The effect of manual removal on movement distances in populations of signal crayfish (Pacifastacus leniusculus)

1. Crayfish are amongst the most frequently introduced non‐native aquatic organisms, with well‐documented negative effects on a large number of freshwater taxa. Many crayfish‐control strategies make use of manual removal by trapping, a method known preferentially to remove the largest individuals, leaving the juvenile population almost entirely untrapped. 2. Removal by trapping may be used in an attempt to delay colonisation of new stretches by invasive crayfish. It is, however, unclear what effects trapping may have on movement distances of crayfish in wild populations. We examine the impacts of removal by trapping on the movements of American signal crayfish in two UK rivers. 3. We studied four 100 m stretches of two rivers, the Evenlode and Thame, comprising two removal and two non‐removal stretches. Each river supported both treatments. Half of the crayfish captured from the removal sections were removed and humanely destroyed by freezing, and half were marked with their trap location and released there. All crayfish captured from the non‐removal sections were marked and returned at the point of capture. 4. Mean movement distances were smaller in the removal stretches than the non‐removal stretches, both within capture sessions (10.8 and 16.0 m, respectively) and between sessions (14.5 and 24.6 m, respectively), suggesting that removal trapping resulted in the remaining crayfish making smaller movements. Larger crayfish under both treatments made substantially larger movements than those with smaller carapace lengths, both within capture sessions (range 7.6–19.6 m) and between range capture sessions (range 8.9–32.6 m). 5. The results of this study are consistent with expectations if removal by trapping lowered population densities, which we speculate may have affected movement distances directly or indirectly through increasing the availability of food and shelter. 6. This study suggests that trapping at the margins of a population may be sufficient to delay colonisation of new stretches by: (i) maintaining low densities and therefore reducing movements, and (ii) preferentially reducing the population of large individuals, which make the largest movements. However, it remains unlikely that any trapping programme can entirely prevent emigration /dispersal, and therefore colonisation, by signal crayfish.     

Immigration rates of signal crayfish (Pacifastacus leniusculus) in response to manual control measures

1. Crayfish are amongst the most frequently introduced non‐native aquatic organisms, with well‐documented negative effects on a large number of freshwater taxa. Many crayfish‐control strategies make use of manual removal by trapping, a method known preferentially to remove the largest individuals, leaving the juvenile population almost entirely untrapped. 2. A predicted outcome of trapping bias in riparian habitats is that removed individuals could be replaced by large crayfish immigrating from surrounding, untrapped, areas. We tested the hypothesis that removal by trapping of American signal crayfish from a U.K. river would result in increased rates of immigration, and increased distances moved, of crayfish from untrapped areas. 3. We studied four stretches of the River Windrush each 1 km in length and divided into three sections; a 250‐m long upstream section, a 500‐m middle section and a 250‐m downstream section. At two sites (removal sites), signal crayfish were trapped and removed from the 500‐m middle sections; at the other two (non‐removal), they were marked and returned. All crayfish captured in the upstream and downstream sections were marked and returned. 4. Probability of capture was higher for larger individuals with both chelae intact, and larger crayfish were more likely to immigrate from the upstream and downstream sections into the middle. The percentage of captured crayfish immigrating into the middle sections was the same (3.7%) in both removal and non‐removal sites. However, the mean distance that crayfish moved when immigrating was significantly greater at removal sites (239 m) than at non‐removal sites (187 m). 5. These results imply that removal of large individuals may have reduced the potential for interference competition by increasing the relative competitiveness of the immigrating individuals and permitting them to make larger movements. Consequently, the impact of manual removal strategies, both on the signal crayfish population and other biota affected by them, is likely to be reduced at the point of removal, but to extend at least 200 m beyond the trapped length of river.     

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

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Metabolic rate in the signal crayfish (Pacifastacus leniusculus) is temporally consistent and elevated at molting

Although the temporal consistency of resting metabolic rate in individual animals is generally considered to be a universal phenomenon, studies on invertebrates are still scarce. Here, we studied the repeatability of standard metabolic rate in the signal crayfish (Pacifastacus leniusculus). We measured oxygen consumption twice on the same individuals, on average in 97-day intervals. At intermolt stage, the standard metabolic rate was a repeatable trait. However, molting increased significantly the minimum metabolic rate, thus emphasizing the role of animal physiological state in determining the rate of metabolism.     

The response to disturbance chemicals and predator odours of juvenile and adult signal crayfish Pacifastacus leniusculus (Dana)

Many species of crayfish have been shown to use disturbance chemicals to detect the presence of predators and other dangers, yet there have been no investigations in adults or juveniles of Pacifastacus leniusculus, a key species, particularly for invasive species study. Juveniles and adults were exposed to conditioned water from stressed and damaged conspecific males. Both responded to these odours, but did not respond to water from stressed or damaged males with blocked nephropores. Urine from stressed and damaged males also elicited a significant response, which was much stronger than that of conditioned water. Odours from urine and water of damaged individuals induced a greater response than stress odours. We propose that chemicals released by stressed or damaged Pa. leniusculus are the same substance, but quantities released in the urine vary in context-specific circumstances. Predator-odour conditioned water from Perca fluviatilis elicited no response, but conditioned Anguilla anguilla water stimulated significant responses in juveniles and adults. Juveniles exhibited the strongest responses to all odours. These results show that Pa. leniusculus utilises both conspecific and heterospecific sources of info-chemicals to assess risk in the environment, which may contribute to its success as an invasive species and give it a competitive advantage.     

The signal crayfish is not a single species: cryptic diversity and invasions in the Pacific Northwest range of Pacifastacus leniusculus

1. We used historical sources, morphology‐based taxonomy and mtDNA sequence data to address questions about the signal crayfish Pacifastacus leniusculus. These included evaluating unrecognised cryptic diversity and investigating the extent to which P. leniusculus may have been introduced within its presumed native range in the Pacific Northwest region of North America. Our study builds and expands on Pacific Northwest phylogeographic knowledge, particularly related to patterns of glacial refugia for freshwater species. 2. Extensive collections (824 specimens) from British Columbia (Canada), Idaho, Nevada, Oregon and Washington (United States) were used to characterise P. leniusculus at the mitochondrial 16S rRNA gene. Genetic groups within the species were elucidated by phylogenetics and amova ; evolutionary relationships within the most common and diverse group were investigated using a statistical parsimony haplotype network, a nested amova , and tests of isolation by distance. Morphological measurements were used to relate findings of molecular analyses to three historically recognised P. leniusculus subspecies and characterise cryptic diversity by morphology. 3. We found substantial cryptic diversity, with three groups highly distinct from P. leniusculus in discrete geographic regions: the Chehalis River glacial refugium, Central Oregon and the Okanagan Plateau. Disjunct distributions of P. leniusculus relative to these cryptic groups and known patterns of Pleistocene glaciation and landscape evolution cast doubt on whether P. leniusculus is native to some areas such as coastal drainages of northern Washington and southern British Columbia. Morphological traits previously used to characterise P. leniusculus subspecies still persist but may be incapable of distinguishing P. leniusculus from newly discovered cryptic groups. 4. Cryptic diversity found within P. leniusculus highlights the pressing need for a thorough investigation of the genus Pacifastacus using data based on more extensive gene and taxon sampling. It also warrants conservation attention, as introductions of P. leniusculus within the Pacific Northwest may carry risks of hybridisation and introgression for cryptic groups. Owing to high genetic diversity and limited dispersal capacity relative to more vagile organisms like freshwater fish, crayfish of the genus Pacifastacus offer powerful potential insights into the geological history and phylogeography of the Pacific Northwest region. Finally, by shedding light on the long‐neglected native range of P. leniusculus, our results should also better inform our understanding of potential source populations for, and the ecology of, this important invasive species in regions including Europe, Japan and elsewhere in North America.     

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

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The artificial incubation of crayfish eggs: review and report from an experimental study concerning the effects of offspring origin (maternal or artificial incubation) on the survival and growth of juvenile signal crayfish (Pacifastacus leniusculus, Astacidae)

The development of artificial incubation techniques in astacid crayfish has attracted attention from scientists in many countries ever since the nineteenth century. It is only in the last few years that these techniques, along with studies on egg storage and transport, have provided reliable options for improving the reproductive phase in farming. The juveniles produced need to be reared until they reach a sufficient size both for restocking and for growing purposes. In view of the current level of knowledge of rearing juvenile astacids, two 80-day experiments were carried out under controlled conditions to compare the survival and growth of Stage 2 juvenile signal crayfish (Pacifastacus leniusculus) from two origins: maternal or artificial incubation. In the first experiment, three treatments were tested: juveniles from artificially incubated eggs with formaldehyde treatments, juveniles from maternal incubation and a mixture from both origins (50% each). Survival rates ranged from 87.8% to 93.3% with no significant differences among treatments. Crayfish from artificial incubation grew significantly faster (11.47 mm carapace length (CL), 373.80 mg weight) than crayfish from maternal incubation. In the second experiment, a bifactorial design included four treatments: the crayfish was derived from artificial or from maternal incubation and was fed once a day or twice a day. Final survival rates ranged from 68.89% to 77.78%, with no significant differences among treatments. Crayfish from artificial incubation grew significantly faster than crayfish from maternal incubation. The highest CL (14.54 mm) and weight (780.13 mg) were reached by the juveniles from artificial incubation that were fed once a day. No significant differences were found between the two feeding frequencies. Results showed that artificial incubation with formaldehyde treatments had no harmful effects and made it feasible to get a better performance from the juveniles obtained.     

Description of intracerebral ocelli in two species of North American crayfish: Orconectes limosus (Cambaridae) and Pacifastacus leniusculus (Astacidae)

Abstract. Among malacostracan crustaceans, intracerebral ocelli were first discovered in Isopoda, but they have been more recently reported from a crayfish ( Cherax destructor ) and a sandhopper ( Talitrus saltator ). This electron microscopic study increases the number of crayfish taxa in which intracerebral ocelli are now known to occur by two: Astacidae and Cambaridae. These photoreceptors are always integrated into the anteromedio-dorsal part of the brain and are not visible externally. Each ocellus is made up of 4–5 photoreceptor cells and is characterized by the presence of a fused rhabdom. The occurrence of different kinds of lysosomes in the cytoplasm is indicative of metabolic activity and perhaps membrane turnover. One typical feature of crayfish ocelli is their extraordinary variability in number. This trait is exemplified by individuals of Pacifastacus leniusculus , where as many as 14 ocelli were identified in a single brain. The arrangement of the ocelli is often not symmetrical with regard to the brain's midline and the ocelli always lack dioptric structures. Thus, it is difficult to see how they are involved in image formation. However, further research is needed to determine the precise role of these "hidden" receptors.     

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.     

New records and distributions of two North American branchiobdellidan species (Annelida: Clitellata) from introduced signal crayfish, <Emphasis Type="Italic">Pacifastacus leniusculus,</Emphasis> in Japan

This is the first report of two North American branchiobdellidans, Sathodrilus attenuatus Holt, 1981, and Xironogiton victoriensis Gelder and Hall, 1990, on the signal crayfish, Pacifastacus leniusculus (Dana, 1852) introduced into Japan from the Columbia River system, northwestern North America. Signal crayfish from 12 localities in eastern and northern Hokkaido, Japan, were examined and each supported S. attenuatus. In addition, an individual of this species was found on preserved material from Ishikawa Prefecture, central Honshu. All of these branchiobdellidans reported in Hokkaido most probably came from the original population of signal crayfish introduced into Lake Mashu, Hokkaido, Japan, in 1930. It is suggested that the use of non-pathogenic branchiobdellidans, when present, provides an easy method for tracing the spread of crayfishes around Japan and could also be applied in other countries and continents. Specimens of X. victoriensis were only found on crayfish in a stream at Akashina in Nagano Prefecture, central Honshu, Japan. Although the signal crayfish appears to be displacing the endemic Japanese crayfish, C. japonicus, no native branchiobdellidans were found on any of the introduced signal crayfish examined.     

Species richness and similarity of metazoan parasite communities in three species of leatherjacket (Oligoplites: Pisces: Carangidae) from the Pacific coast of Mexico

Species richness and similarity in metazoan parasite communities of fishes can be influenced by several biotic (age, body size, vagility, feeding and social behavior, among others), and local abiotic (temperature, salinity, etc.) factors. The parasite communities of three species of Oligoplites, marine fishes from the Pacific coast of Mexico, were quantified and analyzed. Four hundred sixty‐eight leatherjackets (O. altus, n=94; O. saurus, n=260; and O. refulgens, n=114) were collected from February 2016 to June 2017 from five locations. Twenty‐eight species of metazoan parasites were recovered and identified: four species of Monogenea (adults), nine of Digenea (seven adults and two metacercariae); two of Cestoda (larvae); four of Nematoda (two adults and two larvae); four of Acanthocephala (two adults, one juvenile, and one cystacanth); four of Copepoda; and one Pentastomida (larvae). At the component community level, species richness ranged from 9 in O. saurus to 19 in O. altus. Different species of helminth dominated the component communities of each species of host. Community composition and species richness of parasites differed among the three species of host, locations, and sampling years. Host feeding behavior, body size, and vagility had the most influence on these differences.