FIU OCB3043 Lecture Notes

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Zooplankton

Definitions

Zooplankton = all heterotrophic plankton except bacteria and viruses; size range from 2 µm (heterotrophic flagellates, protists) up to several meter (jellyfish)

Zooplankton can be categorized by their life cycle:
Holoplankton: spend entire life in the water column (pelagial)
Meroplankton: spend only part of their life in the pelagial, mostly larval forms of invertebrates and fish

Protozooplankton

Dinoflagellates: heterotrophic relatives to the phototrophic Dinophyceae; naked and thecate forms. Noctiluca miliaris – up to 1 mm or bigger, bioluminescence, prey on fish egg & zooplankton
Zooflagellates = heterotrophic nanoflagellates (HNF): taxonomically mixed group of small, naked flagellates, feed on bacteria and small phytoplankton; choanoflagellates: collar around flagella
Foraminifera: relatives of amoeba with calcareous shell, which is composed of a series of chambers; contribute to ooze sediments; 30 µm to 1-2 mm, bacteriovores; most abundant 40°N – 40°S
Radiolaria: spherical, amoeboid cells with silica capsule; 50 µm to several mm; contribute to silica ooze sediments, feed on bacteria, small phyto- and zooplankton; cold water and deep-sea
Ciliates: feed on bacteria, phytoplankton, HNF; naked forms more abundant but hard to study (delicate!); tintinnids: sub-group of ciliates with vase-like external shell made of protein; herbivores

Forms of radiolarians

Jellyfish

Cnidaria: primitiv group of metazoans; some holoplanktonic, others have benthis stages; carnivorous (crustaceans, fish); long tentacles carry nematocysts used to inject venoms into prey; box jellyfish of Australia kills humans within minutes

Medusae: single organisms, few mm to several meters
Siphonophores: colonies of animals with specialization: feeding polyps, reproductive polyps, swimming polyps; Physalia physalis (Protuguese man-of-war), common in tropical waters, Gulf of Mexico, drifted by the wind and belong to the pleuston (live on top of water surface)

Ctenophores: separate phylum, do not belong to Cnidaria; transparent organisms, swimm with fused cilia; no nematocysts; prey on zooplankton, fish eggs, sometimes small fish; important to fisheries due to grazing on fish eggs and competition for fish food

Invetebrates (other than Crustaceans)

Heteropods: small group of pelagic relatives of snails, snail food developed into a single “fin”; good eyes, visual predators
Pteropods: snail foot developed into paired “wings”; suspension feeder – produce large mucous nets to capture prey; carbonate shells produce pteropod ooze on sea floor

Appendicularia (older: Larvacea): group of Chordata, live in gelatinous balloons (house) that are periodically abandoned; empty houses provide valuable carbon source for bacteria and help to form marine snow; filter feeders of nanoplankton
Salps: group of Chordata, mostly warm water; typically barrel-form, filter feeders; occur in swarms, which can wipe the water clean of nanoplankton; large fecal bands, transport of nano- and picoplankton to deep-sea; single or colonies

Crustacea

Cladocera (water fleas): six marine species (Podon spp., Evadne spp.), one brackish water species in the Baltic Sea; fast reproduction by parthenogenesis (without males and egg fertilization) and pedogenesis (young embryos initiate parthenogenetic reproduction before hatching)

Ostracoda: bivalved shell, usually not important; some species in tropical waters cause bioluminescence; little studied

Amphipoda: less abundant in pelagial, common genus Themisto; frequently on siphonophores, medusae, ctenophores, salps
Euphausiida: krill; 15-100 mm, pronounced vertical migration; not plankton sensu strictu; visual predators, fast swimmers, often undersampled because they escape plankton nets; important as prey for commercial fish (herring, mackerel, salmon, tuna) and whales (Antarctica)

Copepoda most abundant zooplankton in the oceans, “insects of the sea“; herbivorous, carnivorous and omnivorous species;

Calanoida: most of marine planktonic species
Cyclopoida: most of freshwater planktonic species
Harpacticoida: mostly benthic/near-bottom species
Copepod development: first six larval stages = nauplius (pl. nauplii), followed by six copepodit stages (CI to CVI)

Copepod nauplius

Tropical species distinct by their long antennae and setae on antennae and legs (podi)

Copepods from polar and temperate waters (left) have less setae on their antennae and podi than those from tropical waters (right)

Meroplankton - Larvae of benthic animals

Mollusca: clams and snails produce shelled veliger larvae; ciliated velum serves for locomotion and food collection
Cirripedia: barnacles produce nauplii, which turn to cypris
Echinodermata: sea urchins, starfish and sea cucumber produce pluteus larvae of different shapes, which turn into brachiolaria larvae (starfish); metamorphosis to adult is very complex
Polychaeta: brittle worms and other worms produce trochophora larvae, mostly barrel- shaped with several bands of cilia

Decapoda: shrimps and crabs produce zoëa larvae; they turn into megalopa larvae in crabs before settling to the sea floor
Various other forms of larvae occur in the plankton
Pisces: fish eggs and larvae referred to as ichthyoplankton; fish larvae retain part of the egg yolk in a sack below their body until mouth and stomach are fully developed

Links to photograph resources