MAY: EFFECTS OF DELAYED INITIAL FEEDING 



60° C before weighing. The mean weight per 

 nauplius was 1.64 jj-g, almost identical to Paf- 

 fenhofer's (1967) vakie of 1.65 fig per newly 

 hatched Artemia nauplius. Where the larvae 

 had yolk sacs at the beginning of the feeding 

 period, the mean weight of the yolk masses dis- 

 sected off the 10 larvae sampled at the start 

 of the experiment (the dissection technique is 

 described below) was added to the weight of 

 the nauplii consumed to yield the total dry weight 

 of the consumed material. 



The growth and food consumption of indi- 

 vidual larvae during the feeding period were 

 thus known and allowed calculation of the ef- 

 ficiency of food conversion: 



percent conversion efficiency = dry weight goined ^ ,og 

 dry werght consumed 



INCIDENCE OF FEEDING 



The percentage of larvae which fed after 

 progressively longer times without food, termed 

 here the incidence of feeding, was determined 

 in separate experiments in 10-liter containers. 

 Approximately 25 larvae were transferred from 

 one of the "unfed" supply containers to a 10- 

 liter container with filtered seawater late in the 

 afternoon preceding the experiment, and on the 

 following morning large numbers of Artemia 

 nauplii were introduced into the container. One 

 hour later, the anesthetic MS-222 (tricaine meth- 

 anesulfonate) was added to the container to a 

 concentration of 132 mg/liter, and the anesthe- 

 tized larvae were examined under a dissecting 

 microscope for the presence of an orange-col- 

 ored gut indicative of feeding on Artemia naup- 

 lii. Simultaneously, the incidence of feeding 

 among larvae from one of the "fed" supply con- 

 tainers was determined in the same way, to 

 serve as a control with which hitherto unfed 

 larvae could be compared. Experiments of this 

 sort were conducted on the same days on which 

 food intake and conversion experiments were 

 started, beginning with day 4. Owing to mor- 

 tality from starvation, the numbers of larvae 

 available in the "unfed" containers dwindled so 

 that only 13 larvae were available for the feeding 



Cahn Instrument Company, Paramount, Calif. 



incidence experiment on day 13 and 4 on day 16. 

 By day 16, body pigmentation had developed in 

 larvae from the "fed" containers to such an ex- 

 tent that feeding incidence could not be assessed 

 by examining the coloration of the gut, and no 

 value was obtained for previously fed larvae 

 on this day. 



Anesthesis stimulated peristalsis in grunion 

 larvae, as Blaxter (1965) observed in larval her- 

 ring, but the procedure in the present experi- 

 ment was rapid enough that at most only the 

 contents of the rectum were being extruded dur- 

 ing examination and all larvae which had in fact 

 fed were recorded as such. It should be pointed 

 out that, unlike the straight, tubelike gut of 

 clupeid larvae, the gut of larval grunion is al- 

 ready differentiated at hatching into three more 

 or less distinct portions, the last of which, the 

 rectum, is separated from the rest of the gut 

 by an "ileo rectal valve" (Al-Hussaini, 1947) 

 which inhibits rapid defecation of material not 

 in the rectum. 



MORTALITY 



Dead larvae were removed from the contain- 

 ers each morning by pipette. A larva was con- 

 sidered dead when its brain had become opaque 

 and it did not respond to water current or to 

 tactile stimulation. Dead larvae were routinely 

 examined with a dissecting microscope. 



SAMPLING PROCEDURE AND ANALYSIS 

 OF LARVAE 



Larvae were collected by pipette, and their 

 lengths measured with an ocular micrometer 

 from snout to tip of notochord, or, after upward 

 flexion of the tip of the notochord had taken 

 place, to the posterior edge of the hypural ele- 

 ments (standard length). Only free-swimming 

 larvae were sampled, although in "unfed" con- 

 tainers these became increasingly rare toward 

 the end of the experiment. Sampled larvae were 

 rinsed quickly in distilled water and placed on 

 glass microscope slides. Since larvae which 

 were sampled on days 1 and 4 still possessed 

 yolk sacs, they were preserved in 3% 



415 



