CLUTTER and THEILACKER: PELAGIC MYSID SHRIMP 



pace and body wall, it is possible to observe the 

 late stages of oogenesis in live animals without 

 dissecting them. The ovary (cf. Nair, 1939, for 

 description) is situated in the interspace be- 

 tween the alimentary canal and the pericardial 

 floor. Its most obvious feature is the pair of 

 larger tubes that lay side by side. It is in these 

 tubes that the eggs to be extruded into the brood 

 pouch are invested with yolk. The process of 

 yolk formation takes about a week in Metamysi- 

 dopsis and is completed just before the female 

 molts and copulates. By observing the ova in 

 these tubes it is possible to estimate the size or 

 age at first reproduction in maturing females, 

 and to count the number of eggs that will be 

 spawned by reproducing females of all ages. 



Copulation occurs at night within 2 to 3 min 

 after the mature female molts, during which 

 time sperm are passed into the empty brood 

 pouch by the attending adult male. The eggs 

 are subsequently extruded into the brood pouch 

 where they are fertilized. The eggs hatch from 

 the vitelline membrane after 2 to 3 days. Ac- 

 cording to Manton (1928) and Nair (1939) a 

 larval ecdysis occurs in the brood pouch shortly 

 before the larvae are liberated. These late stage 

 larvae have movable appendages and pigmented 

 eyes that show through the transparent ooste- 

 gites of the brooding female. The small quantity 

 of yolk that is present after the larval ecdysis 

 is absorbed, or nearly so, prior to liberation 

 from the brood pouch. 



After liberation the larvae tend to sink, then, 

 according to Nair (1939), they undergo a sec- 

 ond larval ecdysis after which the statocysts 

 appear and they are capable of swimming. The 

 mysids assume this highly mobile juvenile form 

 within a few minutes after liberation. Although 

 we did not attempt to distinguish sexes of larvae 

 and juveniles, the observations of Nair (1939) 

 indicate that dimorphism is exhibited by the ant- 

 ennules and abdominal appendages even though 

 neither the brood pouch nor the penis is de- 

 veloped. 



Incubation time was determined in the lab- 

 oratory. Adult females and adult males were 

 observed in an aquarium during molting and 

 copulation. Ten females were caught after be- 

 ing observed in copulo and were placed in sep- 



arate containers of sea water at the temperature 

 of their natural environment at that time (17°- 

 19° C). Five of them were removed, at var- 

 ious times, to determine the stages of develop- 

 ment of the young. The remaining five all re- 

 leased their young as juveniles on the tenth day 

 after fertilization. 



In addition, a large number of nonpregnant 

 adult females were kept in separate containers 

 for various periods up to 157 days. The range 

 of intermolt periods in 218 observations was 

 5 to 13 days; the median and modal values were 

 both 10 days. There was no obvious temper- 

 ature effect. The adult females molt just before 

 fertilization and just after liberation of the 

 young; therefore, the average incubation time 

 was taken to be 10 days. This is intermediate 

 between incubation times given for Mysidae that 

 live and reproduce at higher and lower temper- 

 atures. Nair (1939) determined the incubation 

 time of Mesopodopsis orientaUs to be 4 days at 

 25° to 29° C. Mauchline (1967) reports a min- 

 imum incubation time of 3 weeks for Schisto- 

 mysis spiritus at 12.5° C. 



MOLTING 



To avoid handling and possible injury of the 

 experimental animals, the growth rates were 

 determined by measuring molts. The molts suf- 

 fered no appreciable decomposition because they 

 were collected on the day following ecdysis. The 

 morphological development of the animals was 

 usually discernable from their molts. But the 

 molts are fragile, split just back of the cara- 

 pace where the animals emerge, and easily 

 stretched out of shape. Therefore, to measure 

 growth it was necessary to measure a part of 

 the molt that always retained its form and bore 

 a consistent relationship to the body length. 



Uropod-Body Length Relationship 



The exopod of the uropod (tail fan) was used 

 to estimate the body length of each animal for 

 its previous intermolt period. The uropods were 

 measured from the base (end of last abdominal 

 segment) to the tip, not including spines, which 

 were sometimes broken, with an ocular micro- 



95 



