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Contributions to Zoology, 80 (2) 133-141 (2011)
Coexistence of surface and cave amphipods in an ecotone environment
Roman Luštrik1, 3, Martin Turjak1, Simona Kralj-Fišer2, Cene Fišer1
1 University of Ljubljana, Biotechnical faculty, Department of Biology, PO Box 2995, SI-1001 Ljublja na, Slovenia
2 Jovan Hadži Institute of Biology, Scientic Research Centre of the Slovenian Academy of Sciences and Arts, PO
Box 30 6, SI-1001 Ljublja na, Slovenia
3 E-mail: roman.lustrik@gmail.com
Key words: Amphipoda, cannibalism, competition, ecotone, microhabitat preference, predation
Abstract
Interspecic interactions between surface and subterranean spe-
cies may be a key determinant for species distributions. Until
now, the existence of competition (including predation) between
these groups has not been tested. To assess the coexistence and
potential role of interspecic interactions between surface Gam-
marus fossarum and subterranean Niphargus timavi, and to de-
termine their micro distributions, we conducted a series of eld
and laboratory observations. We aimed to deter mine: (1) species
substrate preference, (2) whether the presence of G. fossarum
inuences the habitat choice of N. timavi, and (3) possible preda-
tion effects on micro habitat choice of small juveniles. Through-
out a small river in SW Slovenia, N. timavi was predominantly
found in leaf litter and gravel, but rarely in sand. In the sand
however, we exclusively found juveniles. In contrast, surface G.
fossarum sheltered mainly in leaf litter. A similar, body size de-
pendent, micro distribution was observed in G. fossarum, where
small individuals were generally found in gravel and sand. The
presence of G. fossarum affected the micro distribution of juve-
nile, but not adult, N. timavi. In the laboratory we observed pre-
dation and cannibalism in both species. Niphargus timavi, how-
ever, appeared to be a more efcient predator than G. fossarum.
In particular, juvenile N. timavi were most vulnerable to preying
by adults of both species. This probably affected the distribution
of juvenile N. timavi tha t chose ner substrates when placed wit h
adult individuals in an aquarium with granules of different size.
To understand the distribution of subterranean species, the
summed effect of intraspecic interactions, as well as surface –
subterranean species interactions, in particular between indi-
viduals of different size, should be taken into account.
Contents
Introduction ................................................................................... 133
Material and methods .................................................................. 134
Field experiment and data analysis .................................. 134
Laboratory work and data analysis .................................. 135
Results ............................................................................................. 135
Field experiment results ....................................................... 135
Laboratory work results ....................................................... 137
Discussion ...................................................................................... 137
Acknowledgements ...................................................................... 140
References ...................................................................................... 14 0
Introduction
Environmental changes may result in range expansions
of some species (e.g. Jażdżewski et al., 2004). Invasions
due to expanding species ranges inherently lead to ad-
ditional interspecic relationships. These are often
asymmetric and can eventually dramatically change the
local fauna, the survival of which in turn depends exclu-
sively on the number of appropriate refugia (Savage,
1981, 1982; Dick, 2008). Although poorly studied, caves
are not an exception. Sket (1977) reported how mild
pollution of sinking rivers enabled immigration and
survival of surface species deep in the caves, which sig-
nicantly reduced abundance of cave-specialized spe-
cies, so-called troglobionts. It is assumed that surface
species outcompete subterranean species if resources
are abundant, but conversely, subterranean species out-
compete surface species in food-limited environments
(reviewed in Culver and Pipan, 2009). If these hypoth-
eses were true, they might be of key importance for un-
derstanding distributions of subterranean species. First,
surface fauna may be a major factor in preventing move-
ment of subterranean species to the surface (Sket, 1981;
Culver and Pipan, 2009). Second, troglobiotic fauna of
small cave systems may be vulnerable to pollution
mainly on account of changes in competitive strengths
of surface versus subterranean species.
Despite the presumed importance of interspecic
interactions between surface and subterranean species
for their distributions, the existence of competition (in-
cluding predation) between them has not been explic-
itly tested before. A hypothesis about competitive and
predation interactions between surface and subterra-
nean species could be tested in ecotones, such as
springs (sensu Connell, 1980), which was the approach
taken in this study.
Our research focused on two amphipod species.
Amphipods represent a substantial portion of aquatic
134 Luštrik et al. – Coexistence of amphipods
fauna, both in biomass and in species numbers (e.g.
Conlan, 2008). They are widely distributed in both
surface and subterranean waters (Pinkster, 1978; Boto-
saneanu, 1986). The role of negative interactions such
as competition or predation has been studied exten-
sively across surface species (MacNeil et al., 1997)
with particular emphasis on invasive species (Bollache
et al., 2008; Dick, 2008). Moreover, there are also a
few reports indicating negative interspecies interac-
tions between subterranean species (e.g. Culver et al.,
1991; summarized in Culver and Pipan, 2009). Thus,
amphipods may serve as the appropriate model organ-
isms for studying interspecic interactions between
surface and subterranean species.
Recent observations of a spring in SW Slovenia
identied an appropriate natural setting with two am-
phipod species for this study. Gammarus fossarum
Koch, 1835 (Gammaridae) represents surface species
(note that this name might cover a number of cryptic
species). The species Niphargus timavi S. Karaman,
1954 (Niphargidae) can be found in surface (springs)
and subterranean habitats (Fišer et al., 2006; it could
be categorized as eutroglophile sensu Sket, 2008).
Both species coexist along the studied stretch of brook.
The spatio-temporal shares of both species vary sig-
nicantly throughout the year, suggesting that both
species may compete for space and food. We predicted
that G. fossarum, either as a result of its higher repro-
ductive potential or through enhanced predation, could
constrain the distribution of N. timavi in surface habi-
tats (Fišer et al., 2006, 2007). Indeed, the stomachs
contents corroborate the hypothesis that both species
overlap in feeding behaviour (Fišer et al., 2010).
To assess the coexistence and the potential role of
interspecic interactions between surface and subter-
ranean amphipod species, we conducted a series of
eld and laboratory observations. The setup was de-
signed to determine: (1) whether both species show
similar substrate preference, (2) whether the presence
of G. fossarum affects habitat choice of N. timavi, and
(3) whether predation has any effect on microhabitat
choice of younger and weaker juveniles (MacNeil et
al., 2008).
Material and methods
Field experiment and data analysis
We positioned 12 sampling stations along the Kolaški
potok (brook) near Ilirska Bistrica in SW Slovenia.
About 250 m below the primary spring, the brook
sinks and re-emerges as resurgence after about 150 m.
Niphargus timavi is present along the entire brook,
while Gammarus fossarum appears to be conned to
the reaches below the point of resurgence (details in
Fišer et al., 2007). Samples were collected in upstream
(control) and downstream stretches (test).
In February and March 2008, we sampled each site
for four times. We submerged six plastic cups, two for
each substrate: decaying leaf litter (predominately
beech tree leaves), sand (2-5 mm in diameter) and
gravel (5-60 mm) based on unied classication (Buol
et al., 2008). Cups were 12 cm in radius and 7 cm in
height, with 33 drilled holes (d = 10 mm). To prevent
accidental drift of the cups, we lodged them with
rocks. Thereafter, we inspected each plastic cup week-
ly and washed out all animals. Sampled amphipods
were preserved in 70% ethanol in loci, preventing any
loss due to predation after sample collection. Samples
were sorted according to species and size in the labo-
r at o r y.
The following questions were proposed:
1 Does either species show a preference for a particu-
lar substrate? To answer this, we pooled individuals
for each species by substrate categories. We tested
for differences in numbers of individuals present in
different substrate categories.
2 Does the presence of G. fossarum affect the habitat
choice of N. timavi? To answer this question, we
pooled individuals for each species by substrate
type; however, the data for N. timavi from the upper
part of the stream (above the resurgence, where
only N. timavi occurs) were treated separately from
the data collected in the lower stretch (below the
resurgence, where both species are present). We
rst tested whether the substrate preferences of N.
timavi (i.e. gravel vs. sand) in the upper and lower
stretches of the stream differ. Afterwards, we tested
whether the number of individuals of N. timavi
within a selected substrate differed between the up-
per and lower stretches of the stream (i.e. gravel
above versus gravel below the resurgence).
3 For each species we asked whether individuals of
different size invaded the same substrate. We select-
ed subsamples from various sampling sites in order
to attain at least 50 individuals of each species from
all substrate types. We estimated their body size by
measuring body length (measured from the genal
lobes, along the insertia of pereopod coxae – insertia
of pleopods – insertia along uropods) (Fišer et al.,
2009). We opted for non-parametric tests because
135Contributions to Zoology, 80 (2) – 2011
data were not normally distributed. Differences in
substrate preferences of different sized individuals
were analysed using Kruskal-Wallis test; pairwise
comparisons were done by Mann-Whitney U test
(using SPSS 14).
Laboratory work and data analysis
Given that arthropods present a part of both species’
diet (see Fišer et al., 2010), we tested whether either
species showed predatory or cannibalistic behaviour.
Collected animals were separated by species, trans-
ferred to glass aquaria in a speleolaboratory (a dark,
insulated room kept at approximately 10°C) and fed ad
libitum with leaf litter from the study site for four days.
Thereafter, we transferred the animals to Petri dishes.
The number of animals in the dishes was counted eve-
ry 24 hours for four days.
Prey size may affect the prey choice of predatory
amphipods (MacNeil et al., 2008), which implies that
fully developed individuals would prey only on small-
er individuals. To test this, we prepared con- and inter-
specic pairs of large males (>12 mm) of G. fossarum,
N. timavi and N. timavi + G. fossarum, each pair in a
separate Petri dish. All combinations were run in ve
parallels for two weeks. All 30 individuals survived
this testing period as expected, and so only small (<4
mm) and mid-sized (5-7 mm) individuals were used as
the potential prey in the experiments described below.
Furthermore, we placed three individuals of one or
a mix of both species into a Petri dish. The experimen-
tal set up was constrained by limited numbers of avail-
able small N. timavi. Each Petri dish contained either
one large G. fossarum or large N. timavi (predator) and
one out of nine possible pairs of small and/or mid-
sized G. fossarum and/or N. timavi individuals. In do-
ing so, we obtained four categories of prey (small- and
mid-sized individuals of both species), the vulnerabil-
ity of which was estimated by their survival. This ex-
perimental design allowed the estimation of prey vul-
nerability in a pair (i.e. which individual survives long-
er), however, generalizations on vulnerability of prey
category were implicit, since individual predators could
not select among all possible prey categories. Predation/
cannibalism rates were estimated from survival rates,
which were calculated from how many individuals of
each prey category survived to the next day.
We asked whether N. timavi and G. fossarum dif-
fered in predatory behaviour. To answer this, we
pooled the numbers of survivors per day for each pred-
atory species. The data were log-transformed and the
predation rate was then estimated as the slope of the
linear regression line. To test whether predation rates
of the two species were the same, we compared the
slopes of the two regression lines (coefcients) using
the Snedecor and Cochran test (Snedecor and Coch-
ra n, 1976).
To determine whether prey categories differed in
vulnerability, we pooled the data by the focal prey cat-
egories per predator species. For instance, if we were
interested in small N. timavi preyed upon by N. timavi,
we pooled the data from Petri dishes where prey of N.
timavi were two small N. timavi, small N. timavi +
small G. fossarum, small + mid-sized N. timavi and
small N. timavi + mid-sized G. fossarum. To assess
which category was most vulnerable, we pairwise test-
ed the differences in slopes of regression lines for each
prey category as described above (Snedecor and Co-
chra n, 1976).
Furthermore we asked, whether small niphargids
search shelter in ne sand due to cannibalistic pressure
or due to seeking the contact with substrate. We lled
four aquaria (100 mm × 50 mm × 200 mm) with dif-
ferently sized glass pebbles, with each layer being 50
mm thick. Top, middle and bottom layers were con-
structed with pebbles of diameter 10, 5 and 2 mm, re-
spectively. Well-aerated water from the site was used
to ll the aquaria. Due to small number of subjects per
aquarium and short experiment duration (circa one
day), no additional water aeration was provided. We
considered small juveniles to be less than 5 mm in
length and adults larger than 15 mm. We observed the
distributions of one juvenile when a) alone, b) in the
presence of an adult male and c) in the presence of an
adult female. Distribution of juveniles was examined
every 20 minutes for ve hours. Differences in distri-
butions were tested using contingency tables and χ2.
Results
Field experiment results
Both, Gammarus fossarum and Niphargus timavi dif-
ferently inhabited the substrate types (Kruskal-Wallis:
χ2 = 7.83, d.f. = 2, p < 0.05 * and χ2 = 21.150, d.f. = 2, p
< 0.001 ***, respectively). Gammarids more often shel-
tered in leaf litter (Fig. 1, Table 1) than in gravel and
sand (Mann-Whitney U = 376.00, p < 0.05 *; U = 284.00,
p < 0.05 *, respectively). However, they did not distin-
guish between gravel and sand (U = 407.50, p > 0.1). In
contrast, Niphargus timavi showed no differ
ence in
136 Luštrik et al. – Coexistence of amphipods
choosing between leaf litter and gravel (Figs 1-2, Table
2; U = 525.00, p > 0.1), but it preferred either of those
substrates to sand (gravel – sand: U = 142.00, p < 0.001
***; leaf litter- sand: U = 185.00, p < 0.001 ***).
In general, the substrate choice of N. timavi tended
to be effected by the presence of G. fossarum (Kruskal-
Wall is: χ2 = 2.51, d.f. = 1, p > 0.1; Figs 1- 2, Table 2). In
the presence of G. fossarum, N. timavi showed similar
preferences for substrates (Kruskal-Wallis: χ2 =
22.668, d.f. = 2, p < 0.001 ***). It showed no prefer-
ences when choosing gravel or leaf litter (U = 82.00, p
> 0.1), but avoided sand (gravel – sand: U = 3.00, p <
0.001 ***; leaf litter – sand: U = 15.50, p < 0.001 ***).
Furthermore, the numbers of individuals in leaf litter
and gravel in the upper, compared to lower stretch of
the brook, showed no difference (U = 122.00, p > 0.1
and U = 105.00, p > 0.1, respectively). However, in
comparison to the upper stretch, more individuals in-
vaded sand in the lower stretch of the brook (U =
13.00, p < 0.001 ***), on account of smaller individu-
als (see Table 4).
Both species showed similar patterns in size-de-
pendent micro distribution - smaller individuals of
both species invaded their adult stage non-preferential
substrates signicantly more often than larger indi-
viduals. Individuals of N. timavi found in sand were
signicantly smaller than individuals found in gravel
Table 1. Differences in abundances of G. fossarum in different
substrates in the lower stretch of the brook. Mann-Whitney U-
Test, statistically signicant differences in boldface.
Leaf litter Gravel
Gravel U = 376.00
p < 0.05*
Sand U = 284.00 U = 407.50
p < 0.05* p > 0.1
Table 2. Differences in abundances of N. timavi in different
substrates. Values above the diagonal refer to data in the upper
stretch of the brook where N. timavi lives alone, the values be-
low the diagonal refer to data in the lower stretch of the brook
where N. timavi and G. fossarum coexist. Diagonal cells refer to
comparison of abundances of N. timavi within the same sub-
strate in the lower and upper stretch of the brook. Mann-Whit-
ney U-Test, statistically signicant differences in boldface.
Leaf litter Gravel Sand
Leaf litter U = 122.00 U = 525.00 U = 185.00
p > 0.1 p > 0.1 p < 0.001***
Gravel U = 82.00 U = 105.00 U = 142.00
p > 0.1 p > 0.1 p < 0.001***
Sand U = 15.50 U = 3.00 U = 13.00
p < 0.001*** p < 0.001*** p < 0.001***
leaf littergravelsand
substrate
140
120
100
80
60
40
20
0
Gamma rus fossarum
Niphargus timavi
number of individuals
leaf litter
gravel
sand
substrate
140
120
100
80
60
40
20
0
number of individuals
Fig. 1. Number of individuals of G. fossarum and N. timavi in
three different substrates in the lower stretch of the brook (below
the resurgence) where the two species coexist.
Fig. 2. Number of individuals of N. timavi in the upper stretch of
the brook (above the resurgence) where the species appears
alone.
137Contributions to Zoology, 80 (2) – 2011
or leaf litter (sand-gravel: U = 757.00, p < 0.001 ***;
sand – leaf litter: U = 1008.50, p < 0.001 ***), while
body lengths of individuals found in gravel and leaf
litter did not differ (U = 974.00, p > 0.1, Tables 3-4).
For G. fossarum, leaf litter was invaded by signicant-
ly larger individuals than gravel (U = 829.50, p < 0.01
**). Invaders of leaf litter showed a trend with mar-
ginal statistical signicance to be larger than those in-
vading sand (U = 1015.50, p = 0.057). For G. fossarum,
no difference in sizes was detected between the two
types of non-preferential substrates (U = 1222.00, p >
0.1; Tables 3-4).
Laboratory work results
Niphargus timavi appeared to be a superior predator
over G. fossarum. Thirty adult N. timavi consumed
about 60% of available prey items in 24 hours and 21%
of prey survived the full ve days test period. By con-
trast, G. fossarum consumed only 24% of available
prey in the rst 24 hours and consumed only 39% of
available prey items in the ve day test period. This
pattern was consistent across prey categories. In all
cases, N. timavi preyed faster and more efciently than
G. fossarum.
Niphargus timavi preyed more efciently upon
small- over mid-sized individuals of both species. By
contrast, G. fossarum most efciently hunted small N.
timavi, while it preyed on the rest of the prey catego-
ries with roughly the same efciency (Table 5, Fig. 3).
Our data indicate that juvenile N. timavi present the
most vulnerable size (age) class in this particular
brook.
The presence of an adult changed the distribution of
juvenile niphargids. When no adult was present, juve-
niles inhabited mostly the middle layer, whereas it was
evenly distributed in the other two layers. In the pres-
ence of an adult, a juvenile was almost never found in
the layer with large pebbles, being more often found in
the layers with middle and small sized pebbles (large
pebbles: χ2 = 0.24, p < 0.01**; middle sized pebbles: χ2
= 0.235, p < 0.01**; small pebbles: χ
2 = 0.202, p <
0.05**, Table 6).
Discussion
Our study shows that G. fossarum chose leaf litter
more often than gravel and sand, implying only acci-
dental and temporary use of the later two habitats. By
contrast, N. timavi chose no single substrate, but was
found in sufciently large spaces among either particle
grains or leaves. Moreover, the presence of G. fossa-
rum had only little effect on N. timavi microdistribu-
tion where the two populations overlapped (in the
lower stretch of the brook).
Differently sized individuals of both species dis-
tributed non-randomly among substrates. Body size
differences in habitat choice were greater in N. timavi
than in G. fossarum, but in both species smaller indi-
viduals sought shelter in substrates that were preferred
less by adults (gravel, sand).
Table 3. Differences in body lengths in different substrates for N.
timavi (above diagonal) and G. fossarum (below diagonal).
Mann-Whitney U-Test, statistically signicant differences in
boldface, trends in italics.
Leaf litter Gravel Sand
Leaf litter U = 974.00 U = 1008.50
p > 0.1 p < 0.001***
Gravel U = 829.50 U = 757.00
p < 0.01** p < 0.001***
Sand U = 1015.50 U = 1222.00
p > 0.05(*) p > 0.1
Table 4. Body sizes of Niphargus timavi and Gammarus fossa-
rum in three different substrates in the lower stretch of the brook
(below the resurgence), where the two species coexist.
Substrate Descriptive Niphargus Gammarus
statistic timavi fossarum
Gravel min 1.91 0.51
1. quartile 3.57 4.9
median 5.47 7.18
mean ± SD 6 ± 3.08 6.35 ± 2.62
3. quartile 7.89 8.08
max 13.63 11.15
Leaf litter min 1.17 4.3
1. quartile 3.13 5.58
median 3.73 8.37
mean ± SD 5.12 ± 2.94 8.28 ± 2.81
3. quartile 6.54 10.47
max 11.4 13.25
Fine sand min 1.1 0.81
1. quartile 2.32 2.27
median 2.52 6.56
mean ± SD 3.19 ± 2.18 6.76 ± 3.99
3. quartile 2.93 10.3
max 13.2 13.81
138 Luštrik et al. – Coexistence of amphipods
Niphargus timavi was a more efcient predator and
cannibal than G. fossarum. This predatory behaviour
may broaden available food sources, which might be
advantageous in the food-limited subterranean environ-
ment. Juvenile N. timavi, the most vulnerable prey cat-
egory, invaded sand seeking not only contact with sub-
strate, but also in response to predatory pressure from
large N. timavi whose movement through the sand was
constrained. Interpreting the micro distribution of small
N. timavi in this light matches with the eld results,
a b
cd
e
Fig. 3. Effectiveness of predation (cannibalism) by adult G. fos-
sarum (□) and N. timavi (∆) upon differently sized individuals
of both species: a) small N. timavi, b) middle N. timavi, c) small
G. fossarum, d) middle G. fossarum and e) total effectiveness
for all aforementioned groups by adults of both species. See Ma-
terials and methods for class sizes. The slopes of all regression
lines-pairs are statistically different (p < 0.05*).
139Contributions to Zoology, 80 (2) – 2011
which seem to indicate that hiding behaviour of small
N. timavi was most pronounced in the lower stretch of
the stream, where occasional G. fossarum predation ac-
companies cannibalistic acts by larger N. timavi. Simi-
lar segregations of juvenile individuals have also been
observed at least in Niphargus rhenorhodanesis Schel-
lenberg 1937 (Mathieu et al., 1987), Thermosphaeroma
thermophilum (Richardson, 1897) (Jormalainen and
Shuster, 1997), Gammarus pseudolimnaeus Bouseld,
1958 (Williams and Moore, 1986), Gammarus pulex
(McGrath et al., 2007) and Pseudoniphargus grandi-
manus Stock, Holsinger, Sket and Iliffe, 1986 and Pseu-
doniphargus carpalis Stock, Holsinger, Sket and Iliffe,
1986 by Stock et al. (1986).
In contrast, G. fossarum showed less inter- and in-
traspecic interactions. The distribution of differently
sized animals in different substrates may simply reect
rivalry for the preferred substrate, where the largest
individuals outcompete smaller ones. Similar results
were obtained in observations in aquaria (unpublished
observations).
Micro distribution of N. timavi and G. fossarum ap-
parently depends on a broad array of biotic and abiotic
factors. The role of the substrate, interspecic competi-
tion, intraguild predation or a combination of all of the
above have been broadly acknowledged to structure
various communities (Crowder and Cooper, 1982; Po-
lis, 1984, 1989; Skadsheim, 1984; Adams et al., 1987;
Dick, 1996, 2008; MacNeil et al., 1997, 2008; Beisel et
al., 1998; Dick and Platvoet, 2000; Dick et al., 2002;
van Riel et al., 2006; McGrath et al., 2007; Kley et al.,
2009; MacNeil and Briffa, 2009). Nevertheless, at least
in certain cases, intraspecic relationships has been ne-
glected or obscured on account of interspecic rela-
tionships (Sket, 1981; Culver and Pipan, 2009). In this
particular study, the hypothesis of competition between
surface and subterranean fauna remains generally un-
supported. However, juvenile N. timavi were observed
to take refuge in ner substrates (sand) more often in
the presence of cannibalism and predation by their con-
specics and G. fossarum, respectively. Large-scale ef-
fects may arise due to additive effects of predation and
cannibalism upon the smallest (the most vulnerable)
individuals of N. timavi, thereby signicantly decreas-
ing the number of individuals in the next generation.
To conclude, the two species differed in their habi-
tat choice. Smaller and weaker individuals of N. timavi
and G. fossarum inhabited microhabitat less preferred
by adults, probably due to cannibalism and competi-
tion, respectively. Despite the difference in habitat
choice between the two species, the occasional preda-
tion of G. fossarum upon juvenile N. timavi in addition
to cannibalism may accelerate the decrease of
niphargid population along the brook (as observed by
Fišer et al., 2007). To understand distribution of sub-
terranean species, the summed effect of intraspecic
Table 5. Differences in predation preferences by N. timavi (above diagonal), G. fossarum (below diagonal) and the differences between
species. Each cell indicates the most vulnerable prey category and probability that coefcients of the two regression lines are the same.
Statistically signicant values are indicated in boldface, statistical trends are indicated in italics. Diagonal cells list the probability that
both species equally prey the particular prey category.
Small N. timavi Middle N. timavi Small G. fossarum Middle G. fossarum
Small N. timavi N. timavi small N. timavi small N. timavi
p > 0.05 (*) p < 0.05 * p > 0.1 p < 0.05 *
Middle N. timavi small N. timavi N. timavi small G. fossarum
p < 0.01 ** p < 0.01 ** p < 0.05 * p > 0.1
Small G. fossarum small niphargid small G. fossarum N. timavi small G. fossarum
p < 0.05 * p > 0.05 (*) p < 0.01 ** p < 0.05 *
Middle G. fossarum small N. timavi N. timavi
p < 0.01 ** p > 0.1 p > 0.1 p < 0.05 *
Table 6. Distribution of small sized niphargids in layers of arti-
cial substrate of different grain size when alone and in the pres-
ence of adult conspecic under laboratory settings.
Substrate size Alone Adult present
10 mm pebbles 12 2 4
5 mm pebbles 33 47 32
2 mm pebbles 15 11 24
140 Luštrik et al. – Coexistence of amphipods
interactions as well as surface – subterranean species
interactions, in particular between differently sized in-
dividuals, should be taken into account.
Acknowledgements
The paper is dedicated to the memory of Miha Valič, who ac-
companied Cene Fišer in the very rst eld observations in 2004.
The Tibetan ‘Turquoise Goddess’ brought Miha to eternity in au-
tumn 2008. We would like to thank Antonija Bogdan, Melinda
Gal, Maja Herbaj, Mojca Horvat, Melita Korošec, Žana Kovačec,
Boštjan Markelc, Sara Novak, Eva Ogorevc, Luka Predojević,
Mateja Pustovrh, Tina Sečen, Darja Slana, Mojca Škrget and
Anže Zorin for assisting in the eld and laborator y. We are grate-
ful to Peter Trontelj, Yael Kisel, Maar ten de Groot, Boris Sket,
Daiqin Li and two reviewers for giving valuable comments on
our manuscript and language. The work was partially funded by
the Slovenian Ministry of Education and Spor t.
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Received: 24 March 2010
Revised and accepted: 10 January 2011
Published online: 14 April 2011
Editor: R. Vonk
