Meroplankton refers to organisms that live only part of their life
in the plankton. Usually meroplankton has a complete or
incomplete metamorphism. The early stages are usually planktonic and then they emerge either as a land or flying
critter. Eggs of fish or other animals are included in this
group. Other organisms include their larval or nymph stages.
Some groups, like the dragonflies can spend up to 2 years as
plankton and then emerge only to live on land for a few
Consult “Quick Identification of Fresh Water Microorganisms”
for approximate sizes of organisms shown in this section.”
Family Gyrinidae (Whirligig beetles)
Beetles are black,
shiny, oval and ventrally depressed. Adapted for rapid
locomotion with its middle and back legs which are flattened
and paddle-like. Frequently fly but unable to take off
from the surface, but can dive. Larva and pupa stage
tends to be in the mud and not in the water column.
This order includes
many members that are in their larval and pupa stages and
adapted to fresh water. The transition from larval and pupa
may have very different forms. The larval stages may
last several weeks or up to two years depending on species,
temperature, and food availability. The skin usually
molts 3 times. Adults are never truly aquatic. All
aquatic representatives are legless although there may be
pseudopods on the thorax or abdomen. Diptera include the
common mosquito, midges, gnats, crane fly, and the common fly.
This group is referred to as non-biting
midges because the adult mandibles are poorly developed. Larval
chironomid species can help track ecological conditions. This group
can tolerate polluted water. However, low numbers if found within a
diverse population can indicate non-polluted conditions.
dixa midges are small insects that swarm around a pond or stream at
dusk. Females deposit eggs in shallow water. The larvae
have distinct thoracic segments and is U-shaped at rest. The
pupa stage (photograph) is not truly aquatic, as they
attach themselves just above the water.
Family Culicidae (Culicinae)
Mosquito (Anopheles larva)
Culicidae larvae are easily distinguished
because their thoracic segments are fused and thicker than the rest
of the body. They mainly feed on algae, protozoa, and organic
debris by using their bristles. The larvae lie quietly on the
surface and have a characteristic wiggling motion.
Family Culicidae (Chaoborinae)
Similar to mosquito but the adults do not
bite. The larva are called phantom because they are transparent.
They have jerky, lashing movements of the body. The larva is
predatory and catches small crustacean and other larvae with their
Small to medium terrestrial insect
with incomplete metamorphism. Its wings are delicate and held
together at rest. The have large compound eyes with a reduced
mouth because the adult does not eat, as it lives for a few
hours to a week. The nymphs however are herbivores and
browse on the substrate. The nymphs have three occasionally
two distinct cerci (tails). Cerci may be fuzzy or
thread-like. Nymphs are climbers, bottom crawlers, or
Family Corixidae (Water boatman)
are totally aquatic. Nymphs develop through 5 growth stages
or instars. Like all aquatic bugs they lack gills, so they
need to breathe at the surface. Most eat algae while some eat
mosquito larva and other small aquatic organisms.
Boatman are strong swimmers as adults. So technically not
planktonic. We include them in the meroplankton section for
Family Gerridae (Water Strider)
The adult lives
around water. Belly is covered with hair. Short front legs
are used to catch prey while back legs are used to steer and
move. They are predators and scavengers. They eat other
small insects that fall in water or eat larvae. Striders stay
on top of water because of surface tension.
Family Sialidae (alderflies)
10-70 mm with two pairs of wings held over their body at
rest. Head has a long slender antennae and biting mouth
parts. The larva has a single tail filament with distinct
hairs. Segments of abdomen have 6 to 8 filaments on each
side. Not tolerant to pollution.
Argia agrioides (California dancer)
Nymphs are elongated with three
paddle-like tails. Two large eyes on top of head. Nymphs are
carnivorous, feeding on other aquatic invertebrates.