FISHERY BULLETIN: VOL. 78, NO. 2 



sparsely (<4 under 1 m^) from lat. 11° to 20° N 

 where the surface temperature exceeded 26° C and 

 lowest middepth O2 values were <0.05 ml/1 

 through a stratum of 600 m, and where water of 

 westerly origin entered with the Equatorial Coun- 

 tercurrent. Larvae were not observed between lat. 

 2° and 20° N. 



The significant difference in frequency of num- 

 bers of lateral telson spines on late furciliae of the 

 California Current and the South Equatorial and 

 Peru Currents may be evidence of the develop- 

 ment of reproductive isolation between the two 

 populations. The larval evidence was corroborated 

 by a preliminary survey of adult E. eximia which 

 showed a significant difference between popula- 

 tions in the armature of both telson and antennu- 

 lar peduncle. A more thorough examination of 

 adult morphology is necessary, however, for an 

 evaluation of the divergence within the species. 

 The distribution of northern and southern forms 

 observed (Figure 1) suggests that juveniles and 

 adults of E. eximia are carried from the species' 

 reproductive center off Baja California into the 

 oxygen-deficient warm waters of the eastern trop- 

 ical Pacific which may form an effective barrier 

 between the reproductive areas of the two popula- 

 tions. 



Variation in size of larvae at the same stage of 

 development (Figure 11) between areas sampled 

 within each population may be related to the 

 amount of food available among other factors. Le 

 Roux (1974), investigating the effects of diet and 

 temperature on the larval development of 

 Meganyctiphanes noruegica, demonstrated that 

 rhythm of molt, grov^h, and morphogenesis were 

 influenced by quality and quantity of food. With 

 excess food, an elevation in temperature caused an 

 acceleration in rate of molt but not precocious dif- 

 ferentiation and reduction of the number of larval 

 stages; the larvae were found to be a little smaller 

 in a given stage at the higher temperature due to a 

 decrease in growth per molt. 



The relationship of surface temperature to size 

 among E. eximia larvae studied was not consis- 

 tent. Relatively smaller larvae were found at the 

 higher temperature within the South Equa- 

 torial-Peru Current population (22° and 16° 

 C at Stations 21 and 1520/1604) but in cooler wa- 

 ters of the California Current (18° and 24° C at 

 Stations 6 and 10). There was a direct correlation 

 of size with abundance of food, however, among 

 California Current larvae; the volume of zoo- 

 plankton biomass was very low at Station 6 but 



relatively high at Station 10 (Brinton 1979) 

 reflecting displacement upward of recently up- 

 welled waters with high concentrations of nutrients 

 and probably, with relatively green waters, abun- 

 dant food for larval forms. The larger size of larvae 

 from Station 10 may also be related to their posi- 

 tion in the water column; numbers of calyptopes 

 and early furcilia at Station 6 sank below the 

 surface stratum at night while those at Station 10 

 remained almost entirely day and night in the 

 food-rich surface layer. Data on biomass and on 

 larval vertical distribution were not available for 

 Stations in the Peru Current for comparison with 

 those in the South Equatorial Current. 



Most species of euphausiids studied show indi- 

 cations of some downward daytime vertical migra- 

 tion from calyptopis II (Mauchline and Fisher 

 1969) but at Stations 6 and 21 (Tables 9, 11) the 

 position of E. eximia calyptopes in day and night 

 samples appeared to indicate a reverse pattern of 

 movement. After a presumed developmental as- 

 cent from the depths at which nauplii hatched 

 from sinking eggs, the majority of calyptopis I 

 were found in the surface layer in the daytime and 

 in deeper strata at night. The pattern continued 

 through furcilia III at Station 6 and through 

 calyptopis III, to some extent, at Station 21. As 

 noted above, most larvae at Station 10 were found 

 in the surface stratum in both day and night sam- 

 ples through furcilia II; the lack of nighttime sink- 

 ing in early stages may be related to the shoaling 

 of low oxygen water and upwelling conditions ob- 

 served in the area (Brinton 1979). 



The position of calyptopis I in the day-night 

 series at Stations 6 and 2 1 suggests that the larvae 

 were drawn to the surface layer by positive photo- 

 taxis. Sulkin (1973), working with two species of 

 xanthid brachyurans, showed that, in the absence 

 of light, the distribution of larvae varied with on- 

 togeny; the four zoeal and one megalopa stage 

 assumed a differential vertical distribution due to 

 forces of gravity and hydrostatic pressure as well 

 as different sinking rates, with early stages near 

 the surface. In assessing the influence of light on 

 depth regulation in the same species, Sulkin 

 ( 1975) suggested that the observed positive photo- 

 taxis superimposed a diurnal vertical migration 

 on the basic pattern of differential ontogenetic 

 vertical distribution. Larvae of E. eximia appear 

 to show a similar response during the calyptopis 

 phase with modification of their behavior during 

 ontogeny as furcilia develop a pattern of vertical 

 migration similar to that of the adult. 



334 



