Encyclopedia of Inland Waters || Decapoda

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  • 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

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