The sole – Solea solea

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The common sole (Solea solea) is a flatfish that inhabits the coastal waters of Europe, extending to depths of up to 200 meters, although it is more commonly found at around 70 meters. This fish is typically found in sandy or muddy environments but can also be spotted in harbors or estuaries, where it resides at the boundary between fresh and saltwater (tolerating a salinity of 10‰).

Description

The distinctive feature of this flatfish is its oval shape, which sets it apart from other more common flatfish. The length of the common sole typically ranges between 30 and 40 centimeters, although larger specimens can reach up to 70 centimeters, with a weight of 3 kg for individuals of 20 years (two-year-old males can measure 30 cm and weigh 350 g). Adult soles are dextral flatfish, with their eyes located on the right side of the body, lying on the left side. The coloration of this fish can vary significantly, ranging from grayish-blue to brownish-yellow, often with dark stripes and spots on the body. The sole has a rounded snout and a small, arched mouth located at the end of the head. The eyes are small and widely spaced (for a flatfish). The caudal fin often has a rounded edge with a dark fringe at the tip. On the sandy seabed or at the fish market, you can observe a black spot (sometimes golden or brown) toward the upper part of the pectoral fin. It’s important to note that other sole species have different patterns of markings. The dorsal and anal fins are generally bordered in white and connected to the caudal fin by a thin membrane.

Similar Species

There are several species of sole that can resemble each other (such as Solea aegyptiaca, Pegusa lascaris, Microchirus theophila, and Buglossidium luteum, particularly common in the North Sea). It is advisable to consult specialized sources in case of doubt, but Solea solea is the species most commonly encountered by divers along our coasts. Another similar species is Solea senegalensis, which differs from S. solea by having a less distinct and more uniform spot on the pectoral fin and by having 9 pre-caudal vertebrae, compared to the 10 of S. solea (a detail often not observable during dives).

Diet

The common sole primarily feeds on various small crustaceans, mollusks, and worms, which it can detect through both its sense of smell (its olfactory organs are visible) and touch. Under the snout, sensory papillae can be observed.

Reproduction and Life Cycle

These fish reach sexual maturity at around 4 years of age. Reproduction primarily occurs from March to May, although this may vary depending on the region. Plankton studies have shown eggs with a diameter of about 1.5 mm, supported by a lipid droplet that also serves as a food reserve. Females can lay hundreds of thousands of eggs. The larvae initially resemble those of common fish, but after metamorphosis, the young (about 15 mm in size) with an asymmetric shape move toward coastal waters. Research has shown that adults can migrate annually for over 180 km to reach breeding areas.

Biological Aspects

The common sole is a very common species along some Mediterranean coasts and is subject to intensive fishing due to its highly prized meat. This fish is primarily nocturnal in behavior. During the day, it can be observed partially or completely buried in the sediment. For long-distance movements, the common sole approaches the surface and is passively transported by marine currents. During the winter, it moves away from the coast and returns in spring and summer. The black spot on the pectoral fin of the common sole is interpreted as an example of Batesian mimicry, a form of mimicry where a harmless species imitates a poisonous one, such as the venomous dorsal fins of weever fish.

Additional Information

The abundant fishing of this fish, driven by its high commercial value, has led to controversies regarding the allowed mesh sizes for capture. The demand for this fish exceeds the supply. Coastal fishing for gray shrimp often results in the accidental capture of young soles, which is a concern for the populations of this fish.

Conservation Status and Regulations

According to the latest European Union regulation dated 12/21/2006, it is prohibited to capture individuals of common sole with a length less than 20 cm, regardless of the fishing technique used.

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