Xenostrobus securis
(Lamarck, 1819)

Relevant Synonyms
Modiola securis Lamarck, 1819


 photo: S. Gofas / Coll. G. Spada    

Shell equivalve, subcylindrical. Outline modioliform. Periostracum smooth and shining. Dorsal margin usually straight, ventral margin straight or slightly arcuate (distinctly arcuate in older specimens) with posterior end evenly rounded. Umbones nearly terminal. Smooth sculpture. Periostracum shining. Internal margin smooth.

color : shell brown, dark brown in older specimens; pale yellow zigzag lines in younger specimens. Internally the shell is usually purple above and white below the umbonal keel.

common size : 20-30 mm length. Height/length ratio=0.59, width/length=0.35, max. observed length 47 mm (Russo, 2001).

Remarkably similar to X. pulex (Lamarck, 1819), a south Australian species, from which it differs in the following characteristics: exteriorly above the umbonal keel the shell is brown (black in X. pulex), often paler below the keel; the umbones are terminal in X. pulex, nearly terminal in X. securis. Mature specimens of X. securis are significantly narrower than X. pulex.

Found exclusively in brackish waters, this species can tolerate very broad variations in salinity (1‰ to 31‰). X. pulex is strictly a marine species.

habitat : in upper reaches of estuaries where salinity is low for a considerable part of the year. In association with Mytilus galloprovincialis, attached with a byssus on hard substrata or oyster shells in numerous colonies. Specimens are found attached on algae, in masses of timber, stones or dead shells.

1st Mediterranean record
Adriatic Sea, 1994 [1992].

Worldwide: Australia, New Zealand, SE Asia. Mediterranean: recorded first in 1992 in Ravenna lagoon, Italy, Adriatic Sea (Lazzari and Rinaldi, 1994); successive reports in Italy from Venice lagoon as Xenostrobus sp. (Sabelli and Speranza, 1994), Porto Buso (De Min and Vio, 1997), delta of the river Po (Russo, 2001). In French Mediterranean lagoons, massive presence in the canal between Etang du Vidourle and the sea, at Le Grau du Roi (Pelorce, identification confirmed by S. Gofas on the basis of material).

Invasive where established, forming numerous colonies. Extraordinary concentrations have been recorded in the delta valley of the river Po.

speculated reasons for success :
great adaptability to different ecosystems from open lagoons to connector canals; can form massive colonies by attaching to any kind of hard substratum or to pure sand or sandy mud.

Introduced accidentally with aquaculture; its massive presence in the Adriatic lagoons along with Anadara inaequivalvis is related to intense shellfish farming in the area (Occhipinti-Ambrogi, 2000).

May have a negative impact on lagoonal ecosystems when it becomes invasive, covering soft sediments and smothering the infauna.


  • De Min R. and Vio E., 1997. Molluschi conchiferi del litorale sloveno. Annals for Istran and Mediterranean Studies, Koper, Annales 11, Serie historia naturalis, 4: 241-258.
  • 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.
  • Russo P., 2001. Great concentration of Xenostrobus securis (Lamarck, 1819) in different areas of the Po River Delta. La Conchiglia, 298: 49-50.


  • Sabelli B. and Speranza S., 1994. Rinvenimento di Xenostrobus sp. (Bivalvia: Mytilidae) nella laguna di Venezia. Bollettino Malacologico, 29(9-12): 311-318.


Last update : December 2003

©ciesm 2002