40 PROCEEDINGS OF THE NATIONAL MUSEUM VOL. 113 
exception of P. paulus, in the laboratory. Pseudocyclops spends 
most of its time swimming among the algal filaments attached to the 
Thalassia leaves with a smooth leisurely motion that gives it the 
appearance of gliding slowly through the water. The first antennae 
are held closely against the body, directed upward at an angle of 
about 45°, and the swimming legs are bent forward and held motion- 
less against the ventral surface. The urosome is usually bent slightly 
ventrally, but turns to one side when the animal changes direction. 
The gliding movement is brought about by rapid vibratory move- 
ments of the second antennae and mandibular palps, especially the 
former. We were unable to observe the first maxillae im living speci- 
mens, but the second maxillae do not vibrate and are held in position 
to filter food particles brought to it. 
In addition to the leisurely gliding motion, Pseudocyclops can 
also make very quick movements of about 2 to 4 times the body 
length, with a flip of the swimming legs. These movements, made 
to avoid a needle or forceps, are always in a forward direction. Longer 
avoidance movements probably involve several consecutive flips 
of the swimming legs, but we were unable to follow the motion of the 
legs during these movements. 
Pseudocyclops occasionally alights on algal filaments, but does 
not walk along their surfaces as do the harpacticoid copepods. The 
large outer spines on the exopods of the swimming legs must enable 
Pseudocyclops to hold tightly to the substrate; it is very difficult 
to pick up living specimens with an eyedropper. 
The observations of Giesbrecht (1893) on the swimming of Pseu- 
docyclops umbraticus and those reported by Noodt (1958) for P. 
gohari, although less detailed, are in essential agreement with ours. 
Our observations make it possible to characterize the ecological 
niche of Pseudocyclops. It lives in the interstices of the filamentous 
algae attached to the lower parts of the Thalassia leaves, where it 
feeds by filtering the water through which it swims. The small 
size, compact body, and short antennae enable it to maneuver in 
restricted spaces too small for other calanoid copepods. It competes 
neither with the planktonic calanoids in the open water above the 
Thalassia leaves nor with the abundant harpacticoids, which feed by 
browzing on the algal surfaces rather than by filtering the water. 
The presence of 4 congeneric species in the same habitat is of 
considerable interest and raises the question of competition among 
them. Hutchinson (1951) has pointed out that when 2 freshwater 
calanoids coexist in the same habitat, they usually differ im size. 
He suggested that differences in the type of food utilized should 
accompany these size differences and reduce or eliminate competition 
