consisting of a mixture of both these cycles, an internal "clock" mechanism, and other factors such as turbulence, hunger, and magnetic fields.
But with all said and done, the flatfish don't go too far. The anatomy of these organisms is intrinsically negatively buoyant, making them sink in the absence of currents and upwellings. To preserve energy, this is their natural state. They are rather sedentary and the fish spend most of their time on bottom substrates.

Anatomy and morphology

Scophthalmidae is a family of ray-finned fish, so it exhibits any primitive traits of that clade. Being a scientific family, it also has its own set of general traits, as well as a variety of derived traits specific to the different genera of Scophthalmidae. These various features can be distinguished at every level of the scientific hierarchy to best emphasize the differences between the taxon. Being part of the Scophthalmidae family, all scophthalmids share the same fundamental Actinopterygii and pleuronectiformes traits.

''Actinopterygii'' traits

Actinopterygii, or ray-finned fish, share traits will all prior marine vertebrates The main requirements for the taxa are fin-rays and a swim bladder. All genera of Scophthalmidae are Actinopterygii, and as such share this primary features.

''Pleuronectiformes'' traits

share a lot in common with some minor distinctions. Based on the name, it is right to assume that most of genera of pleuronectiformes are 'flat' and have a generally 'low' body profile. Due to this anatomical difference, flatfish are quite unique and have asymmetrical features. In adult individuals, both eyes lye on the same side of the head on whichever side is dominant. Furthermore, flatfish have protrusible eyes and have slowly translated their dorsal fin onto the head. Poking eyes out of the substrate and using this dorsal fin akin to a rudder, these three traits all serve the flatfish in its benthic lifestyle. Lastly, the generalized coloration of flatfish include a very pale underside with a cryptically pigmented dorsal side. This pigmentation serves as a camouflage for the fish and allows them to increase their survivorship via heightened predator avoidance and improved hunting behaviors.

''Scophthalmidae'' traits

The three primary synapomorphies within the Scophthalmidae family have been crucial in defining the clade. Proposed in Interrelationships of Scophthalmid Fishes, the three aforementioned synapomorphies along with two other niche traits help differentiate scophthalmids from the other flatfish. These five anatomical traits are as follows:
"The anterior extension of the two pelvic fins to the isthmus"
"Slightly asymmetric lateral expansions on both pelvic bones"
"An elongated anterior supraoccipital process in contact with the dorsal margin of the right frontal"
"The presence of asymmetrical transverse apophyses on the caudal vertebrae"
"The first neural spine bent at contact with the dorsal margin of the cranium"
Other non-defining traits of scophthalmids that are not specific to the family are their large and prominent lower jaws, sinistral eyes, and the single oil globule present in the yolks of their eggs.

Behavior

Locomotion

Scophthalmids live a fairly sedentary lifestyle in comparison to other ray-finned fish, and due to their negative buoyancy spend most of their time below other taxa. Being asymmetrical and spending most of their time on the bottom substrates, scophthalmids have evolved unique locomotive systems to better fit their lifestyle. Turning their bodies into levers, the flatfish use forward and backward movements to propel themselves across the substrate. Oftentimes, these movements are short, with pauses in between strokes. That being said, the fish can increase their movement speed through "rapid propulsive strokes" that can propel individuals in different directions.
In addition to the "rapid propulsive strokes," scophthalmids can elevate their heads into an alert position. Using both of these adaptations in tandem, the fishes can change the angle of their projection, and shoot themselves into the water column by lifting their heads and then executing a 'rapid propulsive stroke.' This is highly useful to escape predation, initiate gliding, or journey far distances while conserving energy. While the propulsive stroke may be energetically costly, once in the water column the flatfish can exhibit 'swim and glide' behaviors to cruise through the water.
Besides the lever-system that can propel the fish in the transverse plane, scophthalmids also have unique systems to control their vertical and horizontal movements. Due to their asymmetry, their bodies are on a 90 degree rotation and use their fins differently than most Actinopterygii. Flatfish can change their vertical direction by changing the angle of their bodies, with particular emphasis on the angle of their caudal fins. On the contrary, horizontal direction is controlled usnig the pectoral fin.