Characteristics and Taxonomic Sources
381
7.a. Three thoracic dorsal plates; bristly body . . . . . . . . . . . . . . . Hydropsyehe
7.b. One thoracic dorsal plate; body smooth . . . . . . . . . . . . . . . . . . . . . . 8
8.a. Obvious dark plate on top of last full adominal segment. . . . . . . . . .
Agapetus (and legs reduced)
Rhyaeophila (long anal legs)
8.b. No such plate. . . . . . . . ..
. Philopotamidae and Psychomyiidae
Key VIII: Key to the orders of Copepods
l.a. Metasome (cephalothorox) and abdomen (urosome) distinctly different in width
and separated by a distinct movable articulation .
. . . . . . . . . . . . . 2
1.b. Metasome not distinctly narrower than abdomen . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Order Harpacticoida
common genus Canthoeamptus
2.a. Antennae long, usually as long or longer than body, and composed offrom
18 to 25 segments; biramous. . . . . . . . . . . . . . . . Order Calanoida
common genera Epischura, Diaptomus, and Limnoealanus
2.b. Antennae short, usually shorter than the cephalothorax, and composed of
from 6 to 17 segments; uniramous . . . . . . . . . . . . Order Cyc1opoida
.
common genus Cyclops
LIFE HISTORY OF COPEPODS*
Because of the unusual life history and morphological differentiation among common
planktonic copepods, a brief discussion of their development may be helpful.
Freshwater copepods are bisexual. The male attaches a spermatophore to the female.
Eggs are either laid into a membranous case and carried until hatching or are cast into
the water. The larva that hatches from the egg, known as a Stage I nauplius, has only the
three anterior pairs of appendages. There are 11 subsequent moults, each with an
increase in size and development of appendages. Thus there are 12 distinct free-swimming stages after the egg, the last of which is the mature adult (Figs. A.4.1 and A.4.2).
The first six stages are known as nauplii and the last six as copepodids. When the
Stage VI nauplius moults, the changes are greater than the previous ones, and the
shape is considerably altered by the elongation of the anterior part and by the
development of the abdomen. In some copepods, the first nauplius stage may occur
while still in the egg, so that the animal that comes from the egg is actually N-II. In a few
cases, the adult may moult.
The course of development varies somewhat among the groups of copepods; even
within the Calanoids, details differ among genera. The following table shows, in
general, the appendages possessed by each stage of Diaptomus and gives a qualitative
indication of the degree of development. Although none of the appendages is fully
developed until C-VI, the anterior appendages are essentially complete before this
* Based in part on notes of G.W. Comita (Edmondson and Winberg, 1971).
381
7.a. Three thoracic dorsal plates; bristly body . . . . . . . . . . . . . . . Hydropsyehe
7.b. One thoracic dorsal plate; body smooth . . . . . . . . . . . . . . . . . . . . . . 8
8.a. Obvious dark plate on top of last full adominal segment. . . . . . . . . .
Agapetus (and legs reduced)
Rhyaeophila (long anal legs)
8.b. No such plate. . . . . . . . ..
. Philopotamidae and Psychomyiidae
Key VIII: Key to the orders of Copepods
l.a. Metasome (cephalothorox) and abdomen (urosome) distinctly different in width
and separated by a distinct movable articulation .
. . . . . . . . . . . . . 2
1.b. Metasome not distinctly narrower than abdomen . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Order Harpacticoida
common genus Canthoeamptus
2.a. Antennae long, usually as long or longer than body, and composed offrom
18 to 25 segments; biramous. . . . . . . . . . . . . . . . Order Calanoida
common genera Epischura, Diaptomus, and Limnoealanus
2.b. Antennae short, usually shorter than the cephalothorax, and composed of
from 6 to 17 segments; uniramous . . . . . . . . . . . . Order Cyc1opoida
.
common genus Cyclops
LIFE HISTORY OF COPEPODS*
Because of the unusual life history and morphological differentiation among common
planktonic copepods, a brief discussion of their development may be helpful.
Freshwater copepods are bisexual. The male attaches a spermatophore to the female.
Eggs are either laid into a membranous case and carried until hatching or are cast into
the water. The larva that hatches from the egg, known as a Stage I nauplius, has only the
three anterior pairs of appendages. There are 11 subsequent moults, each with an
increase in size and development of appendages. Thus there are 12 distinct free-swimming stages after the egg, the last of which is the mature adult (Figs. A.4.1 and A.4.2).
The first six stages are known as nauplii and the last six as copepodids. When the
Stage VI nauplius moults, the changes are greater than the previous ones, and the
shape is considerably altered by the elongation of the anterior part and by the
development of the abdomen. In some copepods, the first nauplius stage may occur
while still in the egg, so that the animal that comes from the egg is actually N-II. In a few
cases, the adult may moult.
The course of development varies somewhat among the groups of copepods; even
within the Calanoids, details differ among genera. The following table shows, in
general, the appendages possessed by each stage of Diaptomus and gives a qualitative
indication of the degree of development. Although none of the appendages is fully
developed until C-VI, the anterior appendages are essentially complete before this
* Based in part on notes of G.W. Comita (Edmondson and Winberg, 1971).