# Colored lagoon cockle

The colored lagoon cockle (Monodacna colorata), also known as the colored egg cockle or Azov-Black Sea cockle, is a fresh- and brackish-water bivalve mollusc of the family Cardiidae. It has a broadly oval shell, with oblique triangular ribs and a length of up to 40 mm. Its coloration varies from whitish or yellowish to pinkish or grayish-lilac and is deeper on the interior. The native range of this cockle includes freshened zones and limans of the Black Sea and the Sea of Azov, but since the 1950s it has been introduced to the reservoirs of the Dnieper, Don, Manych and Volga rivers, as well as the Volga Delta, northern part of the Caspian Sea and some water systems of Kazakhstan including the Lake Balkhash. The species is a filter feeder that lives on sandy and sandy-silty sediments at depths up to 8–12 m. It is eaten by many commercial fishes and is also edible to humans.

## Description

The shell of Monodacna colorata is broadly oval, with a weakly protruding umbo, thin but solid, moderately convex, gaping posteriorly and anteriorly (anterior gape may be weakly developed), up to 40 mm long and up to 32 mm high with convexity up to 19 mm. It is covered with 21–36 oblique triangular radial ribs which are sharply angled on the posterior margin and in the middle portion of the shell. The exterior is whitish, yellowish, pinkish-white, reddish-brown or grayish-lilac with thin greyish green periostracum. The internal coloration is deeper, often yellowish, brownish or grayish mauve. The hinge of each valve consists of one cardinal tooth developed to varying degrees. The pallial sinus is wide and shallow, extends up to 1/4–1/3 of the shell length or is slightly deeper.

## Distribution

The colored lagoon cockle is native to freshened zones and limans of the Black Sea and the Sea of Azov. It occurs in lakes Kagul, Yalpuh, Kugurlui, and in the Razelm-Sinoe Lake complex of the Danube Delta, as well as in the Kuchurgan, Dniester and Dnieper-Bug limans, in the Taganrog Bay, the Don Delta, the Kirpilsky, Kurchanskiy, Konovalovskiy, Kulikovskiy, and Polyakov limans of the Kuban Delta and in the Temryuk Bay. The species has also spread into the Sasyk Lake when it became less saline after getting connected to the Danube river via a canal in 1978.
In the 20th century land conversion led to the disappearance of M. colorata from the lake Brates of the Danube Delta. In Tylihul Estuary the species went extinct due to salinity increase caused by construction of a canal which connected the liman with the Black Sea. In the early 20th century M. colorata lived in the Berezan Estuary and has since disappeared from some known localities although this area has not been fully studied. Its empty shells have been found in the Lake Varna in Bulgaria where it possibly lived before salinity increase in the first half of the 20th century. In the Yeysk Liman the species was last recorded in the 2000s where it subsequently went extinct due to salinization and eutrophication. In the early 20th century it was also recorded in the Akhtanizovsky Liman, but has not been found there in later studies.
From 1951 to 1956 the colored lagoon cockle was being released into the Veselovsky Reservoir of the Manych river. Since 1959 the species has been recorded in the northern part of the Caspian Sea in front of the Volga Delta where it was presumably introduced via ships after the opening of the Volga–Don Canal. It has since then spread into the reservoirs of the Dnieper river, over the lower course of the Don and to the Tsimlyansk Reservoir, as well as to the Karpovskoye, Bereslavskoye and Varvarskoye reservoirs of the Volga–Don Canal, the Volgograd, and Saratov reservoirs of the Volga, the Bolshoy Irgiz river, and the Volga Delta. From 1965 to 1970 M. colorata has been stocked in the Kuybyshev Reservoir of the Volga where it didn't survive but was found again in the 21st century.
In Kazakhstan the colored lagoon cockle was intentionally introduced to the Lake Balkhash in 1965 and to the Kapchagay Reservoir in the 1970s. Since then it has spread into other water systems of the country such as the Irtysh–Karaganda Canal.

## Ecology

The colored lagoon cockle lives in fresh and slightly brackish waters on sandy and sandy-silty sediments at depths up to 8–12 m. It occurs in waters with salinity of 0,03–7‰ but can also survive for some time at a salinity level of 12–16‰. As a filter feeder it consumes diatoms (Navicula, Coscinodiscus etc.) and detritus. In its native habitats this species forms communities with Dreissena mussels and with brackish water cockles Adacna fragilis and Hypanis plicata.
M. colorata usually starts maturing at the age of two years at a length of 10 mm. Three year old specimens with a length of 20 mm are considered adult. The species reproduces from late April to September. Females produce about 200,000 eggs and release from several dozen to 50–70 eggs at once. At a temperature of 11–24 °C development from embryo to veliger takes one day. After 14–30 days the veliger settles to the bottom.
An unidentified trematode is known to parasitize on M. colorata.

## Uses

Colored lagoon cockles are eaten by many commercial fishes including Azov roach (Rutilus heckelii), common bream (Abramis brama), sturgeons and larvae of Clupeonella. In addition to the above-mentioned deliberate introductions, in 1964 and 1965 the species was released into the Aral Sea where it did not establish. In 1948 and 1960 it has been proposed to introduce M. colorata into the Caspian Sea where it was subsequently accidentally transported by ships.
The species is also considered to be edible to humans.

## Conservation

The conservation status of this species has not been assessed by the International Union for Conservation of Nature. In the Razelm-Sinoe Lake complex M. colorata and other brackish water species are declining due to a salinity decrease caused by closing of Black Sea inlets and rerouting of Danube waters in the second half of the 20th century which in turn has increased the abundance of freshwater species. The Kuchurgan Liman population is included in the Red Book of Moldova under the "endangered" category as it is threatened by pollution and changes in hydrological cycles. In Russia the species is listed in the Red Data Book of Krasnodar Krai (2017) under the "vulnerable" category since the single surviving population in this region was only known from the Kirpilsky Liman. However, it has also been recorded in other areas of Krasnodar Krai in the south-eastern coast of the Sea of Azov, although the current status of these populations is unknown.

## Taxonomy

The species was first described by Karl Eichwald in 1829 as Glycymeris colorata. Its type locality is "Hab. Hypanin fluvium, ad nigrum usque mare" which, according to different authors, either refers to the Kuban River or to the lower course of the Yuzhny Bug River. Type specimens of M. colorata have not been traced.
Monodacna pontica differs from M. colorata by more rounded ribs and less vivid coloration. Wesselingh et al. (2019) treated M. pontica as a synonym of M. colorata attributing differences in shell characteristics to different substrate preferences. J. ter Poorten (2024) tentatively treated M. pontica as a distinct species pending further research.
I. Borcea (1926) described the species Adacna luciae and several varieties of M. colorata including ialpugensis, angusticostata, lucida and razelmiana. In the Black Sea basin most authors recognized 2 species of the genus Monodacna (which was sometimes treated as a subgenus of Hypanis): M. colorata and M. pontica. Scarlato and Starobogatov (1972) also included Hypanis ialpugensis, H. angusticostata and H. luciae while razelmiana was considered to be a partial synonym of H. angusticostata and H. colorata. Munasypova-Motyash (2006) only recognized H. colorata, H. pontica and H. angusticostata separating them by anatomical differences. Molecular studies by Popa et al. (2011) have shown that H. angusticostata is conspecific with H. colorata. All of the mentioned taxa introduced by Borcea as well as the variety tanaisiana described by Milaschewitsch (1916) are currently considered to be synonyms of M. colorata.