Regional Relationships between Development and Growth in Larvae of Antarctic Krill, Euphausia Superba, From Field Samples

Abstract

ABSTRACT Larvae of Euphausia superba were sampled during January-March 1981 in three Antarctic areas: the Scotia Sea (twice), waters near Elephant Island in the South Shetlands (twice), and Bransfield Strait. January-February larvae in the Scotia Sea averaged 250/m3 and 99% were calyptopes, dominated by stage 1. The oldest stages, Furcilia 3 and 4, had grown at slow rates, determined by comparisons of body lengths with larvae from other areas. Progressively younger stages reflected better growth, associated with a contemporary highly variable distribution of chlorophyll. Most rapid growth was inferred in a low density of larvae south of Elephant Island in March, where chlorophyll was at highest observed levels. Intermediate growth had occurred in larvae from north and east of Elephant Island, and Bransfield Strait, where chlorophyll concentrations were intermediate. During March, larvae averaged 3-6 stages older than during January-February. Abundances were 100–500/m3 along 250 km in the same area sampled in January. To the north and east of Elephant Island, principal modes in larval abundances were at younger stages (by 2-4 stages, respectively, = about 16-32 days of growth) than near the South Orkneys. An intermediate region showed an intermediate mode, implying intermediate timing of peak recruitment. Inferred development times of about 8 days per calyptopis and furcilia stage are shorter than previous estimates. In Scotia Sea areas having the highest densities of larvae in March, there were numerous developmental forms (variants) of furcilia stages 1 and 2, without clearly dominant forms. This evidence of indirect pathways during larval development and of slow growth was borne out by body-length measurements. Chlorophyll was near depletion in these areas of high densities of larvae. Data from earlier years indicate that absence of dominant forms in Furciliae 1 and 2 during March in the southern Scotia Sea may occur regularly, possibly contributing to the substantial range of body size of older year classes. Pathways of larval development observed in March were direct in coastal areas, compared with oceanic, in spite of variable ambient chlorophyll. Extremes in growth rate, population density, and food seem to interrelate, whereas intermediate states do not.