An invasive species may be better than none: invasive signal and native noble crayfish have similar community effects

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Etude ultrastructurale de l'ovocyte en vitellogenèse chez les Ecrevisses Astacus astacus et A leptodactylus

In crayfish Astacus astacus and A. leptodactylus, protein yolk materials appear to be derived from intra- and extra-oocytic sources.     

Interactions among juveniles of two freshwater crayfish species and a predatory fish

Two freshwater crayfish species, Astacus astacus L. and Pacifastacus leniusculus Dana, co-occur in some Swedish lakes. Observational studies indicate that the introduced, North American species P. leniusculus may gradually replace the native A. astacus, but the mechanism behind the replacement is not known. This study examined the direct effects of interspecific competition between the crayfish, and indirect effects of competitive interactions and fish (European perch, Perca fluviatilis L.) predation. Three different experiments with young-of-the-year (YOY) crayfish were performed. P. leniusculus was strongly dominant over similar-sized A. astacus in interference competition for shelter in a laboratory experiment. However, in a 35-day experiment in outdoor pools, A. astacus growth and survival were about equally affected by interactions with conspecifics and P. leniusculus. In contrast, P. leniusculus was significantly more affected by intraspecific competition than by competition with A. astacus, suggesting asymmetric competition between the two species. The presence of perch in outdoor ponds with mixed-species groups of the two crayfish species resulted in considerably higher predation rates on A. astacus than on P. leniusculus. Both species showed strong antipredator responses to perch by increasing refuge use. I suggest that higher perch predation rates on A. astacus originate from P. leniusculus being the superior species in interspecific competition for shelter. Because of displacement from refuges, A. astacus individuals become more exposed to the predator. This indirect effect of interactions among the two cray-fish species and the predator may be important in the observed in situ replacement of A. astacus by P. leniusculus.     

Species-specific differences in dynamics of agonistic interactions may contribute to the competitive advantage of the invasive signal crayfish (Pacifastacus leniusculus) over the native narrow-clawed crayfish (Astacus leptodactylus)

Species-specific differences in dynamics of agonistic interactions may influence the outcome of interspecific competition and potentially contribute to competitive advantage of one species over another. In this study, we compared the dynamics of agonistic interactions of one of the most successful crayfish invaders of European freshwater ecosystems, the signal crayfish (Pacifastacus leniusculus) and the widespread native European species currently undergoing range expansion in Croatia and Europe, the narrow-clawed crayfish (Astacus leptodactylus). Comparisons between P. leniusculus pairs and the A. leptodactylus pairs demonstrated significant differences in frequency and duration of agonistic encounters: P. leniusculus dyads engaged in fewer fights, but their duration was significantly longer. In staged interactions between size-matched interspecific pairs, agonistic behaviour of P. leniusculus individuals translated into dominance over their A. leptodactylus counterparts. This indicates that the success of P. leniusculus in agonistic encounters with the native competitor might stem from its readiness to continue fighting for a longer time period, and could lead to ecological advantages during niche competition even when facing a successful native crayfish species.     

Effects of crayfish on leaf litter breakdown and shredder prey: are native and introduced species functionally redundant?

Recent increases in biological invasions frequency may have important consequences on native communities. However, functional redundancy between invasive and native species could reduce non-native species effects on native ecosystems. Despite this, even small differences in functional traits between these species may still have unpredictable effects on colonized ecosystems. Invasive crayfish, as ecosystem engineers, potentially have wide and complex effects on recipient ecosystems, even when replacing a native counterpart. We used laboratory microcosms to test whether native (Astacus astacus) and invasive crayfish species (Orconectes limosus, Pacifastacus leniusculus and Procambarus clarkii) are actually functionally redundant in their effects on prey/shredder density and leaf litter breakdown. Results show that crayfish strongly influenced macroinvertebrate numbers and leaf litter breakdown and indicate that differences in direct (prey and leaf litter consumption) and indirect (prey habitat use and leaf litter breakdown) effects between crayfish species do exist. While the replacement of A. astacus by O. limosus may have induced only minor changes in freshwater ecosystems, invasions by the larger and more aggressive P. clarkii and P. leniusculus will likely have strong effects on invaded ecosystem. Overall, there seems to be no functional redundancy between these four species and outcomes of crayfish invasion will likely be species specific.     

Threat of non-native crayfish introductions into Turkey: global lessons

Introductions of crayfish species from their home range to new environments have been carried out in many parts of the world. The most important introduced crayfish species are Procambarus clarkii, Pacifastacus leniusculus, Cherax destructor, C. quadricarinatus, Orconectes limosus, O. rusticus and Astacus leptodactylus. The environmental impact of crayfish introductions can be positive, negative or neutral. However, native crayfish populations in Europe have been negatively affected by introductions of non-indigenous crayfish species from America. Negative effects of non-native crayfish introductions included displacement of native crayfish species, transfer of disease (crayfish plague), consumption of fish eggs, reduction of fish stocks, consumption of large amounts of macrophytes, indirect and direct effects on other invertebrates and upsetting production in rice fields. As a result of non-native crayfish introductions, the natural harvest and crayfish industry in Europe have been severely affected. Large quantities of Turkish A. leptodactylus were harvested (approximately 7,000 tonnes annually) and exported to Europe before the crayfish plague was observed in these populations. The total harvest of A. leptodactylus in Turkey reduced dramatically to 320 in 1991 after the plague. Therefore, although Turkey currently has no known non-native crayfish species, there is a threat of non-native crayfish introduction in order to increase crayfish productions and subsequent harvest. The North American spiny-cheek crayfish, O. limosus, has been spreading quickly down the River Danube and could soon reach neighboring countries including Turkey. The North American signal crayfish, P. leniusculus is known from Greece and could be a threat to native stocks if it is introduced into Turkey for aquaculture. Additional threats may come from the release of other North American species, which are widely available through the aquarium trade. We conclude that the spread of non-native crayfish introductions throughout Turkey will increase local problems, because introductions of non-native crayfish in many parts of the world have been known to have caused important reductions in population density and numbers of native crayfish species. Furthermore, freshwater ecosystems may be altered by such introductions and the economic viability of native crayfish species fisheries could be severely reduced in Turkey.     

Disturbance and species displacement: different tolerances to stream drying and desiccation in a native and an invasive crayfish

1. Crayfish are among the most threatened taxa in the world and invasive crayfish are the primary cause of the decline of native crayfish. Most research has emphasised biotic interactions as the mechanism by which native crayfish are displaced by invasives, although crayfish occupy variable environments and the role of disturbance in facilitating crayfish invasion and displacement is understudied. 2. We compared tolerance to a disturbance, stream drying, in a native and invasive crayfish as a potential mechanism to explain their distribution. Our experiments and observations were conducted across scales, from laboratory environmental chambers to stream mesocosms to field sampling. We hypothesised that the invasive crayfish would be more tolerant of desiccation than the native, and that this physiological distinction between the two would be reflected in their distribution in relation to stream drying. 3. In the laboratory, the native crayfish Orconectes eupunctus was less tolerant of desiccation than the invasive Orconectes neglectus chaenodactylus, with all native crayfish dying within 2 days without water, while some of the invasive crayfish survived for nearly 2 weeks. Under simulated stream drying in mesocosms, only the native O. eupunctus survived less well than in a control. Field sampling demonstrated a significant negative relationship between O. eupunctus density and low summer flows, while O. neglectus density was positively associated with low summer flows. The greater resistance of O. neglectus to drying could, through priority effects, inhibit recolonisation by O. eupunctus once flow resumes. 4. Abiotic disturbances are potentially important to the displacement of native by invasive crayfish. Disturbance mediated displacement of aquatic species provides both an opportunity to conserve native species by maintaining or restoring habitat and disturbance regimes and is also a challenge due to increasing human water demand, flow regime alteration and global climate change.     

Absence of the crayfish plague pathogen (Aphanomyces astaci) facilitates coexistence of European and American crayfish in central Europe

1. Most European crayfish species are strongly threatened, mainly as a result of the introduced pathogen, Aphanomyces astaci, transmitted by invasive North American crayfish. Long‐term coexistence of American and European crayfish species is therefore regarded as almost impossible, even though some coexisting populations have been observed. 2. In this study, crayfish were collected from presently coexisting populations of the introduced spiny‐cheek crayfish (Orconectes limosus) and the native noble crayfish (Astacus astacus) from nine standing waters in central Europe. Our aim was to resolve whether the coexistence resulted from reduced virulence in local strains of A. astaci, increased immunity in the native crayfish or an absence of the pathogen in these populations. We used highly sensitive A. astaci‐specific real‐time PCR to evaluate the crayfish latent carrier status, combined with transmission experiments to further validate the molecular results. 3. From the total of 523 crayfish tested (490 spiny‐cheek crayfish, 33 noble crayfish), none positive for A. astaci was detected. Transmission experiments confirmed these results: No abnormal mortality or behavioural changes were seen in noble crayfish kept together with American crayfish from the coexisting populations. If we assume a very low prevalence of A. astaci of 10% in a carrier population, there is a 98% probability of disease being absent in five of the nine coexisting populations tested. Hence, a consistent absence, or an extremely low prevalence, of A. astaci seems to allow the coexistence of European and American crayfish in these central European populations. 4. The results are important for native crayfish conservation and management and demonstrate that disease transmission risk may vary substantially between the different populations of spiny‐cheek crayfish in central Europe.     

Replacement of the native crayfish Astacus astacus by the introduced species Pacifastacus leniusculus in a small, enclosed Finnish lake: a 30-year study

The native noble crayfish Astacus astacus L., and the introduced North American signal crayfish Pacifastacus leniusculus Dana, co‐occur in Slickolampi, a small lake in southern Finland. That both species have lived side‐by‐side for 30 yr without any signs of crayfish plague Aphanomyces astaci, indicates that the P. leniusculus population must be plague‐free. According to annual trap catches and population size estimates, A. astacus was clearly dominant in the 1970s and most of the 1980s. At the end of the 1980s, however, there was a shift in the relative abundances of the two species, and in the 1990s, P.leniusculus became dominant. As the 1990s drew to a close, it accounted for >98% of total catches. Originating from a minor stocking (only 900 2nd stage juveniles) P. leniusculus has not augmented the existing fauna in this lake but has almost completely replaced A. astacus. Both species seem to a great extent to prefer the same types of biotope but P. leniusculus is distinctly more demanding and was encountered less often than A. astacus on gently‐sloping soft shores. The proportion of A. astacus with chelae injuries (16 yr, mean 17.3%) was nearly twice that of P. leniusculus (9.3%), suggesting that agonistic interspecific encounters do occur and that P. leniusculus is much more competitive. However, the consistent weakening of A. astacus, even at sites with only a low density of P. leniusculus, indicates that the elimination of A. astacus is not adequately explained by competitive exclusion. We suggest that its disappearance is governed by a combination of several interacting mechanisms, of which harvest (≥100 mm specimens of both species) and interspecific competition with P.leniusculus were initially the main reasons for the decline in the population. The ultimate reason for the collapse of A. astacus seems to have been the almost complete cessation of successful reproduction, presumably due to reproductive interference between the two species. Interspecific mating results in females laying sterile eggs. Although both species suffer from the ensuing loss of recruitment, the consequences are less serious for P. leniusculus, which has a higher capacity for population increase than A. astacus: the smaller the proportion of A. astacus, the greater the role played by reproductive interference as a replacement mechanism.     

The introduced signal crayfish and native noble crayfish have different effects on sublittoral macroinvertebrate assemblages in boreal lakes

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Plasticity in fecundity highlights the females’ importance in the spiny-cheek crayfish invasion mechanism

Invasion is one of the most consequential phenomena affecting the distribution of native species. Few in number of species, European crayfish are losing the competition with introduced North American crayfish. The spiny-cheek crayfish, Orconectes limosus, is an outstanding example, successfully competing against the native narrow-clawed crayfish, Astacus leptodactylus. For four years, we collected data regarding crayfish occurrences, their relative abundance, and the structure of populations in the ongoing colonisation process of O. limosus in the lower Danube. The mature females of both invasive and indigenous crayfish species were analysed with respect to biometry and production of oocytes in relation to the dynamics of invasion. The interspecific comparisons showed no significant differences regarding body size, with an average of approximately 102 mm total length and 31 g wet weight for both species. However, the fecundity of the indigenous species was found to be constant throughout the investigated area, whereas the number of eggs produced by the invasive females was significantly increased at the active front of the invasion. The maximum number of ovarian eggs found was 887 and 1156 in the indigenous species and the invasive species, respectively. We propose the scenario that the invasive species, which carries the deadly crayfish plague, creates an ecological advantage by reducing the populations of indigenous crayfish. Subsequently, the invasive females opportunistically use the available resources to enhance their fecundity, resulting in the acute growth of populations. However, the long-term competitiveness and colonisation success of O. limosus still remain in question.     

Niche differentiation among invasive crayfish and their impacts on ecosystem structure and functioning

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Associations between stream habitat characteristics and native and alien crayfish occurrence

Human mediated introductions of non-indigenous crayfish species (NICS) are responsible for their rapid colonisation of European freshwaters. The introduction of North American crayfish is furthermore linked to the spread of crayfish plague and the decline of indigenous crayfish species (ICS). As the management of ICS and NICS have become necessary, a detailed knowledge on their distribution and ecological requirements is needed. We studied the current range of native noble crayfish Astacus astacus and stone crayfish Austropotamobius torrentium, as well as alien signal crayfish Pacifastacus leniusculus in Carinthia (Austria) and evaluated environmental and physical habitat features in streams with and without crayfish. Meanwhile, the loss of many ICS populations was recorded and alien P. leniusculus was found to be widespread in this region. Most of the habitat features of streams having crayfish differed significantly from sites lacking crayfish for at least one investigated native or alien species. Furthermore, multivariate and regression analyses showed specific differences in the habitat use of the investigated crayfish. Our results showed that the presence of alien P. leniusculus was associated with larger and smoother sloped lowland rivers, while the occurrence of the two native species was confined to smaller streams either at higher altitudes and with distinct physical habitat conditions (A. torrentium) or with moderate water temperatures (A. astacus). This study helps to identify potential refuge areas for the endangered native species and to predict the further spread of the most common non-native crayfish species in European streams.     

Invasive species of crayfish use a broader range of predation-risk cues than native species

The responses of invasive and native species of crayfish to conspecific and heterospecific alarm odors were recorded in the laboratory. Individuals of the North American invasive Procambarus clarkii responded just as strongly to odors from crushed Austropotomobius pallipes as they did to crushed conspecifics. The North American invasive Orconectes limosus also responded as strongly to P. clarkii odor as to conspecific odor. The native Italian species A. pallipes responded more strongly to conspecific alarm than to heterospecific alarm from P. clarkii. The pattern of invasive species of crayfish using a broader range of danger signals than displaced native species appears to be robust.     

Are fish populations in temperate streams affected by crayfish? – A field survey and prospects

Synopsis Fish populations may be affected by predation and competition from various types of organisms, among which crayfish have been suggested as important actors. We here present results from stream surveys, suggesting that neither native noble, Astacus astacus, nor introduced signal crayfish, Pacifastacus leniusculus, necessarily affect fish population densities in temperate stream communities. Comparisons of fish densities within stream sites between years with absence and presence of crayfish showed no effect of either crayfish species. A further analysis of changes in fish densities between periods without and with crayfish in low, intermediate and high densities revealed that crayfish density did neither have an effect on fish densities. Our study is one of exceptionally few that consider the above aspects in long-term perspectives in natural systems, and we discuss that previously reported divergent results of crayfish effects on fish may be highly dependent on specific species and methods used, and that the effects of crayfish on fish populations deserve further attention to enable reliable predictions of community processes in streams.     

Comparative ultrastructure of the spermatozoa of three crayfish species: Austropotamobius torrentium,Pacifastacus leniusculus,and Astacus astacus (Decapoda: Astacidae)

This study reports about the spermatozoal ultrastructure of three species of astacid crayfish, i.e., the stone crayfish Austropotamobius torrentium, signal crayfish Pacifastacus leniusculus, and noble crayfish Astacus astacus. The acrosome is a cup shaped and electron‐dense structure at the anterior of the spermatozoon and comprises three layers of differing electron densities filled with parallel filaments that extend from the base to the apical zone. The acrosome was significantly longer in A. astacus than in P. leniusculus and the shortest acrosome belongs to A. torrentium. The width of the acrosome was significantly narrower in A. torrentium than in P. leniusculus and the widest acrosome belongs to A. astacus. The L:W ratio was significantly greater in A. torrentium than in P. leniusculus and the lowest ratio belongs to A. astacus. Radial arms are visible on each side of the acrosome or nucleus in sagittal view and wrap around the spermatozoon. Each radial arm comprises a parallel bundle of microtubules arranged along the long axis within a sheath. The nucleus, with decondensed material, is located in the posterior of the cell. All parts of the spermatozoon are tightly enclosed within an extracellular capsule. Despite a well‐conserved general structure and similarity of pattern among these spermatozoa, differences in the dimensions of the acrosome within the studied species may be useful to help distinguish the different crayfish species. J. Morphol., 2013. © 2013 Wiley Periodicals, Inc.     

The effect of inter- and intra-specific competition on survival and growth rate of native juvenile noble crayfish <Emphasis Type="Italic">Astacus astacus</Emphasis> and alien spiny-cheek crayfish <Emphasis Type="Italic">Orconectes limosus</Emphasis>

Juvenile noble crayfish, Astacus astacus and spiny-cheek crayfish, Orconectes limosus were reared from the 2nd and the 3rd developmental stage (i.e. since the stage of independence), respectively, until the end of the growing season. The first period revealed very low survival rate of O. limosus in the communal stock (7 ± 6.2%), probably caused by predation of A. astacus. Separately reared O. limosus proved significantly higher survival rate (49 ± 9.2%) than those in the communal stock with A. astacus. The latter species also showed the highest survival rate in this case (90 ± 7.1%). This was not found to differ from the survival rate of separately reared noble crayfish (86 ± 3.1%). In general, both the significant effect of bigger size of A. astacus than that of O. limosus and higher intra-specific cannibalism of O. limosus were reflected during the first phase of rearing. Size and weight of both species were equivalent 11 weeks after hatching (8 weeks of rearing). The higher cumulative survival rate was reached in the intraspecific culture of A. astacus (90%) while low cumulative survival of both species was found in communal stock (less than 50%) in the second phase of rearing. At the end of the growing season, 19-week-old O. limosus attained a higher mean size and weight but were more variable in the group (29.3 ± 5.17 mm and 616.4 ± 412.53 mg) than A. astacus (21.2 ± 1.82 mm and 229.9 ± 59.45 mg). Single and communal stock did not differ. Growth of O. limosus was much faster than that of A. astacus.     

Comparative analyses of <Emphasis Type="Italic">Astacus leptodactylus</Emphasis> morphological characteristics from Croatia and Armenia

Narrow-clawed crayfish, Astacus leptodactylus is a native European freshwater crayfish species, also distributed in Croatian freshwater systems belonging to the Black Sea drainage. Its taxonomical status is still in the process of change and discussion, and the data on morphological, molecular, ecological and zoogeographical characteristics of this species are scarce. Therefore, comparative analyses of morphological characteristics were applied with the aim to contribute to the knowledge on the morphometrical and meristical characteristics of A. leptodactylus. Recent research proved that measurements of a large number of morphometrical characteristics, in combination with multivariate statistical analysis, could provide a good instrument for identification and differentiation between populations. In this research altogether 143 animals were analysed (121 from two Croatian populations and 22 from Armenia). 22 morphometrical characteristics and 4 meristical characteristics, per crayfish, were measured. It was found that males and females differ between populations in measured meristical and morphometrical characteristics. None of recorded meristical characteristics proved itself to be reliable characteristic for distinguishing populations. From measured morphometrical characteristics the most discriminant characteristics for separating males from different populations were those describing carapace shape and for females those that describe shape of the claws.     

The harvest of freshwater crayfish, <Emphasis Type="Italic">Astacus leptodactylus</Emphasis> (Eschscholtz, 1823) in Turkey

Astacus leptodactylus is naturally and widely distributed in lakes, ponds and rivers throughout of Turkey. It has also been stocked in many freshwater systems in Turkey to establish new populations and to restore crayfish stocks devastated by the plague (Aphanomyces astaci) and some other reasons (i.e., farming and culinary purposes). Human consumption of A. leptodactylushas always been very low in Turkey; therefore, it was exported mainly to Europe until 1986. A plague was observed in 1985 in Eğirdir, Apolyont, Sapanca and Manyas lakes which included the main harvest areas, and the fishery was lost within 2 years. Consequently, crayfish harvest was banned in all populations to control the plague. Because of the plague, over-fishing and pollution, the harvest of A. leptodactylusin Turkey declined from approximately 5000 (1984) to 200 (1991) tons annually. In 2000s, there has been a gradual increase in the rate of harvest of A. leptodactylusin Turkey, reaching approximately 1600–1900 tons annually, but the plague is still observed in this species at certain localities. The present study reveals that A. leptodactylus in Turkey needs professional support for recovery, because the present harvest of A. leptodactylusis still nearly 20% of the crayfish catch in 1980s.