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The Nonindigenous Occurrences section of the NAS species profiles has a new structure. The section is now dynamically updated from the NAS database to ensure that it contains the most current and accurate information. Occurrences are summarized in Table 1, alphabetically by state, with years of earliest and most recent observations, and the tally and names of drainages where the species was observed. The table contains hyperlinks to collections tables of specimens based on the states, years, and drainages selected. References to specimens that were not obtained through sighting reports and personal communications are found through the hyperlink in the Table 1 caption or through the individual specimens linked in the collections tables.

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Common name: axe-head mussel

Synonyms and Other Names: pygmy mussel, little brown mussel, Modiola securis Lamarck, 1819

Identification: The bivalve has a periostracum, outermost layer of the shell, that is smooth and shining with a blue to brown color.  The shell is elongated with smooth sculpure. Dorsal margin of the shell, near the hinge, is usually straight, ventral margin straight or slightly curved (distinctly curved in older specimens). The posterior end evenly rounded with the umbones (raised, rounded part where the two shells of a mollusk join) nearly terminal. Internally the shell is usually smooth with purple above and white color below the umbonal keel. Juveniles have characteristic pale yellow zig-zag markings.

Size: 20-30 mm length, maximum observed of 47 mm (Russo 2001)

Native Range: Native to Australia, New Zealand, SE Asia. 

Alaska	Hawaii	Puerto Rico & Virgin Islands	Guam Saipan

Hydrologic Unit Codes (HUCs) Explained
Interactive maps: Point Distribution Maps

Ecology: Xenostrobus securis exclusivley inhabits brackish water and prefers hard substrates, but can be found attached on algae or dead shells, where it often forms extensive colonies. The mussel can be found in the lower reaches of tidal creeks and rivers, as well as the upper reaches of bays and inlet where salinity is low for a considerable part of the year. It can tolerate a salinity range of approximately 1 to 31 parts per thousand and is capable of surviving in freshwater for several months. However, adults tend to remain inactive with their valves closed when salinity drops below about 2 parts per thousand (Wilson 1968).

The mussel reproduces sexually around 15-17mm in size and has demonstrated rapid gametogenesis (sperm and egg development). The buildup of mature gametes in the fall with an extended spawning period with the potential for multiple spawning cycles without pause. Leading to an almost continuous breeding season lasting 10 months in non-native locations, in contrast to the 7 to 8 months observed in its native range (Montes et al. 2020).

Means of Introduction: Ballast water or hull fouling in California. Introduced accidentally with aquaculture in Mediterranean related to intense shellfish farming in the area (Lazzari and Rinaldi 1994) or through the Suez Canal with shipping (Sabelli and Speranza 1994, Mizzan 1999). Introduced to China, Japan, Korea and as larvae in ballast water discharges (Morton and Leung 2015).

Status: Introduced in California in 2024.

Impact of Introduction:

Summary of species impacts derived from literature review. Click on an icon to find out more...

Ecological

References: (click for full references)

Iwasaki, K., and K. Kinoshita. 2004. The Committee for the Preservation of the Natural Environment, the Japanese Association of Benthology 2004b. Range expansion of non-indigenous marine benthos introduced into Japan through human activities. Bull Plankt Soc Jpn 51: 132-144.

Iwasaki, K. 2006. Human-mediated introduction of marine organisms in Japan: a review. Assessment and control of biological invasion risks, 104.

Kimura T., M. Tabe, and Y. Shikano. 1999. Limnoperna fortunei kikuchii Habe, 1981 (Bivalvia: Mytilidae) is a synonym of Xenostrobus securis (Lamarck, 1819): introduction into Japan from Australia and/or New Zealand
Venus, 58, pp. 101-117

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.

Montes, A., Olabarria, C., & Vázquez, E. 2020. Reproductive plasticity in the invasive-Xenostrobus securis (Bivalvia: Mytiloidea) in northwestern Spain. Journal of sea research, 159, 101893.

Morton, B., & Leung, K. F. 2015. Introduction of the alien Xenostrobus securis (Bivalvia: Mytilidae) into Hong Kong, China: Interactions with and impacts upon native species and the earlier introduced Mytilopsis sallei (Bivalvia: Dreissenidae). Marine Pollution Bulletin, 92(1-2), 134-142.

Pastorino, G., Darrigran, G. A., Martin, S. M., & Lunaschi de Redolatti, L. I. 1993. Limnoperna fortunei (Dunker, 1857)(Mytilidae), nuevo bivalvo invasor en aguas del Río de la Plata. Neotropica, 39.

Russo P., 2001. Great concentration of Xenostrobus securis (Lamarck, 1819) in different areas of the Po River Delta. La Conchiglia, 298: 49-50.

Shirafuji, J., & Sato, S. 2003. Benthic communities of tidal flats in Sacheon and Masan, Gyeongsangnamdo, Korea. Investigation report of tidal flat in cooperation with Japan and Korea, 20-25.

Wilson, B. R., 1968. Survival and reproduction of the mussel Xenostrobus securis (Lam.) (Mollusca: Bivalvia: Mytilidae) in a Western Australian estuary: Part I. Salinity tolerance.Journal of Natural History 2: 307-328.

Author: C.R. Morningstar and W. Daniel

This information is preliminary or provisional and is subject to revision. It is being provided to meet the need for timely best science. The information has not received final approval by the U.S. Geological Survey (USGS) and is provided on the condition that neither the USGS nor the U.S. Government shall be held liable for any damages resulting from the authorized or unauthorized use of the information.