Marine invertebrates introduced into the Normano-Breton Gulf since 1920

By Laurent GODET, Patrick LE MAO, Eric THIÉBAUT, Christian RETIÈRE, Louis CABIOCH, Franck GENTIL, Nicolas DESROY, Jérôme FOURNIER

L’Atlas Bleu / Protecting

Geography of the Normandy-Breton Gulf and data on its marine invertebrate fauna

The introduction of an invasive alien species is considered to be one of the major causes of disturbance in biological communities.

Even though much is known about the impact of the proliferation of certain species, the history of their establishment and their dispersion is often difficult to determine, a fortiori in the marine domain, given the observation and monitoring challenges in this environment. The Channel, and particularly the Normano-Breton Gulf, is an ideal study area as it has been explored since the 19th century by naturalists and contains a multitude of anthropogenic vectors that could potentially introduce species (e.g. marine cultures, maritime transport, leisure activities).

In the Channel, approximately 6% of benthic macrofauna species are introduced, the number found in the gulf is probably similar. Among the 60,000 data on benthic fauna assembled in a database by the authors of this article, dating from 1820 to the present day and collected throughout the gulf, several thousand concern exotic species.

We discuss the cases of six of these species here, all of which have different histories (deliberate introduction versus accidental introduction, proliferating or non-proliferating species) and are viewed differently by managers and users of the maritime space.

Some examples of species introduced in the Gulf

(click on the pictures to see the distribution map of each species and its evolution in time)

Crassostrea gigas (Thunberg, 1793)
(Bivalvia : Ostreidae)

Originally from East Asia, the Pacific oyster was introduced on a large scale for oyster farming in France in the 1970s, based on mature oysters imported from Canada and spat from Japan. The first signs of this species in the gulf appeared in 1975 in Guernsey. Its presence was then reported sporadically in Jersey or in the Rance before a major expansion in the 2000s; it currently occupies numerous rocky foreshores along the gulf. It has a strong influence on coastal ecosystem diversity and functioning with various socio-economic benefits (tourism, shellfish aquaculture).

Ficopomatus enigmaticus (Fauvel, 1923)
(Polychaeta : Serpulidae)

Supposedly native to Australia, the vectors responsible for the introduction of this species that has colonised numerous estuaries and lagoons around the world are unknown. It prefers brackish water and moderate tides, and it is found in the gulf in sheltered areas, harbour basins and tide mills of the Rance, in the port of Granville, as well as at Paimpol and in the pond at Matignon’s tide mill. The gulf’s strong tides and low availability of brackish areas explain its geographical confinement and its very moderate environmental impacts.

Crepidula fornicata (Linnaeus, 1758)
(Gastropoda : Calyptraeidae)

Native to the eastern coast of the United States, it was introduced into England at the end of the 19th century through transfers of American Pacific oysters. It arrived in the gulf in 1944-45 with Allied ships that cast their anchors in the port of Cherbourg, and then in 1962 in Paimpol with flat oyster seed. Colonisation took place quite quickly and this species is currently widely distributed over seabeds less than 20 m deep, reaching very high densities locally. It profoundly changes the ecosystems in which it becomes established and may threaten fishing and shellfish farming activities in the medium term.

Monocorophium sextonae (Crawford, 1937)
(Crustacea : Corophiidae)

Native to New Zealand, but perhaps a cryptic species, M. sextonae is an amphipod crustacean measuring approximately one cm that lives in tubes of mud built on various natural or artificial substrata. In 1937, Crawford published the first description from samples collected in 1934-35 in Plymouth Bay. This species was reported on the French coast for the first time in 1936 (Trieux and the Rance estuary). Found on most European coasts, it is currently widely distributed in Saint-Brieuc bay, in the Rance, and more sporadically elsewhere in the gulf.

Elminius modestus (Darwin, 1854)
(Crustacea : Austrobalanidae)

E. modestus is a barnacle originating from Australasia. Identified in Portsmouth in 1945, it was introduced to Europe indirectly during the second world war by ships from Australia and New Zealand and rapidly colonised European coasts. Reported in Brittany in 1950, this barnacle became abundant in the west of the reason from 1957. It is found on all rocky shores of the bay and was recently identified in Paimpol. Its absence from certain localities can be explained by low sampling effort. It is known for competing with the native species Semibalanus balanoides.

Venerupis philippinarum (Adams & Reeve, 1850)
(Bivalvia : Veneridae)

This bivalve, introduced into France in 1972, originated in the Indo-Pacific and was first introduced accidently with spat of the Pacific oyster Crassostrea gigas, then later intentionally for shellfish farming. The first recording in the bay dates from the early 1980s in the Rance basin. Today, its distribution is confined to several shellfish farming areas (e.g. Trieux, Rance, the bays of Mont Saint-Michel and Arguenon, the west coast of Cotentin and Jersey). This species does not have a major impact in our region, except in the areas of high abundance, due to the harvesting method.


The dispersive nature of the marine environment and extensive maritime trade mean that many species were introduced into the gulf fairly early, but the rate accelerated after the Second World War, similarly to reports on various other areas along the European coastline.

Given the lack of major port infrastructures, it seems as though the gulf was not a major point of entry for species introduced into Europe or France; instead it appears to have been a region colonised during the expansion stages of species introduced via a natural route (larval dispersal) or via human activities, particularly shellfish farming.

The historical and geographical context is needed not just to see the contemporary impacts that species introduction has on a biological compartment or human activities, but also to understand the dynamics of this introduction and its repercussions at different spatial and temporal scales, and to prepare suitable management plans if necessary.

Among the species presented, some have remained quite geographically confined (F. enigmaticus) while others have undergone proliferation phases since they were introduced into the gulf, some of which were quite dramatic (common slipper limpet, Pacific oyster). The perception of these species has changed over time. The common slipper limpet gives a good example of the a priori ‘positive’ consequences for local benthic diversity, but when it is very proliferic within a site, it may reduce regional diversity by homogenising communities and may present a risk for certain human activities.

Furthermore, although the introduction of some species can be spectacular in some parts of the world, others may be introduced in a completely localised and non-widespread manner, like F. enigmaticus, which multiplies to such an extent in certain South American areas that it clogs power station cooling systems.

Other species, such as the Manila clam, are also confined to certain areas of the gulf, but their very recent introduction and expansion probably cannot be used to draw any conclusions about their possible future distribution areas.

Finally, the invasion of other species (such as E. modestus and M. sextonae) remains almost imperceptible to most actors as it is tricky to identify them and they are small in size.

Evaluation of the risk presented by the introduction of species in the marine domain is therefore complex and must be carried out individually for each species, each region and each stage of colonisation in time and space. Moreover, the current definition of the good environmental status of coastal zones with regard to the Marine Strategy Framework Directive (MSFD) is partly based on this evaluation.

L’article ci-dessus constitue un travail de recherche antérieur à l’édition de l’Atlas de la faune marine invertébrée du golfe Normano-Breton paru en 2019 :

  • LE MAO et al., 2019.  Atlas de la Faune marine invertébrée du golfe Normano-Breton. 7 volumes en coffret. Ed. Station Biologique de Roscoff, 1200 p.
  • Les 7 volumes de cet atlas sont disponibles en téléchargement sur les archives HAL Cnrs (, voir par ex. vol.1 ici

Laurent GODET, Chercheur au CNRS UMR 6554 LETG-Nantes

Patrick LE MAO, Chercheur à l’Ifremer, Cresco, Dinard

Eric THIÉBAUT, Professeur à Sorbonne Université, Station Biologique de Roscoff

Christian RETIÈRE, Professeur retraité du Muséum national d’Histoire naturelle

Louis CABIOCH, Chercheur retraité du CNRS

Franck GENTIL, Maître de conférences retraité de Sorbonne Université

Nicolas DESROY, Chercheur à l’Ifremer, Cresco, Dinard

Jérôme FOURNIER, Chercheur au CNRS, Station Biologique de Concarneau


Laurent GODET, Patrick LE MAO, Eric THIÉBAUT, Christian RETIÈRE, Louis CABIOCH, Franck GENTIL, Nicolas DESROY, Jérôme FOURNIER, « Invertébrés marins introduits dans le golfe Normano-Breton depuis 1920 », L’atlas Bleu, Revue cartographique des mers et des littoraux. Mis en ligne le 11 janvier 2020,

(version digitale adaptée d’après l’article paru dans L’Atlas Permanent de la Mer et du Littoral n°7 « Risques littoraux et maritimes ». Ed. LETG-Nantes, 2015. pp.52-53)


DOI : 10.35109/atlasbleu-eng.10010