Life in Freshwater

Lecture 2



Subkingdom Protozoa

·        Characteristics

o       Unicellular (a few unicells)

o       Eucaryotic

o       Heterotrophic

·        Phylum Sarcomastigophora (amoeboid and flagellated cells, some have both)

o       Subphylum Mastigophora (flagellates)

§         Properties

·        1 to many flagella

·        many are closely related to algal species

·        food storage as glycogen

·        most are naked (no cell wall)

·        may undergo amoeboid stages

·        strong affinity for solid substrates

§         Representative taxa

·        Peranema (euglena-like, holozoic = predaceous)

·        Chilomonas (cryptomonad-like, saprozoic: dissolved organics)

·        Noctiluca (dinoflagellate-like)

·        Choanoflagellates (filter feeders)

·        Trypanosoma, Giardia (endoparasite in vertebrates)

·        Trichonympha (termite endosymbiont)        

§         Reproduction

·        Asexual: binary fission

·        Sexual: rare

o       Subphylum Sarcodina (amoebae)

§         Properties

·        Move by pseudopodia (flowing extensions of cytoplasm)

·        Generally feed by engulfing particles in pseudopods

·        Some also produce flagella

·        Naked or shelled

§         Representative Taxa

·        Amoeba (large pseudopods, naked)

·        Arcella (chitinous shell)

·        Euglypha (siliceous shell)

·        Difflugia (cemented particle shell)

·        Heliozoans (naked or scaled with axiopods)

§         Reproduction

·        Asexual (binary fission)

·        Sexual: rare

·        Phylum Ciliophora

o       Class Ciliata

§         Properties

·        Cilia or compound ciliary structures for locomotion and/or food gathering

·        Two types of nuclei

·        Most are solitary, but some colonial forms

·        Most are holozoic (particle feeders), but may also be saprozoic

·        Important bacterial grazers in microbial loop

§         Reproduction

·        Asexual: binary fission

·        Sexual: conjugation

§         Representative Taxa

·        Paramecium (figure)

o       Uniform body ciliation

o       Bucal cavity for feeding

o       Food and contractile vacuoles

·        Vorticella (peritrich=cilia concentrated at one end)

o       Attached to a solid substrate

o       No body cilia

o       Bucal cilia create vortex for drawing in food particles

·        Euplotes (hypotrich=few cilia)

o       Body ciliature reduced to fused tufts of cilia called cirri

o       Dorsoventrally flattened

o       Class Suctoria

§         Sessile, stalked “ciliates”

§         Distal end bears tentacles

§         No body ciliature when mature



Phylum Porifera (sponges)

  • Only one family with freshwater sponges
  • 170 species (30 in US) found in freshwater out of a total of 4500
  • Size varies greatly as a function of species, age, and environment
    • Thin mat, few cm2, 1 mm thick
    • 40 m2, 4 cm thick
  • Family Spongillidae
    • Skeleton of monaxial spicules (silicious spines) and sponging
    • Nutrition
      • Particle feeders up to 50 um
      • Water enters trough ostia by current produced by choanocyte cells
      • Bacteria and bits of detritus caught on sticky surface of choanocyte collars and absorbed
      • Then passed on to amoebocytes through mesoglea for digestion
      • Filtration rate: 0.01-0.28 mL/sec/ml sponge
      • Some have zoochloroellae
    • Reproduction
      • Asexual
        • Reduction bodies (blobs of tissue that shrink)
        • Fragmentation, reaggregation
        • Gemmules (spheres with hard outer layer, internal mass of undifferentiated tissue, highly resistant)
      • Sexual
        • Oogonium is amoebocyte that grows by absorbing adjacent cells
        • Sperm may develop from other cells and become flagellated
        • Flagellated embryos released and settle on substrate


Phylum Coelenterata (hydroids, jellyfish)

  • 2 life forms (sessile hydroid, mobile medusa)
  • Class Hydrozoa
    • Only one of the three classes of coelenterates with freshwater representatives
    • 20 species in the US: 16 Hydra/Chlorohydra, 1 fw jelly (Craspedacusta), 1 colonial polyp
    • all sessile except jelly, also may hang from surface film
    • most abundant during summer
    • Representative Taxon
      • Hydra
        • Radially symmetric morphology, 1-25 mmlong
        • External cell layer (epidermis), internal cell layer (gastrodermis)
        • Tentacles with minute stinging cells (nematocysts)
        • Has only polyp stage
        • Nutrition
          • Carnivorous on small metazoans (cladocera, copepods, insects, annelids, occasionally fish fry)
          • Kills prey with nematocysts, prey fed into opening below the tentacles, prey may be 4x diameter of Hydra
          • Some Hydra have zoochlorellae which are passed on in the egg cytoplasm
        • Reproduction
          • Asexual: budding
          • Sexual
            • monoecious or dioecious
            • stimulated by autumn temp declines
      • Craspedacusta (freshwater jelly)
        • Normally found in small lakes/ponds in late summer
        • 5-22 mm in diameter
        • feeds on zooplankton 0.2-2 mm
        • eaten by crayfish
        • also has a colonial hydroid stage


Phylum Playhelminthes

·        Three classes, two of which are exclusively parasitic (Cestoda, Trematoda)

·        Class Turbellaria (flatworms)

o       Many are freshwater

o       Elongate, may be flaggened or cylindrical

o       Anterior end may be differentiated as head

o       Some 5-30 mm long (triclads)

o       Others <4 mm long (rhabdocoels)

o       Body surface more or less covered with cilia

o       Body cavity has a single external opening

o       Reproduction:

§         Asexual: fission or fragmentation

§         Sexual

·        Hermaphroditic (monoecious): both sex gonads on the same individual

·        Not self-fertile, must mate with another individual

·        Sperm held, fertilize eggs as they leave ovary

·        Zygote accumulates yolk cells moving down the oviduct, zygotes and yolk cells aggregate into a capsule or cocoon of 2-20 zygotes and hundreds of yolk cells (2-4 mm dia), leaves body attached to a stalk

·        Cocoon resistant to low T, but not to drying

·        “worms” emerge 1-3 mm long, direct development by enlargement

o       Nutrition

§         Suck in soft material, decaying animals, whole living micrometazoa (rotifers, nematodes, gastrotrichs)

§         May secrete mucous to entangle particles

§         May recycle nematocysts from Hydra


Phylum Nematoda (roundworms)

  • Found in great abundance in sediments/vegetation
  • Poorly known because of small size and difficulties in identification
  • 1500 freshwater species described, probably an underestimate
  • <1 cm long, protein cuticle shed during growth
  • Feeding
    • Cover all food items as a group, but individual species specialize
  • Reproduction
    • Asexual: parthenogenic egg development
    • Sexual
      • Dioecious: separate sexes
      • Male inserts sperm into female
      • Fertilization followed by egg shell formation, embryo hatches in a few days, small, but fully formed and terminal number of cells
      • Undergoes four molts to reach final size
      • Eggs are highly resistant to dessication and temperature extremes for up to 10-20 years
      • Transported by wind, animals


Phylum Rotifera

  • Phylum most characteristic of freshwater: 95% of 2500 species are exclusively fw
  • Two classes
    • Monogononta: single ovary and lorica (shell)
    • Digononta: paired ovaries, no lorica, “bdelloid
  • Nutrition
    • Omnivorous particle feeders
      • Brachionus, Keratella, Filinia
    • Predatory
      • Asplanchna, Synchaeta, Trichocerca
  • Reproduction
    • Asexual: parthenogenesis (diploid females produce diploid eggs)
    • Sexual
      • diploid female undergoes meiosis, produces haploid eggs
      • if haploid eggs unfertilized, produce haploid males which produce haploid sperm
      • if haploid eggs fertilized by haploid sperm, produce zygotes which hatch into diploid females
  • Bdelloid rotifers may undergo dessication and remain viable for up to 25 years


Phylum Gastrotricha

  • Small metazoans related to nematodes, rotifers
  • 70-600 um (most are 100-300 um)
  • body is flattened strip with enlarged head, narrower neck, enlarged abdomen, and two tails
  • body is covered with cuticle which may be thickened to form plates
  • Feeding
    • Browse on solid substrates (plants, surfaces) ingesting periphyton (bacteria, algae, small protozoa, organic detritus)
    • Head cilia induce currents
  • Reproduction:
    • All known species are parthenogenetic females, males are unknown
    • 1-5 eggs/lifetime
    • eggs very large: 70-80% of adult size
    • two types of eggs
      • quick developing (tachyblastic), hatching in 12-70 hrs
      • resistant (opsiblastic)
        • dormancy 2 yrs or more
        • survive desiccation, freezing, high temps
    • animal is large and well-developed at emergence


Phylum Annelida

  • Semented structure, true coelom (tube within a tube), thin cuticle
  • Three classes, one is Polychaeta with is rare in freshwater although found in the tidal Potomac
  • Class Hirudinea (leeches)
    • Dorsoventrally flattened, mouth surrounded by oral sucker, also large caudal sucker
    • All have 34 segments
    • 5 mm to 45 cm long
    • Feeding: scavengers and carnivores mostly (2 US genera suck blood)
    • Reproduction:
      • Sexual: hermaphroditic (not self-fertile), cocoon (2-15 mm long) formed around 1 to several eggs and fastened to substrate, young hatch from cocoon and start to grow
  • Class Oligochaeta (aquatic earthworms)
    • Similar in anatomy to terrestrial earthworms
    • Few setae on segments
    • Families: Aeolosomatidae, Naididae, Tubificidae
    • Feeding: ingest bulk sediment and detritus, a few are carnivorous
    • Reproduction:
      • Asexual: budding in some groups
      • Sexual: hermaphroditic, not self- fertile