Feeding ecology of the American crab Rhithropanopeus harrisii (Crustacea, Decapoda) in the coastal waters of the Baltic Sea

Abstract

The feeding ecology of the American crab Rhithropanopeus harrisii Gould, 1841 from brackish waters of the Baltic Sea was studied by analyses of the stomach repletion index (SRI) and stomach content with regard to sex, size and habitat (Dead Vistula River and the Gulf of Gdańsk). Neither the sex nor the size of an individual crab had a significant (P> 0.05) influence on the SRI or on the diversity of food items found in the stomachs of R. harrisii . But the type of food consumed was significantly (P< 0.05) dependent on the locality inhabited: the greater the biodiversity of the habitat, the richer the dietary composition. In Baltic coastal waters this species feeds on detritus, and also on animal and plant matter. Remains of Chlorophyta, Amphipoda, Ostracoda, Polychaeta, Gastropoda and Bivalvia were found in the stomachs of the specimens analysed.

Full text

Feeding ecology of
the American crab
Rhithropanopeus harrisii
(Crustacea, Decapoda)
in the coastal waters of
the Baltic Sea*
OCEANOLOGIA, 51 (3), 2009.
pp. 361 – 375.

C2009, by Institute of
Oceanology PAS.
KEYWORDS
Rhithropanopeus harrisii
Non-native species
Feeding ecology
Stomach content
Baltic Sea
Joanna Hegele-Drywa∗
Monika Normant
Department of Experimental Ecology
of Marine Organisms,
Institute of Oceanography,
University of Gdańsk,
al. Marszałka Piłsudskiego 46, PL–81–378 Gdynia, Poland;
e-mail: asiahd@ocean.univ.gda.pl
∗corresponding author
Received 12 September 2008, revised 16 January 2009, accepted 19 August 2009.
Abstract
The feeding ecology of the American crab Rhithropanopeus harrisii Gould, 1841
from brackish waters of the Baltic Sea was studied by analyses of the stomach
repletion index (SRI) and stomach content with regard to sex, size and habitat
(Dead Vistula River and the Gulf of Gdańsk). Neither the sex nor the size of
an individual crab had a significant (P>0.05) influence on the SRI or on the
diversity of food items found in the stomachs of R. harrisii. Butthetypeof
food consumed was significantly (P <0.05) dependent on the locality inhabited:
the greater the biodiversity of the habitat, the richer the dietary composition. In
Baltic coastal waters this species feeds on detritus, and also on animal and plant
matter. Remains of Chlorophyta, Amphipoda, Ostracoda, Polychaeta, Gastropoda
and Bivalvia were found in the stomachs of the specimens analysed.
* This research was funded by the Department of Experimental Ecology of Marine
Organisms, grant No. DS/1350-4-0150-8.
The complete text of the paper is available at http://www.iopan.gda.pl/oceanologia/

362 J. Hegele-Drywa, M. Normant
1. Introduction
Since the beginning of the 20th century there has been a significant
increase in the number of non-native species turning up in areas well beyond
their natural range of distribution (Bax et al. 2003, Occhipinti-Ambrogi
& Savini 2003, Thessalou-Legaki et al. 2006). Global in character, this
phenomenon has affected the Baltic Sea to a considerable extent, where
for the last forty or fifty years, annually increasing numbers of new species
of flora and fauna have appeared (Lepp¨akoski 1984, Lepp¨akoski & Olenin
2000, Lepp¨akoski et al. 2002b). On the one hand, this has enhanced
the biodiversity of the Baltic, especially its south-eastern part, which is
naturally fairly poor in plant and animal species; on the other, it may give
rise to a whole range of negative ecological and economic effects (Gollasch
& Lepp¨akoski 1999, Lepp¨akoski et al. 2002a, Lepp¨akoski 2004, Streftaris
et al. 2005, Gollasch & Rosenthal 2006).
One such non-native species in the Baltic is the American crab
Rhithropanopeus harrisii Gould, 1841, which arrived in Europe in ballast wa-
ters from the Atlantic seaboard of North America (Wolff 1954, Lepp¨akoski
2005). The species was first recorded by Maitland (1874) in the Netherlands,
from where it expanded to Denmark – the waters around Copenhagen
(Jensen & Knudsen 2005) – and Germany (Nehring & Leuchs 1999). Since
its first appearance in Poland in the 1950s, R. harrisii has occurred in
the greatest numbers in the Vistula Lagoon (Zalew Wiślany) (Demel 1953,
Żmudziński 1957, Rychter 1999) and in the Dead Vistula River (Martwa
Wisła) (Michalski 1957, Turoboyski 1973, Janta 1996, Normant et al. 2004).
For several years now, increasing numbers of R. harrisii have been turning
up in the Gulf of Gdańsk (Normant, own observations), in places where
earlier it was rare or absent altogether (Żmudziński 1967).
From the moment it arrived in Poland, the American crab aroused the
interest of scientists. But studies of this species dealt primarily with its
distribution (Demel 1953, Michalski 1957, Żmudziński 1957, 1961, 1967,
Czerniejewski & Rybczyk 2008), its biology and ecology (Kujawa 1957, 1963,
Filuk & Żmudziński 1965, Ławiński & Pautsch 1969, Pautsch et al. 1969,
Turoboyski 1973, Janta 1996, Wiszniewska et al. 1998, Normant et al.
2004), and only to a very small extent with its physiology (Bomirski
& Klęk 1974, Rychter 1997, Kidawa et al. 2004, Normant & Gibowicz
2008). Little information is available on the part played by R. harrisii
in the trophic network of the waters that it inhabits. Studies done so
far indicate that in the Dead Vistula R. harrisii feeds mainly on animals
like ragworms Hediste (Nereis) diversicolor, blue mussels Mytilus edulis,
zebra mussels Dreissena polymorpha and hydroids Cordylophora caspia,on
plants like the green algae Cladophora sp. or Enteromorpha sp., and on

Feeding ecology of the American crab Rhithropanopeus harrisii ... 363
dead organic matter of animal origin (Szudarski 1963, Turoboyski 1973).
In the Vistula Lagoon, this crab is a scavenger, feeding on detritus (Demel
1953) or on D. polymorpha (Kujawa 1957). In the Odra estuary it feeds
mostly on detritus, but algae, animal remains and inorganic material were
also found in the gut contents (Czerniejewski & Rybczyk 2008). It is itself
a source of food for eels Anguilla anguilla and flounder Platichthys flesus
(Filuk & Żmudziński 1965), and also for cormorants Phalacrocorax carbo
(Wiszniewska et al. 1998). The biodiversity of Baltic coastal waters is
much greater than that of the Dead Vistula or the Vistula Lagoon, so
R. harrisii may well be involved in numerous trophic interactions with
other organisms. In view of this, a comparative study was carried out in
an attempt to determine the diet of R. harrisii in two ecologically different
environments, namely, the Dead Vistula River and the Gulf of Gdańsk. The
stomach repletion index and stomach contents were analysed in relation to
the sex and size of the individual crabs.
2. Material and methods
The crabs (118 specimens) were collected in the summer months
(July–September) of 2005 and 2006 from two areas. 28 females and 44
males were collected from three sampling points in the Gulf of Gdańsk
(54◦2637N, 18◦3613 E; 54◦2752 N, 18◦3790 E; 54◦2859 N, 18◦3971 E)
and 26 females and 20 males from one point in the Dead Vistula River
(54◦2089N, 18◦4772 E). Both sampling areas lie in the Polish zone of
the Baltic Sea. The animals were immediately frozen at −20◦Ctohalt
digestion. In the laboratory the crabs were sexed on the basis of their
abdominal structure and number of pleopods (De Man 1892), and their
carapace width was measured with slide callipers (±0.1mm) (ECOTONE,
Poland). Next, the stomach of every specimen was excised and analysed
under a stereomicroscope (ECO-VISION – ECOTONE, Poland) at 6.6−45×
magnification in order to assess its repletion index (Albertoni et al. 2003)
and to determine its content. Repletion was analysed using the five stomach
repletion indices (SRIs), where 0 indicates an empty stomach, I a stomach
that is 0–25% full, II one that is 25.1–50% full, III one that is 50.1–75% full
and IV one that is 75.1–100% full. All the food items in the stomachs were
placed in one of the following categories: (1) digested, (2) of plant origin, (3)
of animal origin, and (4) detritus. Plant and animal remains were identified
to the most precise taxonomic level based on the characters given by Pliński
(1980a,b), Jażdżewski & Konopacka (1995) and Kołodziejczyk & Koperski
(2000).
The data normal distribution (Gaussian distribution) was validated
through the application of the Shapiro-Wilk test at a significance level of 5%.

364 J. Hegele-Drywa, M. Normant
The differences in the studied parameters between groups of crabs were
tested using the Mann-Whitney U-test or the Kolmogorov-Smirnov test at
the 5% significance level. The dependence of the SRI and diversity of food
items on the locality of occurrence were determined using the comparative
proportions test at a significance level of P <0.05. Analyses were carried
out using the STATISTICA 6.0 PL program.
3. Results
The carapace width of females from the Gulf of Gdańsk ranged from 3.1
to 16.3 mm (mean 10.5±3.8mm), those of males from 3.2 to 22.8 mm (mean
10.4±4.0mm). The corresponding dimensions for animals from the Dead
Vistula River were from 8.9 to 19.4 mm (mean 11.5±2.6mm) (females),
and from 9.6 to 21.6 mm (mean 15.0±3.6mm) (males). The carapace width
of males from the Dead Vistula River was significantly greater (P <0.05)
than that of males from the Gulf of Gdańsk. In the former area, the most
numerously represented width class was 12.1–15.0 mm; in the latter area it
was 9.1–12.0 mm.
Neither the sex nor the size of the individual crab had any significant
(P >0.05) effect on the stomach repletion index (SRI) in Rhithropanopeus
harrisii. Statistically significant differences (P <0.05) were found between
the SRIs in the crabs from the two areas. In both areas, the greatest number
of specimens was found with an SRI of I, and the smallest number with an
index of III. In the Gulf of Gdańsk the whole range of SRIs was recorded,
but in the Dead Vistula River there was not a single crab with a completely
full stomach (Figure 1). The percentages of crabs with SRI categories I and
II were statistically different (P <0.05) in the two areas.
Unlike the sex and size of individuals (P>0.05), the locality of
occurrence did have a significant (P <0.05) influence on the diversity of food
items in the stomachs of R. harrisii. 50 and 72.7% of the specimens from
the Gulf of Gdańsk and the Dead Vistula River respectively had stomachs
containing only digested matter. In 45% of crabs with full stomachs from
the Gulf of Gdańsk the food items were identified as belonging to one of
three categories (plant matter, animal matter, detritus), and in 14.3% they
were from two categories; in only one single crab were all three categories
found. In the case of the crabs from the Dead Vistula River, 44% had food
items from one category in their stomachs, and only one individual had
items from two. Animal remains were found in 39% of crabs from the Gulf
of Gdańsk with full stomachs, whereas plant remains were found in 34%
of replete crabs. Animal and plant remains were identified in 15 and 21%
respectively of crabs from the Dead Vistula River. Detritus was found in
34% of crabs from the Gulf of Gdańsk and in 15% from the Dead Vistula

Feeding ecology of the American crab Rhithropanopeus harrisii ... 365
120
100
80
60
40
20
0
frequency [%]
Gulf of Gdańsk Dead Vistula River
stomach repletion index
empty 0.1-25% 20.1-50% 50.1-75% 75.1-100%
Figure 1. Frequency of crabs with different stomach repletion indices (SRI)
collected in the Gulf of Gdańsk (n =72) and the Dead Vistula River (n =46)
80
70
60
50
40
30
20
10
0
frequency in stomach content [%]
digested plant origin
categories of food items
animal origin detritus
Gulf of Gdańsk Dead Vistula River
Figure 2. Frequency of different categories of food items in the stomachs of crabs
from the Gulf of Gdańsk (n =56) and the Dead Vistula River (n =34)
River (Figure 2). Only the percentages of crabs with digested matter in
their stomachs differed significantly (P <0.05) in the two areas.
Among plant matter, Chlorophyta remains were found in the largest
number of crabs from the Gulf of Gdańsk and the Dead Vistula River
– 14 and 5 respectively (Figure 3). Among the animal remains found
in crabs from the Gulf of Gdańsk five different taxonomic groups were
recorded: Polychaeta, Amphipoda, Ostracoda, Bivalvia, Gastropoda. The
most frequent were Amphipoda fragments, which appeared in 10 specimens.

366 J. Hegele-Drywa, M. Normant
16
14
12
10
8
6
4
2
0
number of crabs
Chlorophyta Polychaeta
taxonomic group
Gulf of Gdańsk Dead Vistula River
Amphipoda GastropodaOstracoda Bivalvia
Figure 3. Frequency of occurrence of different plant and animal food items in the
stomachs of crabs from the Gulf of Gdańsk (n =27) and the Dead Vistula River
(n =9)
In crabs from the Dead Vistula River only two taxonomic groups were
found: Amphipoda and Bivalvia; the latter was the most numerous and
was recorded in 3 specimens.
4. Discussion
The dietary composition of Decapoda in their natural environment is
frequently determined directly from an analysis of their stomach contents,
even though identification of the food remains is difficult as they are
very finely comminuted. This is due to the structure and function of the
mouthparts and the gastric mill in this order of animals (Hill 1976, Williams
1981, Grabda (ed.) 1985, Choy 1986, Chande & Mgaya 2004).
From studies done so far of Rhithropanopeus harrisii from different
regions, it can be inferred that the species is omnivorous, feeding as it
does on detritus, as well as on animal and plant matter (Mordukhay-
Boltovskoy 1952, Szudarski 1963). But as in other Decapoda (Hill 1976,
Ryer 1987, Parslow-Williams et al. 2002), the frequency of feeding and the
quality of the food ingested depend not only on the locality inhabited by an
individual, but also on the diurnal cycle of activity and foraging (Takahashi
& Kawaguchi 2001, Turra & Denadai 2003). In the literature there does not
appear to be any endorsement of the above statements applying directly to
feeding patterns in R. harrisii. The environments from which the crabs for
the present investigation were taken – the Gulf of Gdańsk and the Dead
Vistula River – differ in their abiotic factors (temperature, salinity, type
of bottom), as well as in the availability and diversity of species of flora
and fauna that are potential food items for R. harrisii (Kruk-Dowgiałło

Feeding ecology of the American crab Rhithropanopeus harrisii ... 367
1994, Żmudziński 1997, Pliński 1999, Osowiecki 2000, Janas et al. 2004,
Janas 2005, Łysiak-Pastuszak et al. 2006, Kruk-Dowgiałło & Szaniawska
2008, own observations). Confirmation of the better trophic conditions for
R. harrisii in the Gulf of Gdańsk is the presence there of specimens with full
stomachs; crabs in this condition were not caught in the Dead Vistula River.
Interestingly, the individuals from the Gulf of Gdańsk were smaller in size,
and the rate of consumption in these smaller crabs may have been limited by
the availability of prey in the vulnerable size classes (Kneib & Weeks 1990,
Cotton et al. 2004). On the other hand, smaller, younger specimens have
a faster rate of metabolism and consumption than larger, older individuals
(Bridges & Brand 1980, Emmerson 1985, Schmidt-Nielsen 1997, Łapucki
et al. 2005). Large individuals of R. harrisii, which have the greatest chance
of capturing prey, do not in fact forage for prey if they have sufficient energy
reserves (Kidawa et al. 2004). In addition, fewer individuals from the Gulf
of Gdańsk had digested matter in their stomachs; more had undigested plant
and animal remains. These latter belonged to six different classes of flora
and invertebrate fauna, whereas in the crabs from the Dead Vistula River
the undigested food remains were from three classes.
This study did not indicate the existence of feeding selectivity in
R. harrisii from the Gulf of Gdańsk, because the numbers of individuals
feeding on plant or animal matter, or detritus, were similar. The choice
of food to be consumed depends not only on its availability in the
environment, but also on its assimilability sensu lato. It has been reported
that crustaceans frequently select small and medium-sized high-energy food
items from which the nutrients are readily assimilated (Morales & Antezana
1983, Juanes 1992, Kennish & Williams 1997). It is important that the
energy value of the food compensates for the energy expended by the
animal in foraging for it. Crabs belong to the mobile benthic fauna, but
they mostly crawl about on the bottom, and sudden movements are not in
their nature: they are not good hunters. They therefore tend to feed on
sessile organisms, macrophytes or detritus (Bourdeau & O’Connor 2003).
Crabs are particularly fond of mussels and snails, the shells of which they
crush with their pincers (Elner 1978, Hughes & Seed 1981, Flimlin & Beal
1993). Evidence for R. harrisii feeding on mussels is supplied by a few
shell fragments, which could have entered the stomach attached to the soft
tissue (e.g. adductor muscle) consumed (Hill 1976). The small proportion
of mussel remains found in the stomachs of crabs from the Gulf of Gdańsk
could be due to the fact that softer-bodied prey like mussels or polychaetes
require considerably less time to be digested than harder prey items like
crustaceans (Parslow-Williams et al. 2002). Hence, the presence of an item
in the gut contents can be taken as positive evidence of ingestion, but

368 J. Hegele-Drywa, M. Normant
its absence cannot be taken as evidence that the item does not occur in
the diet (Kneib & Weeks 1990). Interestingly, the stomachs of crabs from
both the Dead Vistula River and the Gulf of Gdańsk contained carapace
fragments from amphipods, which apparently move much faster than the
crabs. On the other hand, there are large numbers of gammarids in some
parts of the Gulf of Gdańsk (among the mussel beds, for instance), which
improves the chances of a predator capturing them. R. harrisii probably
finds dead organisms with the aid of its very well developed chemoreceptor
sense (Kidawa et al. 2004). Even though a diet of crustaceans has a fairly
low energy value, it does nonetheless supply consumers with the essential
minerals and calcium. R. harrisii is omnivorous, a feeding strategy that is
optimal since a mixed diet provides for the best growth (Buck et al. 2003).
By feeding on animal matter, crabs obtain the necessary proteins and fats
(Takeuchi & Murakami 2007), and the plant matter they ingest supplies
important nutrients (O’Brien 1994, Dahdouh-Guebas et al. 1999).
Interestingly, even though male crabs are usually more active and more
aggressive than females, and though their pincers are more massive, the
SRIs suggest that the foraging frequency is similar in the two sexes (Lee
1995, Takahashi & Kawaguchi 2001, Barki et al. 2003). This observation
may be due to the fact that this investigation was carried out on crabs
caught in summer, i.e. at a time of year when the high temperature of
the water governs feeding intensity and growth (Turoboyski 1973). In this
season, too, reproduction in R. harrisii is intensified, which suggests that
females should have a fairly low SRI, since they consume less food during
the breeding period (Ruiz-Tagle et al. 2002). In any case, ovigerous females
bury themselves in the bottom and spend more time grooming their eggs
than feeding (Turoboyski 1973, Sumpton & Smith 1990). Nevertheless, to
produce eggs females require a lot of energy, which they probably acquire
in their food and then save up for the breeding season (Ruiz-Tagle et al.
2002).
Studies of R. harrisii in the Gulf of Gdańsk done to date suggest
that it does not have many natural enemies in these waters. The crab
was not found either in the stomachs of benthic fish from the Gulf of
Gdańsk or in the diet of cormorants (Ostrowski 1997, Wandzel 2003, Bzoma
& Meissner 2005, Złoch et al. 2005, Karlson et al. 2007). It can itself,
however, affect the existence of the benthic flora and fauna ubiquitous in
the Gulf of Gdańsk, e.g. Chlorophyta, Amphipoda, Ostracoda, Polychaeta,
Gastropoda and Bivalvia species (Wiktor 1990, Osowiecki 1998, 2000,
Jęczmień & Szaniawska 2000a,b, Kruk-Dowgiałło & Szaniawska 2008). In
view of the ever-increasing numbers of R. harrisii in the Gulf of Gdańsk
over the last few years (Normant & Gibowicz 2008, Hegele-Drywa et al., in

Feeding ecology of the American crab Rhithropanopeus harrisii ... 369
preparation), this situation may have serious ecological consequences. The
present investigation is the first on the feeding ecology of R. harrisii in Baltic
coastal waters. Stomach contents often reflect the availability of food in the
environment rather than an animal’s preferences. That is why laboratory
studies are now in progress to try to discover the feeding preferences of
R. harrisii and the rate at which it consumes food. From the results it
should be possible to assess the potential effect of the American crab on the
benthic communities that it inhabits.
Acknowledgements
We would like to thank Mr. Stanisław Parszo from the Biological Station
of Gdańsk University for his assistance in collecting the material for this
study. We also express our gratitude to Dr Katarzyna Bradtke from the
Department of Physical Oceanography, University of Gdańsk, for her help
with the statistical analyses.
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