COMPARATIVE-STUDY OF SYMPATRIC POPULATIONS OF 2 HYPERIID AMPHIPODS, PRIMNO-JOHNSONI AND PRIMNO-EVANSI, FROM THE EASTERN NORTH-ATLANTIC OCEAN

Hyperiid amphipods are common components of oceanic plankton communiti es, and yet information is lacking on the ecology and biology of the m ajority of species. This study compares sympatric populations of two p hrosinids, Primno johnsoni Bowman and P. evansi Sheader, using materia l collected on R.R.S. ''Discovery'' Cruises 121 and 140 in the eastern North Atlantic Ocean. The two species were essentially very similar i n terms of their vertical distribution and diurnal migration, and in t he timing and sequence of reproductive events (onset of maturity, ovar y/oocyte development, marsupial development). Mature female and male s ize ranges were comparable for the two species, males maturing at a sm all body size, with little subsequent growth, and females producing a succession of 5 (P. evansi) or 6 (P. johnsoni) broods maximum. The spe cies differed significantly in egg size and in total egg output. These dissimilarities were accentuated by differences in the patterns of fe male mortality in the two species, such that actual egg output per fem ale was 1.8 times greater (2.4 times in terms of total egg volume) in P. johnsoni than in P. evansi. These differences were reflected in the relative size of populations in the field. Although both species are capable of consuming a wide range of planktonic prey, it is assumed th at gelatinous species are important, especially as hosts for the paras itoid juvenile stages, and it is suggested that host-specificity might act to separate the niches.