Cardita calyculata -

Cardita calyculata is commonly found in the Mediterranean region, as well as around the Canary Islands and the Azores. Further north, its range extends along the Atlantic coasts of Portugal and Spain, with its northern limit roughly at the French coast near the Bay of Biscay ^[2].

First observation in Belgium

The species has not yet been recorded in Belgium to date.

Distribution in Belgium

The species has not yet been recorded in Belgium to date.

Distribution in neighbouring countries

A juvenile specimen of this bivalve was observed for the first time in Zeeuws-Vlaanderen (Western Scheldt, the Netherlands) on April 2, 2021. The species was found attached to the shell of a Japanese oyster (Crassostrea/Magallana gigas) ^[2]. To date, this remains the only recorded observation of a living specimen of this organism in our region.

Since the specimen found in the Netherlands was discovered among a cluster of Japanese oysters, the shellfish industry represents a potential introduction vector, although other methods of introduction cannot be ruled out. Oysters provide an ideal substrate for planktonic larvae to settle on. Therefore, natural spread via migration remains a possibility, as climate change is prompting several species to extend their southern ranges northward. Additionally, shipping (ballast water, biofouling on hulls) also presents a plausible pathway ^[2].

Based on a single observation of a living specimen in Zeeuws-Vlaanderen (the Netherlands), it cannot yet be concluded that the species has established in this region, let alone that it is thriving in our areas.

Although there is currently no scientific evidence of a potentially established presence of this species in Zeeuws-Vlaanderen (the Netherlands), it is true that the milder climate and warmer winters in the southern North Sea region, along with the accompanying rise in sea temperatures, could increase the survival chances of Cardita calyculata in our areas ^[2].

No effects and/or measures against the presence of this species are known.

The animals live in the littoral zone in habitats with hard substrates. They attach themselves to rocks, loose gravel, and shells using byssal threads they produce. The shells of young specimens are initially rounded, then become more rectangular to trapezoidal in shape, and eventually, in mature specimens, they are strongly elongated (stretched) and wider. Adult specimens reach about 3 cm in length ^[2]. The shells are thick, chalky white, often with brown spots. The inside is glossy white. On the outside, they have 17 to 20 radiating ribs, which become wider toward the back, with small scales on them. The left valve has two prominent cardinal teeth and one small lateral tooth, while the right valve has one cardinal and one lateral tooth ^[6].

An analysis of specimens from the coast of Malaga (Spain) showed that all organisms had a bacterial biofilm on their shells ^[3], as previously observed in species belonging to the subclass Archiheterodonta [4]. The observed presence of bacteria in the soft tissues, specifically in the female gonads (on the oocyte), may indicate vertical transmission of bacteria from parents to offspring ^[5], which could serve as an indicator of symbiosis between the two organisms.

[1]          World Register of Marine Species (WoRMS) (2024). Cardita calyculata (Linnaeus, 1758). https://www.marinespecies.org/aphia.php?p=taxdetails&id=139016 (2024-10-18).

[2]          Mulder, G., de Bruyne, R. (2021). Trapezeschelp ontdekt in Nederland. In: Nature Today. Stichting ANEMOON. https://www.naturetoday.com/intl/nl/nature-reports/message/?utm_source=newsletter&utm_medium=e-mail&utm_campaign=user-mailing&msg=27684

[3]          Castro-Claros, J.D.; González-Ruiz, B.; Salas, C. (2019). Bacterial symbiosis in the bivalve Cardita calyculata (L., 1758)?, in: II Congreso de Jóvenes Investigadoros del Mar, Málaga, Spain, 1-4 October 2019. pp. 1-3. [https://www.vliz.be/en/imis?module=ref&refid=391059] 

[4]          Gillan, D.C.; Speksnijder, A.G.C.L.; Zwart, G.; de Ridder, C. (1998). Genetic diversity of the biofilm covering Montacuta ferruginosa (Mollusca, Bivalvia) as evaluated by denaturing gradient gel electrophoresis analysis and cloning of PCR-Amplified gene fragments coding for 16S rRNA. Appl. Environ. Microbiol. 64(9): 3464-3472 [https://www.vliz.be/nl/imis?module=ref&refid=261805] 

[5]          Bright, M.; Bulgheresi, S. (2010). A complex journey: transmission of microbial symbionts. Nat. Rev., Microbiol. 8(3): 218-230. [https://www.vliz.be/en/imis?module=ref&refid=381356] 

[6]          ANEMOON Verspreidingsatlas Weekdieren. Cardita calyculata (Linnaeus, 1758) – Verlengde trapezeschelp. https://www.verspreidingsatlas.nl/S139016 (2024-09-30)

VLIZ Alien Species Consortium (2024). Cardita calyculata. Introduced alien species of the Belgian part of the North Sea and adjacent estuaries anno 2024. Flanders Marine Institute (VLIZ). 4 pp.