NATURA SLOVENIAE 22(1): 29-32 Prejeto / Received: 21. 6. 2020 
FIELD NOTE Sprejeto / Accepted: 27. 7. 2020 
 
Biotehniška fakulteta Univerze v Ljubljani in Nacionalni inštitut za biologijo, Ljubljana, 2020 
Occurrence of fairy shrimp 
(Chirocephalus croaticus) in 
the lake Petelinjsko jezero 
during the 2008–2014 period 
 
Pojavljanje kraškega škrgonožca 
(Chirocephalus croaticus) v 
Petelinjskem jezeru v obdobju  
2008–2014 
 
Tina KIRN, Zagorje 33, SI-6257 Pivka, Slovenia;  
E-mail: kirn.tina@gmail.com 
 
The lake Petelinjsko jezero is the second largest, 
lowest lying and longest lasting of the Pivka 
intermittent lakes (SW Slovenia). In this particular 
lake, a small fairy shrimp crustacean Chirocephalus 
croaticus (Crustacea, Anostraca) lives, adapted to 
temporary wetlands. Although this species is 
declared endemic to the lake Petelinjsko jezero, it 
also occurs in large numbers in puddles of 
Jeredovce (Pipan 2005).  
 
Brancelj & Gorjanc (1999) gave a detailed 
morphological description of the species in Slovenia, 
together with its ecology. The species does not 
survive in stagnant waters inhabited by fish. For its 
normal development it requires a certain period of 
time during which the thick-shelled eggs are 
exposed to drought and low temperatures (Brancelj 
& Gorjanc 2000).  
 
Chirocephalus croaticus has the status of a 
vulnerable species (VU) in the IUCN Red list of 
Threatened Species (Sket 1996). It is also a rare 
species (R) in the Red List of lower crustaceans of 
Slovenia and is one of the protected species. It is 
included in the list of animal species listed in Annex 
2, the habitats of which are protected under the 
Decree on protected wild animal species (Ur. l. RS 
2002, Ur. l. RS 2004). Its population is potentially 
threatened by mechanical interventions and/or 
pollution (including fertilization of meadows) 
(Brancelj & Gorjanc 2000). 
 
Based on multi-year sampling at different seasons 
(Tab. 1), the population appears to be stable. Under 
favourable conditions, the species occurs in large 
numbers (Brancelj & Gorjanc 2000). In addition to 
its occurrence, Jeklar (2019) studied the average 
size and sex ratio of C. croaticus, and how physical 
and chemical parameters of the water and air affect 
its population prevalence and size (Tab. 1).  
 
During several years of our field observations of the 
lake Petelinjsko jezero (Kirn 2016), we occasionally 
noticed C. croaticus. It had never been observed in 
other Pivka intermittent lakes, while the lakes in 
Krajnikov dol and Jeredovce were not inspected. We 
observed fairy shrimps in the lake Petelinjsko jezero 
every year from 2008 to 2014 in ten periods when 
the lake was filled with water (Tab. 1). During our 
fieldtrips, we found fairy shrimps in all seasons; they 
were spotted throughout the spring until early 
summer, after which they were inspected only in 
late autumn and winter. Since fairy shrimps are 
difficult to spot during high water levels, our field 
data cover only the time when the lake water level 
was dropping. For example, in the December 2009–
April 2010 period, when the lake basin was filled 
with water, fairy shrimps were spotted only in April, 
while their previous occurrences remain unknown. 
In general, the monthly occurrence of C. croaticus 
has changed over the years, depending on the lake 
dynamics. 
 
The time when the lake basin is filled with water 
affects the occurrence of fairy shrimps. In the small 
lake, which was partially filled with water in June 
2012 (Fig. 1), fairy shrimps appeared smaller and 
their density was lower than at the time when the 
lake was larger. In June 2010, however, fairy shrimp 
did not occur in the lake, reaching only the first 
filling phase and lasting for less than a week. 
 
At high water levels, fairy shrimps are difficult to 
spot when individual specimens were observed in 
November 2010, specifically in the streambed 
leading from Jeglenk. Otherwise, fairy shrimps were 
observed in a puddle at the bottom of the Jeglenk 
sinkhole, which dries up before the main lake. 
Apparently, fairy shrimps at Jeglenk first separate 
from those in the main lake, and as this lake dries 
up, the population is divided further. Fairy shrimps 
appear longest at the bottom of the lowest parts of 
the lake Petelinjsko jezero, i.e. in the basin under 
estavelles in the southeastern part of the lake, as 
well as at the lowest section of the main cart track 
and in holes of the central basin. Their density 
increases with the water decrease.  
 
  
30 Tina KIRN: Occurence of fairy shrimp (Chirocephaluc croaticus) in the lake Petelinjsko jezero ... / FIELD NOTE 
NATURA SLOVENIAE 22(1): 29-32 
Table 1. Time of occurrence of Chirocephalus croaticus (Crustacea, Anostraca) in the lake Petelinjsko jezero according to 
literature and our field data. Exact dates are given if available in the literature. 
Tabela 1. Čas pojavljanja kraškega škrgonožca Chirocephalus croaticus (Crustacea, Anostraca) v Petelinjskem jezeru iz 
literature in naših terenskih podatkov. Natančni datumi so navedeni, če so na voljo v literaturi. 
Month Brancelj & Gorjanc 
1999*, Brancelj & 
Gorjanc 2000 (autumn 
1999–spring 2000) 
Jeklar 2019 
(spring 2018–spring 2019) 
Our observations in 2008–2014, 
when the lake was filled with 
water  
(indicated in parentheses**) 
Jan 2000: 20.1.  2013: 20. 1.  
(October 2012–January 2013) 
Feb   2011: 12. 2., 19. 2.  
(September 2010–February 2011) 
Mar   2009: 15. 3., 24. 3.  
(December 2008–March 2009)  
Apr 1998: 17. 4., 23. 4.,  
30. 4. 
2000: 19. 4. 
2018: 11. 4., 13. 4., 17. 
4., 
20. 4., 23. 4., 25. 4. 
2010: 13. 4., 21. 4., 24. 4. 
(December 2009–April 2010) 
2011: 5. 4. (March–April 2011) 
May 1998: 5. 5., 8. 5.  2018: 2. 5., 5. 5. 2008: 23.5. (March–May 2008) 
2009: 2. 5., 3. 5., 5. 5., 6. 5.  
(March–May 2009) 
2010: 28. 5., 30. 5. (May 2010) 
2014: 4. 5., 17. 5. (till May 2014) 
Jun  2019: 10. 6., 18. 6. 2012: 23. 6. (Jun 2012) 
Jul    
Aug    
Sep    
Oct    
Nov  2018: 13. 11. 2010: 14. 11.  
(September 2010–February 2011) 
Dec    
* Sampling was performed once or twice per year for a few years before 1998 and weekly in 1998, but the exact dates 
are given only for the late spring 1998. Colour scale: blue – winter, green – spring, yellow – summer, orange – autumn  
** The time when the lake was filled with water is not given by exact dates – the period is defined monthly.  
* Vzorčenje je bilo nekaj let pred letom 1998 opravljeno enkrat ali dvakrat na leto, leta 1998 pa tedensko. Datumi so 
navedeni le za pozno pomlad 1998. Barvna lestvica: modra – zima, zelena – pomlad, rumena – poletje, oranžna – jesen  
** Trajanje pojavov jezera ni podano z natančnimi datumi – obdobje je opredeljeno mesečno.  
 
Figure 1. The Petelinjsko jezero basin with smaller water surface (photo: Tina Kirn). 
Slika 1. Kotanja Petelinjskega jezera ob nastanku manjšega jezera (foto: Tina Kirn).
Jeglenk 
Tina KIRN: Occurence of fairy shrimp (Chirocephaluc croaticus) in the lake Petelinjsko jezero ... / FIELD NOTE 31 
NATURA SLOVENIAE 22(1): 29-32 
The fairy shrimps were initially observed in the 
shallows of the flooded cart track (in the extent of 
the smaller lake) along the northern edge of the 
central basin. Some specimens were found dead on 
the dry cart track along the southern edge of this 
basin (May 2014). We believe that they were 
washed up on the lakeshore. As the water level 
dropped, only the lower part of the lake basin was 
flooded. During this time, the initially single lake 
splits into two smaller lakes. The fairy shrimps were 
found in the lake shallows of the flooded cart track 
at the bottom of the central basin, as well as in holes 
and small basins along the lake. Before the 
disappearance of the lake, we observed fairy 
shrimps only in the lowest parts of the lake 
bottom. Thus, after drying, the patches of dead fairy 
shrimps remained there. If there are filamentous 
algae in the lake, the dead fairy shrimps are pressed 
into the algae paper that remains at the dried up 
bottom (Fig. 2). We believe that based on the 
number and size of the patches we could 
check/track the state of the fairy shrimp population 
(stable, increasing, decreasing) in the lake. 
 
To conclude, C. croaticus occurs in the lake 
Petelinjsko jezero regularly and in large numbers, 
as already stated by Brancelj & Gorjanc (2000) and 
Jeklar (2019). We observed fairy shrimps in all 
seasons, but were the commonest in spring. 
 
 
  
 
  
 
Figure 2. Examples of finds of fairy shrimps (Chirocephalus croaticus) in the lake Petelinjsko jezero (photo: Tina Kirn). 
Slika 2. Primeri najdb kraškega škrgonožca (Chirocephalus croaticus) v Petelinjskem jezeru (foto: Tina Kirn).  
 
 
32 Tina KIRN: Occurence of fairy shrimp (Chirocephaluc croaticus) in the lake Petelinjsko jezero ... / FIELD NOTE 
NATURA SLOVENIAE 22(1): 29-32 
Acknowledgements 
 
I would like to thank Tanja Pipan and Anton Brancelj, to 
whom we sent some photos of the found fairy shrimps for 
determination. I am also very grateful to the editor and 
anonymous reviewer for suggestions that significantly 
improved the manuscript.  
 
 
References 
 
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