Encyclopedia of Inland Waters || Decapoda

Download Encyclopedia of Inland Waters || Decapoda

Post on 08-Dec-2016

214 views

Category:

Documents

0 download

TRANSCRIPT

  • DecapodaA

    Crayfish, crabs, and shrimp are members of thecrayfish create some type of burrow and these bur-rows are often important habitats for other species.Freshwater decapods occur on all continents

    except Antarctica. Crayfish are the dominant group

    into an enlarged limb with a large chela. In freshwatershrimp, if an enlarged limb with a large chela ispresent (e.g., Macrobrachium), it is the second pairof chelae, not the first, that are enlarged. The gillsR Creed, Appalachian State University, Boone, NC, US

    2009 Elsevier Inc. All rights reserved.

    Introduction

    Decapod crustaceans (crayfish, shrimp, and crabs)are the largest invertebrates inhabiting freshwatersystems (Figure 1(a)1(d)). While the adults of mostfreshwater decapods range in length from 2 to 15 cm,some are giants. The giant Tasmanian crayfish (Asta-copsis gouldi) can reach lengths of 40 cm and is thelargest freshwater invertebrate in the world. Densitiesof decapods can also be high. For example, densities ofadult crayfish and crabs are often greater than 3 persquare meter; densities of adults and young combinedcan exceed 10 per square meter. Densities of someshrimp species (e.g., Caridina, Atya, Palaemonetes)can be quite high (50100 per square meter); densitiesof other shrimp species (Macrobrachium) are muchlower (usually

  • (272 Invertebrates _ Decapoda(a)arise from the base of the legs and are covered by thelateral margins of the carapace. The abdomen isclearly segmented. The abdomens of crayfish andshrimp are large and approximately the same size asthe cephalothoracic region. The abdomens of crabsare much smaller and are tucked under the posteriormargin of the cephalothorax. There are also appen-dages on the abdomen called swimmerets or pleo-pods. These are well developed in crayfish andshrimp and can be used for swimming. They are notas well developed in crabs. The pleopods are used forholding eggs in female decapods.Decapods, like other arthropods with an exoskele-

    ton, can only grow by molting. When they molt, theyshed their old exoskeleton. The animal that emergesis extremely soft and highly vulnerable to predators.

    (c) (

    (e)

    Figure 1 Representatives of the major groups of freshwater decashrimp (Macrobrachium eriocheirum, Palaemonetes antennarius), (d)

    the absence of pigment). Photographs (a) and (e) by Robert Wayne v

    reproduced with permission.b)Thus, most decapods molt in a safe environmentsuch as a burrow or under a rock. The new exoskeletonforms beneath the old one. Once they have shed the oldexoskeleton decapods drink a lot ofwater to increase insize. Over the next few days the new exoskeleton hard-ens at this larger size. The excess water is then excretedand the animal now has room to grow until the nextmolt. Young decapods may molt many times in theirfirst year of life. Older individuals may molt only onceor twice a year.Lengths of crayfish and shrimp are determined as

    either carapace length (from the tip of the rostrum ormargin of the carapace immediately behind the eye tothe posterior margin of the carapace) or total length(to the tip of the tail). Crabs are measured differently;carapace width is the commonly used measure.

    d)

    pod crustaceans: (a) crayfish (Cambarus chasmodactylus), (b, c)crab (Potamon potamios), and (e) cave-dwelling crayfish (note

    an Devender; photographs (b), (c), and (d) by Werner Klotz;

  • Walking legs

    12

    3 4 5

    Antenna

    Antennule

    Eye

    RostrumCephalothorax Abdomen

    Pleopods

    Telson

    (a)

    Eyes

    1

    2

    3

    4

    5

    Walkinglegs

    Carapace

    (b)

    Invertebrates _ Decapoda 273Reproduction and Life History

    Crayfish

    The life histories of less than 10% of the knowncrayfish (over 540 species of crayfish have been iden-tified worldwide) have been described in detail. Themembers of the three families of crayfish, the Astaci-dae, Parastacidae, and the Cambaridae, have similarlife histories but differ in terms of whether or notmales exhibit sexual dimorphism (discussed later).Crayfish tend to copulate in autumn, although copu-

    Table 1 The infraorders of freshwater decapod crustaceanswith some prominent families and selected genera

    Infraorder Family Selected genera

    Astacidea(crayfish)

    Astacidae Astacus, PacifastacusCambaridae Cambarus,

    Orconectes,

    Procambarus

    Parastacidae Astacopsis, Cherax,Paranephrops

    Brachyura

    (crabs)

    Grapsidae Metopaulias,

    SesarmaParathelphusidae Barytelphusa,

    Spiralothelphusa

    Potamidae Candidiopotamon,

    PotamonPotamonautidae Potamonautes

    Caridea (shrimp) Atyidae Atya, Caridina,

    Paratya

    Palaeomonidae Palaemonetes,Macrobrachium

    Xiphocarididae Xiphocarislation may occur throughout the year in some species.Male crayfish immobilize the female by grabbing herlarge chelae on walking leg 1 with his large chelae androlling her onto her back. Sperm, in the form of aspermatophore or sperm plug, are transferred viaa modified pair of pleopods (called gonopods) locatedon the first abdominal segment. Females may carry aspermatophore for several months before ovipositionoccurs the following spring. When releasing eggs,females secrete a sticky substance called glair that isused to attach the eggs to her pleopods. Once a femalehas released all of her eggs and the glair has hardenedshe is said to be in berry (Figure 3(a)). The numberof eggs a female can carry while in berry is a functionof body size, egg size, and species. Females will carryeggs for several weeks; the duration of this perioddepends on the species and is also influenced bywater temperature. Crayfish exhibit direct develop-ment; there are no free-living larval stages and theyoung crayfish that hatch from the eggs look likesmall adult crayfish. When the eggs hatch the young

    Abdomen

    (c)Figure 2 Morphology of generalized crustaceans. (a) Majorbody regions and appendages of a generalized crayfish.

    Shrimp are similar to crayfish in having large, obvious

    abdomens. Shrimp differ morphologically from crayfish in

    several ways, including the number of walking legs with chelae(chelae are present on only walking legs 1 and 2), in the size

    and shape of the rostrum, the degree to which the body is

    laterally flattened, etc. Also, shrimp which have enlarged limbs

    with enlarged chelae have them on the second pair of walkinglegs, not the first. (b) Major body regions of a generalized crab.

    (c) Ventral view of crab showing abdomen. Drawings by

    Robert Creed.

  • 274 Invertebrates _ Decapoda(a)crayfish initially remain attached to the female.The young will undergo three molts during thisperiod. It is at this stage that the young leave thefemale and become free-living. They are approxi-mately 1 cm long (total length).Young crayfish are highly vulnerable to a wide

    range of predators and tend to aggregate in habitatsin which they are the safest, e.g., shallow water habi-tats. During their first summer they molt several timesand increase in size considerably. By the end of theirfirst summer they may have quadrupled in size. Withtheir increased size they have outgrown many of theirpotential aquatic predators with the exception oflarge fish and adult crayfish.Unlike astacid and parastacid crayfish, most

    mature male cambarid crayfish exhibit sexual dimor-phism. Specifically, they molt in and out of a sexuallycompetent form. Sexually competent males arereferred to as first form males. Male cambarids moltinto this form in the autumn prior to copulation.They remain in this form throughout the autumnand winter months. Additional copulation may occurin the spring. In late spring, they molt into second

    tonic larvae are released into the water column. One

    (b)

    Figure 3 (a) Eggs attached to the pleopods of a femalecrayfish. Photograph by Chris Lukhaup. (b) A female shrimp

    carrying eggs. Photograph by Werner Klotz.female can release several thousand larvae. Larvalshrimp drift downstream, primarily during thenight, in some cases drifting all the way to estuarinehabitats. They spend part of the juvenile phase inestuarine habitats. Larvae of these species requireexposure to brackish water to complete their devel-opment. After a period of 24months juveniles moveinto freshwater and begin migrating back upriver toheadwater streams. Fish predators can take a toll onshrimp during these migrations.Not all tropical shrimp larvae drift all the way to

    estuarine habitats. In some species, the larvaeform males. The major morphological changes thattake place when male cambarids molt into first formare changes in the length and shape of the gonopods.The large chelae on the first walking leg will alsobecome enlarged. Adult crayfish can reproduce mul-tiple times during their lives.The lifespan of crayfish varies from species to spe-

    cies. Most species found in surface waters live24 years. However, some species that live at higherlatitudes may live 710 years. The giant Tasmaniancrayfish may live in excess of 15 years. Some cave-dwelling crayfish may also live longer than 10 years.

    Shrimp

    Although abundant in many tropical and subtropicalfreshwater systems, the life histories of only a fewspecies of shrimp have been described in detail. Fresh-water shrimp exhibit a greater range of reproductivestrategies than is observed in crayfish. Direct devel-opment has been described in some shrimp species.Other species produce free-living larvae and thelength of the larval period may be short, intermediate,or long. Regardless of reproductive strategy, allfemale shrimp extrude eggs and attach them to theabdominal pleopods (Figure 3(b)).Copulation in shrimp is similar to that described

    for crayfish. Mating may occur seasonally in temper-ate species. It may occur year round in tropical speciesalthough seasonal breeding (e.g., breeding during thewet season) has been described for some tropicalshrimp species.Females with attached eggs may be found through-

    out spring, summer, and early autumn in some sub-tropical/temperate shrimp (e.g., Palaemonetes spp.).Young shrimp molt and grow during their first year atwhich point they reproduce.Adults of several species of tropical shrimp migrate

    to and reproduce in headwater streams, exhibiting adiadromous life cycle that some researchers callamphidromy. Females will extrude eggs and attachthem to their pleopods. When the eggs hatch plank-

  • pods. Females in some species of freshwater crabs

    feeding in streams. Juvenile crabs only feed in aquaticenvironments. While shrimps and crabs consume a

    Invertebrates _ Decapoda 275remain in aquatic environments while they are carry-ing their eggs. In other species, females with eggs leavethe water andmove to the forest floor in the vicinity ofthe water body. These females may spend this periodin burrows, under stones, or in some other moistenvironmentwhile their eggs develop.Theywill returnto water when their eggs are about to hatch. There issome evidence suggesting that this strategy of brood-ing eggs in the terrestrial environment and returninglater to release the hatching young crabs may be astrategy to reduce loss of young crabs during floodsin streams during the rainy season.In some species, the newly hatched crabs will

    remain with the female for 12weeks. This maternalcare undoubtedly increases the survival of the youngcrabs. Free-living young crabs appear to live exclu-sively in aquatic environments. Adult crabs may alsoonly drift a short distance downstream and thelife cycle is completed entirely in freshwater. Theselarvae do not require exposure to brackish water todevelop successfully. An important determinant ofwhether or not a shrimp species will exhibit an estua-rine larval phase is the length of the river and thedistance a larval shrimp will have to drift to reachan estuary. Tropical shrimp having life historiesthat include an estuarine stage occupy shorter riversand their larvae can reach the estuary in 12 nights ofdrifting.Shrimp lifespan appears related to body size.

    Smaller species (e.g., Caridina spp., Palaemonetesspp.) may only live 12 years. However, this is notalways the case, as some Caridina species may live610 years. Larger species (e.g., Macrobrachium sp.or Atya sp.) may live 24 years.

    Crabs

    While there are more than 600 described species offreshwater crabs the reproductive biology of fewerthan 10 species has been studied in detail. Mating infreshwater crabs appears to be similar to crayfish.The male immobilizes the female by grabbing herlarge claws, flips her on her back, and then copulates.Unlike crayfish and shrimp, crabs have to relax theirabdomens in order for the male gonopods to comeinto contact with the female opening. Spermato-phores are transferred from the male to the femalevia the gonopods.Female freshwater crabs produce many fewer eggs

    than do marine crabs; their eggs are also much larger.Like crayfish, many freshwater crabs exhibit directdevelopment; a few species, however, have free-livinglarvae with an abbreviated larval period. Femalefreshwater crabs extrude their eggs onto their pleo-wide range of food types, like crayfish, they too mayturn out to be largely carnivorous.

    Predators

    Decapods are consumed by many different types ofanimals. Potential predators include aquaticbe fully aquatic. However, in some species, adultsmay become amphibious moving onto land to feed,often at night.There is little information on the lifespans of fresh-

    water crabs. Available information suggests that theylive 24 years.

    Ecology

    Diet

    Crayfish are omnivores. They consume algae, vascu-lar plants, invertebrates, and fish. They will also read-ily consume plant detritus and carrion. The majorityof crayfish appear to feed by using the chelae onthe smaller walking legs to pick food items off of thesubstrata. Some species (e.g.,Orconectes cristavarius)appear to shovel fine sediments into their mouths,i.e., they do not appear to be particulate feeders.Despite the fact that the majority of crayfish sto-machs often contain detritus and sediment, recentstable isotope analyses for several species suggest that-much of what crayfish assimilate is animal matter,i.e., they are carnivorous. It is possible that much ofthe detritus and sediment found in crayfish stomachsis in fact consumed incidentally as they search foranimal prey.There has been some speculation that crayfish diets

    may change over the course of their lives. Small cray-fish are thought to be primarily carnivorous, whileadults are thought to be primarily herbivorous ordetritivorous. However, all crayfish of all sizes andages appear to be fairly opportunistic with respect tothe prey they consume. Again, this idea of a diet shiftwith age appears to be primarily based on stomachcontent data. As more stable isotope analyses areconducted on more species of crayfish, we may findthat most species and both young and adult crayfishare largely carnivorous.Similar comments apply to shrimps and crabs.

    These animals feed in a variety of ways. Some grabindividual food items (invertebrates, algae, detritus),while others appear to brush fine sediments and detri-tal particles off of substrata. Some tropical shrimp arealso filter feeders. Many freshwater crab adults areamphibious and feed on the forest floor in addition to

  • invertebrates, fish, amphibians, reptiles, mammals

    (Ambloplites rupestris, all members of the family Cen-

    particularly a small crayfish, will be exposed to

    move around the entire exoskeleton other species

    276 Invertebrates _ Decapodatrarchidae)]. While the large body size of adult cray-fish can reduce their vulnerability to most aquaticpredators, they become more vulnerable to a varietyof terrestrial predators (e.g., raccoons, minks, otters,wading birds, and kingfishers). Small crayfish appearto be relatively invulnerable to these predators, as theyare more difficult to see or can take refuge under agreater range of substrates.This difference in the relative vulnerability of large

    and small crayfish to terrestrial and aquatic predatorscan influence their distribution in some freshwaterhabitats. Small crayfish are often found in shallowhabitats where they are safe from most aquatic pre-dators, primarily larger fish and adult crayfish. Largecrayfish, on the other hand, tend to be more abundantin deeper habitats where they are relatively safe fromterrestrial predators. Size-related segregation byshrimp and crabs in different habitats has receivedless attention, although it is likely to occur amongthem as well since they are exposed to comparablepredators.

    Competitors

    Decapods appear to compete primarily with otherdecapods. Of course, this conclusion may be theresult of a lack of studies evaluating competitionbetween decapods and other species. It is entirelyfeasible that decapods compete with other species(e.g., insects, fish) for food but this has received littleattention.The best studied aspect of competition among

    decapods is shelter competition among crayfish.Larger, dominant crayfish may evict smaller,subordinate individuals from shelters. This is a poten-tially important interaction because being evictedfrom a shelter increases the chance that an individual,(including humans), and birds. Other decapods canalso be important predators under some circum-stances. Body size is an important factor in determin-ing vulnerability of decapods to various types ofpredators. For example, young crayfish inhabitingtemperate streams and ponds can be consumed by awide range of organisms, including insects (e.g., drag-onfly larvae, water bugs, beetle larvae), salamanders,water snakes, and many species of fish. As they grow,the number of animals capable of consuming themdecreases. With few exceptions, most adult crayfishare invulnerable to almost every aquatic predator theyencounter, with the exception of some large fish [e.g.,largemouth bass (Micropterus salmoides), small-mouth bass (Micropterus dolomieui), and rock bassare found at specific locations. Some species are alsoknown to enter the gill chambers. Some taxa liveexclusively inside the gill chamber.The association between crayfish and branchiob-

    dellids was originally considered to be parasitic.However, only a few branchiobdellids appear to betruly parasitic. The current consensus is that mostbranchiobdellids are commensals, i.e., they have nodiscernable negative effect on their crayfish host.Recent research suggests that at least one species ofbranchiobdellid may be involved in a mutualisticpredators. Although shelter competition may occuramong shrimp and crabs, it has received littleattention.There is no good experimental evidence demon-

    strating that decapods compete for food in naturalsettings. Assertions that decapods compete for foodare based largely on correlative data. Competition forfood does appear to occur in aquaculture pondswhere crayfish are stocked at very high densities.Studies evaluating competition for food in shrimpand crabs are needed.

    Ectosymbionts

    A variety of organisms colonize the hard exoskeletonof freshwater decapods. Bacteria, protozoa, diatoms,and other organisms attach to their exoskeleton andepibiont density tends to increase with time since thelast molt of their decapod host. While organisms thathave colonized the outer surface of the exoskeleton aremost obvious, bacteria, protozoa, and some wormswill also readily colonize the surfaces of the gills.Small (generally

  • and Myanmar. Unlike branchiobdellids, they do not

    Invertebrates _ Decapoda 277(a)association, specifically a cleaning symbiosis, withone or more of its crayfish hosts. In an experiment,the growth of crayfish hosts decreased and mortalityincreased when branchiobdellids were removed. Thehypothesized mechanism by which these branchiob-dellids affect their hosts is by cleaning bacteria andprotozoa from crayfish gills. Keeping crayfish gillsfree of bacteria and protozoa should allow forincreased gas exchange across the gill epithelia aswell as increased ammonia excretion. These mechan-isms have yet to be determined experimentally,however. These results suggest that the associationbetween crayfish and other species of branchiobdellidshould also be evaluated experimentally.Temnocephalid flatworms (Platyhelminthes) are

    another common ectosymbiont of freshwater decapods(Figure 4(b)). These worms are found on decapodsprimarily in South America, Central America,Australia, New Zealand, Madagascar, Indonesia,

    resources can also be affected.Crayfish do not just eat algae and plants. In fact,

    (b)

    Figure 4 (a) A North Carolina crayfish and associatedbranchiobdellid worms. The small white spheres on the

    exoskeleton are branchiobdellid egg cases or cocoons.Photograph by RobertWayne vanDevender. (b) A temnocephalid

    worm on a crayfish. Photograph by Chris Lukhaup.even though crayfish eat everything, they appear toprefer and grow fastest on a diet of animal prey. Thus,direct predation by crayfish on various invertebratescan be an important process affecting invertebratedensity in a habitat. Slow moving taxa like snails areparticularly vulnerable, although snails with thickershells may be able to coexist with crayfish. Thinshelled snails, however, are at considerable risk.They often avoid or are excluded from habitatsappear to be specialized to just decapods. They havealso been reported on snails, insects, and turtles.Temnocephalans feed on protozoa and small inverte-brates they capture on or near their host. Likebranchiobdellids they lay their eggs on decapod exos-keletons. Some temnocephalans appear to be com-mensals while others are parasitic. There appears tobe little experimental work evaluating their effects ontheir decapod hosts.

    Importance of Decapods in AquaticCommunities

    Studies conducted over the last 25 years suggest thatvarious species of crayfish and shrimp have markedimpacts on their communities and on ecosystem func-tion. It is surprising that it took aquatic ecologists solong to appreciate the effects of decapods on theircommunities, given their large size and frequentlyhigh densities.Studies conducted in the 1980s, demonstrated that

    crayfish could have marked effects on the abundanceof filamentous algae and plants in streams and lakes.By reducing the abundance of filamentous algal coveron rocks in streams and the extent of plant beds inlakes, crayfish could indirectly influence the distribu-tion and abundance of other taxa. In streams, forexample, when crayfish cleared rocks of large algae,smaller algae, which were unable to grow beneath thedense canopy of filamentous algae, were able to pro-liferate. Small insects which feed on these smalleralgae increased in abundance as well. In lakes, thereduction of plant beds reduced an important feedinghabitat for various invertebrates such as snails andgrazing insects. These species grazed small algae offof the surfaces of the larger plants. Plant beds are alsoan important habitat for many species of fish. Theloss of plant beds can influence the survivorship ofsmall fish that use plant beds as a refuge from theirown predators. Moreover, many fish also feed on theinvertebrates inhabiting the plant beds so their food

  • freshwater decapods should not be introduced into newhabitats. This means new lakes and streams as well as

    278 Invertebrates _ Decapodacontaining crayfish. Research has shown that crayfishpredation can actually affect the life histories of somesnail species; snails living with crayfish grow fasterand reproduce at a smaller body size.Decapods will also readily consume detritus, par-

    ticularly decaying leaves of various terrestrial plants.At present, we do not know whether decapodsare consuming dead leaves because they require nutri-ents in the leaves or if they are feeding on the morenutritious bacteria, fungi, and invertebrates asso-ciated with the leaves. Regardless of the reason theyconsume leaves, a number of studies have demon-strated that decapods (primarily crayfish and shrimp)can accelerate the rate at which large leaves arereduced to small particles of detritus. As small detritalparticles are more easily suspended by currents,decapods can increase the downstream transport ofsmall detrital particles in temperate and tropicalstreams.Another way decapods can indirectly affect the

    abundance of other species is by affecting the amountof sediment in a habitat, particularly in streams. Asdecapods move across the bottom of a stream they candisturb sediments and cause them to be washed away.As mentioned above, some crayfish intentionally feedon fine sediments which may further reduce accumula-tions of silt and sand on substrates. For organisms thatprefer habitats with lots of accumulated fine sedi-ments, decapods have a negative effect on their abun-dance. However, organisms which prefer substrateswith little if any fine sediment will benefit from thiseffect of decapods. A number of studies in temperateand tropical streams have demonstrated the effects ofchanges in fine sediment abundance by decapods onthe distribution and abundance of algae and inverte-brates.Recent experimentalwork has shown that deca-pods can even influence the distribution of largersubstrate particles (e.g., gravels) in streams.Many decapods can also influence environmental

    structure by creating burrows. This is particularlytrue of the crayfish. Many crayfish construct a sim-ple burrowbeneath a rock.Others construct elaborateburrow systems. These burrows may serve as habitatfor various invertebrates. In wetland habitats prone todrying, crayfish burrows may serve as an importantrefuge for other aquatic taxa during dry periods. Stud-ies conducted in the north central part of NorthAmerica suggest that the larvae of an endangereddragonfly species, the Hines Emerald dragonfly,actively takes refuge in crayfish burrows duringdroughts. Overall, the survival of larvae is higherduring droughts when they have access to crayfishburrows even though crayfish may consume someindividuals. Because of their wide-ranging effects onhabitat structure and the availability of various kindsnew continents. Their ability to alter invaded commu-nities as well as introduce new diseases could havesignificant negative effects on the habitats they invade.of resources, decapods may be influencing the abun-dance and survival of many other rare and endangeredspecies.

    Effects of Introduced Decapods

    Many decapods are popular food items. Others areroutinely used as bait. For these and other reasons anumber of decapod species have been introduced intonew habitats or even to new continents, often withdisastrous consequences. In North America, the rustycrayfish (Orconectes rusticus) has been introducedinto lakes and streams outside its native range, mostlikely as a result of its use as bait. Rusty crayfish canreduce the abundance of aquatic plants in invadedlakes, which indirectly affects various invertebratesand fish. It also appears to displace native crayfishspecies. Some native crayfish species appear to beout-competed by the rusty crayfish; others appear todecline through the process of introgression wherenative crayfish species preferentially mate with therelated, introduced rusty crayfish.North American crayfish (e.g., Procambarus clar-

    kii, Pacifastacus lenisculus,Orconectes limosus) havebeen introduced into Eurasia. These introduced cray-fish often have different effects on invaded commu-nities than native crayfish species have. For example,the introduced P. lenisculus can have a greater impacton aquatic macrophyte abundance than native cray-fish, i.e., it can have a greater effect on habitat struc-ture than native species. As a result, this species couldaffect invertebrate and fish distributions to a greaterextent than native crayfish. The introduction of NorthAmerican crayfish also resulted in the unintentionalintroduction of a fungus-like organism (Aphanomycesastaci) to Eurasian freshwater systems. Aphanomycesastaci does not appear to harm North American cray-fish species, but is lethal to a number of native Eur-asian crayfish, and appears to be responsible for theelimination of native Eurasian crayfish from much oftheir range. This fungus is not the only disease poten-tially transmitted by introduced crayfish and it maynot be the only pathogen to cause dramatic crayfishdeclines. Other introduced decapods may also trans-mit new diseases to new habitats which have thepotential to decimate local populations of native dec-apods. A better understanding of the diseases thataffect freshwater decapods is needed.The results of numerous studies strongly suggest that

  • Acknowledgments

    I am grateful to Bryan Brown and Werner Klotz fortheir comments on the manuscript. Any mistakes thatremain are mine. I also wish to thank Wayne VanDevender (Appalachian State University), Werner Klotz(Crusta10.de), and Chris Lukhaup (Crusta10.de) forgiving me permission to use their beautiful photographs.

    See also: Annelida, Oligochaeta and Polychaeta; AquaticInsects Ecology, Feeding, and Life History; BenthicInvertebrate Fauna, Lakes and Reservoirs; BenthicInvertebrate Fauna, River and Floodplain Ecosystems;Benthic Invertebrate Fauna; Biodiversity of AquaticEcosystems; Competition and Predation; Conservationof Aquatic Ecosystems; Green Algae; Invasive Species;Regulators of Biotic Processes in Stream and RiverEcosystems; Streams and Rivers as Ecosystems; Sub-terranean Aquatic Ecosystems - Groundwater Ecology;Trophic Dynamics in Aquatic Ecosystems.

    Further Reading

    Creed RP Jr (1994) Direct and indirect effects of crayfish grazing in

    a stream community. Ecology 75: 20912103.Creed RP Jr and Reed JM (2004) Ecosystem engineering by crayfish

    in a headwater stream community. Journal of the North Ameri-can Benthological Society 23: 224236.

    Crowl TA and Covich AP (1990) Predator-induced life historyshifts in a freshwater snail. Science 247: 949951.

    Edgerton BF, Henttonen P, Jussila J, et al. (2004) Understanding thecauses of disease in European freshwater crayfish. ConservationBiology 18: 14661474.

    Gelder SR (1999) Zoogeography of branchiobdellidans (Annelida)

    and temnocephalidans (Platyhelminthes) ectosymbiotic on fresh-

    water crustaceans, and their reactions to one another in vitro.Hydrobiologia 406: 2131.

    Hobbs HH III (2001) Decapoda. In: Thorp JH and Covich AP

    (eds.) Ecology and Classification of North American FreshwaterInvertebrates, 2nd edn. San Diego: Academic Press.

    Liu HC and Li CW (2000) Reproduction in the fresh-water crab

    Candidiopotamon rathbunae (Brachyura: Potamidae) in Taiwan.Journal of Crustacean Biology 20: 8999.

    Lodge DM, Kershner MW, Aloi JE, and Covich AP (1994) Effectsof an omnivorous crayfish (Orconectes rusticus) on a freshwaterlittoral food web. Ecology 75: 12651281.

    March JG, Benstead JP, Pringle CM, and Scatena FN (1998)

    Migratory drift of larval freshwater shrimps in two tropicalstreams, Puerto Rico. Freshwater Biology 40: 261273.

    Nystrom P (2002) Ecology. In: Holdich D (ed.) Biology of Fresh-water Crayfish, pp. 192235. Oxford: Blackwell Science.

    Pintor LM and Soluk DA (2006) Evaluating the non-consumptive,

    positive effects of a predator in the persistence of an endangered

    species. Biological Conservation 130: 584591.

    Invertebrates _ Decapoda 2792nd ed. San Diego: Academic Press.Brown BL, Creed RP, and Dobson WE (2002) Branchiobdellid

    annelids and their crayfish hosts: Are they engaged in a cleaning

    symbiosis? Oecologia 132: 250255.Statzner B, Fievet E, Champagne J-YC, Morel R, and Herouin E(2000) Crayfish as geomorphic agents and ecosystem engineers:

    Biological behavior affects sand and gravel erosion in experimen-

    tal streams. Limnology and Oceanography 45: 10301040.Brinkhurst RO and Gelder SR (2001) Annelida: Oligochaeta and

    Branchiobdellida. In: Thorp JH and Covich AP (eds.) Ecologyand Classification of North American Freshwater Invertebrates,

    DecapodaIntroductionClassification, Morphology, and GrowthReproduction and Life HistoryCrayfishShrimpCrabs

    EcologyDietPredatorsCompetitorsEctosymbionts

    Importance of Decapods in Aquatic CommunitiesEffects of Introduced DecapodsAcknowledgmentsFurther Reading