Large Crayfish Research Articles

Ontogenetic Variation in Food Consumption of Rusty Crayfish (Orconectes rusticus) in a Central New York Stream

ABSTRACT We examined feeding periodicity of three size groups of the rusty crayfish (Orconectes rusticus) at four-hour intervals over a 28-hour period during July in a headwater stream of the Susquehanna River drainage in central New York. Feeding activity was expressed as the ratio of stomach weight divided by the crayfish wet weight. The diel food consumption patterns of all three size groups of rusty crayfish (i.e., ≤ 10 mm, 11–20 mm, and > 20 mm carapace length) were significantly different. Peak feeding of the smallest crayfish occurred during crepuscular periods. Food consumption of the intermediate size crayfish was highest at 2000 h, and feeding of large crayfish was consistently high from 1200 h to 0400 h. Feeding intensity of both small and intermediate size crayfish was highest when feeding intensity of large crayfsh was lowest. Ontogenetic differences in feeding periodicity may be associated with predation pressure from large rusty crayfish on smaller individuals.

Size-dependent impacts of invasive alien crayfish on a littoral marsh community

The invasive signal crayfish ( Pacifastacus leniusculus) may undergo ontogenetic diet shifts with increasing size, but the consequences of crayfish size–structure changes for aquatic food webs are little known. We evaluated size-dependent impacts of signal crayfish on a littoral marsh food web using in-situ enclosures in a northern Japanese marsh. During the 60-day experiment, large crayfish (<30 mm orbital carapace length) rapidly eliminated submerged macrophytes through mechanical destruction while the comparable impacts of small crayfish (>30 mm) appeared after more time had passed. Benthic algal biomass was reduced in the presence of large crayfish but was affected little by small crayfish, suggesting that large crayfish were significant bioturbators. On the other hand, total invertebrate biomass was reduced while total invertebrate density was enhanced in the presence of both crayfish size classes. Potential crayfish impacts on invertebrates are: (1) predation on large vulnerable taxa such as Trichoptera and predatory invertebrates, (2) mechanical destruction of macrophytes, thereby reducing microhabitats for invertebrates, (3) heightened emigration of invertebrates in response to bioturbation and/or (4) predation or competition release on small invertebrates as a result of crayfish reducing their predators/competitors. Invertebrate taxa richness was nearly halved in the presence of large crayfish, probably reflecting reductions of rare taxa. Overall, functional roles of signal crayfish as omnivores and ecosystem engineers remained similar during the ontogenetic stage but the magnitude and rate of crayfish impacts intensified with increasing crayfish size. Thus, removal of large invasive crayfish alone may reduce detrimental impacts on aquatic biodiversity.

Shelter Occupancy by Mixed-Species Pairs of Native Signal Crayfish and Non-Native Red Swamp Crayfish Held in Enclosures

ABSTRACT Sheltering ability does not appear to confer an advantage to non-native red swamp crayfish (Procambarus clarkil) in a small, western Washington lake where the invader outnumbers the native signal crayfish (Pacifastacus leniusculus.) When paired with similar-sized non-native P. clarkii, P. leniusculus was adept at monopolizing limited artificial shelter inside enclosures placed on the bottom of Pine Lake. In these contests, the dominant crayfish or winner was typically the one with longer chelae, or, in the case of P. clarkii, the one that was sexually mature. Female crayfishes of both species also were adept at monopolizing the artificial shelter. Furthermore, irrespective of species and sex, when paired with smaller heterospecifics, large crayfish readily monopolized the artificial shelter. These results suggest that additional mechanisms besides shelter competition are contributing to the possible displacement of P. leniusculus in Pine Lake.

Population dynamics of the invasive crayfish (Procambarus clarkiiGirard, 1852) at two marshes with differing hydroperiods

The population dynamics of the red swamp crayfish (Procambarus clarkii Girard, 1852) was stud- ied from May 2005 to May 2006 in a pond (PCD) subjected to drought between June and December and in a continuously flooded rice field drainage channel (PMG) located in the Tejo river basin, Portugal. The pro- portion of females in the population was not independent from the season and was negatively correlated with both female and male maturation status. Male and female maturation was synchronous but males remained in a reproductive form after recruitment while females reverted quickly to lower maturation indexes. At PCD and PMG, respectively 2 and 5 cohorts were detected by modal analysis. Large crayfish were abundant in spring, summer and early autumn and juveniles were abundant in autumn, especially at PCD. Crayfish den- sity at PCD was lower than at PMG and mortality rate (Z) was 3.99 at PCD and 2.43 at PMG. The average lifespan of the crayfish populations was 91 days at PCD and 150 days at PMG. Our study indicated faster growth and larger maximum sizes at the site with the more stressed environment and lowest relative crayfish densities (PCD), suggesting that any exploitation or control strategies for these two populations should con- sider management of the hydroperiod and of maximum crayfish densities.

Invaders eating invaders: potential trophic interactions between the amphipodDikerogammarus villosusand juvenile crayfishOrconectes limosus

We investigated potential interspecific predation between two invasive crustacean species, currently widespread and co-occurring in European inland waters: the Ponto-Caspian amphipod Dikerogammarus villosus (Gammaridae) and the North American spiny-cheek crayfish Orconectes limosus (Cambaridae). We evaluated interactions of adult specimens of D. villosus and juvenile O. limosus (from the 3rd developmental stage – DS) under conditions with and without feeding. We used two different experimental setups: short-term (one-week) rearing in aquaria each containing 30 specimens of either single-species or mixed stock, and 20-daylong rearing in small circular plates with one individual of each species. In the aquaria, a significant effect of D. villosus presence on survival of O. limosus stocks was found, with stronger influence on unfed stocks. Survival of D. villosus was not influenced by either O. limosus presence or feeding. Direct predation of D. villosus on juvenile O. limosus, predominantly on the 3rd DS, was often observed in the small plates. The 5th DS O. limosus was killed only a few times and was already able to feed on adult D. villosus. Our results show that both species are able to affect each other negatively through intra-guild predation: D. villosus may successfully feed on juvenile O. limosus (3rd and 4th DS), but larger crayfish can resist predation by the amphipod.

Microhabitat use by foraging white‐clawed crayfish (Austropotamobius pallipes) in stream pools in the NE Iberian Peninsula

AbstractThe white‐clawed crayfish (Austropotamobius pallipes) is an endangered species across most of its distribution range, and information on its ecological requirements is needed to implement effective conservation measures. Its habitat use has been studied in different areas and at various spatial scales. However, being a nocturnal species, there is scarce information on its habitat selection during foraging periods. In this work we analyse nocturnal habitat use of white‐clawed crayfish in pools of a small stream in the northeastern Iberian Peninsula at two different scales: (1) microhabitat selection and (2) pool characteristics. Large crayfish showed a clear positive selection for deeper microhabitats, a selection pattern that was weaker for medium‐sized crayfish and absent for small ones. On the other hand, crayfish of all sizes avoided cobble and boulder microhabitats and positively selected fine substrate and more exposed microhabitats. Crayfish abundance in pools was positively influenced by pool area, pool depth and the availability of fine substrates, especially silt. While studies on white‐clawed crayfish habitat use have often stressed the importance of rough substrates as crayfish refuge, our results show that fine substrates are positively selected by foraging crayfish of all size classes and promoted active crayfish abundance in pools. These apparently contradictory results may be due to the differences in microhabitat preferences exhibited by active and inactive crayfish. Thus, our results help to better complete the picture of white‐clawed crayfish habitat requirements.

Grain size and sorting effects on size-dependent responses by lotic crayfish to high flows

We examined how sediment grain size and sorting mediate flow-induced dislodgment, movement (i.e., time spent crawling), and postural changes of small (10.5–19.9 mm carapace length [CL]), medium (20–29.9 mm CL), and large (30–42.4 mm CL) crayfish (Orconectes obscurus) subjected to incrementally increased flows in a laboratory flume. We also compared size-specific distributions of O. obscurus among three habitat types (deep pools, shallow pools, and riffles) with different current velocities and grain properties in a 4th-order stream in northeastern Ohio, USA. Number of crayfish dislodged increased with current velocity and crayfish body size on all grain sizes (small pebble, large pebbles, and small cobbles) and all sorting treatments (well, moderately, and poorly sorted) in the flume. Small crayfish were dislodged at significantly higher average current velocities than medium and large crayfish on all grain sizes and sorting treatments, in part because they were small enough to crawl into stone interstices to avoid high hydraulic stress. Movement by all crayfish decreased as current velocity increased on all grain sizes and sorting treatments. However, activity responses differed in a size-specific fashion. Crayfish of all sizes flattened their bodies against grains, flattened or tucked their telson under their abdomen, and attempted to use their chelipeds to grasp grains to resist dislodgment or aid in movement on the grain patch at increased velocities. Field collections showed that small crayfish occupied riffle habitats characterized by faster current velocity and larger grain size, whereas larger crayfish occupied deep and shallow pools characterized by slow current velocity and smaller grain sizes. Our results imply that body size is a critical factor in determining crayfish habitat distributions and resistance to high-flows in streams with stony beds.

The influence of productivity and width of littoral zone on the trophic position of a large-bodied omnivore

Omnivory is common in many food webs. Omnivores in different habitats can potentially change their feeding behaviour and alter their trophic position and role according to habitat conditions. Here we examine the trophic level and diet of the omnivorous signal crayfish (Pacifastacus leniusculus) in gradients of trophic status and lake size, both of which have been previously suggested to affect trophic position of predators separately or combined as productive space. We found the trophic position of omnivorous crayfish to be positively correlated with lake trophic status, but found no evidence for any influence of lake size or productive space on crayfish trophic position. The higher trophic position of crayfish in eutrophic lakes was largely caused by a shift in crayfish diet and not by an increase in trophic links in basal parts of the food web. Hence, our results support the "productivity hypothesis," suggesting that food chains can be longer in more productive systems. Furthermore, stable isotope data indicated that larger crayfish are more predatory than smaller crayfish in lakes with wider littoral zones. Wider littoral zones promoted the development of intrapopulation differences in trophic position whereas narrow littoral zones did not. Hence, differences in habitat quality between and within lakes seem to influence the trophic positions of omnivorous crayfish.

Habitat characteristics of crayfish (Paranephrops planifrons) in New Zealand streams using generalised additive models (GAMs)

Quantitative data on the habitat characteristics of stream crayfish have been generally lacking and competing demands on water resources has created a need to address this knowledge gap. We investigated day-time habitat relationships of stream crayfish (Paranephrops planifrons White) from 793 quadrats at 30 rivers and streams in the North Island, New Zealand to develop models of koura presence–absence and abundance. The model (stepwise GAM) included width, cover, median substrate size, edge location, velocity and depth, and correctly predicted presence–absence of crayfish (8–39 mm OCL) at 73.4% of quadrats and of young-of-the-year (YOY) ≤8 mm OCL at 83.4% of quadrats. Streams ranged from 1.6 to 11.5 m in width and the probability of finding both crayfish size classes reduced sharply as streams became wider than 6 m and as the substrate became large (i.e., boulder > 256 mm). Crayfish, particularly YOY, were most likely to be found in association with cover and at the stream edge. YOY were associated with shallow depths and fine substrates, whereas larger crayfish showed a preference for cobble substrate. Undercut banks, leaf litter, tree roots, and woody debris were strongly related to the presence–absence of crayfish. The model for crayfish abundance (log-linear Poisson GAM) explained 50% of the variation between quadrats with cover, velocity, edge location, depth, and the overall crayfish abundance at each particular stream being significant variables. Highest crayfish numbers were recorded in still or slow flowing water, with the majority occurring where velocities were below 0.4 m/s. Water depths up to 0.7 m were sampled, but highest numbers were found in depths of 0.2–0.3 m. Our presence–absence model determined variables that were significant over all streams, whereas our abundance model determined variables that were significant within streams. Use of the GAMs models enabled us to untangle the multiple factors contributing to habitat selection. Cover, velocity, and locations at the stream edge were important determinants of both presence–absence and abundance. Generally, substrate was important when comparing between streams, but not within streams, whereas depth was a significant determinant of abundance within streams, but not presence–absence between streams.

Does size matter? Relating consumed prey sizes and diet composition of otters in South Iberian coastal streams

We analysed seasonal changes in the sizes of prey [grey mullets (Mugilidae), flatfish (Soleidae), eelAnguilla anguilla and crayfishProcambarus clarkii] consumed by ottersLutra lutra Linnaeus, 1758 in a Mediterranean-climate coastal area and relate them to the frequencies of occurrence of each prey species in otter diet. The sizes of over 1500 otter prey were estimated from measurements of key pieces found in 814 otter spraints, which were collected in lower stream stretches in a sandy coastal area. Clear relationships between mean prey size and frequency of occurrence were observed for the four prey types. These relationships were positive for grey mullets, flatfish and crayfish (ie they occurred more in otter diet when more large individuals were predated), but was negative in the case of eels. Results suggest that these patterns could be related to seasonal changes in habitat use. Previous works in the study area showed that otter concentrate its predation efforts in freshwater stream stretches during spring and summer, when more and larger crayfish are available. Freshwater stretches have neither grey mullets nor flatfish, while eels are larger and scarcer there than near streams’ mouths, where crayfish is absent.

Activity patterns, behavioural repertoires, and agonistic interactions of crayfish: A non-manipulative field study

AbstractAgonistic behaviour of crayfish has been studied extensively in laboratory settings where pairs or groups of individuals are allowed to interact within an experimental arena. Crayfish agonistic behaviour within its natural context, however, has received little attention to date. The present, non-manipulative field study explored activity patterns, behavioural repertoires, and agonistic encounters of rusty crayfish (Orconectes rusticus) during the summer months using continuous, 24-hour, underwater video recording at a series of representative field sites. Following the filming, crayfish within the vicinity of the camera site were captured and measured. Individual densities were high, reaching a maximum of 68 individuals/m2 at some sites. Large crayfish predominantly inhabited deeper sections of the river and were mostly active at night, whereas small crayfish generally utilized the shallows and were active outside their burrows during day and dusk. Time outside their shelter was mainly used for feeding. Individuals frequently returned to the same shelter they had emerged from. Agonistic encounters were common events and generally occurred in the context of shelter acquisition or defense. Dyadic fighting progressed with escalating sequences of stereotyped aggressive acts. Furthermore, high intensities with unrestrained use of claws were seen in encounters between size-matched opponents. The results of this study allow us to root laboratory findings of crayfish aggression within a comprehensive, ethological framework and to consider ultimate consequences for individual fighting decisions and strategies.

Growth of redclaw crayfish ( Cherax quadricarinatus) in a three-dimensional compartments system: Does a neighbor matter?

Rearing crayfish in individual compartments could be a means of overcoming social-dependent density limitations in order to increase yields per unit area in intensive culture. Two experiments were conducted to evaluate the impact of neighbors on the growth of redclaw crayfish ( Cherax quadricarinatus) males in three-dimensional units, consisting of 144 individual compartments, designed for intensive battery culture. The first experiment revealed significantly lower growth rates in crayfish surrounded by neighbors on all sides than in those reared with no neighbors in adjacent compartments, or with neighbors only in compartments on the same level but not above or below them. The second experiment revealed significantly lower growth rates in relatively small crayfish surrounded by large crayfish in a mixed-size unit than in those surrounded by similar-sized small crayfish in a uniform-size unit. No such effect was found for relatively large crayfish surrounded by small crayfish. The results clearly indicated interactions between crayfish in our system, despite them being housed in separate compartments. Growth was adversely affected by the presence of neighbors in adjacent compartments, especially vertically adjacent ones, and the presence of larger individuals in adjacent compartments. This social effect was attributed to partial tactile contact between neighbors in our system. The consequences of these findings for battery culture of crayfish are discussed.

Social control of growth in the redclaw crayfish, Cherax quadricarinatus: testing the sensory modalities involved

The magnitude of and the sensory involvement in social control of growth in Cherax quadricarinatus were analyzed in the laboratory. In the first experiment 80 pairs of juvenile males, each comprising a relatively small (2.0 g) and a relatively large (3.5 g) crayfish, were raised for 66 days under six different types of social contact. The pairs were raised in full contact with either intact or immobilized claws, separated by a partition with the allowance of transmission of chemical and/or optical cues, and without any contact. Any type of contact with the small one did not affect the growth of the large individual. However, the growth of the small individual was reduced by 50% only when in full contact with the large one, because of both increased time between successive molts and a reduction in the size increment per molt. In the second experiment small juvenile males were reared for 98 days in chemical and/or optical contact with much larger adult males. As in experiment 1, no effect of the chemical and/or optical cues on growth was found. The relevance of these findings to C. quaricarinatus culture is discussed.

Growth and population size of crayfish in headwater streams: individual‐ and higher‐level consequences of acidification

Summary1. Environmental stress may have indirect positive effects on population size through modification of food‐web interactions, despite having negative effects on individuals. Here we evaluate the individual‐ and population‐level effects of acidification on crayfish (Cambarus bartonii) in headwater streams of the Allegheny Plateau (PA, U.S.A.) with field experiments and survey data. Median baseflow pH of 24 study reaches in nine streams varied from 4.4 to 7.4, with substantial variation found both among and within streams.2. Two bioassays were conducted to evaluate the relationship between stream pH and crayfish growth rates. Growth rates were always higher in circumneutral reaches than in acidic reaches. Crayfish originating in acidic water grew less when transplanted into neutral water than did crayfish originating in neutral water, providing some evidence for a cost of acclimation to acidity.3. Stream surveys showed that fish were less abundant and crayfish more abundant in acidified streams than in circumneutral streams. Crayfish density was sixfold higher in reaches with the lowest pH relative to circumneutral reaches. Large crayfish made up a higher proportion of crayfish populations at sites with high fish biomass, consistent with the hypothesis that fish predation on small individuals may be limiting crayfish population size at these sites.4. Although individual crayfish suffered lower growth in acidified streams, increased acidity appeared to cause an increase in crayfish population size and shifts in size structure, possibly by relieving predation pressure by fish.

Ecosystem engineering by crayfish in a headwater stream community

AbstractWe conducted an enclosure/exclosure experiment in a forested, headwater stream to determine if crayfish acted as ecosystem engineers in this community. Three sets of channels (4 channels per set) were placed in separate pools. All channels contained cobble-filled baskets. Large crayfish (Cambarus bartonii) or a 5 g chestnut oak (Quercus prinus) leaf pack were added to some channels. The 4 treatments in the experiment were: 1) no crayfish/no leaf pack, 2) no crayfish/leaf pack, 3) crayfish/no leaf pack, and 4) crayfish/leaf pack. This design allowed us to assess the effects on the benthic community of adding crayfish and a leaf pack. It also allowed us to assess the effect of large crayfish on leaf pack breakdown. Leaf pack breakdown rate was significantly faster in the presence of crayfish. Crayfish did not have a significant effect on the abundance of any invertebrate taxon associated with leaf packs. However, few large (≥3.0 mm long) chironomids were found in leaf packs exposed to crayfish after...

SCALE-DEPENDENT ASSOCIATIONS AMONG FISH PREDATION, LITTORAL HABITAT, AND DISTRIBUTIONS OF CRAYFISH SPECIES

To predict how species establish and disperse within novel communities, the spatial scale at which competition, predation, and habitat interact must be understood. We explored how these factors affect the distribution and abundance of the exotic crayfishes Orconectes rusticus and O. propinquus and the native O. virilis at both the site-specific and whole-lake scales in northern Wisconsin lakes. During summer 1990, we quantified crayfish, fish predators, and fish diets in cobble and macrophyte sites in Trout Lake, com- paring resulting patterns to those in 21 lakes surveyed during summer 1987. Within and across lakes, fish abundance was unrelated to habitat. Within Trout Lake, O. rusticus and O. propinquus were common in both cobble and macrophyte. Orconectes virilis was re- stricted to macrophyte, probably due to strong displacement by the invaders in cobble. Across lakes, O. rusticus increased where habitat was more than 16.7% cobble, O. pro- pinquus was generally rare, and O. virilis abundance was unrelated to cobble. Crayfish were generally small in cobble and large in macrophyte, perhaps because of habitat-specific, size-selective fish predation or because large crayfish leave cobble when it no longer pro- vides refuge. Orconectes virilis, the largest of three congeners, may have a size refuge in macrophyte but not in cobble. Across lakes, O. rusticus was only abundant when fish biomass was low; O. virilis abundance varied positively with fish. Effects of fish predation and habitat on the ability of invaders Orconectes rusticus and O. propinquus to establish and replace O. virilis appear to be scale dependent. At local (site-specific) scales, cobble likely interacts with selective predation for O. virilis to allow the invaders to establish and replace the native. At the lake-wide scale, high cobble facilitates invaders but predation may curb their successful dispersal and establishment at new sites. Models of community assembly and invasions need to incorporate scale dependencies in habitat availability and biotic interactions to effectively assess the invasion potential of novel species.

Effects of Experience and Body Size on Refuge Choice in the Crayfish Orconectes immunis

ABSTRACT We investigated whether refuge size or experience with a refuge affected the refuge use of male Orconectes immunis crayfish. Individuals were given choices among seven refuges for 10 consecutive days. Refuges were formed from equal length but different diameter PVC pipe and placed in an array in a random sequence. Three treatments were used. In the Novel Refuge treatment, individuals were placed in a new test arena with a new arrangement of cleaned refuges every day. In the Nonremoval treatment, individuals were left in the same arena with the same set of refuges each day. In the Removal treatment, individuals were removed from the refuges each day but placed back in the same arena with the same set of refuges after the refuges had been cleaned. We found that refuge occupation was correlated with an individual's size; smaller crayfish tended to use smaller refuges than larger crayfish, even though all crayfish could fit in all of the different sized refuges. When first tested, individuals initially chose larger refuges than they would subsequently settle in, suggesting that under duress, they were not as particular about refuge characteristics. Individuals in the Nonremoval and Removal treatments were significantly more consistent in their refuge use than those in the Novel Refuge treatment, suggesting that experience with a particular refuge increased use of that refuge. Individuals from the Novel Refuge treatment that were housed for a month with a single refuge did not increase their use of that sized refuge more than those that were housed without a refuge, indicating that simply occupying a refuge of a given size did not affect refuge preference.

Heart Activity in the Crayfish Astacus astacus during Early Ontogenesis

Using a non-invasive method of recording of heart activity in crayfish Astacus astacus in early ontogenesis (1–9 months) and analysis of parameters of cycles of laser light cardiogram, it is shown that cardiac contraction rate (CCR) decreases with increases of weight and size of the animals. The range of changes of CCR parameters in air and in water is wider in larger crayfish of the same age, than in crayfish retarding from them in size and weight. This seems to allows the larger crayfish to be better adapt to environmental changes. It is shown that in crayfish, at change of environmental conditions, inotropic parameters of the cardiogram, amplitude of diastole and systole (DA and SA), change more, than chronotropic parameters, duration of diastole and systole (DD and SD). This may indicate that systems of regulation of inotropic parameters of the cardiac activity are the first to become mature in ontogenesis of the crayfish cardiovascular system; later, maturation of system of regulation of chronotropic parameters of the heart activity occurs. This is confirmed by results of experiments on adult crayfish. Analysis of correlations between parameters of cardiogram cycles in crayfish has shown that regardless of crayfish weight, higher correlation coefficient values are revealed between inotropic parameters of the heart activity cycles, than between chronotropic parameters. In larger crayfish, DD values are higher, than SD, whereas in stress, SD is longer than DD. During stress, the highest negative values of the correlation coefficient are revealed between CCR and DA, CCR and SA regardless of in which medium (air, water) the crayfish is placed.

MANAGEMENT STRATEGIES, YIELD AND POPULATION DEVELOPMENT OF THE NOBLE CRAYFISH ASTACUS ASTACUS IN LAKE STEINSFJORDEN

Lake Steinsfjorden is the most important noble crayfish locality in Norway. The crayfish population has been monitored annually since 1979 including data on total trapping effort, yield and population composition before and after the catching season. The harvest has ranged from 0.7-4.7 kg.ha-1. The catch per trap night decreased prior to 1991 and the yield was reduced by some 50% over the years 1987-1992 as compared to 1979-1986. This was due to removal of small crayfish, high exploitation and the establishment of dense stands of Canadian pondweed (Elodea canadensis). Baited traps catch a large fraction of crayfish &lt; 95 mm total length minimum size. These should according to the regulations immediately be released into the lake. However, it is apparent from the size composition that release of these small crayfish was rather incomplete. In 1983, trap mesh size was increased from 17.5 mm to 21 mm to reduce the fraction of undersized crayfish in the trap catches, yet no effects of this increased mesh size on crayfish size distribution were observed. The legal season has been reduced three times during the study period. In 1983, the closing date was changed from 31 December to 15 September. In 1989 the legal season was further reduced to two weeks and finally in 1995 to 10 days. This reduced total trap effort by 45%. The shorter season allow many crayfish to moult twice instead of once between seasons and the fraction of large crayfish has thus increased and so has the catch per trap night. Canadian pondweed has established dense annual stands and thus has made large parts of the shallow areas unsuitable for crayfish, causing an overall decrease in crayfish population size and production.

THE SIGNIFICANCE OF THE CRAYFISH PARANEPHROPS ZEALANDICUS AS SHREDDERS IN A NEW ZEALAND HEADWATER STREAM

Abstract Despite their prevalence in headwater streams, crayfish are largely ignored in most leaf decomposition studies. We conducted a field survey in 30 pools along a headwater stream to document the biomass of various shredder invertebrate species and subsequently compared their leaf processing and particulate organic matter (POM) production rates in a laboratory experiment. The crayfish Paranephrops zealandicus dominated the shredder functional feeding group, comprising 99% on average of the total biomass of shredder invertebrates. This was followed by the stonefly Austroperla cyrene, which made up only 0.62%. A laboratory experiment, using stoneflies and two size classes of crayfish, showed that processing rates of large crayfish (normalised by body weight) were lower than those of small crayfish and stoneflies. However, large crayfish had the greatest impact on leaf decomposition and POM production. It is their large body size and dominance of invertebrate biomass that determines the significance of...