Distribution and abundance of the Louisiana red swamp crayfish Procambarus clarkii Girard at Lake Naivasha,Kenya between 1987 and 1999

The abundance and habitat distribution of Procambarus clarkii, an exotic herbivore at Lake Naivasha, Kenya, was studied between 1987 and 1999. Its growth and morphometrics were similar to other locations in the world where the species have been studied. P. clarkii was found primarily amongst the floating, exotic vegetation where it achieved densities of over 500 m^–2 by the end of the study. It is suggested that these densities became possible because the habitat offers protection against predation and an adequate food supply of bacterial-enriched detritus exists in the absence of green plant material. Both adults and juveniles forage on the lake bed at night.     

Feeding of the exotic Louisiana red swamp crayfish,Procambarus clarkii (Crustacea,Decapoda), in an African tropical lake: Lake Naivasha,Kenya

The Louisiana red swamp crayfish, Procambarus clarkii, has had a major impact on aquatic ecosystems in Europe and America where it has been introduced for human consumption. In Lake Naivasha, where it was introduced in 1970, it is suspected of causing the complete disappearance of native floating-leaved and submerged plants by 1982 and again by 1996; recovery of submerged plants occurred in between, concurrent with a decline in P. clarkii. Examination of the feeding of P. clarkii by microscopic analysis of gut contents collected from the lake confirmed information from both its native and exotic ranges; that it is an omnivore with green plants and insects the predominant items recognised. Feeding experiments confirmed this omnivory, with a preference from herbivory over carnivory. They further showed the importance of native plants as opposed to exotic water hyacinth Eichhornia crassipes. It is concluded that P. clarkii could well have accounted for the observed elimination of native plant species in Lake Naivasha; the cyclical nature of populations of submerged plant species and of P. clarkii in inverse proportion to each other lead us to hypothesise that P. clarkii is a `keystone' species in the lake ecosystem.     

Eleven polymorphic microsatellites isolated from red swamp crayfish, Procambarus clarkii

Eleven microsatellites were isolated from a genomic DNA library enriched for CA- and GA-repeats, and were characterized in 48 individuals of red swamp crayfish Procambarus clarkii. All 11 microsatellites were polymorphic with an average of allele number of 6.6 per locus. Genotypic distributions of these 11 markers were found to be in conformance with the expectations of Hardy-Weinberg equilibrium. All 11 markers were unlinked. These markers are being used to study the invasion routine, genetic diversity and population structure of the species P. clarkii.     

The topological organization of primary afferents in the terminal ganglion of crayfish,Procambarus clarkii

Summary The central projections of primary afferents in the terminal ganglion of the crayfish can be seen when an axonal filling with nickel chloride with subsequent silver intensification was used for identification. We describe here the topological relationships of the projections to the landmark structures of the neuropil.The terminal ganglion has five pairs of sensory nerves associated with the mechanosensory hairs and internal proprioceptors. The projection fields of the primary sensory neurons in the nerves Rl and R2 are almost entirely restricted to the ipsilateral half of the ganglion, whereas those of the nerves R3, R4 and R5 cross the midline to form three sensory commissures, A6SCI, A7SCI and A7SCII. The projection fields are segregated from each other, although all are restricted to the ventral neuropil which lies under the ventral intermediate tract (VIT). The intersegmental projections that ascend via the connective ipsilateral to their origins could be observed. This pattern of projection correlates well with the receptive fields exhibited by several mechanosensory interneurons on the body surface of the final segment.     

Multimodal responses of the nonspiking giant interneurons in the brain of the crayfish Procambarus clarkii

Three pairs of nonspiking giant interneurons (NGIs; G1, G2, and G3) of the crayfish brain responded with depolarizing and hyperpolarizing graded potentials to body tilt in roll to the ipsi- and contralateral sides in the dark. The higher and the larger the angle of body tilt, the larger was the amplitude of the geotactic responses. In ipsilaterally statocystectomized animals, all the NGIs responded with hyperpolarizing potentials only to the contralateral side-down tilt, whereas in contralaterally statocystectomized animals, they responded with depolarizing potentials only to the ipsilateral side-down tilt. In bilaterally statocystectomized animals, none of the NGIs responded to body tilt in the dark, but in the presence of an overhead light, they exhibited depolarizing and hyperpolarizing potentials in response to body tilt to the ipsi-and contralateral sides, respectively. All the NGIs responded with depolarizing and hyperpolarizing graded potentials to illumination of the contra- and ipsilateral eyes, respectively. The amplitude of these visual responses, however, varied in association with the amplitude of the geotactic response produced by body tilt. These results indicate that the NGIs integrate the sensory inputs from eyes and statocysts and that the interaction between sensory inputs from the left and right sensory organs with either the same modality or with different modalities enhance the directional sensitivity of NGIs as premotoneurons in the compensatory oculomotor system.     

Distribution of synapses on two types of ascending interneurons in the crayfish,Procambarus clarkii

Summary About 60 pairs of ascending interneurons are present in the terminal ganglion of the crayfish Procambarus clarkii (Girard). Some of these interneurons have been impaled intracellularly, characterized physiologically, and then labeled with horseradish peroxidase (HRP) to examine the distribution and ultrastructure of synapses. A close relationship between ultrastructure and physiological properties has been found between two types of interneurons, which either have a pre-motor effect upon motor neurons or have no such effect. In one interneuron with a pre-motor effect (6D2), input and output synapses are intermingled on thicker branches, whereas only input synapses are found on small diameter branches. Only input synapses have been observed on the branches in another interneuron with-out a pre-motor effect (6B1). No differences in branch morphology are found in these two interneurons. Interneuron 6D2 contains large numbers of small round agranular vesicles, but the same type of synaptic vesicles is rarely seen in interneuron 6B1, which has no output synapses. Our results indicate a good correlation between the synaptic distribution and pre-motor effects of interneurons in the terminal ganglion.Abbreviations A6, 7 Sixth and seventh abdominal segment of the terminal ganglion - AVC anterior ventral commissure - DC I dorsal commissure I - DIT dorsal intermediate tract - DMT dorsal medial tract - eLG extra lateral giant interneuron - LVT lateral ventral tract - LG lateral giant interneuron - LVT lateral ventral tract - MDT median dorsal tract - MG medial giant interneuron - MoG motor giant neuron - MVT median ventral tract - PVC posterior ventral commissure - R1s sensory fiber tract of nerve root 1 - R3m motor fiber tract of nerve root 3 - R4–7 nerve roots 4–7 - SC I,II sensory commissure I,II - VC I,III ventral commissure I, III - VIT ventral intermediate tract - VLT ventral lateral tract - VMT ventral medial tract     

Diel vertical distribution of zooplankton in Lake Naivasha,Kenya

Diurnal and diel vertical distribution of limnetic zooplankton species in relation to temperature and dissolved oxygen profiles was examined at a central station in Lake Naivasha. During calm days thermal stratification developed gradually from late morning to reach maximum formation at mid-day. Dissolved oxygen concentrations showed similar vertical profiles to temperature. These stratifications were, however, short lived and were broken up in late afternoons by the wind induced poly-holomictic nature of the lake. During the day most zooplankters aggregate at the top 3–4 metre zone of the water column coincident with maximum photosynthetic activity. The pattern of diel vertical distribution of zooplankton in Lake Naivasha is undefinedly even. The absence of significant diel changes in the distribution of the limnetic zooplankton may be related to the absence of permanent physico-chemical boundaries and lack of predation pressure in the open water.     

Organochlorine and organophosphorus pesticide concentrations in water,sediment, and selected organisms in Lake Naivasha (Kenya)

Water, sediment, red swamp crayfish (Procambarus clarkii) and black bass (Micropterus salmoides) from Lake Naivasha were analyzed for selected organochlorine and organophosphorus pesticide residues. The mean p,p'-DDT, o,p'-DDT and p,p'-DDE residue levels recorded in black bass (28.3 (± 30.0), 34.2 (±54.0) and 16.1 (±16.1) g kg^–1, respectively) and crayfish (4.6 (±5.1), 3.2 (±2.8), and 1.4 (±1.1) g kg^–1, respectively), were higher than previously recorded. This indicated recent usage of technical DDT in the lake's catchment. Levels of p,p'-DDT, higher than those of p,p'-DDE further emphasized this. Mean lindane, dieldrin, -endosulfan and aldrin concentrations in black bass were 100.5, 34.6, 21.6 and 16.7 g kg^–1, respectively. The same residues were detected at lower concentrations in crayfish at 2.0, 2.0, 2.0 and 1.9 g kg^–1, respectively. The higher fat content (3.7 ± 2.7% SD) in black bass (compared to 0.6 ± 0.3% in crayfish) accounted for the significantly higher residue concentrations in black bass. Organophosphate pesticides were the most commonly used pesticides in the lake's catchment, but none was detected in any of the samples. The results indicate that there is need for further work to identify sources and fate of pesticide contaminants, as well as to improve monitoring of pesticide use throughout the catchment.     

Invasion of the shelter snatchers: behavioural plasticity in invasive red swamp crayfish,Procambarus clarkii

1. In many freshwater systems, competition for shelter plays an important role in determining the persistence of both native and alien species. The red swamp crayfish, Procambarus clarkii, is currently invading the native habitat of the signal crayfish, Pacifastacus leniusculus, in southern Oregon, and interspecific competition for shelter may be driving the species replacement in this region. 2. We designed a 2 × 3 factorial mesocosm experiment, with shelter density and species combination as factors, to investigate shelter occupancy and resource competition. Contrary to our predictions, the two crayfish species are equal competitors for shelter. Further, the invasive P. clarkii modified its shelter occupancy behaviour in the presence of the native P. leniusculus and has broader microhabitat preferences. 3. Specifically, we found that P. clarkii alters shelter occupancy and space use patterns when the two species occurred together, such that shelter use was identical between P. clarkii and P. leniusculus in mixed‐species treatments. In such treatments, both species increased their use of shelters when shelter density increased. When P. clarkii was alone, however, individuals did not alter shelter use as a function of shelter density, whereas P. leniusculus exhibited similar density‐dependent behaviour in both mixed‐ and single‐species treatments. 4. In a complementary field survey, we employed an ‘epicentre‐based’ design to sample two field sites. We observed patterns of microhabitat use and breadth for each species similar to those in our mesocosm experiment: the invasive P. clarkii was more abundant across different habitats and used a broader range of microhabitats than the native P. leniusculus. As such, we found that P. clarkii was more abundant across both field sites than the P. leniusculus, occupying microhabitats within and beyond the preferred range of P. leniusculus. Both field sites were affected by urban development and agriculture. 5. The use of microhabitats by both species was similar in the laboratory and the field. This study confirms that P. clarkii individuals can, and do, successfully occupy microhabitats preferred by P. leniusculus in the Willamette Valley. The results from our study may be relevant to other freshwater systems inhabited by P. clarkii and contribute to the understanding of ‘niche opportunity’, a concept which defines the environmental conditions that promote biological invasions.     

Circadian rhythm does not affect responses to chemical cues in the red swamp crayfish, Procambarus clarkii

Studies of crayfish chemical ecology have been conducted in both day and night conditions. This variation may hinder the comparison of data among studies, if the responses by crayfish to chemical cues are dependent upon the time at which the cues are encountered. We tested the hypothesis that responses to chemical cues are dependent on observation time using the red swamp crayfish, Procambarus clarkii. Procambarus clarkii is known to exhibit a light-regulated circadian rhythm, with nocturnal activity peaks. Habitat use differed significantly between non-stimulated periods and periods of exposure to a food stimulus, but no effects of photoperiod (normal vs. reversed) or laboratory conditions (dark vs. light) were observed. The results suggest that, all else being equal, (1) studies of crayfish chemical ecology can be successfully conducted in a variety of experimental conditions, and (2) previous studies conducted at various times of the day should have comparable results.     

Molecular characterization of the sarcoplasmic calcium-binding protein (SCP) from crayfish Procambarus clarkii

Sarcoplasmic Calcium-binding Protein (SCP) is believed to function as the invertebrate equivalent of vertebrate parvalbumin, namely to “buffer” cytosolic Ca²⁺. We have cloned and characterized a novel SCP from axial abdominal muscle of crayfish Procambarus clarkii (referred to as pcSCP1), and have examined tissue specific distribution and expression as a function of molting stage in non-epithelial and epithelial tissues. The complete sequence of pcSCP1 consists of 1052 bp with a 579 bp open reading frame, coding for 193 amino acid residues (molecular mass of 21.8 kDa). There is a 387 bp 3′ terminal non-coding region with a poly (A) tail. The deduced pcSCP1 protein sequence matched most closely with published SCP sequences from another crayfish Astacus leptodactylus (92.8%) and from shrimp (78.6–81.2%) and fruit fly (53%). Real-time PCR analysis confirmed that pcSCP1 is ubiquitously expressed in all tissues tested (gill, hepatopancreas, intestine, antennal gland, muscle); however it is most abundant in muscle particularly in the axial abdominal muscle. The real-time PCR analysis revealed that pcSCP1 expression is downregulated in pre- and postmolt stages compared with intermolt. Epithelial (hepatopancreas and antennal gland) SCP expression exhibited a more dramatic decrease than that observed in muscle. Expression trends for pcSCP1 paralleled published trends for sarco/endoplasmic reticular calcium ATPase (SERCA), suggesting that their cellular function in regulating intracellular Ca²⁺ is linked.     

A Study of a population of Micronecta scutellaris Stal (Hemiptera : Corixidae) in Lake Naivasha,Kenya

Micronecta scutellaris is one of the most abundant macro-invertebrates found in Lake Naivasha, Kenya and forms an important component of the diet of juvenile large mouthed bass.Its distribution in the lake and breeding biology are described and considered in relation to the lake's food chains.     

Changes in fish species composition in Lake Naivasha,Kenya

The fish species composition in Lake Naivasha has been modified during recent years. Out of seven introduced and one indigenous species reported in 1962, only three introduced ones are found today. The elimination of species has been brought about by predation and changing ecological conditions. The most common fish species today is Sarotherodon leucosticuus. The hybridization found by previous workers between S. leucostictus x S. spilurus niger has stopped, as S. s. niger is no longer found in the lake. Most of the hybrid characters have been lost by the few surviving hybrids due to backcrossing between hybrids and between hybrids and S. leucostictus. The hybrids now mostly resemble S. leucostictus.     

Shelter competition between resident male red swamp crayfish Procambarus clarkii (Girard) and conspecific intruders varying by sex and reproductive status

The shelter defense dynamics of reproductively active (Form I) male red swamp crayfish Procambarus clarkii (Girard) were investigated by staging serial intrusions by male, maternal or non-maternal female conspecifics after one day of residence. The male residents showed a direct contest advantage only against non-maternal females, and won a significantly higher proportion of the encounters against non-maternal females than against either male or maternal female intruders. However, there was no significant difference in success against male or maternal female intruders. Further experiments against male intruders showed that increasing male prior residence to either 2 or 4 days did not significantly improve the residents' proportion of successful encounters. A final experiment revealed extremely low shelter fidelity in male P. clarkii, the most likely reason why male shelter defense against conspecific males and maternal females is not successful. These results, combined with those of past research, suggest that non-maternal females, juveniles and recently molted conspecifics are most vulnerable to predation and cannibalism, since males and maternal females easily out-compete non-maternal females for shelter, and adults out-compete juveniles. These findings may have implications for P. clarkii aquaculture and general management.     

Assessing mate size in the red swamp crayfish Procambarus clarkii: effects of visual versus chemical stimuli

1. Mate assessment requires an efficient system of information exchange between the sexes and often relies on several forms of communication. Both sexes of the crayfish, Procambarus clarkii , select a mate on the basis of its body size, independently of chelar size/symmetry and dominance status, large size being a proxy for a high quality partner. Here, we explored the effects that visual and chemical stimuli, emitted alone or in combination, exert on the assessment of size of a potential mate.
2. We followed a binary choice test paradigm, in which two male or female 'targets', of either large or small size, were simultaneously presented to a female or a male 'chooser'. We recorded the first target visited, the total duration of the visits per target, and the behaviour of the chooser when approaching the target area.
3. Our results show that females require a combination of visual and chemical stimuli to select the larger male. A more complex pattern was found when males were the choosers. While visual and chemical cues together rendered them willing to mate with the larger female, their initial choice and the total time spent near her depended only on smell. In mate assessment by P. clarkii , visual and chemical information seem to act as 'non-redundant signals'.     

Purification and properties of alanine racemase from crayfish Procambarus clarkii

Fresh water crayfish Procambarus clarkii is known to accumulate d-alanine remarkably in muscle after seawater acclimation, accompanied by an increase in alanine racemase activity. We have purified alanine racemase from crayfish muscle to homogeneity. The enzyme is a monomeric protein with a molecular mass of 58 kDa. It is highly specific to alanine and does not racemize l-serine, l-aspartate, l-glutamate, l-valine and l-arginine. The enzyme shows the highest activity at pH 9.0 in the conversion of l- to d-alanine and at pH 8.5 in the reverse conversion. Properties such as amino acid sequence, quaternary structure, pyridoxal 5′-phosphate (PLP)-dependency, pH-dependency and kinetic parameters seem to be distinct from those of the microbial alanine racemases. Various salts including NaCl at concentrations around seawater level were potently inhibitory for the activity in both of l- to -d and d- to -l direction.     

The diet of largemouth bass, Micropterus salmoides, in Lake Naivasha, Kenya

Lake Naivasha is a freshwater lake situated in the eastern rift valley of Kenya. Only five species of fish are present, all of which have been introduced. Of these, Oreochromis leucostictus, Tilapia zillii and Micropterus salmoides (largemouth black bass) support an important gillnet fishery with bass also being taken for sport. Until bass reached 260 mm f.l. they depended upon invertebrate food organisms. Thereafter crayfish, fish and frogs became increasingly important the larger the size of the bass. The most important invertebrate prey species was the water boatman, Micronecta scutellaris , followed by chironomid and culicid pupae. Zooplank-ton was consumed but only in large quantity by fish smaller than 80 mm. For bass over 260 mm the crayfish, Procambarus darkii , was the principal food. The largemouth bass in Lake Naivasha are generalized macro-predators, feeding principally on free-living animals of a kind most likely to be found in the littoral zones.     

Structure and dynamics of an invasive population of the red swamp crayfish (<Emphasis Type="Italic">Procambarus clarkii</Emphasis>) in a Mediterranean wetland

This study analyzes the population structure and dynamics of an invasive population of Procambarus clarkii (Girard, 1852) in a Mediterranean wetland using the Bhattacharya’s and Von Bertallanfy’s analytical methods. The main purpose was to collect biological data necessary for the management of this nuisance species. A maximum of five age classes were identified for both sexes, three of which being composed of a few or zero individuals. Age classes were classified into two subgroups––spring (SpL) and summer (SuL) lines––on the basis of the different hatching periods. Individuals of SpL showed a faster growth rate and reached a larger body size than those belonging to SuL, probably because they were able to grow for a longer time. No between-sex differences were found in growth patterns except for the asymptotic length (L_∞), which was reached faster by the females. Other population properties were analyzed, such as a high mortality rate, a maximum longevity of 4 years, and a low mean life-time (<12 months). Finally, a relatively small fraction of individuals seemed to survive after the first reproductive peak in spring. Consequently, the structure and dynamics of the study population seem to reveal its stability and spreading potential, as a confirmation of the invasiveness of P. clarkii in Mediterranean wetlands. Handling editor: P. Viaroli