arc clams

  Anadara inaequivalvis
(Bruguière, 1789)

Relevant Synonyms
Arca inaequivalvis Bruguière, 1789
Scapharca inaequivalvis (Bruguière) [Ghisotti and Rinaldi, 1976]

Scapharca cornea (Reeve, 1844)

 photo: E. Rinaldi    

Shell solid, heavy, inflated. Strongly inequivalve in juveniles, less so in adults. Highly variable in shape, shell thickness and convexity of the valves. Height/length ratio = 0.69-0.88, width/length = 0.53-0.84 (Cesari and Pellizzato, 1985). Umbonal area smooth and dark in juvenile specimens. Sculpture of 31 to 34 radial ribs with a mean value of 32.6 ± 1.0 ribs (Morello and Solustri, 2001). Taxodont hinge. Internal ventral margin strongly crenulate. Periostracum persistent.

color : shell white; a velvety, dark brown to black periostracum present near the margins.

common size : 70-75 and up to 80 mm in length, up to 61 mm in height.

Easy to distinguish because of its valves being different in size. However, as in other Anadara, the shells become equivalve with age (Rinaldi, 1994b). Can be confused with A. diluvii (Lamarck, 1805), which differs in the number of radial ribs: 31-34 in A. inaequivalvis versus 26-28 in A. polii and the bigger size (A. diluvii does not exceed 50 mm in length). Differences from A. demiri.

Species eurytherm, euryhaline, that can withstand extreme conditions. Comparative biometric studies in marine and lagoonal populations revealed that under extreme conditions of salinity and temperature (in lagoons) the shell is more fragile - weight about 14% less - than in marine environments and the population density limited (Lazzari and Rinaldi, 1994). Maximum occurrence corresponds to areas with salinity 30 and sandy seabeds (Mizzan, 1999).

habitat : brackish lagoons in the Adriatic. From inshore brackish waters down to 30 m on sand, on rocks; mud and sand with Zostera nana and Cymodocea nodosa.

1st Mediterranean record
Adriatic Sea, Italy, 1973 [1969].

Worldwide: Indo-Pacific, but not in the Red Sea. Mediterranean: reported first in 1969 as Scapharca cfr. cornea from the Adriatic Sea, near Ravenna, Italy (Ghisotti, 1973) from where it rapidly spread.

Invasive. Following a heavy storm, millions of shells were washed out, along with shell of Rapana venosa near Ravenna (see photo below). Both accidentally introduced species, had a major impact on the ecosystem, fast expanding and outcoming local species.

photo: E. Rinaldi

speculated reasons for success :
opportunistic species, resistant to a broad range of conditions.

An accidental introduction into the Adriatic, most likely by shipping. Also accidentally introduced into Venice lagoon. Possibly the population in Lake Faro (Sicily) was first imported from the Adriatic with big stocks of Chamelea gallina (Di Natale, 1982).

The species edible in Japan is not appreciated as food in Italy (Rinaldi, 1994b).


  • Ghisotti F., 1973. Scapharca cfr. cornea (Reeve), ospite nuova del Mediterraneo. Conchiglie, 9(3-4): 68.
  • Lazzari G. and Rinaldi E., 1994. Alcune considerazioni sulla presenza di specie extra Mediterranee nelle lagune salmastre di Ravenna. Bollettino Malacologico, 30(5-9): 195-202.
  • Rinaldi E., 1985b. Alcuni dati significativi sulla proliferazione di Scapharca inaequivalvis (Bruguière, 1789) in Adriatico lungo la costa Romagnola. Bollettino Malacologico, 21(1-4): 41-42.


  • Rinaldi E., 1994b. Molluschi di origine extra Mediterranea entrati a far parte della malacofauna della costa Romagnola. Pagine de Museo Ornitologico e di Scienze Naturali di Ravenna, 19(1-2): 104-108.


Last update : December 2003

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