Invasive Ascidians and Marine Biodiversity Dynamics

Ascidians, commonly known as sea squirts, are sessile tunicates that have become among the most pervasive marine invaders worldwide. Their success stems from a combination of rapid reproduction, robust larval dispersal and strong tolerance of variable environmental conditions. Ascidians often attach to artificial substrates such as ship hulls, aquaculture gear and harbour structures, leading to extensive biofouling that alters native benthic communities. Invading populations can outcompete indigenous filter feeders, modify nutrient cycles through altered filtration rates and even change the physical complexity of benthic habitats. Human activities—particularly global shipping and the transfer of live shellfish—have accelerated range expansions, resulting in recurrent introductions across temperate and tropical coastlines. The ecological consequences are multifaceted, including reduced native species richness, facilitation of secondary invasions and economic impacts on fisheries and aquaculture. Understanding the drivers of establishment and spread, as well as the ecological interactions between introduced ascidians and recipient communities, is crucial to inform effective management and mitigation strategies.

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Invasive Ascidians and Marine Biodiversity Dynamics publication trend

The graph below shows the total number of articles in invasive ascidians and marine biodiversity dynamics across all publications each year (not limited to Nature Index journals).

Technical terms

Ascidians: Sessile marine tunicates characterised by a sac-like body and filter-feeding through an incurrent and excurrent siphon.

Non-indigenous species (NIS): Organisms introduced by human activities beyond their native range, often establishing self-sustaining populations.

DNA barcoding: A molecular identification technique using a standard gene fragment to distinguish between species and detect cryptic diversity.

Phenotypic plasticity: The capacity of an organism to alter its physiology, morphology or life-history traits in response to environmental variation.

Biofouling: The undesirable accumulation of organisms on submerged surfaces, affecting vessel performance and local ecosystems.

References

1. Effects of color variation and physiological state on ascidian microbiomes. MicrobiologyOpen (2024).
2. Cryptic Diversity and Database Errors Challenge Non-indigenous Species Surveys: An Illustration With Botrylloides spp. in the English Channel and Mediterranean Sea. Frontiers in Marine Science (2019).
3. Ongoing expansion of the worldwide invader Didemnum vexillum (Ascidiacea) in the Mediterranean Sea: high plasticity of its biological cycle promotes establishment in warm waters. Biological Invasions (2015).