KNIGHT: LARVAL DEVELOPMENT OF EUPHAUSIA EXIMIA 



Brinton (1975). Among species which have two 

 lateral denticles on the carapace (group lA), E. 

 eximia is most closely related ioE. krohnii and E. 

 americana by the conspicuous pectinate dorsal 

 lappet on the first segment of the antennular 

 peduncle. The larvae oi E. krohnii have been de- 

 scribed from the North Atlantic and Mediterra- 

 nean by Frost (1934) and Casanova (1974) and, 

 while it is not possible to compare the species in 

 detail, the development oiE. eximia appears to be 

 very similar to that of the North Atlantic popula- 

 tion of E. krohnii. The timing of acquisition of 

 pigmentation in abdominal photophores may dif- 

 fer; Frost notes that all are pigmented in furcilia 

 ni but this condition was not normally seen in E. 

 eximia until furcilia V. Development of the dis- 

 tinctive antennular lappet appears to begin at 

 about the same stage (in furcilia V of £. eximia at 

 4.5-5.7 mm TL and in 5.9-6.1 mm larvae of North 

 Atlantic E. krohnii) and with variable form. Ac- 

 cording to Soulier (1963) the lappet of Mediterra- 

 nean E. krohnii develops in a fixed pattern, and 

 >2 spines are not seen until 10 mm TL. There was 

 greater variability in number of telson spines in 

 furcilia IV of £. eximia, and one more stage in the 

 furcilia phase, than noted by Casanova ( 1974) in 

 E. krohnii from the Mediterranean. Larvae of the 

 California Current population of £■. eximia were 

 intermediate in size between the large North At- 

 lantic larvae of E. krohnii and the smaller 

 Mediterranean population, while larvae of E. 

 eximia from the South Equatorial Current tended 

 to be similar in overall length to Mediterranean E. 

 krohnii. 



Larval forms appear to be very similar within 

 Euphausia species group lA which, besides E. 

 eximia, E. krohnii, and E. americana, includes E. 

 recurva, E. mutica, E. brevis, and E. diomedeae. 

 Their larvae share the following characters: 

 spines on anterior margin of carapace in 

 metanauplius, calyptopis phase, and early furcilia 

 stages; a posterodorsal spine on carapace in calyp- 

 topis I-III and furcilia I; telson with middle pos- 



terolateral spine relatively long until midfurcilia 

 phase; and a fixed pattern of pleopod development 

 which progresses from 1 pair nonsetose to 1 pair 

 setose plus 4 pair nonsetose, and finally to 5 pairs 

 setose pleopods. Talbot (1974) noted variation in 

 number of telson spines developing beneath the 

 cuticle of furcilia III in E. recurva- mutica of the 

 Agulhas Current, and Casanova (1974) described 

 timing of events among larvae ofE. brevis (e.g., 

 modification of antenna and mandible, and devel- 

 opment of legs), which is similar to the pattern 

 discerned inE. eximia. 



Developmental events may vary considerably, 

 however, between the species groups of 

 Euphausia. Euphausia eximia (group lA) and E. 

 gibboides (group IB) differ in several details, some 

 of which are listed in Table 13, although they 

 share all the general features of group lA larvae 

 except pattern of pleopod development. Larvae of 

 E. gibboides are larger than those ofE. eximia, on 

 the average, in the metanauplius and calyptopis 

 phases but they are similar in the furcilia phase 

 due to a slightly higher rate of growth per stage in 

 furciliae of E. eximia. The telson is wider in E. 

 gibboides from calyptopis I through furcilia I, and 

 the carapace is wider from calyptopis I-III, than in 

 E. eximia, with no overlap in range of measure- 

 ments. 



The morphological differences observed within 



E. eximia, between larvae from the California 

 Current terminus and the South Equatorial-Peru 

 Current populations, appear related to the geo- 

 graphical separation of reproductive centers de- 

 scribed by Brinton (1979) in his study of the dis- 

 tributional adaptations of euphausiids to the 

 oxygen-deficient eastern tropical Pacific. He found 

 that E. eximia achieved the highest densities 

 (>500 beneath 1 m^) in the South Equatorial Cur- 

 rent and across the California Current-eastern 

 tropical Pacific transition off Baja California, the 

 productive zones marginal to the low oxygen wat- 

 ers. The species occurred consistently across a 

 transect of the eastern tropical Pacific but only 



Table 13. — Some differences in leirval development of Euphausia eximia and E. gibboides. 



Feature 



£ eximia 



E. gibboides 



Pleopods: pattern of development from furcilia I (' = nonsetose. " = setose) 

 Telson: dominant pattern of terminal spine reduction from furcilia III to VI 

 innermost posterolateral spine, inner margin 



number of lateral spines 

 Carapace: stage when anterior median spine develops 

 Antennule: stage when dorsal lappet develops 

 Antenna: stage when modified to juvenile form 

 Mandible: stage when modified to juvenile form 

 lateral knob in calyptopis phase 



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