Flatfish Biology: Adaptation, Anatomy, and Major Families

Flatfish Asymmetry: Anatomy and Adaptation

Called flatfish, these organisms have a laterally flattened body, characterized by a profound loss of symmetry compared to other vertebrates. This asymmetry arises because they lie on one side (the blind side). Consequently, the organs on the supporting side atrophy or migrate toward the side that remains facing upward (the ocular side).

The flat, supporting side is blind and unpigmented, while the slightly convex ocular side is fitted with two eyes and endowed with color.

Flatfish larvae are often similar to those of symmetric fish. It is during advanced stages of metamorphosis that the eye moves from the blind side to the ocular side, ending the pelagic life stage and initiating the benthic life stage.

Principal Flatfish Families

The principal families of this group are Scophthalmidae, Pleuronectidae, and Soleidae.

Scophthalmidae (Left-Eyed Flatfish)

Flatfish in this family have eyes on the left side, a prominent lower jaw, a terminal mouth, and a large caudal fin. This family includes the turbot and megrim (roosters).

Turbot (Psetta maxima)

• Distribution: From Spain to Norway and the Mediterranean.
• Habitat: Lives in coastal waters, from shallow areas down to 80–100 m, typically on sandy or gravel bottoms.
• Characteristics: Lacks scales, possessing abundant spiny tubercles, which are smaller on the head. Dark-colored spots are variable due to its high mimetic capacity.
• Size: Reaches 40–45 cm.

Gallo (Megrim)

These fish live buried in the sand most of the time. Adults inhabit depths between 200 and 300 m, while juveniles are found in shallower waters (30–60 m).

Pleuronectidae (Right-Eyed Flatfish)

Asymmetrical fish whose eyes are on the right side. They typically have an inferior terminal mouth and a prominent jawline. They live in coastal waters, though some species may exceed 1,000 m depth. This family includes flounders and halibut.

Flounder (European Flounder)

The only European flatfish species that enters estuaries and lives in fresh water for short periods.

• Scales: Small scales on the upper side; along the lateral line, it shows toothed scales.
• Habitat: Common in coastal waters up to 50 m deep. It shifts toward the coast in summer and returns to deep water during the winter.
• Growth: Spawns in shallow waters. Growth depends on temperature, reaching maturity at age 3 years.

Halibut (Atlantic Halibut)

The largest flatfish species, reaching over 2.5 meters and weighing more than 300 kg. They are found in Arctic and Subarctic waters of the Atlantic.

They inhabit waters between 1 and 5°C, preferring sandy or muddy bottoms. Sexual maturity is reached late, around age 12.

Halibut migrate to depths of up to 1,000 m in late winter and early spring. They are found in the North Sea, Scotland, Ireland, and Norway.

They are suffering from overhunting because sexual maturity comes too late.

Soleidae (Soles)

Fishes of the continental shelf, generally living in shallow sandy or muddy bottoms. They inhabit warmer waters, so their distribution is more southern than the previous flatfish families.

The body shape is oval, unlike the previous families.

Common Sole (Lenguado)

A common example of the Soleidae family.