Alien marine heterobranchs in the Mediterranean Sea

What is an alien species?

An alien species, also called a non-indigenous species (NIS), is a species that occurs outside its natural geographical range as a result of human activity, either directly or indirectly. In the marine environment, this means that it has reached a region where it would not normally be present without human assistance.

Alien species must be distinguished from native species that naturally expand their distribution. For example, a species may spread from the Atlantic into the Mediterranean through the Strait of Gibraltar. If this movement occurs through natural dispersal, the species is not necessarily alien. By contrast, a species originating in the Red Sea that reaches the Mediterranean through the Suez Canal is generally considered alien because its entry has been facilitated by a human-created connection.

The distinction is not always straightforward. Historical records may be incomplete, the original distribution of a species may be poorly known, and taxonomic revisions can reveal that specimens previously considered to belong to a native species actually represent a different taxon. Consequently, the alien status of a species should be supported by biogeographical, taxonomic and, whenever possible, molecular evidence rather than inferred from its recent appearance alone.

Occasional records, established species and invasive species

Not all alien species become established, and not all established species become invasive. These terms describe different stages or characteristics of a biological introduction.

• Casual or occasional species: individuals have been observed, but there is insufficient evidence that the species reproduces successfully and maintains a population over time. A single specimen, or even several isolated observations, does not demonstrate establishment.
• Established species: the species reproduces in the new region and maintains a self-sustaining population without requiring repeated introductions from its original range. Repeated observations, juveniles, egg masses and records spanning several years can support this interpretation, although no single indicator is conclusive on its own.
• Invasive species: an alien species that spreads and causes, or is likely to cause, significant ecological, economic or other harm. Establishment and invasiveness are therefore not synonyms: a species can be established without having demonstrated a substantial negative impact.
• Cryptogenic species: a species whose native or alien origin cannot be determined confidently with the available evidence.
• Questionable or unconfirmed records: records for which identification, provenance or other essential evidence remains uncertain. These should not be treated as confirmed alien occurrences until the uncertainty has been resolved.

These categories are particularly important for marine heterobranchs. Their small size, cryptic appearance, short life cycles and sometimes seasonal occurrence make them difficult to detect consistently. Furthermore, a species may be common in one locality but represented by only a few records elsewhere. Its status can therefore differ between localities or change as new evidence becomes available.

How do alien species reach the Mediterranean?

The Suez Canal: Lessepsian migration

The opening of the Suez Canal in 1869 created a direct marine connection between the Red Sea and the Mediterranean. The subsequent movement of organisms through this corridor is known as Lessepsian migration.

Many species originating in the Red Sea and Indo-Pacific region have entered the eastern Mediterranean through this route. Some have subsequently spread along the coasts of the Levant, Turkey, Greece and the central Mediterranean, with a smaller number reaching the western basin.

The canal allows organisms to move between connected marine environments. Their initial entry is associated with a human-created corridor, while subsequent dispersal may occur naturally through larval transport, currents or the movement of adults.

Maritime transport

Ships can transport marine organisms between distant regions. Two important mechanisms are:

• Ballast water: water taken aboard to stabilise a vessel can contain larvae and other small organisms. These may be released at the destination.
• Biofouling: organisms attached to hulls, sea chests, propellers and other submerged structures may survive a voyage and be released into a new environment.

For sea slugs and their relatives, transport may involve not only the animals themselves but also their egg masses, prey organisms or the algae and other surfaces on which they live. Ports, marinas and other heavily trafficked coastal areas can consequently act as introduction hotspots.

Aquaculture and the movement of marine organisms

The transfer of oysters, mussels, clams and other cultured organisms can unintentionally move associated species between regions. Small heterobranchs, egg masses and their food organisms may be transported with the cultivated material or its associated equipment.

This pathway is particularly relevant when an alien species appears at several geographically separated sites where shellfish transfers or other human-mediated movements have occurred.

Other possible pathways

The aquarium trade, transport of live marine organisms and movements associated with maritime equipment may also contribute to introductions. However, a plausible pathway is not proof of the actual route followed by a particular species. Unless a study provides supporting evidence, the pathway should be described as possible or suspected, rather than confirmed.

Why can a species survive in a sea where it was previously absent?

Arrival is only the first step. For an alien species to establish itself, it must survive the environmental conditions, find suitable food and habitat, reproduce successfully and produce enough offspring to maintain its population.

Environmental tolerance and temperature

Temperature influences metabolism, feeding, growth, reproduction and larval development. Many tropical and subtropical species are associated with warmer waters, and the eastern Mediterranean can offer conditions suitable for their survival.

Climate change and rising sea temperatures can make previously unsuitable areas more favourable to warm-water species. This process may facilitate establishment and expansion, particularly near the cooler limits of a species’ potential distribution. However, warming alone does not explain every introduction, and it does not guarantee that a species will establish itself. Salinity, oxygen availability, depth, currents and seasonal temperature extremes can remain important constraints.

Food availability and habitat suitability

Heterobranchs often have highly specialised diets. Nudibranchs may feed on particular sponges, cnidarians, bryozoans or other invertebrates, while sacoglossans generally feed on algae and some cephalaspideans exploit other specific food resources.

An alien species can establish itself if its prey or host organisms are already present in the new environment. In some cases, the introduction of an appropriate prey species may indirectly facilitate the arrival of a predator. Conversely, even a species capable of surviving the local temperature may fail to establish if its food requirements cannot be met.

The availability of suitable microhabitats is equally important. Harbours, artificial structures, seagrass meadows, rocky reefs and shallow coastal habitats provide different opportunities for colonisation.

Reproduction and dispersal

The reproductive biology of a species can strongly influence its establishment success. Some heterobranchs produce numerous eggs, while others have more specialised reproductive strategies. Their larvae may develop within the egg mass or enter the plankton before settling.

In species with planktotrophic development, larvae feed on plankton while developing. This can allow a dispersive larval phase, although successful dispersal depends on currents, larval duration and survival. In species with lecithotrophic development, larvae rely primarily on yolk reserves supplied by the egg. Their dispersal potential is often more limited, although it varies among species. Direct development, in which juveniles emerge without a free-swimming larval stage, may further restrict natural dispersal.

These distinctions help explain why some species spread rapidly along coastlines while others remain restricted to a few localities. Nevertheless, transport by ships or aquaculture can overcome the dispersal limitations of species with short-lived or non-planktonic larvae.

Ecological interactions and natural enemies

A species entering a new environment may encounter fewer predators, parasites or competitors than it does in its original range. This is sometimes described as enemy release.

Some heterobranchs also possess chemical defences, acquire defensive compounds from their food or use warning colours to discourage predators. Such characteristics may contribute to survival in a new environment, but their importance must be assessed for each species rather than assumed.

Successful establishment therefore results from the interaction of several factors: the characteristics of the species, the environmental conditions, the available ecological resources and the circumstances of its introduction.

Ecological consequences: does every alien species cause damage?

The arrival of an alien species does not automatically imply ecological damage. Some species remain rare or localised, and their effects may be negligible or simply insufficiently studied. Others become abundant, alter food webs, compete with native organisms or affect habitats and ecosystem functioning.

For heterobranchs, possible effects include:

• Predation: an alien species may consume native organisms or compete with native predators for prey.
• Competition: species with similar food or habitat requirements may compete for limited resources.
• Changes in food webs: a new predator, grazer or prey species may alter interactions among existing organisms.
• Habitat modification: effects may arise indirectly through changes in the abundance of algae, sessile invertebrates or other organisms.
• Introduction of associated organisms: transported animals may carry parasites, pathogens or other species, although this risk needs to be demonstrated rather than presumed.

The ecological effect of an alien species depends on its abundance, distribution, feeding ecology and interactions with the recipient community. A widespread species is not necessarily highly damaging, just as a localised species is not necessarily harmless.

It is also important to distinguish a biological introduction from its consequences. Alien status describes biogeographical origin and introduction history; invasiveness refers to the capacity to spread and cause harm. These are related but separate questions.

Why is the Mediterranean particularly relevant?

The Mediterranean is a semi-enclosed sea connected to the Atlantic through the Strait of Gibraltar and to the Red Sea through the Suez Canal. It also supports intense maritime transport, aquaculture, tourism and other activities that facilitate the movement of marine organisms.

The eastern Mediterranean has been especially affected by the arrival of species from the Red Sea and Indo-Pacific region. Some of these species subsequently spread towards the central and western Mediterranean. Shipping and aquaculture have also introduced species independently of the Suez Canal, meaning that not every alien heterobranch in the Mediterranean is a Lessepsian migrant.

The process is not uniform throughout the basin. The eastern Mediterranean, central Mediterranean, western Mediterranean and Adriatic Sea differ in temperature, circulation, habitats, shipping activity and survey effort. A species recorded in one sub-basin may not yet have reached another, or it may be present but overlooked.

Consequently, the apparent number of alien species depends not only on biological introductions but also on the intensity of research, the accessibility of records and the taxonomic expertise available. New observations may reveal genuine range expansions, but they can also correct previous misidentifications or clarify the status of long-known species.

How should alien heterobranch records be interpreted?

A reliable inventory should combine published records, taxonomic revisions, validated observations and, where available, molecular evidence. Photographs are extremely useful for documenting distribution, but they do not always allow confident identification, particularly in species complexes with similar external appearances.

Records should ideally include the locality, date, identification evidence and original source. Repeated observations at the same site can indicate persistence, but proof of establishment is stronger when observations also demonstrate reproduction, juveniles or populations maintained over several generations.

The same care is needed when assigning a Mediterranean sub-basin. A record from the Aegean Sea, for example, should not automatically be interpreted as evidence that the species occurs throughout the eastern Mediterranean. Likewise, a species established in Italy may still be only occasional elsewhere.

Checklist, biogeographical status and taxonomic uncertainties

Scope and classification criteria

The Mediterranean Sea hosts an increasing number of non-indigenous marine heterobranchs. A reliable checklist must assess taxonomic identity, native distribution, introduction pathways and establishment evidence separately. Historical misidentifications, uncertain provenance and incomplete distribution data can otherwise inflate estimates of alien diversity.

This inventory is a critical working list rather than a complete census of every alien heterobranch reported from the Mediterranean. It uses three mutually exclusive categories:

1. Main checklist: taxa with sufficiently supported Mediterranean alien status, reliable identification and confirmed occurrence. Establishment status is reported separately.
2. Auxiliary checklist: taxa reported as alien or non-indigenous in regional inventories or recent studies but requiring further assessment of distribution, establishment, record quality or introduction pathway. These taxa are excluded from the main checklist count.
3. Pending verification: taxa for which identity, Mediterranean occurrence or alien status remains unresolved. They are excluded from confirmed alien counts until the relevant uncertainty is resolved.

A taxon is included in the main checklist only when its identification and Mediterranean occurrence are adequately supported and an alien interpretation is more plausible than a native, naturally expanding or cryptogenic one. Establishment is assessed independently. Repeated records, juveniles, reproduction and persistence support establishment, whereas a single observation or shell record demonstrates presence only.

The classification follows general principles used in Mediterranean inventories of non-indigenous species while recognising that terminology and evidence thresholds vary among publications (Crocetta et al., 2017; Zenetos & Galanidi, 2020).

Terminology

An alien species, also called a non-indigenous species (NIS), occurs outside its natural geographical range as a result of human activity, either directly or indirectly. A species reaching the Mediterranean through the Suez Canal may be considered alien because the canal created a human-mediated connection between previously separated marine regions. Species entering through the Strait of Gibraltar by natural dispersal are not necessarily alien.

An established alien species maintains a self-sustaining population in the recipient region. Repeated observations, juveniles and reproductive activity can support this classification, although evidence of reproduction and persistence is stronger than repeated observations alone.

A casual alien species has been recorded outside its native range but has not been shown to maintain a self-sustaining population. An isolated observation may confirm presence without demonstrating establishment.

A cryptogenic species has uncertain native or alien status and is excluded from the confirmed alien count.

A taxonomically uncertain record is one for which identification is insufficiently secure. Such records are placed in the pending-verification category and are not counted as confirmed species occurrences.

The status assigned below refers to the Mediterranean as a whole only when the evidence supports that interpretation. Otherwise, local establishment, casual occurrence or uncertain establishment is stated explicitly.

Main checklist of alien marine heterobranchs

The following table contains taxa with sufficiently reliable identification, occurrence and alien status for inclusion in the confirmed inventory. Establishment is reported separately and should not be inferred from inclusion in the table.

Group	Species	Mediterranean sub-basin	Establishment status	Key references
Anaspidea	Aplysia dactylomela	Mediterranean-wide, with records from several sub-basins	Confirmed alien; establishment supported in examined populations	Alexander & Valdés, 2013; Valdés et al., 2013
Anaspidea	Bursatella leachii	Mediterranean-wide in the available population-genetic evidence	Alien; establishment supported in examined populations	Bazzicalupo et al., 2018
Anaspidea	Syphonota geographica	Eastern Mediterranean; additional records in other sub-basins	Established	Zenetos, 2005; Zenetos & Galanidi, 2020
Cephalaspidea	Biuve fulvipunctata	Eastern Mediterranean; records extending into the Adriatic	Established locally	Zenetos & Galanidi, 2020
Cephalaspidea	Bulla arabica	Eastern Mediterranean; additional records require regional validation	Established locally	Malaquias & Reid, 2008; Crocetta et al., 2013
Cephalaspidea	Haloa japonica	Central Mediterranean; additional western Mediterranean records require verification	Established locally	Crocetta et al., 2013
Cephalaspidea	Lamprohaminoea ovalis	Eastern Mediterranean; western Mediterranean	Established locally	Yokeş & Rudman, 2004; Crocetta et al., 2013
Nudibranchia	Baeolidia moebii	Eastern Mediterranean; expanding towards the central and western Mediterranean	Established and spreading	Paz-Sedano et al., 2019; Zenetos & Galanidi, 2020
Nudibranchia	Coryphellina rubrolineata	Eastern Mediterranean	Established	Zenetos, 2005; Zenetos & Galanidi, 2020
Nudibranchia	Goniobranchus annulatus	Eastern Mediterranean; additional records in the central and western Mediterranean require verification	Established locally	Yokeş & Rudman, 2004; Crocetta et al., 2013
Nudibranchia	Goniobranchus obsoletus	Eastern Mediterranean, particularly the Levant	Established locally	Yokeş & Rudman, 2004; Zenetos & Galanidi, 2020
Nudibranchia	Goniodoridella picoensis	Western Mediterranean, including the Alboran Sea and Malta; widespread throughout the western basin	Confirmed alien; established and common throughout the western Mediterranean	Pola et al., 2019; subsequent Mediterranean records
Nudibranchia	Hypselodoris infucata	Eastern Mediterranean	Established	Yokeş & Rudman, 2004; Zenetos & Galanidi, 2020
Nudibranchia	Melibe viridis	Eastern and central Mediterranean; wider distribution reported	Established	Zenetos, 2005; Crocetta et al., 2013
Nudibranchia	Plocamopherus ocellatus	Eastern Mediterranean	Established locally	Yokeş & Rudman, 2004; Zenetos & Galanidi, 2020
Nudibranchia	Plocamopherus tilesii	Eastern Mediterranean, including southern Turkey	Established locally	Zenetos & Galanidi, 2020
Nudibranchia	Polycera hedgpethi	Central Mediterranean and Adriatic; additional records elsewhere	Established locally	Crocetta et al., 2013; Poursanidis & Cattaneo-Vietti, 2016; Toso et al., 2025
Sacoglossa	Elysia nealae	Eastern Mediterranean; reported from additional Mediterranean localities	Establishment requires assessment	Yokeş & Rudman, 2004; subsequent regional records require verification
Sacoglossa	Oxynoe viridis	Eastern Mediterranean	Establishment requires assessment	Yokeş & Rudman, 2004

Geographical entries are deliberately conservative. A record from one locality should not be interpreted as evidence of occurrence throughout an entire sub-basin. The distributions attributed to Bulla arabica, Elysia nealae, Goniobranchus annulatus and Lamprohaminoea ovalis should therefore be checked against the original locality records before quantitative analyses.

Auxiliary checklist: reported alien taxa requiring additional assessment

The following taxa are retained as reported alien or non-indigenous records but are excluded from the main checklist count. They are included when identification and occurrence are sufficiently credible for continued consideration, while establishment, geographical extent, record history or pathway remains unresolved. Shell-only records demonstrate the occurrence of material, not a living established population.

Group	Species	Mediterranean sub-basin	Reason for auxiliary treatment	Key references
Cephalaspidea	Acteocina mucronata	Eastern Mediterranean, including the Aegean coast of Turkey	Living specimens are reported, but shell records and basin-wide establishment require separate assessment	van Aartsen et al., 1990; Bitlis & Öztürk, 2022
Cephalaspidea	Camachoaglaja africana	Central Mediterranean, including Salento	Recorded at Tricase Porto; establishment is uncertain	Toso et al., 2025
Cephalaspidea	Mnestia girardi	Eastern Mediterranean	Living specimens are reported, while some older records concern shells	Eastern Mediterranean alien-species inventories
Cephalaspidea	Pyrunculus fourierii	Eastern Mediterranean	Records include shells; establishment must be assessed independently	Eastern Mediterranean alien-species inventories
Nudibranchia	Anteaeolidiella lurana	Central Mediterranean, including Salento	Recorded in ports; extent of establishment remains uncertain	Toso et al., 2025
Nudibranchia	Bermudella polycerelloides	Central Mediterranean, including Salento	Confirmed in port surveys, but broader establishment remains unresolved	Toso et al., 2025
Nudibranchia	Caloria indica	Mediterranean records require locality-by-locality confirmation	Reported in regional literature; records and establishment require review	Mediterranean regional inventories; original records to be checked
Nudibranchia	Cuthona perca	Adriatic Sea	Historical record; current occurrence and establishment are unclear	Zenetos et al., 2016
Nudibranchia	Dermatobranchus rubidus	Adriatic Sea	Two published localities; a self-sustaining population is unconfirmed	Knapič et al., 2024
Nudibranchia	Godiva quadricolor	Central Mediterranean, including southern Italy	Local establishment is reported, but the full distribution requires checking	Mediterranean regional records; original localities to be verified
Sacoglossa	Thuridilla mazda	Western Mediterranean, including southern Spain	Occurrence is reported, but the first record alone does not demonstrate establishment	Mediterranean alien and cryptogenic species literature

The five non-indigenous taxa identified in the 2025 survey of Salento harbours were Anteaeolidiella lurana, Bermudella polycerelloides, Camachoaglaja africana, Polycera hedgpethi and Stiliger cf. auarita. Polycera hedgpethi is included in the main checklist because its identity and alien status are sufficiently supported. Stiliger cf. auarita remains in the pending-verification category because its identification was explicitly treated as provisional (Toso et al., 2025).

Pending verification: unresolved identity, occurrence or alien status

The following records are excluded from the confirmed alien count. They remain separate because the principal uncertainty concerns identity, occurrence or biogeographical status rather than merely the extent of establishment.

Group	Species	Issue	Decision rule	Key references
Anaspidea	Aplysia parvula	Mediterranean and eastern Atlantic specimens assigned to this name belong to Aplysia punctata according to molecular evidence	Exclude from the confirmed alien list	Golestani et al., 2019
Cephalaspidea	Liloa mongii	Records are affected by identification problems and questionable locality data	Exclude until reliable specimens and localities are documented	Romani et al., 2021
Cephalaspidea	Notodiaphana atlantica	Native or alien origin cannot be established confidently	Classify as cryptogenic, not confirmed alien	Romani et al., 2021
Nudibranchia	Armina rubida	Relevant Adriatic records were published under Dermatobranchus rubidus	Do not count both names; follow the primary source’s accepted name	Knapič et al., 2024
Nudibranchia	Halgerda willeyi	The historical Adriatic specimen was reidentified as Halgerda paulayi	Exclude H. willeyi from the confirmed Mediterranean alien list	Donohoo et al., 2023
Nudibranchia	Stiliger cf. auarita	Salento specimens differ from the original description of S. auarita	Exclude pending integrative taxonomic analysis	Toso et al., 2025
Sacoglossa	Elysia grandifolia	Mediterranean specimens may have been confused with Elysia ornata	Do not count until the material is reassessed	Pasternak & Galil, 2012
Sacoglossa	Elysia tomentosa	The name has been applied to a complex of genetically differentiated species	Do not count without reviewing Mediterranean specimens and applying a resolved taxonomy	Krug et al., 2013
Sacoglossa	Stiliger fuscovittatus	A Mediterranean record has not been verified against a reliable primary source	Treat as an unverified record, not a confirmed alien	Lance, 1962; Mediterranean record requires verification

Biogeographical cases requiring particular attention

Aplysia dactylomela: This species is included in the main checklist as a confirmed alien. Molecular studies indicate that Mediterranean specimens belong to the Atlantic lineage of Aplysia dactylomela, rather than to the Indo-Pacific species now known as Aplysia argus. The evidence supports an Atlantic origin for the Mediterranean populations examined, although the exact introduction pathway remains unresolved (Alexander & Valdés, 2013; Valdés et al., 2013).

Aplysia parvula: Mediterranean records attributed to Aplysia parvula are excluded from the confirmed alien count. Molecular analyses associated the Mediterranean and eastern Atlantic specimens examined with Aplysia punctata. This case demonstrates that published names must be checked against current taxonomic evidence before records are incorporated into alien-species inventories (Golestani et al., 2019).

Bursatella leachii: This species is included in the main checklist as alien. Population-genetic analyses support an Atlantic origin for the Mediterranean populations investigated, rather than an automatic attribution to a Lessepsian introduction. Because the relevant study lacked Red Sea samples, alternative pathways cannot be excluded with complete certainty (Bazzicalupo et al., 2018).

Notodiaphana atlantica and Liloa mongii: These species illustrate two different reasons for exclusion from the confirmed inventory. Notodiaphana atlantica is cryptogenic because its native or alien origin cannot be established confidently. Liloa mongii remains pending verification because its Mediterranean occurrence is affected by identification problems and potentially incorrect locality data (Romani et al., 2021).

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