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ANNALES · Ser. hist. nat. · 27 · 2017 · 1
short scientifi c article                 DOI 10.19233/ASHN.2017.01
received: 2016-12-23
UNUSUAL EXPANSION OF LAURENCIA OBTUSA (HUDSON) J.V. 
LAMOUROUX IN THE ZAMBRATIJA BAY (NORTHERN ADRIATIC SEA)
Martina ORLANDO-BONACA 
Marine Biology Station, National Institute of Biology, SI-6330 Piran, Fornače 41
E-mail: martina.orlando@nib.si
Roberto ODORICO
SHORELINE Soc.Coop., AREA Science Park, IT-34149 Trieste, Padriciano 99
e-mail: roberto.odorico@shoreline.it
ABSTRACT
The paper reports an unusual expansion of the red alga Laurencia obtusa in shallow waters of the Zambratija 
Bay (northern Adriatic Sea). The large population of L. obtusa was never observed during underwater surveys 
conducted in recent years. In this area, benthic communities dominated by brown algae from the genus Cystoseira 
were present. The authors therefore assume that some environmental factors have probably changed and have 
favoured the extension of this red alga. The hypothesis for this phenomenon takes into account the possibility that 
the establishment of the invasive green alga Caulerpa cylindracea in the Zambratija bay has had a negative impact 
on native macroalgal assemblages.
Key words: Laurencia obtusa, monospecifi c population, Zambratija Bay, Adriatic Sea
ESPANSIONE INSOLITA DI LAURENCIA OBTUSA (HUDSON) J.V. LAMOUROUX 
NELLA BAIA DI ZAMBRATTIA (ADRIATICO SETTENTRIONALE)
SINTESI
L’articolo riporta un’insolita espansione dell’alga rossa Laurencia obtusa in acque poco profonde della baia di 
Zambrattia (Adriatico settentrionale). Ampi popolamenti di L. obtusa non sono mai stati osservati durante le indagini 
subacquee effettuate negli ultimi anni. In quest’area erano presenti comunità bentoniche dominate da alghe brune 
del genere Cystoseira. Gli autori pertanto suppongono che alcuni fattori ambientali siano cambiati e abbiano favorito 
l’estensione di quest’alga rossa. L’ipotesi per questo fenomeno prende in considerazione la possibilità che l’insedia-
mento dell’alga verde invasiva Caulerpa cylindracea nella baia di Zambrattia abbia avuto un impatto negativo sulle 
associazioni macroalgali autoctone.
Parole chiave: Laurencia obtusa, popolamento monospecifi co, baia di Zambrattia, mare Adriatico
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Martina ORLANDO-BONACA & Roberto ODORICO: UNUSUAL EXPANSION OF LAURENCIA OBTUSA (HUDSON) J.V. LAMOUROUX IN THE ..., 1–6
INTRODUCTION
The red alga Laurencia obtusa (Hudson) J.V. Lam-
ouroux is commonly found in the Adriatic Sea, from 
the northern to the central and southern part of the 
basin (Avčin et al., 1973, 1974; Matjašič et al., 1975; 
Giaccone, 1978; Munda, 1979; Vukovič, 1980, Gómez 
Garreta et al., 2001; Antolić et al., 2011; Curiel et al., 
2012). The species usually grows on rocky substrata 
moderately exposed to wave actions in the lower me-
diolittoral belt, and with other photophilous taxa in the 
association Cystoseiretum crinitae Molinier 1958 in the 
upper infralittoral belt (Giaccone et al., 1994). However, 
it was found also on rocky outcrops in the lower infralit-
toral and circalittoral belts in the northern Adriatic Sea 
(Curiel et al., 2012). 
According to the available literature, L. obtusa was 
never found forming dense and extended monospecifi c 
populations in the Adriatic Sea. However, dense stands 
of Laurencia species were reported for the Florida Bay 
(Zieman et al., 1989) and for the Gulf of Siam in the 
South China Sea (Latypov, 1986). The aim of the present 
paper is to report the fi nding of a dense monospecifi c 
population of L. obtusa in the Zambratija Bay (northern 
Adriatic Sea) and to discuss the possible factors that led 
to this phenomenon.
MATERIAL AND METHODS
The Zambratija Bay is located in the northwestern 
part of the Istrian peninsula, near Cape Savudrija, in 
Croatia (Fig. 1). The sea bottom is predominantly rocky, 
locally mixed sandy-rocky (Sladonja & Banovac-Kuča, 
2014). A recent study concerning the reduction of 
chlorophyll a concentrations in a wider area, consistent 
with the decrease in concentrations of phosphate and 
ammonia (Solidoro et al. 2009; Mozetič et al. 2012), 
underlined the oligotrophication of the whole northern 
Adriatic basin over the last decade (Mozetič et al. 2010). 
In September 2016, during a scuba diving survey 
in the upper-infralittoral belt of the Zambratija Bay, a 
dense monospecifi c population of L. obtusa were found. 
The area covered by the species was photographed and 
fi lmed. Its extension was measured by the use of satellite 
imagery (Google Earth) and the GIS program Manifold®. 
Fresh samples were taken to the laboratory of the 
Marine Biology Station (NIB) in Piran and fi xed in 4% 
formaldehyde solution. The species identifi cation was 
made using a stereomicroscope Olympus SZH-ILLK, 
and a microscope Olympus BX51, and was in accor-
dance with Falace et al. (2013) and Rodríguez-Prieto et 
al. (2013). 
RESULTS AND DISCUSSION
In the middle part of the Zambratija Bay, a dense 
monospecifi c population of the invasive non-indigenous 
species Caulerpa cylindracea Sonder was found in 2016, 
but the record is not new, since the Zambratija Bay is 
currently considered the northernmost site where the 
establishment of C. cylindracea was confi rmed earlier 
(Sladonja & Banovac-Kuča, 2014; Iveša et al., 2015). 
But at two sites, which can be considered the entrance 
and the exit points of the bay, a dense monospecifi c 
population of L. obtusa (Fig. 2) were recorded for the 
fi rst time. The two sites were characterized by carpets 
of interwoven thalli of this red alga, where holes made 
by fi sh species were evident (Fig. 3). In these cavities the 
presence of blennies was observed, mostly Parablennius 
tentacularis (Brünnich, 1768) and Parablennius rouxi 
(Cocco, 1833). The extension of the area covered by L. 
obtusa was estimated to be about 3215 m2 at site 1 and 
11618 m2 at site 2 (Fig. 1), in a depth range from 1.0 m 
to 2.5 m. When thalli of L. obtusa were manually lifted 
from the substrate, thalli of Halimeda tuna (J. Ellis & 
Solander) J.V. Lamouroux were found at the sea bottom. 
The segments of thalli of this green alga were mostly 
whitish, which is normal after the release of gametes, 
when the outer involucres remain light due to the 
presence of calcium carbonate in cell walls (Lipej et al., 
2016). However, in this case the whitish colour of thalli 
could also be related to the overgrowing by L. obtusa 
and the consequent reduction of the light that reaches 
the green alga. 
Fig. 1: The map of the Zambratija Bay with the sites 
(1 and 2) where a dense monospecifi c population of 
Laurencia obtusa were found. 
Sl. 1: Zemljevid zambratijskega zaliva z označenima 
lokalitetama (1 in 2), kjer je bila ugotovljena gosta 
monospecifi čna populacija vrste Laurencia obtusa.
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Martina ORLANDO-BONACA & Roberto ODORICO: UNUSUAL EXPANSION OF LAURENCIA OBTUSA (HUDSON) J.V. LAMOUROUX IN THE ..., 1–6
The wide population of L. obtusa was never ob-
served during underwater surveys conducted in recent 
years. In this area, benthic communities dominated by 
brown algae from the genus Cystoseira (mainly C. com-
pressa (Esper) Gerloff & Nizamuddin)), Padina pavon-
ica (Linnaeus) Thivy, and H. tuna were present (pers. 
obs.). Therefore, we assume that some environmental 
factors have probably changed and have favoured 
the extension of this red alga. Our hypothesis for this 
phenomenon takes into account the possibility that the 
establishment of the invasive green alga C. cylindracea 
in the Zambratija bay has had a negative impact on 
native macroalgal assemblages, as reported by Bulleri et 
al. (2010). According to Bulleri et al. (2016), the species 
is able to enhance the competitive ability of algal turfs in 
respect to canopy-forming and encrusting macroalgae, 
by the alteration of some abiotic factors, like the en-
hanced trapping of sediments. Therefore, the spreading 
of C. cylindracea in the Zambratija bay, may have led to 
the disappearance of Cystoseira spp. and other canopy 
forming macroalgae, with the consequent dense growth 
of the lower vegetation cover. Such signifi cant changes 
at the community level may have favoured the unusual 
formation of monospecifi c stands of L. obtusa. Addition-
ally, Bulleri et al. (2016) reported that at pristine sites, 
the prevention of the re-invasion of C. cylindracea on 
experimental cleared plots, had favoured the recovery 
of canopy-forming and encrusting macroalgae.
However, in order to test this hypothesis, other 
sites along the Istrian coast, where the establishment 
of C. cylindracea was already confi rmed (Sladonja & 
Banovac-Kuča, 2014; Iveša et al., 2015), should also be 
surveyed in the nearby future. This is needed to verify 
all the changes in the composition and density of native 
macroalgal assemblages. 
ACKNOWLEDGEMENTS
The authors would like to thank dr. Monica Moras 
and her students Elisabetta Borlina, Antonio Callegaro, 
Giacomo Cattaruzza, Elena Del Pup, Anthea Isola, 
Veronica Mariuz, Stefania Mazzega, Francesco Porracin 
Fig. 2: Details of the apical portion of Laurencia obtusa; scale bar: 5 mm (photo: Martina Orlan-
do-Bonaca). 
Sl. 2: Podrobnosti apikalnega dela vrste Laurencia obtusa; velikostna lestvica: 5 mm (foto: Martina 
Orlando-Bonaca).
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ANNALES · Ser. hist. nat. · 27 · 2017 · 1
Martina ORLANDO-BONACA & Roberto ODORICO: UNUSUAL EXPANSION OF LAURENCIA OBTUSA (HUDSON) J.V. LAMOUROUX IN THE ..., 1–6
from the Liceo Scientifi co Vendramini (Pordenone, Italy) 
that collaborated in the fi eld work during their summer 
campus. Special thanks go to Milijan Šiško for the prepa-
ration of Figure 1, and to two anonymous reviewers for 
their suggestions and comments. 
The fi rst author acknowledges the fi nancial support 
from the Slovenian Research Agency (research core 
funding No. P1-0237).
Fig. 3: A dense monospecifi c population of Laurencia obtusa were found in the Zambratija Bay 
(northern Adriatic Sea) in September 2016. The holes in the carpet of interwoven thalli were made 
by fi sh species, mainly blennies (photo: Monica Moras). 
Sl. 3: Gosta monospecifi čna populacija vrste Laurencia obtusa, opažena v zambratijskem zalivu 
(severni Jadran) septembra 2016. Rove v preprogi steljk so naredile ribe, predvsem babice (foto: 
Monica Moras).
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NENAVADNO RAZŠIRJANJE ALGE LAURENCIA OBTUSA (HUDSON) J.V. LAMOUROUX 
V ZAMBRATIJSKEM ZALIVU (SEVERNI JADRAN)
Martina ORLANDO-BONACA 
Marine Biology Station, National Institute of Biology, SI-6330 Piran, Fornače 41
E-mail: martina.orlando@nib.si
Roberto ODORICO
SHORELINE Soc.Coop., AREA Science Park, IT-34149 Trieste, Padriciano 99
e-mail: roberto.odorico@shoreline.it
POVZETEK
Avtorja poročata o nenavadnem razširjanju rdeče alge Laurencia obtusa v plitvinah zambratijskega zaliva (severni 
Jadran). V zadnjih letih nista nikoli imeli priliko opazovati velike populacije te alge. V tem predelu so prevladovale 
rjave alge iz rodu Cystoseira. Avtorja domnevata, da so spremembe v okoljskih dejavnikih verjetno spodbudile 
razširjanje te rdeče alge. Ta hipoteza vključuje možnost, da je do tega pojava prišlo zaradi naselitve invazivne zelene 
alge Caulerpa cylindracea v zambratijskem zalivu, ki je imela negativen vpliv na samoniklo skupnost makroalg.
Ključne besede: Laurencia obtusa, monospecifi čna populacija, zambratijski zaliv, Jadransko morje
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