than 1491 m m
-2 of stolon with 16125 fronds m
-2
(Žuljević et al., 2003). It drastically displaces
native species, especially in photophilic macroalgae habitat and coralligenous assemblages.
Dense colonies develop in meadows of Cymodocea nodosa and in sparse meadows of
Posidonia oceanica, where it drastically reduces epiphytes on rhizomes (Antolić et al., 2008). It
overgrows and displaces sessile invertebrates, particularly sponges. The alga was observed
growing on the bivalve mollusk noble pen shell, Pinna nobilis, and on colonial coral, Cladocora
caespitosa. On circalittoral sandy bottoms, it develops an extensive and dense network of thalii,
changing the bottom into a meadow of C. cylindracea. Below its dense matte, the increased
accumulation of sediment with organic detritus is usually accompanied by anoxic conditions
and the development of Beggiatoa spp. bacteria (Matijević et al., 2013). The alga probably
reproduces only by fragmentation and spreads primarily using currents. It can be consumed by
Paracentotus lividus, but this sea urchin does not have a significant impact on algal colonies.
The second most invasive macroalga is Acrothamnion preissii. It has been detected in two
distant areas, around the city of Dubrovnik in 2008 and later in the Kornati National Park. In
2022 in Dubrovnik, it developed extremely dense settlements, mainly affecting the rhizomes of
Posidonia oceanica along > 30 km of coastline, from a depth of 5 to 30 meters. Almost no
rhizomes free of A. preissii now exist in the area of Dubrovnik. Its development has
dramatically changed the biodiversity of the Posidonia meadows, though concrete
measurements of its impact are still missing. The current situation indicates its potential further
expansion and high impact on the southern part of the Adriatic. The third most invasive
macroalga is Stypopodium schimperi. Discovered sporadically in 2020 in Vis Island in the
central Adriatic, it still has a limited spatial distribution. However, it has exhibited exceptional
local spreading capabilities. The number of thalii on monitored transects increased from 10 to
several thousands in two years. This has resulted in the predominance of S. schimperi on rocky
bottoms between 8 and 15 m depth. With 40 cm in length and covering 100% of the bottom,
this species totally displaces erect native macroalgae. Considering documented fast spread,
increase in coverage and impact on macroalgae, S. schimperi might soon become the worst
invasive benthic organism in the central and southern parts of the Adriatic.
Acknowledgments
This work was supported by the Croatian Science Foundation under the project HRZZ-IP -
2019-04-6702 and project HIDROLAB, funded by the European Union through the European
Structural and Investment Funds 2014-2020 under contract number KK.01.1.1.04.0053.
Bibliography
ANTOLIĆ B., et al. (2008) - Impact of the invasive green alga Caulerpa racemosa var.
cylindracea on the epiphytic macroalgal assemblage of Posidonia oceanica seagrass
rhizomes in the Adriatic Sea. Nova Hedwigia, 86 (1-2): 155-167.
MATIJEVIĆ S., et al. (2013) - Biogeochemical characteristics of sediments under the canopy of
invasive alga Caulerpa racemosa var. cylindracea (Pelješac Peninsula, Adriatic Sea).
Fresenius Environ. Bull., 22 (10a): 3030-3040.
TSIAMIS K., et al. (2019) - Non-indigenous species refined national baseline inventories: A
synthesis in the context of the European Union's Marine Strategy Framework Directive. Mar.
Poll. Bull., 145: 429-435.
ŽULJEVIĆ A., et al. (2019) - Decline of Caulerpa taxifolia in the Adriatic sea. UNEP/MAP –
SPA/RAC, 2019. Proceedings of the 1 st Mediterranean Symposium on the Non-Indigenous
Species (Antalya, Turkey, 18 January 2019). LANGAR H., OUERGHI A., edits, SPA/RAC
publi., Tunis, 116 p: 82-87.
ŽULJEVIĆ A., ANTOLIĆ B, ONOFRI V. (2003) - First record of Caulerpa racemosa
(Caulerpales: Chlorophyta) in the Adriatic Sea. J. Mar. Biol. Assoc. U. K., 83 (4): 711-712.
2nd Mediterranean Symposium on the Non-Indigenous Species (Genoa, Italy, 22-23 September 2022)
130
-2 of stolon with 16125 fronds m
-2
(Žuljević et al., 2003). It drastically displaces
native species, especially in photophilic macroalgae habitat and coralligenous assemblages.
Dense colonies develop in meadows of Cymodocea nodosa and in sparse meadows of
Posidonia oceanica, where it drastically reduces epiphytes on rhizomes (Antolić et al., 2008). It
overgrows and displaces sessile invertebrates, particularly sponges. The alga was observed
growing on the bivalve mollusk noble pen shell, Pinna nobilis, and on colonial coral, Cladocora
caespitosa. On circalittoral sandy bottoms, it develops an extensive and dense network of thalii,
changing the bottom into a meadow of C. cylindracea. Below its dense matte, the increased
accumulation of sediment with organic detritus is usually accompanied by anoxic conditions
and the development of Beggiatoa spp. bacteria (Matijević et al., 2013). The alga probably
reproduces only by fragmentation and spreads primarily using currents. It can be consumed by
Paracentotus lividus, but this sea urchin does not have a significant impact on algal colonies.
The second most invasive macroalga is Acrothamnion preissii. It has been detected in two
distant areas, around the city of Dubrovnik in 2008 and later in the Kornati National Park. In
2022 in Dubrovnik, it developed extremely dense settlements, mainly affecting the rhizomes of
Posidonia oceanica along > 30 km of coastline, from a depth of 5 to 30 meters. Almost no
rhizomes free of A. preissii now exist in the area of Dubrovnik. Its development has
dramatically changed the biodiversity of the Posidonia meadows, though concrete
measurements of its impact are still missing. The current situation indicates its potential further
expansion and high impact on the southern part of the Adriatic. The third most invasive
macroalga is Stypopodium schimperi. Discovered sporadically in 2020 in Vis Island in the
central Adriatic, it still has a limited spatial distribution. However, it has exhibited exceptional
local spreading capabilities. The number of thalii on monitored transects increased from 10 to
several thousands in two years. This has resulted in the predominance of S. schimperi on rocky
bottoms between 8 and 15 m depth. With 40 cm in length and covering 100% of the bottom,
this species totally displaces erect native macroalgae. Considering documented fast spread,
increase in coverage and impact on macroalgae, S. schimperi might soon become the worst
invasive benthic organism in the central and southern parts of the Adriatic.
Acknowledgments
This work was supported by the Croatian Science Foundation under the project HRZZ-IP -
2019-04-6702 and project HIDROLAB, funded by the European Union through the European
Structural and Investment Funds 2014-2020 under contract number KK.01.1.1.04.0053.
Bibliography
ANTOLIĆ B., et al. (2008) - Impact of the invasive green alga Caulerpa racemosa var.
cylindracea on the epiphytic macroalgal assemblage of Posidonia oceanica seagrass
rhizomes in the Adriatic Sea. Nova Hedwigia, 86 (1-2): 155-167.
MATIJEVIĆ S., et al. (2013) - Biogeochemical characteristics of sediments under the canopy of
invasive alga Caulerpa racemosa var. cylindracea (Pelješac Peninsula, Adriatic Sea).
Fresenius Environ. Bull., 22 (10a): 3030-3040.
TSIAMIS K., et al. (2019) - Non-indigenous species refined national baseline inventories: A
synthesis in the context of the European Union's Marine Strategy Framework Directive. Mar.
Poll. Bull., 145: 429-435.
ŽULJEVIĆ A., et al. (2019) - Decline of Caulerpa taxifolia in the Adriatic sea. UNEP/MAP –
SPA/RAC, 2019. Proceedings of the 1 st Mediterranean Symposium on the Non-Indigenous
Species (Antalya, Turkey, 18 January 2019). LANGAR H., OUERGHI A., edits, SPA/RAC
publi., Tunis, 116 p: 82-87.
ŽULJEVIĆ A., ANTOLIĆ B, ONOFRI V. (2003) - First record of Caulerpa racemosa
(Caulerpales: Chlorophyta) in the Adriatic Sea. J. Mar. Biol. Assoc. U. K., 83 (4): 711-712.
2nd Mediterranean Symposium on the Non-Indigenous Species (Genoa, Italy, 22-23 September 2022)
130