Zooplankton, continued

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Zooplankton, continued Arthropods (jointed
legs) Crustaceans (distinct feeding
structures) Ostracodes the coolest
crustaceans! Shrimp-like animal with a
two-valved calcite carapace A few mms in
length or smaller Use antennae to swim, but
weak swimmers No circulatory system Eye
tubercles Brood pouches Largest reproductive
organs (relatively) among animals
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• Larger Zooplankton
• Copepods
• Crustaceans
• 0.5 to 10 mm
• Feed by creating eddies

Simple median eye (lacking in some)
Simple head shield
Frilly appendages to avoid sinking
6 thorax segments, 1 or 2 fused to head
5 abdomen segments telson
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• Krill
• Crustaceans, related to shrimp
• Two eyes
• Tough organic carapace
• External gills
• 4 to 15 cms long
• Densities may exceed 1000 animals per cubic meter
• Important food for whales, squid, fish, and birds
• Human consumption?

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• Arrow worms
• 0.5 to 12 cm
• Present at all water depths but very common at a
  few hundred meters or less
• Streamline, fins, some transparent
• Effective predator grasping spines and teeth
  like a leg hold trap, have neurotoxins
• Not efficient digestion waste products rich in
  nutrients
• Hermaphrodites

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• Tunicates
• Chordates, but lack backbone (not vertebrates,
  urochordates)
• Larger ones, called salps, are between 1 and 15
  cm in length
• Smaller ones are less than 1 cm
• Swimming larvae
• Some build hollow gelatinous homes, transparent
• Can form dense mats on sea surface.

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By-the-Wind-Sailor Man o War
Gas-filled floats
Hydra (hydrazoans)

• Cnidarians
• Include true jellyfish, hydra and corals.
  Jellyfish and some hydra are plankton.
• Include solitary and colonial forms.
• Alternating polyp and medusa generations. In
  jellyfish, medusa stage dominant in hydra polyp
  and medusa stage are about equal longevity.
• Feed using nematocysts

Jellyfish (scyphozoans)
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• Alternation of generation in the cnidarians
• Medusa stage (planktonic) is a product of asexual
  reproduction (genetic material from one parent),
  but it reproduces sexually.
• Polyp stage (sessile benthic attached to the sea
  floor) may be a product of sexual or asexual
  reproduction, but it only reproduces asexually

How a nematocyst (stinging cell) works When the
cnidocil is bumped, a trap door (operculum) is
opened. It held back a tightly coiled stinger.
The stinger snaps. The stinger has barbs and
spines so that it stays in the victim. It also
contains toxins.
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• Ctenophores (comb jellies)
• Live from the surface down to about 3 km of
  depth.
• Transparent, gelatinous body with neutral
  buoyancy.
• Cm to m in size.
• Predominant zooplankton in some areas
• Use ctenes (rows of cillia) to adjust position in
  water column
• Can eat cnidarians and ingest nematocysts without
  causing nematocysts to fire. Nematocysts get
  incorporated in ctenophores skin and are then
  used for defense.
• Predators. Tentacles have colloblasts (suction
  cup that secretes sticky substance).
• Simultaneous hermaphrodites (contain male and
  female sex organs at the same time). Yup, they
  can self-fertilize.

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• Pteropods (sea butterflies)
• A strange group of snails in which the foot
  (parapoda) has been modified into a sail or
  parachute like device.
• Many have calcareous shells.
• Foot covered with sticky mucus--useful in
  capturing zooplankton

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• Meroplankton
• These are organisms that are planktonic for only
  part of their lives.
• The other plankton that we have been discussing
  are holoplankton (plankton for entire life)