\ceanological and Hydrobiological Studies, VOL. 51, NO. 2 | JUNE 2022 
(atharina Romoth. Mavva Goaina. Kol: Beisienel. Alexander Darr, Michael Lothar Zettle 
identification of the specimens found and identified as 
€. PAppOSUS. 
A seventeen-year study by Carlson and Pfister 
1999) found that Crossaster papposus exhibits a 
relatively high longevity. Estimates based on growth 
and survival rates showed that some individuals 
live longer than 20 years and reach more than 20 
zm in diameter (Carlson & Pfister 1999; up to 34 cm 
according to Wilson 2008). In our case, age estimation 
remains difficult due to the fact that the brackish 
anvironment of the Baltic Sea imposes stressful 
asmotic conditions on marine organisms, often 
resulting in a reduced growth rate and body size 
‚Theede 1996). 
In a suitable habitat, C. papposus is a highly mobile 
species known to be a dominant predator feeding on 
sea urchins, as well as on many other invertebrates, 
including echinoderms, bivalves, cnidarians, and 
tunicates (Himmelman & Dutil 1991). In the Fehmarn 
Belt, the substrate is dominated by patches of coarse 
and mixed sediments with rock outcrops, stones, 
cobbles and shell gravel, while fine-grained silt and 
mud prevail in deeper zones (Diesing & Schwarzer 
2006). The area is known to be periodically and 
irregularly inhabited by marine species migrating 
actively or floating passively as adults or larvae from 
the Belt Sea. This pattern is triggered by aperiodic 
saline water inflows from the North Sea. The last 
axtreme major Baltic inflow event occurred in 2014 
Mohrholz 2018). While, inter alia, the great spider 
zrab Hyas araneus was observed in comparably 
high density in the year after the inflow event and 
is still present in the Fehmarn Belt (M.L. Zettler, 
IOW-benthos-database, unpublished), the occurrence 
9f the sun star in this area is not directly linked to such 
an inflow event. However, in 2019, saline bottom water 
body with a maximum salinity over 20 was detected in 
the Fehmarn Belt in early February; weak barotropic 
inflows occurred in the Baltic Sea in March and 
April, also bringing saline waters from the North Sea 
‚Naumann et al. 2020). The maximum bottom salinity 
af 19.3 was measured close to the study site during the 
EMB211 cruise farther east at a comparable depth. 
The observation of Crossaster papposus in the 
Fehmarn Belt represents a very rare find in the Baltic 
Sea. It is the second easternmost observation ever, 
with only one documentation located less than 
18.5 km farther east and dating back 150 years. The 
last known records of this species in the region were 
D5y Möbius (1873). His finds in 1871 were located close 
to the island of Fehmarn and in the south-western part 
9f Kiel Bay (Bülk, Kiel Fjord). Since then, C. papposus 
was documented only from the Kattegat, and rarely 
from the Great and Little Belt. The most recent 
Wu 
w.oandhs.ug.edu.pil 
findings were made south of the Danish island of 
Anholt in 2016 (Gonschior 2016) and by the IOW in the 
Great Belt in 2010 and 2011 (M.L. Zettler, IOW-benthos- 
database, unpublished). 
In addition to other factors, one major reason 
for the rare occurrence of C. papposus in the Baltic 
Sea may be the challenging osmotic conditions. 
"he natural steep salinity gradient in the Baltic Sea, 
ranging from fully marine conditions near the narrow 
connection with the North Sea to almost freshwater 
values in the north-eastern parts (Schiewer 2008), 
strongly affects and limits the occurrence and 
distribution of species (Telesh et al. 2013; Zettler et al. 
2014; Snoeijs-Leijonmalm & Andren 2017). Crossaster 
papposus is regarded as native to the Baltic Sea 
/HELCOM 2020). The species has a larval stage and the 
potential for long-distance dispersal, but appears to 
favor full salinity conditions above 30 PSU, suggesting 
that physiological constraints are the most important 
factor affecting colonization in the region. 
When species occur at their tolerance limits in very 
low densities, routine monitoring (based on point 
sampling once a year) cannot conclusively assess 
cheir distribution and status, therefore imaging has an 
obvious advantage of covering larger seafloor areas. 
The amount of data collected in various sub-basins 
affects the number of species found, simply because 
the possibility of finding more species increases 
with the amount of data collected. Even trawling 
surveys that cover larger areas have not reported the 
oaccurrence of C. papposus, supporting the fact that 
the species is extremely rare. However, whether our 
detection of C. papposus in the Fehmarn Belt in 2019 
and 2021 remain isolated findings needs to be proven 
by future monitoring efforts. 
Acknowledgemen? 
Our observations were made during mapping 
and monitoring as part of the project ”Life along 
gradients - Analysis of the impact of environmental 
parameters on the distribution, diversity and function 
of benthic communities and their habitats in the 
southern Baltic Sea” (Grant number 3 519 532 202)” 
funded by the Federal Agency for Nature Conservation 
(BfN). We gratefully acknowledge the valuable work 
of all colleagues involved in sampling campaigns and 
laboratory analyses, including the staff of our research 
vessel Elisabeth Mann-Boragese. 
ournal owner: Facultv of Oceanoaraphv and Geoaraphv, University of Gdanisk, Poland