The ctenophore Pleurobrachia rhodopis, unlike the recent
invaders Mnemiopsis leidyi and Beroe ovata, has been
living in the Black sea for a long time. Pleurobrachia
casts its sticky tentacles to catch tiny
plankters and particles of detritus.
Among the large zooplankton species of
the Black Sea there are Scyphozoan
jellyfish Aurelia aurita and Rhizostoma
pulmo, and ctenophores Pleurobrachia rhodopis,
Mnemiopsis leidyi, Beroe ovata (the latter two
were participants of the most dramatic recent story of
alien marine species introduction into the Black
Sea: Evolution of the Black Sea
Usually, during the warm season, the
gelatinous plankton biomass near the Black Sea shoreline
can be measured by tens or hundreds of grams (sometimes
over 1kg) per cubic meter of water, whereas the biomass
of non-gelatinous zooplankton rarely exceeds 10 g.m-3.
And here are microscopic animals inhabiting
the Black Sea water column:
The largest of the small ones are crustaceans
of the Copepoda order. They are the main phytoplankton
microalgae grazers – the herbivores of the Sea. Though
there is an important difference from land ecosystems:
the plankton "grass" can escape – swim away
from the grazer. Copepod movements look like a chain
of impetuous hurls in all directions: they see the prey,
hurl, then stifle for a moment, eating.
Quick darting rushes even of the
smallest transparent Copepoda can be seen without the
microscope when looking through the dense plankton
sample: the animals are invisible, but their movements are
noticeable! Wild mobility of the plankton crustaceans
can be extinguished by a drop of formaldehyde put into
the sample; otherwise they are hard to follow in the
crustacean Oithona sp.
Long antennulae of many
Copepoda act as
Most Copepods have very long antennules which serve
as a locomotion organs: with the help of these elastic
oars’ rows they make their rushing leaps.
Orange reproductive glands can be discerned within
their transparent abdomens; females bear bunches of
fertilized eggs in the two bags hanging on the sides of
the thin abdomen.
Copepod crustaceans have only one eye in the centre
of their head; that’s where the name of the common
freshwater Copepod Cyclops comes from.
There are also many nauplii, crustacean larvae
of the early development stage in the plankton. The
majority of them are Copepod nauplii again.
These tiny shaggy monsters are none the less mobile
and more voracious than the adult Copepods – they eat as much
as they can to grow.
After many moultings, they turn into an adult animal.
When grown up, this numerous nauplii most
probably can be identified as Oithona, Calanus or
Acartia, the most common genera of the Black Sea
- early stage planktonic larvae of Crustaceans
devours dinophycean alga of genus Protoperidinium
Infusoria, the ubiquitous protozoa living in
almost all marine habitats, play an important role
within the plankton community as well; there are many
different species. They are densely trimmed with
relentlessly rowing cilia pushing the unicellular
This infusorium has just caught a large
dinoflagellate of genus Protoperidinium, and is trying
to pull it inside. Usually, infusoria become the first
ones to attack the overgrown plankton vegetation.
There are very common and amazing planktonic
infusoria called Tintinnidae. A Tintinnid’s body of
one cell is hidden in a transparent wineglass-like
little house called theca. Thecal edge is furnished by a ciliar
funnel; furious flickering of the cilia forms a
whirlpool driving seston particles inside the funnel,
right to the infusorian “mouth”.
Here is a really good catch: the smallest Metazoan -
multicellular organism – rotifer. These tiny
animals can be as small 50 microns long - smaller than
many phytoplankton algae! This one is about 100 microns.
This microscopic body contains muscles and digestive
system. Right beside the rotifer is a small diatom, as if
specially put there for scale.
Another interesting creature got into the microscope
light field. May be this is some larva? No, it’s not an
animal at all – it’s pine pollen! Pollen grain of the
dark-green, long-needle Crimean pine that decorates
mountain slopes descending to the Black Sea. It was
carried by off-shore wind to the sea and got caught in
our net. Terrestrial plant pollens are typical
participants of the coastal plankton community.
Black Sea anchovy larva
The largest organisms we find in the microscopic
plankton samples are fish larvae. This one most
probably is the larva of hamsi, the Black Sea anchovy
Engraulis encrasicholus ponticus, or a similar small
planktivourous fish; there are many of them in the
plankton samples taken in May. Although this prospective
fish already have fins, it’s still incapable of escaping
even a crustacean nauplius. And all the animals we find
in the plankton sample may become a prey of ctenophore’s
sticky tentacles or scyphoid nematocysts.
The larvae will grow up and become adult fish; they start
swimming faster. The new opportunities and their new
lifestyle determine their ecological niche within the
nekton - midwater inhabitants able to swim in the
direction chosen by them and not the currents.
Not only fish, most sea creatures spend at least part of
their lifetime within the plankton: seaweed gametes and
spores, eggs and larvae of many benthic invertebrates, like
bivalve molluscs and decapod crustaceans, such as shrimps
Various future benthic animals inhabit the Black Sea coastal
waters from April to October; most common of them are
trochophores – early larvae of Polychaeta and Mollusca. They
swim using whisks of cilia assembled in rows edging contours
of the larval body. Pilidium, larvae of Nemertini, another
group of benthic worms, is a very rare catch; it reminds
a helmet bearing tuft of cilia on top.
As the larvae grow they acquire the shape of an adult
animal; this is a microscopic larvae of bivalve mollusc, soon
to settle on the bottom.
Late stage trochophore of
Late stage of planktonic
larval development of bivalve
Pilidium, larvae of
Adult benthic organisms and their planktonic larvae
have completely different ways of living: they move
different ways, and in different environments; they eat
different food and are hunted by different predators. That
is, they occupy different ecological niches.
This helps survival of the population: if, for example,
sea storm kills bivalves living on sandy bottom in shallow
water, their populations is soon regenerated by the
planktonic larvae of the same species, brought by currents
from other location.
Mobility and excessive amount of planktonic larvae is the
base for the mussel aquaculture: sea farmers remove the harvest
of shellfish from collecting ropes, but next year they are
again covered by newly settled bivalves.
What the crab is?
A bottom dweller coated in hard cuticle, having 10
legs (two of them are its claws), feeding on remains
of other animals and seaweeds.
Also, it is a microscopic planktonic larvae called
in midwater, feeding on smaller plankton; it has a long
horn on its head, and a long thin abdomen. It does not
remind an adult crab neither by its appearance nor by
lifestyle. At this stage it is even hard to say if this
zoea is a future crab or a shrimp.
of a crab
Zoea grows and turns into megalope,
larger planktonic larvae, again looking different: it
has very big eyes (which is where the name megalope
comes), and swims using its abdomen as an oar. Megalope
grown to a macroscopic size of 3 mm settles on
the bottom, and finally starts to acquire the characters
of an adult crab.
a common crustacean
sandy bottom habitat
in the Black
Another most common and very conspicuous marine animal found
in all types of nearshore benthic habitats in the Black Sea,
predatory marine gastropod Rapana is a part of plankton too:
it has a planktonic larvae called veliger - see the
life cycle of Rapana venosa
at the Black Sea Molluscs page.
There are marine organisms that, on the contrary, spend
most of their life and reproduce in the plankton, whereas
the benthic stage of their life cycle is reduced. For
example Scyphozoan Aurelia
aurita is known to us as a most usual
jellyfish in the Black Sea; this is an adult, reproductive
stage of this species. Its cilia-covered larva called
planula settles on the bottom and gives rise to a small
branched polyp that produces new tiny medusas by gemmation.
Aurelia aurita life cycle
Plankton is a food for benthic sestonophags – bivalves,
sponges, actiniae, many others, and - for some fishes as well.
Most important planktivourous fish species in the Black
Sea are hamsa (khamsi), atherina, and sprat.
Hamsi, a gaunt fish of up to 20cm with the dark
metallic blue sides and back would attack plankton
during the night, and eat all of them – diatoms,
dinoflagellates, crustaceans, eggs, larvae – including
its own ones.
Night is the time when most zooplankters migrate
up the water column, and hamsi follow them. Small
zooplankton avoids being at the sea surface during the
day – these small animals are too easily spotted in the
sunlit upper layer by those who would eat them.
Phytoplankton, on the contrary, needs light as the
source of its life, and tends to be closer to surface,
however not at the very surface where sunlight can be so
intense that it destroys photosynthetic structures of
microalgae. In the off-shore waters of the Black Sea in
summer, the highest phytoplankton density usually is
observed at 30 meters depth.
Hamsi, Black Sea anchovy Engraulis
encrasicholus ponticus swim with its mouth wide
open, and filter plankton from the water passing through
the gill raker. The fish swallow accumulating lump of
plankton from time to time. All planktivourous fish
- from anchovy to whale shark - feed this way
Atherina Atherina mochon
pontica, most common planktivourous fish in
the Black Sea coastal waters
Schools of atherina Atherina mochon pontica
patrol coastal Black Sea waters; this is a
planktivourous fish that can be most often observed near
shore, starting from the very surfline. Atherina can be
distinguished by elongated semi-transparent body with
golden dorsal scales. Another plankton-eating fish usual
in Black Sea is sprat Sprattus sprattus falericus; it
differs from atherina by a higher, less oblong body with
a silvery back.
Though many plankton species are very small, there is one
special way to see them without using the microscope:
plankton emits light in the night water. The biochemical
base for the plankton bioluminescence is the
luciferine-luciferase reaction: one molecule of luciferine
emits one quantum of green light during luciferase-catalyzed
oxidation. The same shining chemistry takes place in the
abdomen of fireflies; these insects are common at the Black
Sea coast in summer. In most plankters bioluminescence
reaction turns on as a reaction to mechanical irritation:
tiny green flashes serve to scare off small predators.
Not all plankters can give light: for example diatoms and
abundant large scyphozoan jellyfish of the Black Sea don’t
emit light. Dinoflagellates are most the most noticeable
bioluminescent microalgae. During the August-September
phytoplankton peak in the Black Sea, disturbed water often
shines with a steady light, swimming fish, dolphins and men
become visible in the dark water being outlined with green
glow. The most intense biolumenscence is observed in the Black
Sea during Noctiluca blooms.
Many plankton crustaceans emit light – they are seen as many
little separate flashes when disturbed. Large ctenophores
Mnemiopsis leidyi and Beroe ovata shine like lamps; they are
the largest and brightest luminescent plankters in the Black