LARVAL DEVELOPMENT OF EUPHAUSIA EXIMIA (CRUSTACEA: 



EUPHAUSIACEA) WITH NOTES ON ITS VERTICAL DISTRIBUTION 



AND MORPHOLOGICAL DIVERGENCE BETWEEN POPULATIONS 



Margaret D. Knight^ 



ABSTRACT 



Larval development of Euphausia eximia includes the following stages: nauplius I-II, metanauplius, 

 calyptopis I-III, and furcilia I- VI. The larvae are similar to those of congener £. gibboides but differ in 

 both morphological detail and timing of developmental events. 



A comparison of larvae of £. eximia from across the species' range showed significant differences in 

 morphology between forms from the California Current terminus off Baja California and from the 

 South Equatorial and Peru Currents. This variation may be evidence of genetic divergence between 

 populations and perhaps indicates that the oxygen-deficient warm waters of the eastern tropical Pacific 

 form an effective barrier between reproductive centers of the species. Significant differences in mor- 

 phology were found as well during a preliminary survey of adults; the southern limit of the northern 

 form of E. eximia was about latitude 2° north. 



The vertical distribution of larval stages in day and night samples from two locations off Baja 

 California and one in the South Equatorial Current showed development of diurnal vertical migration 

 in the second half of the furcilia phase after acquisition of the full complement of setose abdominal 

 pleopods. A "reverse" migration pattern was seen among calyptopes at two stations with the majority of 

 larvae occurring in the surface stratum during the day and below the surface layer at night; larvae at 

 the third station were found, both day and night, in the surface stratum until the onset of vertical 

 migration. Variation in growth rate between areas within the range of each population may be 

 correlated with relative abundance of food and duration of stay in food-rich surface waters. 



Euphausia eximia Hansen is endemic to the east- 

 ern tropical Pacific, ranging from lat. 32°-34° N to 

 30° S and with areas of relative abundance in 

 waters of the California Current terminus off Baja 

 California and the Gulf of California, in the South 

 Equatorial Current, and in the Peru Current 

 (Brinton 1962; Antezana-Jerez 1978). In a recent 

 study of the horizontal and vertical distribution of 

 euphausiids along a transect from ca. lat. 23° N, 

 long. 115° W to lat. 3° S, long. 88° W, Brinton 

 (1979) observed that E. eximia occurred sparsely 

 between lat. 11° and 20° N but achieved high den- 

 sities in the productive zones marginal to the 

 oxygen-deficient portion of the eastern tropical 

 Pacific. He noted that "Reproduction, as deter- 

 mined by presence of larvae, was not observed 

 between 2° and 20° N; occurrences of juvenile and 

 adult £. eximia in this zone, therefore, appear due 

 to meridional advection from the northern (21° to 

 25° N) and equatorial population centers." This 

 agreed with earlier observations o{ E. eximia in 

 these areas (Brinton 1962). 



'Scripps Institution of Oceanography, University of Califor- 

 nia, La Jolla, CA 92093. 



Within the genus Euphausia, E. eximia is most 

 closely related in adult morphology to E. 

 americana, an Atlantic species, and to E. krohnii, 

 found in both the Atlantic and Mediterranean 

 (Mauchline and Fisher 1969). The larvae of E. 

 krohnii have been described (Frost 1934; 

 Casanova 1974) but those of E. americana have 

 not yet been identified. The literature on larval 

 development within the Euphausiacea has been 

 reviewed by Gopalakrishnan (1973). 



The purpose of this paper is to describe the lar- 

 val development of £■. eximia in the reproductive 

 area of the California Current terminus popula- 

 tion, to note the differences observed in morphol- 

 ogy of larvae from the South Equatorial-Peru Cur- 

 rent population and apparent variation within 

 each population in rate of growth, and to provide 

 information on the vertical distribution of larval 

 stages. 



METHODS 



Larvae ofE. eximia were sorted from preserved 

 samples of plankton taken in the eastern Pacific 

 (Figure 1) during Scripps Institution of Oceanog- 



Manuscript accepted December 1979. 

 FISHERY BULLETIN: VOL. 78, NO. 2, 1980. 



313 



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