TIME OF SPAT/i^v'ING 



As there are no actual observations on the time of day pilchards 

 spawn in nature, this could be determined only by indirect evidence. 

 The time of day newly-spawned eggs appear in the plankton should roughly 

 establish the time of spawning. Such newly-spaivned eggs vrould be pre- 

 cleavage eggs that had not had time to begin cell division, i.e., stage 

 I (cf. appendix). The times when pre-cleavage eggs occurred in plankton 

 samples is summarized in tables 1 and 2. According to both tables about 

 87 percent of pre-Cleavage eggs vrere taken during the four-hour period, 

 8:00 p.m. to midnight. At that time there were practically no eggs in 

 the early stages of cleavage. After midnight very many pilchard eggs 

 undergoing cleavage were in the samples taken, while only an occasional 

 egg v>ras of precleavage stage. 



Eggs of that age taken during the four-hour period before midnight 

 are considerably smaller in diameter than are those with some embryonic 

 development. 4Since the yolks are of similar size in both groups, the 

 difference lies in the width of the perivitelline space^ which is nearly 

 wanting in pre-cleavage eggs taken during this period; such eggs averaged 

 only 1.20 mm. in diameter (range 1.02 - I.38 mm.). This is slightly 

 smaller than the average size of 1.21; mm. that F. N. Clark (193U, p. 3) 

 reported for mature pilchard . eggs from ripe females. The difference in 

 size between eggs newly-spawned and those undergoing development (average 

 size l-,70 mm.) could result from the latter having absorbed water, swell- 

 ing the perivitelline space up to the size characteristic of planktonic 

 pilchard eggs. 



A few pre-cleavage eggs were taken in the saiiqples after midnight. 

 The majority of these were much larger than pre-cleavage eggs taken before 

 midnight (cf. tables 1 and 2), in fact, vfere about the same size as pil- 

 chard eggs in the same samples undergoing cleavage. Hence, they must have 

 been in the water for some time, and, consequently, must either have started 

 development very slowly or had remained unfertilized. Hovrever, a few 

 pre-cleavage eggs taken as late as mid-day have a very narroiv perivitel- 

 line space. Either these had been spawned at the normal time but did not 

 attain the typical planktonic egg size, or they resulted from isolated 

 instances of deposition during the day. In numbers such eggs were neg- 

 ligibly fev^. 



Further support for the thesis that spavming is rhythmical, occurring 

 during a brief ,*relatively fixed time each day and not sporadically, is 

 furnished- by the time distributions of eggs undergoing cleavage. Thus, 

 eggs in early stages of cleavage v/ore taken immediately after midnight 

 and not during any other period of the day, while eggs in the last stages 

 of cleavage were collected in samples around noon. This could occur only 

 if spawning took place during a brief period each day. 



The following account of the length of time of development assumes 

 that spawning is confined to the period, 8:00 p.m. to midnight, by taking 

 the midpoint of this period, 10:00 p.m., as the hour when development is 

 initiated. True, using the time of fertilization rather than of spavvTiing 

 vTOuld be more precise, but the two processes are believed to occur close 

 enough together to justify neglecting the time difference betvieen them. 



135 



